Paleoceanography

Is a study of development of ancient ocean system based on the information available from the sedimentary material including microfossils.

Material

• Most paleo-information taken from sediment cores• Extremely expensive to collect, hence valuable• Deep Sea Drilling Program (DSDP), Ocean

Drilling Program (ODP)• International Ocean Drilling Program (IODP),

Glomar Explorer • Ocean sediment record limited to about 180

million years.• Study of paleoceanography is based on microfossils

especially foraminifera, nannofossils and isotope (Oxygen and carbon).

History

• Beginning in 1968, an enormous amount of effort in this Beginning in 1968, an enormous amount of effort in this subject, especially through the work of the CLIMAP subject, especially through the work of the CLIMAP group in Pleistocene oceanography, and through the group in Pleistocene oceanography, and through the extensive drilling in the deep sea by Glomar Challenger. extensive drilling in the deep sea by Glomar Challenger.

Nature of paleoceanography

• Paleoceanography includes:-– Study of water circulation (surface and

bottom currents)– Planktic and benthic development and

evolution– History of biogenic productivity and its effect

on sediment distribution– History of Carbonate and silica deposition and

dissolution.

Proxies in paleooceanography• Microfossils

• Remnants (inorganic structures or organic chemicals) of specific species.

• Modern day activity of these species is extrapolated to the past (warm-water species vs. cold, specific diversity, coiling directions and productivity of microfossils.

• Isotopic Composition of carbon and oxygen isotopes

• - Measure how much of the heavy and light isotope there is in the sediments .

• -The relative abundance of the heavy and light isotope (i.e. the isotopic composition) depend on ocean conditions….especially climate

• -

Oxygen Isotopes and climate• Perhaps no tool has been more highly utilized for

paleoclimate reconstruction than δ O18. Early work by Sam Epstein at Caltech opened the field of quantitative paleoclimate research.– O16 - 99.8% of the oxygen present– O18 accounts for most of the rest. O18/O16 ~ 1/400.

• The changes in Oxygen isotope 18 is expressed by• δO18 = [(O18/O16)sample – (O18/O16)standard] X 1000

(O18/O16)standard• Standard is taken from CaCO3 from Cretaceous

belemnite obtained from Pee Dee Formation, South Carolina

• The utility of oxygen isotopes as paleothermometers has to do with the fractionation that occurs during evaporation and condensation of water and in the formation of CaCO3.

• During evaporation, δO18 of the vapor is less than that of the source because the heavier isotope is preferentially retained.

• When this water condenses, again, the heavier isotope is preferentially condensed and the vapor becomes ever lighter.

• The net movement of water vapor poleward by the atmospheric hydrological cycle produces a latitudinal gradient in δ O18 with the heavier isotope preferentially retained in the tropics. Snow falling at high latitude is very depleted in O18 and thus formation of glaciers leads to enrichment in oceanic O18 while melting of the ice caps reduces this enrichment.

Paleothermometry – using carbonates

• Epstein Equation:• T= 16.9 – 4.2 (δO18c – δO18w)• where δO18c is from the calcite shells of foraminifera• δO18w is the mean value of ocean water when the shells

formed.

• t (ºC) = 16.9 – 4.14(18Ocalcite – 18Owater) + 0.13(18Ocalcite – 18Owater)2

Paleoclimate curve

δO18c in benthic foraminifera record the longtermcooling of the deep ocean. K.G. Miller et al.,Paleoceanography 2, 1, 1987.

Types of climate

• There are two modes of climate– Warm (green-house) – Cold (ice-house)

– Paleoceanographic conditions can be divided into two types.

Polytaxic

Oligotaxic

Polytaxic condition

• Develop during the warm period, No glacier in the polar regions.

• Ocean less stratified • Poorly developed thermocline• Paleogeographic provinces not well-developed• Bottom currents weak• No thermohaline circulation• Ocean bottom poorly oxygenated• Development of black shale due to reducing

condition at the sea-bottom.• Relatively higher number of species

Oligotaxic condition

• Develop during the glacial period the oceans are stratified warm water near the surface and cold water at the bottom.

• Well developed thermocline• Paleogeographic provinces are well developed.• Strong cold dense bottom currents developed.

Oxygenated bottom of the ocean.• Development of thermohaline circulation. Strong bottom

currents• A lot of erosion and hiatuses• Relatively low number of species. • Nunber of species higher in Tropic and reducing towards

the poles

• This increase reflects both growth of ice on Antarctica and a world wide cooling of abyssal waters.

• A substantial carbon isotope excursion toward heavy values

• Synchronous with the onset of abundant sedimentation of organic-rich sediments in the margins of the Pacific.

A drastic increase in oxygen isotope values in A drastic increase in oxygen isotope values in benthic foraminifera between 15 and 13 million benthic foraminifera between 15 and 13 million years ago. years ago.

Paleogeography All through the Tertiary, changes in geography due

to plate motions affect the configuration of exchange between ocean basins.

The gateways control access to the Arctic Ocean (east and west of Greenland), connnect the global ocean along the Equator ("Tethys Ocean" between Africa and Eurasia, Panama Straits, Indonesian Seaway between Borneo and New Guinea), and control the evolution of the Circumpolar Current (Tasmanian Passage, Drake Passage).

The Great Partitioning

Fig.9.12 Geography of the middle Eocene (ca 45 Ma) and major critical valve points for ocean circulation. Tropical valves are closing (filled rectangles), high latitude valves and opening up (open rectangles) throughout the Cenozoic.[Base map from B. U. Haq, Oceanologica Acta, 4 Suppl.:71]

Example of using microfossils to ‘reconstruct’ change in ocean current pattern

Warm water surface circulation cut off between N. and S. America.

Tertiary Oxygen Isotope Record

• An overall cooling trend since the Cretaceous, from the increase of oxygen-18 in benthic foraminifera.

ice sheets

1070 60 50 40 30 20 0

Age Ma

Plio-

Pleistocene

MioceneOligoceneEocenePaleo.

Cretaceous

possibly ice free

"ice free"

2

4

6

8

10

12

14

modern

0

4

6

8

2

Modified after Miller et al.,Paleoceanography, 2, 1-19, 1987.

Antarctic Ice Sheets

N. Hemisphere Ice Sheets

4

3

2

1

0

-1

18 O

äT°C

(A) 1-My means for planktic foraminiferal assemblage sizes95/5 from tropical and subtropical sites (squares) and from temperate and subpolar sites (triangles). The vertical line shows the mean size95/5 (389 µm) of all assemblages. Light blue and orange shading shows ±1 standard deviations for mean sizes95/5. Plio., Pliocene; Plt., Pleistocene. (B) The global deep-sea oxygen isotope record representing Cenozoic polar cooling and ice accumulation (23). (C) The total number of planktic foraminiferal species globally known per 1-My interval (18).

Correlation based on foraminifera

Fig.9.18 Reconstructions of CCD fluctuations for various oceanic regions. Solid and dashed lines. Reconstructions of Tj. H. van Andel et al., 1977, J Geol 85:651. Dotted line Reconstructions of W. H. Berger, P. H., 1975, Rev Geophys Space Phys 13:561. The reconstructions agree in the general patterns of the fluctuations, which appear correlated with sealevel changes on the whole.

Atlantic and Pacific CCD Fluctuations

• The CCD stood high in the late Eocene, dropped in the earliest Oligocene, rose in the Miocene when it reached a peak between 10 and 15 years ago, and then fell to its present depth near 4.3 km.

CCD Fluctuation• An overall similarity in the CCD fluctuations of

Pacific and Atlantic: a sign that the chemical climate of the ocean is changing on a global scale.

• Eocene and Miocene have a shallower CCD than Oligocene and Plio-Pleistocene.

• A certain parallelism of this pattern with oxygen isotope variations and with sea-level variations.

• Periods of high sea level are characterized by shallow CCD and warm high latitudes; periods of low sea level have a deep CCD and cold high latitudes (and deep waters).

Fig.9.21 "Oceanic Anowic Events" of Schlanger and Jenkyns, and δ13C record of pelagic marine limestones, showing coincidence of the "events" with positive δ13C excursions (generated through the lock-up of 12C-rich carbon).  Width of blank band reflects uncertainty in the value of δ13C . Lower part; Cretaceous stages from Berrassian (right) to Maastrichtian (left). OAEs are centered in the Aptian, Cenomanian/Turonian and Santonian/Campanian

"Anoxic Events" and Volcanism • The widespread occurrence of Cretaceous organic-rich

sediments • Distinct positive excursions in the δ13C record

Two Modes of Circulation

• Warm Mode– N. Atlantic Deep Water– Shallow Compensation

• Cold Mode– N. Atlantic

Intermediate and Upper Deep Water

– Deep Compensation