milankovitch fluctuations in sea level and recent trends in sea-level change… · 2013-07-19 ·...

CHAPTER 11

Milankovitch Fluctuations in Sea Level and Recent Trends in Sea-Level Change: Ice may not always be the answer

David K. Jacobs and Dork L. Sahagian

ABSTRACT: Studies of short term sea-level change have emphasized the effects of climate on the volume of water tied up in continental ice. Here we discuss two different time scales of non-ice-related storage of water on the continents and their implication for sea-level change. Human activities generate a flux of water from continental reservoirs, such as aquifers and wetlands, to the sea. Our calculations suggest that this flux is currently in excess of 113 of the sea-level rise rate inferred from tide gauge records. This observation has implications for interpretation of 20th century sea-level rise. Secondly, on time scales of orbital variations, climatically driven changes in non-ice-related continental water storage can produce geologically significant cyclic change in sea-level. This mechanism of sea-level change may have been the dominant source of Milankovitch frequency eustatic fluctuation during periods of Earth history that lacked continental scale ice sheets. In a final comment we consider the impact of fluctuation in lake area on climate models, and on the abundance of modern lake related fauna.

1. Introduction

In the absence of the dramatic changes in ice volume that characterize the Quaternary record, changes in intensity of monsoonally driven precipitation provides the strongest natural driving mechanism for changing water storage on the continents. Lake basin filling chronologies and global climate models indicate that precession of the equinoxes had a controlling influence on the waxing and waning of monsoonal intensity and areal distribution in the Quaternary. We used this Quaternary record of changing monsoonal influence as a proxy to evaluate the possible magnitude of sea-level change caused by fluctuations in monsoonal intensity. In this initial work (Jacobs and Sahagian, 1993) our intent was not to reconstruct events in the Quaternary, but to assess the limits of continental water storage pertaining to

329

B. U. Haq (ed.). Sequence Stratigraphy and Depositional Response to Eustatic. Tectonic and Climatic Forcing. 329-366. © 1995 Kluwer Academic Publishers. Printed in the Netherlands.

330 D.K. Jacobs, D.L. Sahagian

periods of time when the Earth lacked continental ice. Our calculations suggest that 2 to 8 meters of sea-level change could be produced periodically via Milankovitch driven changes in water storage in the absence of changing ice volume. The magnitude of this change appears to be sufficient to explain the meter scale eustatic cycles observed in near-shore carbonate sediments during periods of Earth history lacking continental ice sheets. For example, Triassic lacustrine deposits document monsoonal fluctuation of water storage. These fluctuations in continental water storage correlate with carbonate packages generated by eustatic fluctuation. Spectral analyses indicate a precession dominated Milankovitch signature from lake deposits as well as in littoral carbonates. Thus the Late Triassic provides strong support for this ice-free mechanism of Milankovitch sea-level change.

In this work we revisit the Quaternary proxy and the Late Triassic example, and explore some of the variables critical to this mechanism of documenting Milankovitch frequency sea-level fluctuation, including: the absence of ice, a near shore carbonate record of sea-level fluctuation, tectonic generation of large internally drained basins, and the continental configurations necessary to enhance monsoonal flow. We then suggest that more work can be done demonstrating similar phenomena in other geologic periods. Much of the Mesozoic, especially the Early Cretaceous, as well as the Late Permian and Devonian look promising in terms of evidence for cyclic sedimentation potentially driven by the monsoonal eustatic mechanism. We then discuss how human activities may change the amount of water stored on the continents and thereby influence sea-level. We also briefly discuss the implications of Quaternary fluctuation in lake area for climate models, and for conservation biology. The large scale of change in lake environments may have strongly influenced the evolution and distribution of the modern fauna.

2. Ice

In the Quaternary and late Tertiary changes in ice volume generated sea-level change at Milankovitch frequencies, and during this period ice sheets reaching the sea generated ice-rafted debris resulting in diamictite deposits over very broad areas. Continental glacial deposits or ice-rafted debris provide evidence for continental ice sheets in the Late Ordovician-Early Silurian as well as the Carboniferous. However, no strong evidence of this type is available for the Cambro-Ordovician, much of the Devonian, or the Late Permian through the Early Eocene (Hambrey and Harland, 1981). In addition, uniformitarian arguments based on the temperature requirements of modern taxa provide positive evidence for equable climates at the poles. For example, crocodile fossils are known from Ellesmere island in the Eocene. Such evidence strongly suggests the absence of glacial-scale ice for the entire Mesozoic and early Tertiary. However, during the Cambro-Ordovician, Devonian, Late Triassic and Early Cretaceous, small scale Milankovitch frequency sea-level changes are well documented by nearshore carbonate deposits which show repeated cycles of flooding and subaerial exposure (Fig. 1). These cyclic changes in sea-level have often led researchers to infer fluctuations in ice volume during these time periods. However, if Milankovitch-driven fluctuations in the

Sea-Level Change - No Ice? 331

Schematic of Peritidal Carbonate Cycle Indicating Subaerial Exposure

T

T

Subtidal-Oolites, Peloides, Thrombolites

Flooding surface-Rip up clasts, Lags Subaerial exposure-Desication Cracks, Quartz sands, Regoliths Tidal Flat-Algal Laminates, Stromatolites

Fig. 1. Sedimentary cycles documenting subaerial exposure and subtidal flooding provide the best documentation of sea-level fluctuation. Such cycles are often well preserved on carbonate platforms. In such cycles flooding of the platform is often documented by rip up clasts or other lag deposits, followed by any of a number of subtidal carbonate facies, such as oolites, peloides, ribbon rock or thrombolitic algal heads. These are usually followed by intertidal stromatolitic or algal laminated deposits, and then ideally, by features documenting subaerial exposure, such as mud cracks, regolith, or a quartz sand sheet. Obviously, sea-level also strongly effects sedimentary processes in shallow marine environments that do not experience exposure, and deep water facies may also be effected by sea-level change through changing sediment bypass. However, flooding and subaerial exposure cycles provide the strongest evidence for sea-level fluctuations prior to the Quaternary (after Osleger and Read, 1991: Goldhammer and others, 1990).

monsoon can also produce sea-level changes, the glacial explanation for rapid sea-level fluctuation may not always be appropriate.


3. Quaternary Proxy

Monsoonal l circulation results from summertime insolation over large subtropical and mid-latitude continental areas. During northern hemisphere summer the continental configuration of Eurasia and Africa generates the largest monsoonal effect on Earth today, resulting in the deflection of wind patterns and dramatic summertime precipitation referred to as the Asian monsoon. Solar heating of elevated surfaces, such as Tibet, accentuate the effect, leading to lower pressure and greater convection than would occur otherwise (Kutzbach and others, 1993). The orbit of the Earth is elliptical. When proximity of the Earth and Sun, perihelion, occurs during northern hemisphere summer the strength of the Asian monsoon dramatically increases. Perihelion last coincided with the northern hemisphere summer in the early Holocene, roughly 9,000 years ago. Lake basin filling chronologies and global climate models indicate an early Holocene expansion of monsoonal precipitation over large areas of North Africa and South Asia (Fig. 2). Much greater precipitation occurred in these desert regions at that time than occurs today. Perihelion coincides with the northern hemisphere summer solstice at roughly 21,000 year intervals as a consequence of the precession cycle of the Earth's orbit. Consequently, fluctuations in monsoonal intensity are expected to recur with an approximately 21,000 year repeat time. Thus, evidence from both modeling and the early Holocene geologic record document a mechanism by which periodic expansion of the monsoon could result in fluctuating continental water storage. However, the relevance of this mechanism depends on the magnitude of sea-level change it could generate. In particular, were sea-level changes generated by the mechanism large enough to produce the meter scale eustatic carbonate cycles observed during periods of earth history lacking evidence for continental ice sheets? To explore the potential size of the effect, we assessed the water storage potential associated with the region of increased monsoonal precipitation produced in the early Holocene (Jacobs and Sahagian, 1993).

3.1. EVIDENCE FOR INCREASED EARLY HOLOCENE MONSOONAL PRECIPITATION

The 9,000 year ago COHMAP (1988) model indicates increased monsoonal precipitation over North Africa and South Asia, roughly 1/4 of the Earth's land surface area (Fig. 2). Note that much of this area is internally drained (Fig. 3). Increased precipitation in these regions tends not to flow to the sea, but forms lakes and increases water storage in aquifers. Several kinds of geologic evidence support these model results. Lake level chronologies in closed basins provide the strongest evidence. In a research program given early impetus by the work of Butzer and others (1972) strand line deposits in African lakes have been dated using Carbon 14.

IThe term monsoon comes from an arabic root and was originally associated with seasonal wind shifts on the Indian Ocean. Some authors use the term just to refer to seasonal shifts in wind or only to the seasonal climate of South Asia. We use the "monsoon" in a more general sense refer to convection and precipitation associated with continental heating in the summer. We do not restrict our usage as to time or place, but are most interested in large effects that involve displacment of the Intertropical Convergence Zone (ITCZ) from the equator

Sea-Level Ch ange-No~ ? ceo 333

() .-

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60

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(l. ~

()~ . .. ;-~

~ Fig 3 A 0

.. reas of' ~frica are curren~nte~nal drainage on the mfluence in the e~~ mternally drained. Te::th tod~y. Note that lar .

thot ocou"", ot tlmt;' HoI.",,", ,"d wooid " regmn< f,11 wlthl" ~~ ,nd '"'" of Eo",I, , d Ime m lakes and aq .~ necessarily have st d e area of expanded n North UI ers (after Str ore much of th monsoonal eet and Grove, 1979). e extra precipitation


Using these and other dating techniques lake level chronologies for a large number of lake basins have been developed. Virtually all North and Central African Lakes tabulated by Street and Grove (1979) and Street-Perrott and others (1990) were at highstand about 9,000 years ago. Before 10,000 years ago and since 5,000 years ago fewer than 20% of these lakes were at highstand. Much ancillary evidence supports the increased precipitation and water storage evident from higher lake levels in the early Holocene. Charophyte algae, now only found in Holarctic lakes, document the presence of cool oligotrophic lakes in locations which are now some of the hottest and driest areas on Earth (Kropelein and Soulh~-Marsche, 1991). Buried channels evident from Landsat images demonstrate runoff in currently arid regions of the Sahara (McCauley and others, 1986; Breed and others, 1979). Ancient human activity documents wetter conditions in North Africa and South Asia. Petroglyphs of giraffes and other organisms typical of savannas are found in the center of the Sahara and Stone Age artifacts abound in areas where today even camel caravan cannot travel due to the absence of water (Petit-Maire and others, 1990). Early agrarian civilization flourished in regions of the Indus vaHey that are desert today.

Evidence from the Atlantic Ocean, as well as from the Arabian, Red and Mediterranean Seas, documents increased precipitation over an area extending from North Africa through South Asia. Marine cores containing freshwater algae and forest pollen document wetter conditions in the Niger drainage (Hooghiemstra, 1989). Cores from the eastern margin of the Arabian Sea contain early Holocene spikes of mangrove suggesting more freshwater influx in the coastal regions (Van Campo, 1986). Marine cores from the north-eastern Arabian Sea document greater upwelling in the early Holocene as well as in earlier Milankovitch generated periods of increased northern hemisphere summertime insolation. Such upwelling results from increased strength of monsoonal circulation (Prell and Van Campo, 1986). In the Red Sea corals from Ethiopia and Aqaba show fluorescent banding dating to the early Holocene (Klein and others, 1990). Humic acids derived from the water produce this fluorescence suggesting increased terrestrial runoff. Increased influx from the Nile generated sapropels in the Mediterranean. These organic rich sedimentary layers correlate not only with the early Holocene expansion of the monsoon, but also with model predictions of earlier increases in monsoonal intensity (Rossignol-Strick, 1983). Thus, many terrestrial and marine observation confirm the interpretation of expanded monsoonal precipitation inferred from lake highstand data.

If one restricts the analysis to lake basin chronologies, data from Asia are less uniformly distributed, and less well documented than those in Africa. However, several lakes in the region that have been studied had higher water levels in the early Holocene. Of special interest is Qinghai Lake (Lister and others, 1991) where lake levels rose rapidly from 10,000 years ago and subsequently declined after 6,000 years ago. Qinghai Lake's drainage area at the northeast extremity of the Tibetan Plateau is intimately associated with rivers that flow into Quidam, Tarim and several large Tibetan basins. Consequently, substantial flow to these other basins must also have occurred in the early Holocene. In addition, cores from lake basin sediments in westernmost Tibet also document pluvial conditions in the


TABLE 1 Volumes of some of the larger empty closed basins in the area of expanded monsoonal influence in early Holocene (after Jacobs and Sahagian, 1993). Volumes were calculated using topographic data from the Defense Mapping Agency (ETOP05U) and represent the currently unfilled volume of the basin up to the outflow elevation. East Iran, Afghan-Zabol and one of the basins in Baluchistan proved to be contiguous and are treated together in the analysis.

Basin

1. Tarim

2. Caspian

3. East Iran

4. Afghan-Zabol

5. Baluchistan

6. Chad

7. Urs Nor

8. Quidam

9. Balkash-Zungar

10. Esfahan

Area (I 010

87

232

57*

22

16

30

5.1

Volume

m2) (1013 m3)

39

I3

7*

67

5.2

3.3

1.9

5.8

Sea Level Outflow elevation

(cm) (m)

108 1500 (NE rim)

36 100 (Don River)

19" 900 (NW)*

5.9 16400 (Benue River)

14 1600 (Samaltagatay)Quidam

9 3500 (Lapiquan)

5.3 400 (Caspian)

1.6 1800 (NE to E. Iran)

early Holocene (Gasse and others, 1991; Van Campo and Gasse, 1993). Thus, the available data are consistent with the widespread early Holocene pluvial conditions in South Asia suggested by the COHMAP (1988) climate models (Fig. 2).

3.2. QUANTIFICATION OF LARGE BASIN VOLUMES

To determine how much water could be stored in the region of expanded monsoonal influence 9,000 years ago, we first calculated the volume often of the largest basins in the region. For this analysis we contoured digital topographic data (Table 1), and calculated the volume of each basin from the bottom of the basin, or current lake level if a lake was present, to the overflow elevation. The ten basins considered could hold a volume of water equivalent to 2 meters of sea-level. The Tarim basin alone, if filled, would lower sea-level I meter.

In addition to these large basins, many other basins of moderate size occur throughout the region of the expanded early Holocene monsoon. Graben structures form many large internally drained basins in Tibet. The orogenic belt extending from Anatolia south through the Zagros and Makran into Baluchistan contains many internally drained basins of substantial size, such as Lake Tuz and Lake Van in Turkey and large dry basins further south. Many internally drained tectonic features are associated with the East African Rift, including basins in Kenya, such as lake Natron and Lake Turkana, as well as basins extending north through Ethiopia, ultimately including related structures such as the Dead Sea. Even Lake Tanganyika was as much as 350 m below highstand during late Pleistocene precipitation minima


(Gasse and others, 1989). Many broad shallow basinal features extend across North Africa. These include the Quattara depression, Chott Melhrir in Tunisia, and the Tanzrouft, Taodenni and Quarane regions of Algeria, Mali and Mauritania. These basins approach in size the smaller basins we examined. Thus many of them may accommodate volumes equivalent to a few centimeters of sea-level. The majority of these basins were clearly wetter in the early Holocene when numerous smaller North African lakes were at highstand (for example,Gasse and others, 1987; StreetPerrott and others, 1990).

3.3. IMPORTANCE OF GROUNDWATER

A number of field studies link highstands in medium and small lakes with large aquifer systems. In Lake Natron in the rift valley of Kenya highstand elevation, documented by dating of stromatolites, appears to be controlled by interaction with the groundwater table in the local volcanic terrain (Hillaire-Marcel and Casanova, 1987). At Wadi Howar (Kropelein and others, 1991), Selima oasis (Haynes and others, 1989) and Oyo (Ritchie and others, 1985) in western Sudan, lakes formed in depressions in porous Mesozoic sedimentary rocks during the early Holocene pluvial period suggesting the possibility of large groundwater responses associated with changes in lake level. Presumably this is also true of other basinal oasis systems such as El Kharg, the Fayum and Dakleh of Egypt. Perhaps most intriguing are inter-dune lakes in large dune systems. Clear documentation is available that bodies of water were associated with the Great Western Erg of Algeria (Fontes and others, 1985; Gasse and others, 1987) and the Grand Erg of Bilma (Grunnert and others, 1991) in the early Holocene. Landsat images document lacustrine deposits associated with dune fields throughout North Africa and the Empty Quarter of Arabia (for example, Breed and others, 1979). This evidence is consistent with the interpretation that all these dune fields had higher water tables in the early Holocene. Today, the grass-covered Sand Hills of Nebraska harbor numerous interdunallakes providing an example of a dune field in a hydrated condition.

Lakes are essentially a surface expression of the ground water table, and many of the lakes in the region of increased early Holocene monsoonal precipitation interact with large oasis or dune field aquifer systems. We modeled this complex system of lake basins and groundwater as a continuous layer of uniform porosity across the region of increased monsoonal precipitation in the early Holocene (Jacobs and Sahagian, 1993). The actual water storage reservoir is not uniform because of variation in porosity and thickness of the hydrated layer. Within the area of increased precipitation in the early Holocene there are empty basins which have a porosity of unity; large areas of eolian deposits, such as loess and sand, with porosities in excess of 0.4; Tertiary fluvial and lacustrine deposits, as well as Mesozoic sediments with porosities in excess of 0.3; and small areas of basement rock with porosities of about 0.1. Assuming a conservative average porosity of 0.3 over the area of monsoonal variations, hydration of a layer 65 and 200 meters thick would lower sea-level 2 and 6 meters, respectively. In combination with the result from the calculation of volumes of the ten large basins, these calculations suggest a potential sea-level change of 4 to 8 meters.


3.4. CONTEXT OF THE VOLUME CALCULATIONS

Sea-level reduction approaching the calculated amount may well have occurred in the early Holocene. However, the large sea-level movements associated with changes in Quaternary ice volume overwhelm such small changes. In addition, the presence of continental ice in the Quaternary makes it difficult to isolate the effects of precession-related monsoonal effects from the climatic consequences imposed by the ice sheets. For example, although the internal basins of the Aral and Caspian may have been influenced by the increased monsoonal activity in the early Holocene through the Amu Darya and other drainages, these basins may also have received glacial meltwater through the Volga or Turgai during the late Pleistocene and earliest Holocene. This sort of complexity further increases the difficulty of assessing the influence of Quaternary pluvials on sea-level.

As we have indicated, this exercise is not intended to provide an explanation for actual sea-level change in the Quaternary, rather it is intended to illustrate the potential sea-level change that could be induced by this mechanism during periods of Earth history when continental ice was not a factor. During many periods of Earth history, the potential for periodic water storage via this monsoonal mechanism was likely to have been much greater than it is today. Larger continental areas in the subtropics were typical of the late Paleozoic and Mesozoic. These could have generated more extensive monsoonal circulation, and more broadly ranging fluctuations of monsoonal influence than occurred in the Quaternary. Sedimentary cover with high porosities, such as eolian deposits, as well as regions of basinal topography were also more extensive in the past than they are today. These differences in configuration of continents and water storage may have resulted in greater potential for fluctuating water storage than the Quaternary evidence supports.

4. Orbital Variation, Continental Configuration and the Monsoons: Theoretical Considerations

Monsoons result from solar heating of continental areas. The intensity of the monsoon depends on continental size and arrangement. Here we explore some of the possible interactions between orbital variation and continental configuration that could conceivably generate periodic fluctuations in monsoonal intensity. Berger (1978) extrapolated from recent Solar System dynamics to produce the Milankovitch spectra pertinent to the Quaternary. More recently Berger and others (1989) have examined the question of how these cycles varyed through the Peanerozoic. As one might expect, given the slowing of Earth's rotation documented by increasing day length, these astronomical cycles have also slowed over geologic time. For example, the 41 ka obliquity cycle had approximately 32.9 ka period at around 300 Ma in the Pennsylvanian and the 23 ka precession cycle component had an approximately 20.5 ka periodicity at that time (Berger and others, 1989). These calculations suggest that, with only minor adjustments, Milankovitch periods can applied in a uniformitarian manner to the interpretation of cyclic sedimentary deposits throughout the Phanerozoic.


CONTINENTAL CONFIGURATIONS AND PERIODICITY OF MONSOONAL WATER STORAGE

Continental Area and Water Storage Subtropical One Hemisphere Precession (21,OOOy) Dominant Eccentricity (100,OOOy) Influential

Continental Area and Water Storage Tropical Double Precession Cycles (@10,OOOy) Forced by Perigee at Equinoxes Eccentricity (100,OOOy) Influential

Continental Area and Water Storage Temperate One Hemisphere Precession (21,OOOy) and Obliquity (41,OOOy) Influential

e D ~~~

JJJ

Continental Area and Water Storage Subtropical Both Hemispheres Double Precession Cycles (@10,OOOy) Forced by Perigee at Solstices Eccentricity (100,OOOy) Influential

WATER STORAGE

LAND

SEA

Fig. 4. End member continental configurations expected to respond to particular Milankovitch parameters. Precession driven eustatic cycles are well documented in the Quaternary and Triassic and may have occurred in other times. Monsoonal changes driven by double beats, c, may also have occurred in the Cretaceous. See text for more details.

4.1. PRECESSION, ECCENTRICITY, AND OBLIQUITY AND IDEALIZED CONTINENTS

Prior to examining the Phanerozoic record for sea-level fluctuations generated by the monsoon, we consider Milankovitch-monsoonal responses for several idealized continental configurations (Fig. 4A-4D). The three Milankovitch orbital variables of precession, eccentricity, and obliquity interact with one another. As discussed previously the precession cycle has an impact on the monsoon by modulating the intensity of the seasonal cycle. Given the modem configuaration of the continents, when perihelion coincides with the northern hemisphere summer solstice, greater summertime heating generates stronger monsoonal flow. This precessional response should generally pertain when continents are aggregated in the low and middle latitude of a single hemisphere (Fig. 4A). Solar system dynamics generate a complex set of precessional harmonics. Spectral peaks occur at 23.7, 22.4 and 18.9 ka (Berger, 1978). As a practical matter, these multiple peaks are conceived of, and tend to resolve in the record as, a general 21 ka periodicity.


Although multiple precessional harmonics have been recognized in isotopic data from Pleistocene age marine cores (Hays and others, 1976).

Major periods of the eccentricity determined from celestial mechanics occur at 412.8 ka, 94.9 ka, 128.2 ka and 99.5 ka, as well as additional smaller peaks (Berger, 1978). For practical purposes, peaks in eccentricity are said to occur at about 100 ka and 400 ka. Although eccentricity has only a small impact on total insolation, the 100 ka eccentricity cycle has a dominant effect on changes in ice volume in the Pleistocene (Hays and others, 1976). Eccentricity modulates the intensity of the precession signal. The more oblate the Earth's orbit, the larger the precessional effect. Given the importance of precession to the monsoon, eccentricity would be expected to influence the intensity of the monsoonal fluctuations. As a consequence of this interaction between precession and eccentricity, a precession driven climatic signature, such as the storing of water on large subtropical continents, should include frequency components of eccentricity as well (Fig. 4A). The generalized relationship sought in the rock record is the repeated grouping of @21 ka precession cycles into bundles of 4 or 5. Spectral analyses showing eccentricity and precessional peaks combined with sedimentary units showing consistent bundling are often viewed as confirmatory of a Milankovitch signature (for example, Schwarzacher, 1993), and have been observed in association with monsoonal activity in the Late Triassic (Olsen, 1986).

The angle of Earth 's rotational axis relative to the ecliptic, obliquity, is currently at 230 and varies from 22.10 to 24.4 o • There are mUltiple spectral peaks for obliquity variation periods of 41.0 to 39.7 to 53.6 ka (Berger, 1978). The largest peak at 41 ka is the feature of geologic interest. Large angles of obliquity result in greater insolation at high latitudes in the summer and less in the winter. High angles of obliquity, in association with precessional cycles, could lead to more extreme poleward excursions of the monsoon. If large continents and water storage potential were located in the temperate region, an expanded monsoon, and consequent sealevel change, might be limited to times with high angles of obliquity (Fig. 4B). The 21 ka precession cycle and 41 ka obliquity cycle come in and out of phase with a periodicity of about 800,000 years so any pattern generated could be long term and complex. Sea-level fluctuations that depended on the obliquity cycle would require large water storage capacity at the poleward edge of monsoonal influence. An anomalous highstand is evident in Qinghai Lake well above the last precessional highstand (Lister and others, 1991). This suggests that more intense or more extensive poleward migration of monsoonal precipitation may have occurred earlier in the Quaternary.

4.2. NULL MONSOONS AND DOUBLE PRECESSION BEATS

It is somewhat counterintuitive, but equatorial continents could also generate periodic monsoon related signatures. These effects would be due to accentuation of the Hadley cell or intertropical convergence (lTCZ) circulation at or near the equator. Normally there is some monsoon related departure of the ITCZ from the equator, however, such departure would be minimized, and equatorial precipitation accentuated, when perihelion coincides with either the spring or fall equinox. At these


points in time insolation in the tropics should be greatest, focusing convection and precipitation. The two equinoxes in the annual cycle should produce a double precessional beat. Rainfall, and potentially water storage, would occur roughly every 10,000 years (Fig. 4C). This concentration of rainfall in the tropics could then be referred to as an "anti" or "null monsoon".

Climate models based on modern geography show double beat precession peaks in temperature over equatorial continental areas (Short and others, 1991). In the Cretaceous double beat precessional cycles have also been documented in marine cores (Park and others, 1993). These results suggest that double beats may be a frequent response of large continental areas in the tropics. Double beats could also be generated if large continental areas were located in the subtropics both north and south of the equator (Fig. 4D). In this case precessional forcing would be associated with the coincidence of perihelion with each solstice. Pangean continental configuration of the late Paleozoic and early Mesozoic times had considerable continentality both north and south of the equator. Consequently, double precessional beats could have occurred at these times. However, to be relevant to our discussion of sea-level fluctuation, sufficient continental water storage capacity would have to be located both north and south of the equator in the region of monsoonal influence. We will return to the various frequencies at which monsoons may have stored water in the past when we consider the evidence for such behavior from the pre-Quaternary geologic record.

4.3. OROGRAPHIC EFFECTS

In addition to continental configuration, orographic effects can greatly influence the monsoon. The Tibetan Plateau provides a high elevation surface which when heated by the sun generates anomolously high temperatures for that altitude. Instability and convection result from this high elevation heating. This intensifies the Asian summer-time low pressure systems strengthening the monsoon (for example, Kutzbach and others, 1993). The high topography of South Asia also serves to separate temperate and tropical air masses preventing larger scale mixing. This maintains temperature gradients localizing and focusing convective processes. Monsoonal flow over other continents is accentuated to varying degrees by topography (see Meehl, 1992).

5. Evolution of Internal Drainage

Large areas of internal drainage are necessary if large amounts of water are to be stored on the continents. The maintenance of internally drained basins can be viewed as a competition between processes that disrupt drainage systems, and those processes, such as stream capture, that increase the efficiency of drainage networks. Many modern high latitude lakes result from the relatively recent disturbance of drainages caused by glacial advance and retreat. In a world without continental ice sheets, tectonics should be the primary generator of internally drained basins. Eolian processes may provide a secondary mechanism of drainage disruption. Drainage will organize faster near the sea or other relevant base level, and when


frequent precipitation generates substantial and relatively frequent runoff. Atmospheric transport of marine moisture is limited by continental size and mountain barriers. In addition, in the arid interiors of large continents eolian processes are more active. Intermittently active dune fields disrupt drainage in the Sahara today. Due to these factors, evolution of effective drainage systems should take longer on larger continents. Strength of monsoonal circulation also relates to continental area. Collisions generate large continents and orogenic thickening of continental crust. These factors increase the volume of ocean basins, lowering sea-level and exposing continental shelves, effectively increasing continental area and elevation. As pointed out by Hay and Leslie (1990) the margins of continents exposed at lowstands contain a disproportionately large amount of porous sedimentary rocks ideal for aquifer water storage. The relative elevation of the continents can accentuate the aridity of continental interiors and may accentuate monsoonal activity (Meehl, 1992). Thus, a number of factors effecting the ability to store water on continents via fluctuations in monsoonal intensity closely correlate with continental size.

Many of the tectonic processes likely to produce closed internal basins are active during the periods of continental collision that produce large continents, or during the initial break up of such continents. Tectonic processes associated with the accretion of continental blocks into larger continents generate large closed basins in a variety of ways. We tabulated modern and ancient regions of closed or nearly closed contours on the basis of the tectonic processes that produce them (Table 2). Plate convergence can generate a number of basin types. For example, as continents approach each other collision is often initiated at separated points along the margins. Areas of oceanic crust are trapped between these loci of collision. This mechanism formed the Mediterranean, Black and Caspian Seas (Table 2). Of these, only the Caspian basin is partially empty today. However, in the Messinian (6 Ma) the Mediterranean was completely isolated from the other oceans of the world. Its volume equivalent of 15 meters of sea water evaporated (Hsu and others, 1973). Lower sea levels during the Pleistocene similarly isolated the Black Sea. Tethyan closure may have generated closed basins in the late Mesozoic and Cenozoic as portions of the Asiatic land mass accreted. The colliding terranes include the continental blocks that make up China, Afghanistan, Iran, and India. In the Paleozoic trapping of ocean crust may have isolated basins in the early stages of the Acadian and Alleghanian orogeny. Island arc formation isolates oceanic basins today. Examples include the Aleutian Basin, Sea of Okhotsk, and the Caribbean (Table 2).

Continental collision, and other convergent tectonic processes, produce regions of basinal storage in two additional ways. Thrust loading produces basins as material from one continental block overrides another, and crustal thickening results in plastic deformation and extensional tectonics in the thickened region. Thrust loading due to the collision of India and Asia, by Tibet from the south and by the Tien Sien from the north, has depressed Tarim producing the largest empty basin on Earth today. Thrust loading by foreland fold and thrust belts has generated extremely large basins, such as the Western Interior Seaway of Cretaceous age, and the Late Paleozoic Appalachian Basin of the Eastern US. Comparable basins are few on Earth today, although the Gangetic Plain, Tigris Valley-Persian Gulf, and


TABLE 2 Basins and Water Storage Potential. Modern and ancient basins and other regions of water storage potential. Most basins are, or were closed at some point in the past, other modern basins that are nearly closed are included for comparison. Basins are grouped by tectonic type, and area, as well as volume in meters of sea-level change, are reported in the following columns. In the last column the time of basin closure or desiccation is reported, and if the basin is partially empty today an estimate of the sea-level equivalent is reported in parentheses. Calculations are rough and are based on a number of almanacs and atlases.

Basin Type

Basin

Convergent

Collision Trapped Ocean crust

Mediterranean

Black Sea

Caspian-Aral Basin

Arc Trapped

Caribbean

Bering Sea

Sea of Okhotsk

East China Sea

Thrust loaded basins

Tarim

Balkash-Zungar

Zungar

Persian Gulf

Karoo

Green River

Chinle (Owl Rock)

Transpression

Central Valley

Quidam

Crust dilation

Tibet

Andes

Basin and Range

Falkland-Aghulas

E. North Am.

Keuper

E. Siberian Lowlands

Area Volume DesiccationlIsolation

(106 km2) ~ Sea-level

(m)

2.51 10.4

0.51 1.68

2.32 1.42

2.52 17.95

2.26 9.34

1.39 3.75

0.66 0.34

0.87 1.08

0.30 0.053

0.30+ 0.42

0.23 0.06

1.9? 2.1O?

0.36 0.30

1.0+ 0.55+

0.10 0.14

0.16 0.09

1.22 1.66?

0.12 0.17?

0.86 1.2.?

2.5 3.46?

7.0? 5.8+

6.0? 5.0+

4.3 3.6

Messinian desiccation

Pleistocene lowstand

Currently (0.36)

Jurassic, Currently (1.08)

Currently (0.53)

Permian, Trias, Maim, Dogger

Pleistocene lowstand?

Beaufort-Permian Lakes

Eocene-Precession cycles

Norian Lake Deposits

Eocene-Recent (0.09)

Currently (1.5?)

Currently (0.114)

Miocene-Quaternary, Bonneville

Mid Jurassic-Late Jurassic

Late Triassic-Early Jurassic

Late Triassic

Triassic


TABLE 2 Continued

Basin Type Area Volume DesiccationlIsolation

Basin (106 km2) ~ Sea-level

(m)

Divergent

Rifts

Red Sea 0.45 0.67 Miocene-Pliocene Desiccation

Tanganyika 0.32 0.13 Late Pleistocene (0.31)

Parana Benue 2.0 3.46 Aptian-Albian

Zechstein and rifts 4.1 3.4 Late Permian

Rift Shoulder bounded Basins

Victoria 0.07 0.015

Congo 1.44 1.20 Early Cretaceous,

K wango Lake beds

Pull Apart

Gulf of Mexico 1.51 6.74 Dogger, Luanne SaIt

Shanxi 0.24 0.33 Triassic-Tertiary

Orcadian Basin 0.80 0.88 Devonian, Old Red

Back Arc Basins

Andaman 0.57 1.75

Sea of Japan 1.01 4.68

Transform Traps

Walvis Ridge 1.25 \0

Hotspot Trap

Arctic Ocean 13.23 38.05 Isolated in Oligocene

Chad 0.67 0.16 Currently (0.16)

Non Tectonic

Reefs

Permian Basin

Ergs

N. Africa-Arabia 5.4 0.75? Currently (0.75?)

Navaho-Kayenta 1.0+ 0.20+ Norian-Lias

Botucatu-Sambaiba 3.8+ 0.53+ Aptian-Albian

Coconino, Lyons. 3.0+ 0.60+ Permian

Rotliegendes 3.5+ 0.72+ Permian

Barun Guyot Cretaceous


the Adriatic Sea provide some examples. Both the Adriatic and the Persian Gulf have submerged closed contours forming discrete closed basins (Table 2). Loading of the South African craton by the Cape Fold Belt in the Pennian produced very large lake basins (Yemane, 1993; Yemane and Kelts, 1990). Thrust loading associated with the inversion of Uintan trough during Laramide compression produced the Green River and other Eocene lake basins. Interestingly Green River deposits have long been thought to contain Milankovitch period fluctuations in sedimentation (Bradley, 1929; Fischer and Roberts, 1991). Similar thrust loading can occur along transform margins. Transpression produces subsidence and closed contours in the southern portion of the Central Valley of California (Buena Vista Lake).

Continental collision and subduction related compressional tectonics can result in large regions of thickened continental crust. As such crust reaches thennal equilibrium, it undergoes ductile deformation, spreading out and producing many large graben structures in the thinned brittle layer of overriding crust. The Tibetan plateau contains numerous large extensional structures of this type with substantial water storage potential (Table 2). Similarly, the axial graben of the Andes contains Lake Titicaca and large areas of dry lake beds. With the dissipation of compressional stress in the latest stages of orogeny, extension may accelerate producing even larger regions of dilated crust with complex basinal topography. Post-Laramide extension of the western US produced the basin and range topography of the Great Basin and adjacent areas. This region contained two very large lakes, Lahontan and Bonneville, as well as many smaller ones, as recently as early Holocene times. Similar regions of extension followed Paleozoic Pangean orogenies. In Gondwana active extension produced basin and range topography in thinned crust that now constitutes parts of Mozambique, coastal South Africa, the Aghulas Bank and Falkland Plateau. A similar extensional regime followed the late Paleozoic Appalachian orogeny; many basins formed in the early Mesozoic along the Eastern North American coastal plane, shelf, and rise. A roughly contemporaneous extensional regime formed in West Siberia adjacent to the Urals.

During continental break up, graben formation, rift shoulders, back arc basins, and pull-apart structures associated with transforms can all form isolated basins. The East African rift provides a modern analog for continental break up. Lake levels in all the large lakes, including Malawi (Scholz and Finney, 1994), Tanganyika (Gasse and others, 1989), Natron (Hillaire-Marcel and Casanova, 1987) and Turkana (Johnson and others, 1991) have fluctuated in response to climate changes associated with the precession cycle. The Red Sea provides an example of an even larger rift basin of closed contours (Table 2). Similar basins were present in the early stages of most continental rifting events. Elevated shoulders of adjacent rifts can also isolate basins; Lake Victoria occupies such a basin today and the Congo Basin was similarly isolated by rift shoulders in the Cretaceous.

In later stages of rifting in vol ving oceanic crust, spreading ridges, hotspots and "leaky transforms" interact with other features to produce closed contours. The Arctic Ocean is perhaps the largest region of well defined closed contours today. It was isolated from the Atlantic by hotspot volcanism in the Iceland region in the Oligocene (Table 2). Other basalt edifices that have played a role in isolating basins include the Walvis Ridge in the South Atlantic, and Florida which is founded on


transform associated basalts. Thus, a variety of distinct tectonic processes have played a role in isolating basins. We discuss specific examples in further detail when we examine the potential for basinal response to monsoons in the various periods of the Phanerozoic.

6. Carbonate Models and Sea Level Change

Nearshore carbonate deposits document periodic subaerial exposure and provide the most compelling evidence for periodic small scale sea-level change in the Milankovitch frequency spectrum (Goldhammer and others, 1990; Osleger and Read, 1991). Following the work of Fischer a number of workers generated synthetic sequences of nearshore carbonates to compare with those they find in the field (Fig. 1) (for example, Read and others, 1986; Read and Goldhammer, 1988). These models require subsidence rates, carbonate production rates, and sea-level curves as input parameters. This approach strongly supports a causal linkage between the nearshore carbonate packages observed and orbital variation driven fluctuations in sea-level. As such, they tend to confirm power spectra analyses that document orbital forcing periods for many carbonate sedimentary units. However, ten meters of relative sea-level rise is often invoked in models that produce meter-scale carbonate packages. These amplitudes result from the interplay between carbonate production terms and the sinusoidal shapes of the sea-level curves employed in such modeling. A number of measurements of modern reefs suggest that carbonate production can be very rapid (for example, Schlager, 1981). With large carbonate production terms in the models it becomes difficult to flood a surface because carbonate production keeps pace with the sea-level rise. If this were indeed the case only tidal flat facies would be preserved. The models could not produce the subtidal facies evident at the bases of meter scale beds (Fig. 1). We argue that carbonate production need not have been uniformly high, and that climatic responses and consequent sea-level change may have been relatively abrupt. If either were the case, the rock record could have been produced with smaller amplitude sea-level rises than previously recognized.

6.1. CARBONATE PRODUCTION RATE

Carbonate production rates in sedimentary models derive from modern reefs (for example, Schlager, 1981). However, carbonate production rates may not pertain uniformly to times in the past because modem reef building taxa that produce carbonate at high rate were not present during the time period in question, for example scleractinian corals were not present in Paleozoic reefs. Alternatively, changes in ocean chemistry may have altered calcification rates (for example, Holland, 1984). Temperate climate, clastic influx, salinity change, and organic input are all known to limit carbonate production today. All these factors changed in association with the early Holocene monsoon. Thus, carbonate production need not be constant. It is likely to change periodically in response to the same climatic factors governing sea-level. In addition, hurricanes transport sequestered carbonate material to deeper water in many reef systems today. The assumption of a high


carbonate production and retention on an ancient reef or platform indicates that the researcher can recognize in the record, and discount, those variables that are known to limit carbonate production at present. If carbonate production and retention on the platform was in fact lower than values assumed in the models, then amplitudes of sea-level change need not be as high to produce the eustatically-generated meter-scale carbonate beds observed in the rock record.

6.2. RATE OF PLUVIALLy-INDUCED SEA LEVEL CHANGES

Even if carbonate production values were high, sea-level change may not conform to the sinusoidal curve employed in many carbonate models. Ice core evidence suggests that climate changes of considerable magnitude can occur relatively abruptly, with dramatic changes in hemispheric temperatures occurring in tens of years (for example, Dansgaard and others, 1993; Anklin and others, 1993). Tropical precipitation decreased abruptly in the early Holocene, and in the last century, in response to perturbation of the Atlantic thermohaline circulation (Street-Perrott and Perrott, 1990). Thus, relatively rapid fluctuations in monsoonal precipitation is to be expected. If climate change is abrupt the rise and fall of sea-level as a consequence of sequestration of water by the monsoons could also be relatively rapid. Isotopic data from some North African lakes are taken to indicate extremely rapid filling, a long high stand period, followed by very rapid emptying, resulting from dramatic changes in monsoonal rainfall (Mckenzie, 1993). Very large flux rates of water are also suggested by early Holocene flood stages of the Nile. Thus, water storage and small-scale sea-level change associated with changes in monsoonal conditions could have been quite rapid (see Gasse and Vancampo, 1994).

Figures quoted by Hay and Leslie (1990) indicate that annual precipitation on the continental areas of Earth today exceeds 105 km3 per year. This volume equates to roughly a 30 cm change in sea-level. With dramatic changes in climate it may be possible to store a large fraction of this amount, lowering sea-level by several centimeters a year. Evaporation rates from continents are roughly 2/3rd of the precipitation rate. Annual fluxes of water into and out of aquifer storage are currently about l/Sth the continental precipitation rate and a little more than 1I4th the evaporation rate (Hay and Leslie, 1990). Hsu and others (1973) calculated that modern evaporation rates would desiccate the Mediterranean in about a thousand years, leading to 1 cm sea-level rise per year. The area of increased precipitation in the early Holocene was 4 xl 07 km2, roughly 16 times the area of the Mediterranean. If decline in rainfall in this area was sudden, the resulting sea-level rise rate could have been substantially larger than that calculated by Hsu. After a dramatic climate change initially high sea-level rise rates should decline as the wetted surface area where evaporative loss could occur decreases and deeper subsurface aquifer layers drain with falling water tables. Water storage would similarly be initially rapid and then slow as more and more basins came to equilibrium with evaporation or overflowed to the sea. The processes leading to water storage on and water loss from the continents are not identical. Consequently, some asymmetry between rise and fall might be expected. However, a rapid «100 y) change in sea-level of one to


a few meters following shifts in monsoonal climate appears consistent with current flux rates and our understanding of the rapid onset of climate change in the past. Carbonate modelers (for example, Hinnov and Goldhammer, 1991) have assumed maximum sea-level change rates on the order of 1 to 2 meters per thousand years. It may be that pluvially-induced sea level changes of an order of magnitude higher rate occurred regularly during some period of the Phanerozoic.

If carbonate productions were lower than those used by modelers, or if sealevel rise rates were higher, much smaller changes in sea-level, on the order of three meters, could be sufficient to account for meter scale carbonate beds with algal laminated or subaerially-exposed tops, rather than the ten meters of relative sea-level change that has often been invoked.

7. The Late Triassic

Several lines of evidence support a Milankovitch driven monsoonal water storage mechanism for Late Triassic sea-level fluctuations. During the Triassic the land masses of the Earth were still associated into one large continent, Pangea; a large percentage of the land mass was located in subtropical to temperate latitudes; sea-level was low resulting in a large continental area; and there was a large tropical seaway - the Tethys - adjacent to Pangea. All these factors are thought to contribute to monsoonal circulation. In addition, widespread sedimentologic evidence indicates a strongly seasonal climate (Dubiel and others, 1991). Computer models of global climate based on Triassic continental configurations also indicate very strong monsoonal flow (Kutzbach and Gallimore, 1989) leading to use of the term "Megamonsoon".

During the Triassic, post orogenic (Alleghanian-Variscan) rifting and incipient continental breakup produced vast regions of graben and pull-apart basins. These basins extend from what is now the Gulf of Mexico along the east coast of North America to northern Europe and the Arctic (Holzer and others, 1988; Ziegler, 1982, 1988; Uchapi, 1989). Triassic evaporites are more extensive than those of any other period (Robinson, 1973) testifying to the frequent desiccation of these basins.

That filling and emptying of basins was subject to both seasonal and 104-year fluctuations attributable to Milankovitch forcing is more directly evident from annual varves in lacustrine basins in the Newark Supergroup (Van Houten, 1964). Spectral analyses of these sedimentary cycles were performed on Carnian age rocks of the Locatong Formation and Norian age Rocks of the overlying Passaic Formation. These analyses suggest a dominant cyclicity of approximately 20,000 years, as would be expected if precession of the equinoxes (Olsen, 1986) was the primary driving Milankovitch parameter. In addition, lower frequencies, attributable to eccentricity, are also observed. Olsen (1986) attributes these spectral peaks to Milankovitch controlled variation in the monsoon and consequent variation in fluvial processes and lake level.

The Dachstein carbonates of the Northern Alps record small scale changes in eustasy termed Lofer cycles. These deposits were first interpreted as cyclic in the 1930's; subsequently, Fischer did his classic work on cyclicity and eustasy on these


sections (Sander, 1936; Fischer, 1964). In the Dolomite mountains of northern Italy meter-scale "Latemar cycles" also record sea-level changes (Goldhammer and others, 1987, 1990; Hinnov and Goldhammer, 1991). The meter scale units in both regions consist of progradational packages with subarially exposed caps thought to be indicative of repeated eustatic flooding in conjunction with relatively continuous subsidence. Spectral analysis (Goldhammer and others, 1987) indicate a dominant 20,000-year periodicity and bundling of these cycles units of 4 or 5, a result similar to the spectral signature in the Newark supergroup (Olsen, 1986).

The Lofer cycles roughly correlate with the cyclic Norian age Passaic Formation of the Newark Supergroup. More Van Houten cycles are reported from the preceding late Carnian age of the Locatong Formation (Olsen, 1986). Latemar eustatic cycles occur in the Ladinian stage that precedes the Carnian (Hinnov and Goldhammer, 1991). Thus the observations on eustatic Lofer and Latemar cycles bracket and overlap observations of the lacustrine Milankovitch cycles in the Newark Super Group. The potential for water storage in modem basins in the region of fluctuating monsoonal influence is sufficient to produce four to eight meter changes in sea-level, Table 1, Fig. 2 (Jacobs and Sahagian, 1993).

Extraordinarily large areas of internal drainage, evaporite deposition and eolian deposits are evident in reconstructions of Late Triassic paleogeography. Graben structures containing lake and fluviatile deposits extend from the Gulf of Mexico to Europe with associated basins in Morocco (Brown, 1980). These basins may have covered 13 millions square km (Uchapi, 1989; Ziegler, 1982). Additional basinal settings of Late Triassic age occur on the extended crust of East Siberia and the Donetz Basin (Ziegler, 1988) as well as large areas of lake (Owl Creek Member) and eolian deposits in the Chinle Formation of the southwestern U.S. (Dubiel and others, 1991). Thus rift graben, lakes, and evaporite deposits had a far greater areal extent in the Late Triassic than they do today (Table 2). Given the larger areas involved, storage potential may have been considerably in excess of the 4 to 8 meter potential calculated for the Quaternary (Jacobs and Sahagian, 1993). In addition, storage potential was concentrated in the Northern Hemisphere, particularly in the subtropics and would have been effected by the megamonsoonal circulation predicted for this time period (Kutzbach and Gallimore, 1990; Dubiel and others, 1991). Given precessional forcing of the monsoon, fluctuation of sealevel sufficient to accommodate meter scale eustatic packages would appear to be relatively easy to generate. Standard carbonate models (e.g. Goldhammer and others, 1990) suggest 10 meters of relative sea-level change, 7 meters with isostatic adjustment. Eustatic fluctuations of this magnitude may have been produced in the Late Triassic. However, as we argued earlier, smaller sea-level changes may have been sufficient to produce these carbonate packages.

To claim that ice did not play a role in Late Triassic sea-level change necessitates a negative argument, always a dangerous task in geology.2 However,

2Just ask Darwin: in his negative or exclusionary argument, "Observations on the Parallel Roads of Glen Roy, and other parts of Lochaber in Scotland, with an attempt to prove that they are of marine origin", Darwin overlooked the possible effects of glacial ice in this geomorphologic question. Developments in glaciology convinced the scientific establishment of the glacial origin of these features. This led Darwin to later disparage exclusionary argument (Hull, 1973).


the Greenland Ice Cap contains only enough water to raise sea-level 5 meters (Rowley and Markwick, 1992). Currently, ice sheets that reach the sea distribute dropstones over wide high latitude regions. No such characteristic deposits occur in the Late Triassic (Hambrey and Harland, 1981). Thus there is no evidence for a continental ice sheet at all, not even one a fraction of the size of Greenland. Today, mountain glaciers contain only the equivalent of a 70 cm rise in sea-level, and previous expansions of mountain glaciers in the Quaternary were closely tied to cold conditions at the poles (for example, Dawson, 1992). In the Late Triassic, cycads grew at high paleolatitudes demonstrating equable climates at the poles (Ash and Bassinger, 1991). Thus there is a large body of evidence suggesting cyclic lacustrine water storage in the Late Triassic, and no evidence indicating glaciation. Clearly, glaciation is the dominant influence on sea-level in the Quaternary; however, fluctuations in lake level and groundwater storage appears to provide an alternative mechanism for small scale sea-level fluctuations in periods of Earth history which are thought to lack continental glaciers such as the Late Triassic.

8. Other Time Periods

The Late Triassic meets all the expectations of a time period when Milankovitch driven monsoonal water storage generated high frequency sea-level change. However, in several other periods evidence suggests Milankovitch-monsoonal sea-level fluctuations.

8.1. CAMBRIAN-ORDOVICIAN

From the Late Cambrian to Middle Ordovician times there is little evidence of ice (Hambrey and Harland, 1981), and a well documented record of Milankovitch driven sea-level cyclicity recorded in carbonates on the massive platforms of eastern North America (for example, Goldhammer and others, 1993; Osleger and Read, 1991). During this period a precession driven monsoonal cycle could have developed in response to the Gondwanan continental landmass in the Southern Hemisphere. Evaporite deposits are reasonably abundant, especially in Siberia (for example, Frakes and others, 1992). However, we are not aware of any cyclic fluviatile or lacustrine sediments that document periodic pluvial conditions or water storage on the continents in this time period.

8.2. DEVONIAN

In the Early and Middle Devonian there is no evidence for glaciation and climates were warm and equable (Hambrey and Harland, 1981; Frakes and others, 1992). Early Devonian carbonates of the Helderberg Group contain meterscale thrombolitic, stromotoporoid rich beds toped by flat laminated or stromatolite caps. These deposits have been interpreted as products of a precession dominated Milankovitch frequency oscillation in sea-level (Goodwin and Anderson, 1985; Anderson and others, 1984, Goodwin and others, 1986; Brett, 1986). Upper Frasnian carbonates from western Canada also contain meter scale shallowing upward


cycles, some of a subtidal origin and others with biolaminated tops that exhibit subaerial exposure (Fejer and Narbonne, 1992).

Middle Devonian lake deposits from Scotland contain repeated 8 meter thick transitions from lacustrine to fluviatile deposits inferred to be precession dominated on the basis of spectral analyses (Kelly, 1992; Rogers and Astin, 1991). Henrik Olsen (1990) argued that Late Devonian sediments of the Kap Grath Group in eastern Greenland document cyclic fluctuations in stream hydraulics resulting from precession-mediated fluctuation of monsoonal precipitation. Late Devonian fluvial deposits from southwest Ireland record periods thought to equate with the eccentricity cycle (Sadler and Kelly, 1993). These deposits suggest periodic storage of water in the Orcady Basin. Ziegler's (1982) European reconstructions indicate areas approaching 1 million square kilometers of fluviatile and eolian deposits with some evaporites. Given the distribution of continents in the sUbtropics and the uplift of the Acadian Orogeny a strong monsoon seems likely. Old Red facies and evaporite deposits suggest aridity on a continental scale (Frakes and others, 1992). Continental interactions between Gondwana and Laurasia (Scotese, 1987) may have trapped ocean crust or generated basins through crustal dilation and pullapart structures. Little evidence of these basins would have survived subsequent orogenic activity. Additional basins could have formed via rifting along the South Tethyan margin during the Devonian (Scotese, 1987).

In summary, there is evidence for Milankovitch frequency fluctuations, in sea-level in the Early and Late Devonian, and evidence for similar frequency fluctuations in fluvial and lacustrine sediments in the Middle and Late Devonian from the Orcady Basin and Greenland. Precessional signals of both a pluvial and eustatic nature occur in the Late Devonian. However, there is also limited evidence for glacial ice in the Famminian (Frakes and others, 1992). Thus, the case for monsoonally generated sea-level fluctuations in the Devonian is provocative, but not comprehensive.

8.3. LATE PERMIAN-EARLY TRIASSIC

Gondwanan ice sheets presumably dominated any Milankovitch driven sea-level fluctuations in the Early Permian. By the Late Permian abundant plant remains are evident from the same regions covered by glacial ice sheets in the lower Permian (Crowell, 1978). Thus, high frequency sea-level fluctuations late in the Permian could be due to fluctuation in water storage rather than ice sheet advance and retreat. As discussed previously, there is no evidence for large scale glacial activity in the Triassic. The Late Permian Zechstein salts and associated deposits suggests the intermittent filling of basins in a large region. Including associated graben structures containing terrigenous deposits, these basins total over 4 million square kilometers in Europe alone (Ziegler, 1982). Anderson (1982) documented precession driven cycles of evaporite deposition in the Castile Formation of the Permian Basin of West Texas. Lake basins of Permian age occur in the Zungar region of North China (Binjie and others, 1980) and very extensive lake basins are evident in the Karoo Deposits of Southern Africa (Yemane and Keltz, 1990). First order sea-level was low in the Late Permian, and Pangean continental configurations


appear conducive to Monsoonal activity. Despite these supporting factors, there are few reports in the literature of high frequency sea-level fluctuations. Perhaps given low overall sea-level and high productivity few carbonate platfonns had developed in which to record eustatic fluctuations.

As was discussed previously, the Late Triassic has considerable potential for monsoonally driven water storage and very strong evidence indicating that such a phenomenon actually occurred in the Carnian and Norian. Earlier in the Triassic continental water storage potential also seems large. However, it is not until the Ladinian that Tethyan carbonate platforms develop and provide a clear record of sea-level fluctuation.

8.4. JURASSIC

The Early Jurassic transgression flooded much of the European region conducive to water storage. However, basins in the extensional terrain along the eastern margin of North America contain Hettangian and Sinemurian fluviatile and lacustrine deposits, as well as substantial evaporites. The Middle Jurassic Louann salt documents that the Gulf of Mexico as an evaporite basin. By Middle and Late Jurassic times the incipient break up of Gondwana generated large areas of extensional crust in what has become coastal Mozambique, the Aghulas Bank and Falkland Plateau (Dingle and others, 1983). Rifting also took place along the Tethyan margin of Gondwana. Eolian deposits of the western U.S. extend in to the Early Jurassic, and lake deposits of Jurassic age in China also suggest additional water storage potential (Table 2). Although not as large as in the Late Triassic, Early Jurassic storage capacity appears to be substantial. However, this capacity may not have been ideally located to respond to the monsoon, some being in the Northern Hemisphere and some in the Southern. In addition, the opening of the North Atlantic in the Middle Jurassic, broke up the continentality that drove Late Triassic northern hemisphere monsoonal circulation. Milankovitch cycles have been reported from the Liassic (Weedon, 1985) and spectral analyses of limestone marl couplets from the Kimmeridgian of southern Iberia (Ol6riz and others, 1991, 1992) suggest precessional and eccentricity forcing. Bosellini and Hardie (1985) indicate that cyclic carbonate sedimentation extends from the Triassic to the Eocene in some of the carbonate platforms of Northern Italy. However, spectral analyses, or other work detailing the eustatic-Milankovitch nature of these cycles, have not been reported. Thus, monsoon ally driven eustatic cycles may have occurred in the Jurassic, but have not been well documented.

8.5. EARLY CRETACEOUS

Next to the Late Triassic, the Early Cretaceous exhibits the best support for eustatic cycles generated by a monsoonal mechanism. Basinal water storage is substantial and the marine record contains many precession related cyclic deposits and most authors agree that there is no evidence of continental glaciation from the Late Permian to the Early Tertiary. However, some poorly dated drops tones in Northern


Australian deposits suggest seasonal climates at some point in the Jurassic or Early Cretaceous (Frakes and Krassay, 1992).

A large amount of basinal storage potential was generated by rifting activity associated with the opening up of the South Atlantic. In the early stages of opening the a basaltic edifice known as the Walvis Ridge isolated the northern portion of the South Atlantic, generating a basin containing evaporites of Aptian age (for example, Uchapi, 1989). Burke and Sengor (1988) calculate the volume of this basin, and argue that catastrophic marine flooding of the basin lowered sea-level by 10 meters in the middle Aptian. Rifts contemporaneous with, and presumably genetically-related to South Atlantic opening are widespread in South America and Africa. These include the Parana and Maranhao Basins which contain fossiliferous lake deposits of the Aptian and Albian Santana Formation (Maisey, 1991). These lake sediments are locally interbed with eolian deposits that may have covered as much as 3.8 million square miles in South America (McKee, 1979). In Africa, the Benue Trough of Nigeria contains Aptian lake deposits. The Benue is in tum connected to large structures of similar age in Chad and Niger (Petters, 1991) as well as several en echelon graben structures in the Sudan that contain lake deposits of Early Cretaceous age that exhibit cyclicity (Wycisk and others, 1990). Activity in these rift zones, opening of the South Atlantic, and incipient uplift along the western branch of the East African Rift isolated a large lake basin, the remains of which form the Congo drainage today. Tectonically this basin was similar to Lake Victoria in that it formed between uplifted rift shoulders rather than in a rift graben. The Congo Basin is 20 times the areal extent of Lake Victoria and overflowed to the Benue graben, possibly at relatively high elevation, in the Cretaceous. The extent of the K wango beds, lake deposits of Early Cretaceous age (Cahen, 1983), suggests that a large volume of water was stored in the Congo Basin.

Gilbert (1895) may have been the first to argue for orbital control of sedimentation in the Cretaceous. Examination of marine cores has led other workers to infer Milankovitch forcing of sedimentary processes in the Cretaceous (see Schwarzacher, 1993 for review). Many of these deposits are suggestive of monsoonally generated changes in productivity that occur at precessional intervals in the Quaternary of the Arabian Sea (Prell and Van Campo, 1986), or the Quaternary age precession driven sapropel deposits of the Mediterranean (Rossignol-Strick, 1983). The Scisti a Fucoidi, Aptian-Albian marine sediments ofltaly, record precession frequency modulated delivery of eolian material, an observation consistent with the alternating wet and dry conditions expected from precession driven changes in the monsoon (Pratt and King, 1986). Direct evidence for Milankovitch frequency sea-level fluctuation is available from the Berriasian of southern Iberia where meter scale carbonate cycles with submarine bases are capped by supratidal deposits. Spectral analyses of these sections (De Cisneros and Vera, 1993) suggest precession cycle forcing.

Tropical continents could produce an increase in rainfall twice with each precession cycle as each equinox coincides with perihelion resulting in maximal tropical heating. In the Early Cretaceous such a double beat precession cycle might be expected. The proximity of North Africa and Northern South America in the Early Cretaceous provides a large equatorial continent region that could have re-


sponded to perihelion at the equinoxes. In addition, a large portion of Gondwana extends into southern sub-tropical and temperate latitudes. So one might expect a tropically-generated equinox-related double beat, or subtropically-generated single beat, precession response. In fact, there is evidence in the rock record for both. As previously discussed, there are number of precession driven deep marine records from Cretaceous deposits. In one study of Lower Cretaceous rhythmic black shales and limestones of the Maiolica formation of central Italy, precession cycle forcing is associated with the limestone-shale couplets, but the limestones show an additional shale parting suggestive of an additional half precessional 1 0 ka signature (Herbert, 1992). More concrete evidence for double periodicity is available from the Campanian of the Central Atlantic where cores show a clear double precession 12 and 10 ka peaks (Park and others, 1993). At this point the Atlantic is still narrow in the tropics and this behavior may reflect the large amount of continent near the equator. As discussed previously, such equatorial continentality may generate double beats (for example, Short and others, 1991; Fig. 4C).

The evidence from the Early Cretaceous strongly suggests that the precession driven monsoonal water storage mechanism may be in operation. Studies of cyclicity in lake deposits of Cretaceous age from South America and Africa could greatly strengthen the argument. The Early Cretaceous, especially up through the Aptian, seems promising in terms of water storage (Table 2). In the Upper Cretaceous there is evidence of Milankovitch forcing of deep marine deposits and there are large eolian deposits with associated lakes in Mongolia (McKee, 1979). However, with higher first order sea-level, and Gondwanan break up, large continents are no longer available to generate monsoons and store water. So the Late Cretaceous may not be as promising for monsoonally-generated water storage.

9. Anthropogenic Sea Level Change

Human impact on climate has the potential to induce sea-level change. However, human activities can more directly effect sea level without global climate mediation. Sahagian and others (1994a) performed some preliminary calculations documenting activities that ultimately result in transfer of water to the sea including: the pumping of non-recharging aquifers, draining of wetlands, deforestation, desertification, and diversion of surface waters. Unquantified activities that could also lead to sea-level rise include: construction of poulders, removal of fluid in oil fields, and burning of fossil fuels. On the other hand, dams retain water that would otherwise flow to the sea. Dam construction has a net negative effect on sea-level (Chao, 1988, 1991; Newman and Fairbridge, 1986). Clearly it is important to quantify the human generated fluxes of water between the continents and the oceans. Such an exercise is necessary before the true effects of climate change on sea-level can be assessed.

Sahagian and others (1994a) emphasized the current rates or fluxes of water between land and sea. Other authors have commented on the flux rates calculated, considered other hydrologic sources and sinks, or have focused on total sea-level change rather than current rate of sea-level rise (Chao, 1994; Rodenburg, 1994; GruelI, 1994; Gornitz and others, 1994) leading to replies from Sahagian and others


(1994b,c). We will discuss the fluxes to the sea, and the controversy surrounding dam construction and the need for further assessment.

9.1. FLUXES OF WATER TO THE SEA

Pumping of non-recharging aquifers constitutes mining of water. Some aquifers filled during earlier periods of wetter climate. In other cases, the pumping of water simply exceeds natural recharge rates even though some precipitation and runoff occurs in the region. This is the case in the High Plains Aquifer where 200,000 pumps currently operate (Weeks and Gutentag, 1988). Depression of the groundwater table, beginning in the 1930s, clearly indicates that aquifer pumping rates have exceeded recharge rates. Mining of three U.S. aquifers has already produced a 3.2 mm rise in sea-level: the High Plains regional aquifer, inclusive of the Ogallah Formation; the Southwest U.S. aquifer, encompassing the alluvial basins of southern Arizona and California and northern Sonora (Anderson and others, 1988); and the Central Valley of California (Fig. 5; Table 3). Aquifers in Arabia are being exploited at an increasing rate. The Arabian aquifers (Balek, 1977; AI-Ibraham, 1991) have recoverable reserves sufficient to change sea-level over 1 meter each, and Africa in general has recoverable reserves in non-recharging aquifers equivalent to over 4 meters of sea-level change (Castaney, 1991; Table 3). Exploitation of the Arabian aquifer is currently rising, and may have implications for sea-level change in the next century.

In the Aral Basin, the Amu Darya and Syr Darya rivers have been diverted to the Karakum Desert for the cultivation of rice and cotton (Korlyakov, 1991). The greater evaporation and transpiration resulting from these diversions has effectively removed large volumes of water to the sea. The Aral Sea now occupies half the surface area it did in 1960 (Micklin, 1992). This desication has resulted in ecological disaster and economic dislocation. Groundwater levels in the region are tied to declining lake levels. Continuing rapid water removal from the Aral Basin generates a flux to the sea equivalent to a sea-level rise rate of 0.18 mm of sea water per year (Table 3). This is the largest single flux from the continents to the oceans; however, this contribution to sea-level change will diminish as the Aral approaches complete desiccation early in the next century.

The Caspian has also suffered water loss from diversion of the Volga leading to lowering of the level of the Caspian Sea. Water sufficient to raise sea-level 4 mm was diverted from the Caspian Basin during the middle part of this century. However, presumably as a consequence of climatic changes, this trend has reversed and Caspian lake and groundwater level are currently rising. Other than these large basins, the largest sources of water extraction from continents involves the burning of tropical forests. This activity is currently raising sea-level at 0.14 mm per year. Draining of wetlands also makes a contribution to current sea-level rise (Sahagian and others, 1994a). The rate of sea-level rise currently generated by these activities combined is approximately 0.54 mm per year. This is roughly 113 of the sea-level rise rate inferred from the 20th century global tide gauge records (Fig. 5).

TA

BL

E 3

Ant

hrop

ogen

ic c

ontr

ibut

ions

to

sea-

leve

l ri

se.

Tot

al r

emov

able

vol

ume

is t

he a

mou

nt o

f w

ater

tha

t ca

n po

tent

iall

y be

wit

hdra

wn

from

eac

h aq

uife

r w

ith

curr

ent

tech

nolo

gy

give

n re

ason

able

eco

nom

ic a

ssum

ptio

ns.

Met

hods

of v

olum

e es

tim

atio

n an

d ec

onom

ic a~s

umpt

ions

var

y. s

o th

ese

figu

res

shou

ld b

e co

nsid

ered

rou

gh a

ppro

xim

atio

ns.

Sea

leve

l eq

uiva

lent

is

the

amou

nt o

f eu

stat

ic s

ea-l

evel

ris

e pr

edic

ted

if th

e vo

lum

e in

que

stio

n w

ere

adde

d to

the

oce

ans

(are

a o

f th

e oc

eans

tak

en t

o be

3.6

x 1

01 4

m2

). P

rese

nt n

et

extr

acti

on r

ate

is t

he r

ate

of

wat

er r

emov

al a

fter

acc

ount

ing

for

rech

arge

rat

es o

f 6.7

5 x

109

m3 /

yr.

1.05

x 1

09 m

3 /yr

and

7.8

x 1

08 m

3 /yr

for

the

Hig

h Pl

ains

. S

W U

.S .•

and

Cal

ifor

nia

aqui

fers

. re

spec

tive

ly.

Sah

aran

and

Ara

bian

aqu

ifer

s ar

e no

t re

char

ging

sig

nifi

cant

ly.

"Pro

ject

ed s

ea-l

evel

cha

nge"

is t

he v

olum

e o

f w

ater

that

wou

ld b

e w

ithd

raw

n in

50

yea

rs i

f th

e ex

trac

tion

rat

e re

mai

ned

the

sam

e as

it

is a

t pr

esen

t. "S

ea le

vel

chan

ge t

o da

te"

is t

he c

ontr

ibut

ion

of e

ach

aqui

fer

to th

e 20

th c

entu

ry s

ea-l

evel

ris

e. F

or m

ost

case

s. t

his

was

cal

cula

ted

by a~suming a

lin

ear

incr

ease

in e

xtra

ctio

n ra

te f

rom

0 t

o th

e pr

esen

t ra

te. T

he inc

rea~

e is

a~sumed t

o ha

ve b

egun

in 1

930

for

Am

eric

an a

quif

ers.

195

0 fo

r S

ahar

an a

nd A

rabi

an a

quif

ers.

196

0 fo

r S

ahel

des

erti

fica

tion

. an

d 19

40 f

or d

efor

esta

tion

. D

efor

esta

tion

fig

ures

inc

lude

onl

y lo

sses

of

trop

ical

for

ests

. W

etla

nd r

educ

tion

in

clud

es t

he t

otal

glo

bal

wet

land

are

a in

tot

al r

emov

able

vol

ume.

but

the

rat

es a

nd p

roje

ctio

ns i

nclu

de o

nly

the

loss

of

wet

land

s in

the

U.S

. If

the

rate

of

redu

ctio

n in

the

res

t o

f th

e w

orld

is

equa

l to

the

U.S

. ra

te.

thes

e fi

gure

s sh

ould

be

doub

led.

The

Ara

l an

d C

aspi

an h

isto

ries

are

wel

l re

cord

ed b

y th

eir

fluc

tuat

ing

leve

ls.

We

assu

me

a sp

ecif

ic y

ield

o

f 0.

2 in

the

san

ds i

n th

e su

rrou

ndin

g de

sert

. ov

er a

n ar

ea o

f ab

out

5 ti

mes

the

are

a o

f th

e la

ke i

tsel

f fo

r th

e A

raI.

and

3 t

imes

the

are

a fo

r th

e C

aspi

an.

At

the

pres

ent

rate

of

redu

ctio

n. t

he A

ral

wil

l be

gon

e in

abo

ut 2

0 ye

ars.

The

neg

ativ

e va

lues

for

dam

ed r

eser

voir

s re

flec

t an

thrp

ogen

ic w

ater

sto

rage

on

land

. an

d as

sum

es t

hat

all

rese

rvoi

rs a

re

cons

tant

ly f

illed

to

capa

city

. H

owev

er s

ee d

iscu

ssio

n on

dam

s in

the

tex

t.

Wat

er

Tot

al v

olum

e S

ea l

evel

R

eser

voir

re

mov

able

eq

uiva

lent

(xlO

J2

m3

) (c

m)

Hig

h P

lain

s 4.

0 1.

1 S

WU

.S.

3.0

0.83

C

alif

orni

a 10

.0

2.7

Sah

ara

60

0

167.

0 A

rabi

a 5

00

14

0.0

Ara

l (l

ake)

19

60

1.1

0.3

1990

0.

3 0.

08

Ara

I (g

roun

d w

ater

) 2.

2 0.

6 C

aspi

an (

lake

) 56

.0

15.4

C

aspi

an (

grdw

tr.)

22

0.0

61.2

S

ahel

(so

il w

ater

) 0.

1 0.

03

Def

ores

tati

on

3.3

0.9

Wet

land

red

ucti

on

8.6

2.4

Dar

ns

-1.9

-0

.52

TO

TA

L

1406

.7

392.

1

Pre

sent

net

S

ea le

vel

extr

acti

on r

ate

rise

rat

e

(x 1

010

m

3 /yr

) (m

mly

r)

1.2

0.03

1.

0 0.

03

1.3

0.04

1.

0 0.

03

1.6

0.04

2.7

0.08

3.7

0.1

0.77

0.

02

0.47

0.

01

0.34

0.

01

4.9

0.14

0.

2 0.

006

19.2

0.

54

Pro

ject

ed

Sea

leve

l ch

ange

ne

xt 5

0 yr

s (m

m)

1.6

1.5

1.9

1.4

2.2

3.0

5.1

1.1

0.65

0.

5 6.

8 0.

3

26.1

Est

imat

ed

Sea

leve

l ch

ange

to

dat

e (m

m)

1.1

0.92

1.

2 0.

56

0.89

2.2

3.1

1.3

0.78

0.

28

3.4

1.3

-5.2

11.8

~

Q) ~ ~ Q

Q) ~ I ~ ~

(1) .~

w

VI

VI


12.---------------------~----------------~

9

- x Sahara E E • Arabia -6 Q)

... Sahel C) + Deforestation c:: cu 0 Wetlands J: (,) 3

t:. Dams Q) ... total > ~ · US Aquifers cu 0 Q) c Aral

(J)

• Caspian -3

-6+---~--~----~--~--~--~----~--~---r 1900 1920 1940 1960 1980

Fig. 5. Anthropogenic contributions to sea-level change in the 20th century after Sahagian and others, 1994. The largest contributions to anthropogenic sea-level rise include: diversion of water from the Aral and Caspian basins, tropical deforestation, and pumping of Aquifers in the U.S. Note that the Caspian is now refilling perhaps due to climatic change. Dam construction generates an opposite effect. lowering sea level. Total sea-level rise attributable to anthropogenic sources has been about 12 mm, although other authors disagree as to the total volume associated with dams. Assuming that dam construction is now virtually nil, the sea-level rise rate due to these sources is now 0.55 mm/year. A figure roughly one third of the sea-level rise rate inferred from tide gauge data.

9.2. DAMS

Impounding water behind dams reduces sea-level. Sahagian and others (l994a,c) argued that current dam building activities are much diminished from activities at mid century. On this basis they assume that dams currently are having little effect on sea-level rise rate. Other authors disagree (Chao, 1994; Rodenburg, 1994) and feel there has been greater storage of water behind dams than recognized, and that there may be continued increase in storage capacity. This may be the case, however, there is a wide range in storage capacities cited by the authors and the actual timing of reservoir filling is difficult to assess. We agree with Chao (1994) that the global water storage in impoundments needs to be reexamined. However, the relationship of manmade impoundments to sea-level is likely to be more complex. Sedimentation reduces water storage in reservoirs. On the other hand, if the impoundment had not been constructed, that same sediment might have found its way to the sea and displaced sea water raising sea-level. On yet other time scales the absence of those sediments in the nearshore environment may


reduce coastal subsidence, possibly reducing water storage in the sea. These factors in combination with infiltration and evaporation suggest that we need more than just a better understanding of impoundment volumes. These additional theoretical considerations must be addressed before the influence of impoundments on sealevel is completely understood.

9.3. TOWARD MORE COMPREHENSIVE ANALYSES

Sahagian and others's (1994a) analysis was not intended to be all inclusive. Only the best documented instances in each category were examined, for example: only the Sahel was considered in desertification; only the U. S. was considered in wetland reduction; and only tropical, not temperate, forests were considered in calculating the effects of forest destruction on sea-level. Gornitz and others (1994) point out additional fluxes of water not considered by Sahagian (1994a). In particular, they argue that a 10% fraction of evapotranspiration from irrigated plants and evaporation from reservoirs is deposited permanently in the atmosphere on an annual basis. In other words they suggest an atmospheric sink receives the equivalent of more than 0.6 mm of sea-level per year. This number is 158% of our figure (Table 3) for annual mining of all aquifers. In fact, it seems exceedingly unlikely that a flux of water of this magnitude to an atmospheric sink can be sustained. In addition, the loss of atmospheric moisture from desertification, burning of tropical forests and loss of surface water, in the Aral Basin for example, were not considered and may be of opposite sign to the feature they chose to examine. This analysis points out the need for more comprehensive and inclusive analyses than those performed to date. Many variables could be analyzed further, for example: the pattern of forest destruction and regrowth in temperate regions is not simple, and the amount of water retained may depend on details of precipitation, soil type and plant community; erosional downcutting effects water tables removing surface water to the sea; and complex interactions ensue when sediment delivery to the sea changes with changing land use and flood control practices. Thus, more analyses are necessary before direct anthropogenic effects on sea-level are understood.

10. Lakes, Climate Models and Environmental Concerns

10.1. LAKES AND CLIMATE MODELS

Expansion and contraction of lakes provides robust documentation of climate change. Lakes influence climate on local and regional scales. Lakes may also effect climate on a global scale. Late Permian lakes are thought to have ameliorated the effects of the Gondwanan landmass (Yemane, 1993). Climate models that do not include these lacustrine features generate much colder winters and tend to produce glaciers (Kutzbach and Ziegler, 1993). Thus, incorporation of large lakes appears essential to adequate modeling of paleoclimate. Street-Perrot and others (1990) incorporated albedo of the paleo-lake and vegetation distribution in a reconstruction of the early Holocene climate of North Africa. Actual lake distribution in Asia is less well known (Dawson, 1992), but may be more critical for Holocene


climate reconstruction. The chronology of ice-dammed lakes in large regions such as the West Siberian lowlands as well as a good chronology for filling of the Caspian-Aral system could have critical implications for climate reconstruction. The late onset of the Asian Monsoon relative to modeling expectations has been attributed to late melting of glacial ice in Tibet (Socci, 1991; Lautenschlager and Santer, 1991). However paleo-lakes have not been incorporated into these climate models. Reconstruction of the history oflakes in Asia might permit a more detailed and realistic modeling of Quaternary climate

10.2. LAKES, FAUNAS AND ENVIRONMENTAL POLICY

The effects of ice sheet advance and retreat is thought to have strongly influenced the biogeography and community structure of modern floras leading to the communities of plants and animals we know today (for example, Webb, 1992). Waxing and waning of lake area in response to the precession cycle may have had a comparable effect on the population size and distribution of modern organisms (Jacobs and Hertel, 1994). Large lakes at temperate and subtropical latitudes were pervasive throughout much of the Quaternary. Today such lakes are at a minimum (for example, Street and Grove, 1979). Many organisms presumably evolved in the context of this larger average lake area in the Quaternary. White pelicans inhabiting Pyramid Lake in the Great Basin provide an example. Pyramid lake is a remnant of much larger lakes that occupied the region during much of the Pliestocene. Lakes were as much as three orders of magnitude more extensive in the Pleistocene of the Great Basin than they are today. Presumably the white pelican population became established in the basin when lakes were more extensive, and the pelican population was much larger at that time as well.

Biologists often presume that large aquatic birds, such as the Whooping Crane, were never numerous based on historic records of there abundance and distribution (for example, Matheissen, 1967). These historic observations form the basis for conservation objectives. However, on average during the Quaternary, the habitat available to these birds was much larger than it has been in historic times. Thus the population reductions from the long term average has been much more severe than generally recognized. Presumably populations of pelicans, flamingos, storks and cranes, groups which include some of the most endangered species of birds on Earth today, waxed and waned with the lake environments. When human activity destroys these environments as is currently the case in the Aral Sea, Mono Lake, and Pyramid Lake, and the Tigris delta marshes, it accentuates processes already at natural minimum on time scales of 104 to 106• Most species of crane are endangered by precisely this combination of natural loss of habitat and accompanied by human habitat destruction and depredation. Recognition that certain taxa were already in bottlenecks prior to the effects of human activities, and that they may suffer continued pressure from natural dynamics, suggests that much larger areas of wetlands and other comparable habitats must be preserved if viable populations of taxa such as Storks, Cranes, and Flamingoes are to be preserved.


11. Conclusions

1) Fluctuations of the monsoon driven by the precession cycle can store water sufficient to influence sea-level during geologic periods that lacked continental ice. This mechanism of sea-level change is very likely to have produced sealevel fluctuations documented in Late Triassic carbonate packages of the Alpine region. Similar sea-level changes produced at other time periods when evidence for continental ice is limited may also have been generated in this way. Such times include the Early Cretaceous and Devonian, and possibly the Cambro-Ordovician, Late Permian-Triassic, and Jurassic.

2) Human activity can directly effect sea-level as a result of dam building, aquifer pumping, diverting surface water for irrigation and destruction of forests, among others. These activities change the relative balance and flux of water between the land and sea. Preliminary estimates of sea-level change resulting directly from human activities suggest a current flux equivalent to at least 0.54 mm/year of sea-level rise per year. More research on this topic is needed if the climate driven component of sea-level change is to be isolated climatic.

3) Policy decisions are often based on the premise of a stable, unchanging system. Low and mid latitude lake environments were widespread during the Quaternary. They are now rare as a consequence of climate change driven by orbital fluctuations. Recognition of this may lead to a better understanding of the critical nature of the remaining habitats.

Acknowledgments

We greatly appreciate the effort and insight of the reviewers Bill Hay and Chris Kendall, as well as comments by F. Gasse.

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