neogene and quaternary development of the neotropical rain

8/11/2019 Neogene and Quaternary Development of the Neotropical Rain

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.Earth-Science Reviews 44 1998 147183

Neogene and Quaternary development of the neotropical rainforest: the forest refugia hypothesis, and a literature overview

Henry Hooghiemstra a,), Thomas van der Hammen a,b

a (Hugo de Vries Laboratory, Department of Palynology and PaleorActuo-ecology, Uniersity of Amsterdam The Netherlands Centre for

)Geo-ecological Research, ICG Kruislaan 318, 1098 SM Amsterdam, Netherlandsb

Tropenbos-Colombia, Apartado Aereo 036062, Bogota, Colombia

Received 13 March 1998; accepted 26 June 1998

Abstract

The upheaval of the northern Andes in Miocene and Pliocene time changed the drainage system in northern South

America significantly and caused the present-day rain forest areas of Choco and the Lower Magdalena Valley becameseparated from Amazonas. Plant diversity may have reached the highest level in the Miocene or Pliocene, and excessive

present-day phytodiversity may be regarded as a legacy of the Tertiary, rather than an evolutionary product of the

Quaternary. In the Quaternary strong temperature oscillations, related to the series of ice-ages, were superposed on the LateTertiary forest dynamics, which included river displacement and latitudinal migrations of the equatorial rain belt caloric

.equator with the rhythm of the precession cycle of orbital climate forcing. The hypothesis that claims a permanent rain

forest cover all over the Amazon basin during the last glacial is in contrast with the forest refugia hypothesis, which

accepts replacement of rain forest by savanna, or savanna forest, during dry climatic intervals. Both scenarios have beenevidenced by pollen records. In this paper, it is suggested that both hypotheses are not necessarily conflicting and apparently

did occur in different parts of the Amazon basin, and in different periods, depending on the climatological constraints. A

compilation of the most important literature concerning the vegetational, climatic, and environmental history of the rain

forest areas of Amazonas and Choco, and surrounding dry ecosystems has been included. q 1998 Elsevier Science B.V. Allrights reserved.

Keywords:Neogene; Quaternary; rain forest; forest refugia; Amazonas

1. Introduction

Apart from paleoclimatological studies using ma-

rine sediments, palynological and paleoecologicalstudies from the continents contributed immensely

during the last decades to the understanding of the

history of our environment and climate. Most atten-

)

Corresponding author. Tel.: q31-20-5257857; Fax:q31-20-

5257662; E-mail: [email protected]

tion has been dedicated to the temperate zones,

whereas the history of climate, environment, and

ecosystems of the tropical and subtropical areas is

less understood. Spatial studies covering the globe ofthe past decade, highlight the unacquaintedness with

the tropical part of the earth system.

Tropical rain forests are well known for their high .biodiversity Groombridge, 1992; Davis et al., 1997 .

But it is unclear which conditions in the past have

permitted the evolution of such high degree of diver-

0012-8252r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. .P I I : S 0 0 1 2 - 8 2 5 2 9 8 0 0 0 2 7 - 0


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( )H. Hooghiemstra, T. an der HammenrEarth-Science Reiews 44 1998 147183148

sity and, apparently, also such an effective conserva-

tion of species which developed during geological

time. Originally, the high biodiversity was ascribed

to the stability of the rain forest ecosystem during

the Quaternary. But gradually, it became clear that at

least parts of the tropical rain forest also had experi-

enced a dynamic history as a result of precipitation .change explaining the forest refugia , temperaturechange e.g., Van der Hammen, 1974; Gates, 1976;

.Colinvaux et al., 1989c, 1996; Bush et al., 1990 andriver dynamics Salo et al., 1986; Salo, 1987; Rasanen

.et al., 1987, 1992; Puhakka et al., 1992 . In fact, it is

unclear if environmental stability, or dynamic condi-

tions, have contributed to the present high biodiver-

sity of the tropical rain forests.

The objective if this paper is to provide a concise

history of the development of the neotropical rain

forest during the Neogene, to address the two hy-

potheses concerning the ice-age Amazon, and toprovide a bibliography concerning these topics.

2. Development of the South American tropical

rain forest since the Miocene

2.1. Upheaal of the Andes and deelopment of the

Amazon rier

The upheaval of the different Cordilleras of the

Andes during the Tertiary significantly changed the

river systems, atmospheric circulation and rainfallpatterns in northern South America. Locally, partial

upheaval occurred during certain intervals of theTertiary Van der Hammen, 1961; Case et al., 1971;

Van der Hammen et al., 1973; Kroonenberg et al.,

1990; Helmens and Van der Hammen, 1994; Cooper.et al., 1995 . There was relative subsidence of the

area immediately east of the Andes, giving rise to ahuge accumulation of sediments Rasanen et al.,

.1995; Paxton and Crampton, 1996 . Apart from un-

published studies of deep bore holes by oil compa-

nies, these sediment sequences are still little ex-

plored.

During the Middle Miocene, there were still con-

nections between the present-day Amazon basin and

the Caribbean through the Maracaibo area, and prob- . ably also with the Pacific Hoorn et al., 1995 Fig.

.1 . The rivers of northwestern Amazonia were run-

ning westward, to the Pacific, and northward to the

Caribbean, forming what might be called a paleo-

Orinoco river system reaching the Caribbean in theMaracaibo area. During the Middle Miocene 1610

.million years BP marine incursions occurred in the

Amazon basin during periods of high sea level stands,

leading to extensive mangrove vegetation in the pre-sent-day rain forest area Hoorn, 1993a, 1994a,b,c;

.Hoorn et al., 1995; Rasanen et al., 1995 . Estuarinelacustrine phases occurred when sea level

was lower, giving way to many types of swamp

vegetation and to seasonally inundated forest. Thesetemporal and spatial alternations between salt and

fresh water ecosystems caused a dynamic and di-

verse history for different geographical areas, possi-

bly stimulating floral evolution and biodiversity in

some areas and extinction in others. At places where

lower montane forest was near, altitudinal and latitu-

dinal shifts of vegetation belts may have stimulated

exchange of flora elements and subsequent ecologi-

cal adaptation. Therefore, it may be expected that

vegetation communities have changed continuously

through time. The present-day communities shouldbe regarded as the result of a very long and diverse

history, and a reflection of the present time-slice. As

an effect of the upheaval of the Eastern Cordillera of

the northern Andes, the mouth of the paleo-Orinoco

River is closed during the Late Miocene and the

present Orinoco River was formed. The rivers in the

northwestern part of the present-day Amazon basin

have changed their course toward the east, forming

part of the Amazon river system as we know it

today.

. . .Fig. 1. Paleogeographic maps of the early Middle Miocene a , late Middle Miocene b , and Late Miocene to Present c of northern South

America. The development of the northern Andes, the separation of Choco and Lower Magdalena rain forest areas from the main area ofAmazonian rain forest, and the Middel Miocene fluviolacustrine system in the present-day central Amazon basin is shown. At various

places in the present Amazon basin mangrove vegetation developed under brackish water conditions during periods of high sea-level stands.

Between the Late Miocene and the Quaternary the modern Orinoco River system developed from the paleo-Orinoco River, and the Amazon .River developed as a transcontinental drainage system towards the Atlantic. Modified after Hoorn et al. 1995 .


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( )H. Hooghiemstra, T. an der HammenrEarth-Science Reiews 44 1998 147183 149


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The rain forest area of the Amazon basin became

separated from the rain forests along the PacificOcean, extending from Ecuador to Panama Choco

.biogeographic area , and in the Magdalena Valley .Hoorn et al., 1995 . The neotropical flora of north-

ern South America developed during the entire Ter-

tiary and got its final touch during the Pliocene andQuaternary Van der Hammen, 1974; Gentry, 1982a;

Van der Hammen and Cleef, 1986; Duque-Caro,.1990b; Hooghiemstra and Cleef, 1995 . Phytogeo-

graphic studies e.g., Cleef, 1979; Van der Hammen.and Cleef, 1986 show the source areas of that part

of the neotropical flora that reached northern South

America by migration.

2.2. Impoerishment of Amazonian flora since the

Miocene?

As far as the pollen flora in sediment cores from

the Amazon basin represent the species richness of .the vegetation phytodiversity correctly, we may

.draw some tentative conclusions. Hoorn 1994c

found ca. 280 pollen types in river valley sediments .of Miocene age, whereas Urrego-Giraldo 1994

found 140 pollen types in Holocene sediments of a

comparable environmental setting. It seems that

Miocene plant diversity may have been considerably

higher than today, and that we have to take into

account the possibility of later periods of significant .extinction. Also Wijninga 1996 speculates on the

basis of his palynological and paleobotanical studiesof sediment sequences of Miocene, Pliocene and

Quaternary age that an impoverishment of the

Neotropical flora could have occurred. More in gen-

eral, aspects of the history and ecology related to

biodiversity in the northern Andes and in Amazonas

were summarized and discussed in Van der Hammen .in press, a .

3. Impact of Quaternary climatic change

The final uplift of the Eastern Cordillera of

Colombia to its present-day elevation took place

mainly between 6 and 3 Ma. Upheaval was docu-

mented on the basis of palynological, paleobotanical,and geological studies Van der Hammen et al.,

1973; Helmens, 1990; Helmens and Van der Ham-.men, 1994; Wijninga, 1996 .

The climate and vegetation history of the Pliocene

and Quaternary of the northern Andes is relatively

well known and numerous papers deal with thisaspect e.g., Van der Hammen and Gonzalez, 1960,

1964; Van der Hammen et al., 1973, 1980r81;

Hooghiemstra, 1984, 1989; Kuhry, 1991; Kutzbach

et al., 1993, 1998; Hoorn, 1994c; Hooghiemstra and

Cleef, 1995; Hoorn et al., 1995; Van der Hammen

and Hooghiemstra, 1995, 1997; Wijninga, 1996;.Mommersteeg, 1998 .

At the end of the Pliocene, between 3.2 to ca. 2.5

Ma, there is a significant decrease of temperature on

a global scale. The Pleistocene starts with a major

cold period, considered as the first glacial of theQuaternary stage 100 of the marine oxygen isotope

.record . This cooling is also clearly registered in the

deep bore hole of Funza-II between ca. 470 m and400 m core depth Hooghiemstra and Ran, 1994;

Van der Hammen and Hooghiemstra, 1995, 1997;

.Hooghiemstra and Cleef, in prep. . Pleistocene tem-perature fluctuations at the high plain of Bogota .2550 m , were in the order of 88C.

XThe Funza-I pollen record from Bogota 4850 N;X . 74812 W shows that the 20,000-years rhythm pre-

.cession cycle of the Milankovitch climate forcing

was continuously present, whereas the 100,000-years .rhythm eccentricity cycle occurred only during the

.last ca. 0.8 Ma Hooghiemstra et al., 1993 . Climate

forcing in the precession band is of great importance

as it relates to the latitudinally shifting Intertropical

.Convergence Zone ITCZ , the major system thatdetermines the geographical distribution of precipita-

tion near the equator. As tropical rain forest requires

minimally 1500 1800 mm annual precipitation with-

out dry season, oscillations of the ITCZ in the past,

and the history of the related monsoon system, are of

crucial importance for the history of tropical rain

forest.

3.1. Quaternary climatic conditions in Choco

Air masses from the Pacific Ocean were continu-

ously forced to ascend above the narrow area be-tween the Pacific coast and the Western Cordillera of

Colombia causing heavy cloud formation and con-

vective rains. There is little reason to suppose that

this area with rain forest experienced significant

changes during the Quaternary. The impact of the

precession-forced migrations of the ITCZ-related rain

belt, as discussed for the Amazon basin, are most


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possibly masked by the heavy convective rains at the

western side of the Andes. For that reason it is

hypothesised that the continuous high precipitation

rates during the Quaternary facilitated a continuous

rain forest cover in the Choco biogeographic area.We assume that during the Quaternary Choco did notexperience significant perturbations of a magnitude

that could have impact on the geographical distribu- .tion of rain forest. Gentry 1986a,b mention the fact

that montane forest taxa, such as Podocarpus,

Hedyosmumand Ilex, are present at low elevation in

the very wet Pacific forest. These observations warn

us not to use the presence of montane forest trees at

low elevation as firm evidence of climatic cooling at .sea level Colinvaux, 1996 . Pollen records from

Choco that cover pre-Holocene time are not avail-able.

3.2. Quaternary climatic conditions in AmazonasDuring the Neogene, the neotropical montane and

Amazonian flora was enriched by migration of taxafrom southern source areas austral-antarctic ele-

.ments . After the connection between Central and

South America was established around 54 million .years ago Keigwin, 1982 , the flora was also en-

riched by elements from the holarctic phytogeo-graphic area Van der Hammen, 1974; Cleef, 1979;

Gentry, 1982a; Van der Hammen and Cleef, 1983,.1983r1984, 1986 .

The rich neotropical lowland flora, which devel-oped during the Miocene and Pliocene under alter-

nating conditions fresh water swamps and periods.with marine incursions suffered during the Quater-

nary the series of ice-ages. During ice-ages tempera-

tures at sea-level were some 58C lower and such

conditions lasted several times longer than the

warmer interglacial periods. In fact glacial conditionsare more normal than the present-day ones Colin-

.vaux, 1996, 1997 . These interglacialrglacial tem-

perature changes caused altitudinal shifts of vegeta-

tion belts in areas near mountains, e.g., along the

foot of the Andes and in centralreastern Brazil. This

may have caused exchange of elements between

floras of different altitudinal belts which may have

stimulated biodiversity in the periphery of the rain

forest area.

Northern South America was also influenced by

precession driven climatic change, which was evi-

denced in the long continental pollen record of Funza .for the Quaternary Hooghiemstra et al., 1993 . A

considerable part of the Amazon basin experienced

probably the above mentioned 20 kyr rhythmic

change in precipitation. During relatively dry inter-

vals of the precession cycle only the wettest parts ofthe Amazon basin ) 15001800 mm annual pre-

.cipitation without dry season kept its rain forest

cover, leading to the occurrence of forest refugia .sensu Haffer 1969 . Effects of orbital forcing on the

vegetation were also evidenced on the Yucatan .Peninsula Leyden et al., 1994 . Pollen diagrams

from the Colombian Andes and southern Amazonas

show that precipitation maxima are reflected as

forest phases in the Amazon basin, and as phases

with wet vegetation in the Colombian Andes and

occur at intervals of ca. 20,000 years distance, i.e.,

ca. 55,000 BP, ca. 35,000 BP, and ca. 6000 BP

.Hooghiemstra, 1995; Fig. 2 . Also the pollen record .of e.g., Salitre, south of Amazonia Ledru, 1993 ,

and the pollen record from Lake Valencia in .Venezuela Leyden, 1985 show similar precipitation

fluctuations.

4. Conflicting scenarios of the ice-age Amazon?

Paleoecological data from pollen records on the

Pleistocene history of the Amazonian rain forest is

still scarce, whereas first evidence from ColombianChoco rain forest is now available Behling et al., in

.press . Amazonian evidence is based on geomorpho-logical and geological studies e.g., Irion, 1976a,b,

1984, 1989; AbSaber, 1982; Bigarella and Ferreira,. 1985 , on palynological studies e.g., Van der Ham-

men, 1972, 1974; Absy, 1985; Colinvaux, 1987c,

1989c, 1996; Bush and Colinvaux, 1988; Liu and

Colinvaux, 1988; Absy et al., 1991; Van der Ham-

men and Cleef, 1992; Ledru, 1993; Van der Ham-

men and Absy, 1994; Urrego-Giraldo, 1997; Ledru.

et al., 1998a,b and biogeographical patterns Haffer,1969; Prance, 1973, 1978, 1982b; Andersson, 1979;.Brown, 1982; Gentry, 1982b; Beven et al., 1984 .

During the past years, there has been considerable

discussion about these data and hypotheses, and

several new theories have been put forward, to ex-

plain the very high biodiversity of the Amazonianrain forest and the diversity patterns Colinvaux,


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. . Fig. 2. Pollen records of Carajas Brazilian lowland forest , Lake Fuquene Colombian montane forest belt , Lake La Primavera .Colombian paramo . Vegetation indicative of high precipitation is present around 27,000 BP and 6000 BP: forest in Carajas and a

and TPN 21B. Vegetation indicative of low precipitation is present around 18,000 BP: savanna in Carajas and dry paramo in Fuqu . . . .Geel and Van der Hammen 1973 , Melief 1985 and Salomons 1986 , respectively. Taken from Hooghiemstra 1995, 1997 .


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.1996, 1997 . Here we will discuss briefly the palyno-

logical evidence.

4.1. Late Quaternary pollen records of Amazonas

and the forest refugia hypothesis

.When Haffer 1969 put forward his forest refugia

hypothesis on the basis of zoological evidence, no

pollen records existed from the Amazonian rain for-

est. Based on distribution patterns of Amazonian

forest birds, Haffer recognized centres of dispersal,

which were assumed to reflect refugia where the

tropical fain forest ecosystem survived climatologi-

cal dry conditions. Areas outside such refugia as-

sumedly experienced so much reduction in precipita-

tion that rain forest was replaced by savanna and

savanna forest vegetation.The first pollen record of Amazonas Katira; 98S;

.638W was published in 1972 by Van der Hammen.Subsequently, more records became available. Paly-

nologists working in areas where during the last

ice-age rain forest was replaced by savanna, e.g., inRondonia Van der Hammen, 1972, 1974; Van der

.Hammen and Absy, 1994 supported the forest refu-

gia hypothesis. But palynologists working in other

areas of the Amazon basin, e.g., in the Ecuadorian XAmazonas and the area of Lake Pata 0816 N;

X . 66841 W Colinvaux, 1987a,b, 1996; Bush et al.,.1990 did not find evidence for a replacement of rain

forest by savanna. The controversy about Haffersforest refugia hypothesis was born. Papers discussing

and supporting the forest refugia hypothesis are e.g., . .Simpson-Vuilleumier 1971 ; Prance 1973, 1982a ;

. .Brown et al. 1974 ; Van der Hammen 1974 ; An- . .dersson 1979 ; Brown and AbSaber 1979 ; Haffer

. .1979, 1982, 1987a,b ; Steyermark 1979, 1982 ; . .Andrade-Lima 1982 ; Gentry 1982c, 1992 ; Simp-

. .son 1982 ; Mayr and OHara 1986 ; Aguilar-Del- . .gado 1987 ; Van der Hammen and Absy 1994 ,

.and Hooghiemstra 1997 . Papers claiming the forest

refugia hypothesis cannot be supported by data are .e.g., Colinvaux 1979, 1987c, 1996, 1997 ; Endler

. . .1982 ; Livingstone 1982 ; Nelson et al. 1990 , and .Haberle 1997 .

4.2. Two scenarios

According to our view, both of the above-men-

tioned scenarios did occur. In fact, four aspects are

important when the forest history of the Amazon

basin is considered. .1 The annual migration of the caloric equator

.ITCZ between about 88N in July and 38S in Jan-

uary causes an annual latitudinal shift of the equato-

rial rain belt, leading to seasonal variations in pre-

cipitation. Most of the area has two dry and two

humid periods, whereas the distal areas experience

only one dry and one humid season. . 2 The precession cycle of orbital forcing e.g.,

.Hays et al., 1976; Berger, 1989 causes an oscillation

of the equatorial rain belt as described under the first

point with a period of about 20,000 years. Under the

present-day orbital configuration the southern hemi-

sphere is tilted towards the sun and the caloric .equator ITCZ lies south of the geographical equa-

tor. This configuration brings most precipitation to

the central and southernmost part of the Amazon

basin. Half a precessional cycle in the past 11,000.years ago , the northern hemisphere was tilted to-

wards the sun and the caloric equator was north of

the geographical equator. This configuration brings

most precipitation to the northernmost part of the

Amazon basin and adjacent Caribbean. .3 The temperature record of the Quaternary was

modulated by the glacialrinterglacial cycles. Long

pollen records from the Colombian Andes at 2550 melevation, such as Funza-I and Funza-II e.g.,

.Hooghiemstra and Ran, 1994 , Funza IIArFuquene-

. X

VII Mommersteeg, 1998 , and Fuquene-II 5827 N;X . .73846 W Van Geel and Van der Hammen, 1973

show with high resolution the temperature record of

northern South America with changes of some 88C.

But it is widely accepted now that the tropical

lowlands experienced also temperature oscillations

of some 58C during the last glacialrinterglacial cyclee.g., Colinvaux, 1987c, 1996; Bush et al., 1990;

.Van der Hammen and Absy, 1994 . It seems plausi-

ble that the composition of the Amazonian lowland

forest changed during the Quaternary. Montane arbo-

real taxa could reach lower elevations under condi-tions of lower temperature andror high precipitation .Gentry, 1986a,b . The modern forest composition

should be seen as the situation of the present time-

slice rather than a constant characteristic of the

Quaternary. .4 The concave shape of the eastern slopes of the

Andes between 58N and 158S act as a trap for humid

Atlantic air masses causing continuous convective


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rains in northwestern Amazonas, irrespective of pre-

cession forcing. Therefore, the northwestern part of

the Amazon basin most possibly received continu-

ously high precipitation and explains in this area a

continuous cover of rain forest during the Quater-

nary. Also other minor areas may have experienced a

continuous rain forest cover related to a complex,

and yet poorly understood, pattern of precipitation

related to Walker circulation and an Amazonian

Convergence Zone.

Considering the four aspects mentioned above, it

is very plausible that the paleo-ecological history

was markedly different depending on the geographi-

cal location in the Amazon basin. A continuous

forest cover, expansion of savannas and dunes, and

lowering of temperatures occurred. The hypothesis

claiming a continuous rain forest cover in the Ama- .zon basin put forward by the school of Colinvaux ,

and the forest refugia hypothesis put forward by theschool of Haffer, Prance, Gentry, and Van der Ham-

.men do not exclude each other but reflect two

extremes out of a spectrum of different regional

paleo-ecological histories which are summarized in

the following section.

4.3. Regional egetation histories of the Amazon

basin

( )4.3.1. Site Katira 98S; 638W

Among the first palynological papers on the vege-

tation history of Amazonas was the pollen record of . .Katira Rondonia by Van der Hammen 1972, 1974

.and Absy and Van der Hammen 1976 . This site is

located in the rain forest area and the sequence

represents the sediment infill of a minor valley. The

interpretation of this sequence was improved when

some years ago AMS radiocarbon dates became

available. The pollen record shows rain forest vege- .tation around 50,000 BP Middle Pleniglacial age

and vegetation dominated by grass savanna around .18,000 BP Pleniglacial age Van der Hammen and

.Absy, 1994; Van der Hammen, in press, a . Todaythe site is in the rain forest area and recent sediments

from the area show that pollen grains of rain forest

elements dominate.

( X X )4.3.2. Site Carajas 15832 S; 47840 W

Another important pollen record from lake sedi-

ments comes from the top of table mountains at ca.

700 m elevation in the rain forests of the Carajas

area in eastern Amazonia. The vegetation on the

table mountains consists of edaphically determined

low forest and shrub and open savanna like vegeta-

tion. Rain forest covers the area around the tablemountains and their slopes Absy et al., 1991; Van

der Hammen, 1992; Van der Hammen and Absy,. .1994 . The radiocarbon dated sequence Fig. 2 rep-

resents the last approximately 60,000 years and the

pollen record shows three intervals during which

savanna vegetation is dominant: around ca. 65,000

BP, ca. 40,000 BP and the period of ca. 25,000 to

10,000 BP. These periods of savanna vegetation

alternate with three periods in which forest vegeta-tion dominate around 55,000 BP, 35,000 BP, and

.6000 BP . Three dry savanna periods and three wet

forest periods are evidenced and do show a rhythm

of ca. 20,000 years, related to the precession cycle

.Hooghiemstra, 1995 .

4.3.3. Sites Pantano de Monica and Araracuara

Another relevant pollen diagram is from sedi-

ments of a forest-marsh on top of the lower terrace

of the Caqueta River, south of Mariname Island X X .0844 S; 72804 W in the Araracuara region of

Colombia Amazonas Urrego-Giraldo, 1994, 1997;.Van der Hammen, in press, a . Radiocarbon dates of

this approximately 4-m sequence indicate Upper

Pleniglacial, Late Glacial and Holocene age. The

Late Pleistocene part is dominated by forest vegeta-tion, but the record shows a specific composition

which is considerably different from the Holocene

forest composition. During the Upper Pleniglacial

part Myrtaceae dominate the forest, followed by

Ilex; the presence of some Gramineae pollen grains

and especially of some grains of Podocarpus is

notorious. In this area, there was apparently no Late

Pleniglacial replacement of rain forest by dry sa-

vanna-like vegetation. The local forest composition

was partly different and there might have been lo-

cally more open vegetation, or caatinga type ofvegetation, on sands of the higher terraces. So, it

seems that under climatic marginal conditions,

edaphic factors play an important role in changes in

the composition of forest. This was also observed in X X .pollen record Araracuara-1 0840 N; 72830 W , lo-

cated in the Bonnetia-dominated shrub savannas near

the air strip of Araracuara: a sharp transition from


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Rapateaceae and Xyridaceae dominated open sa-

vanna into Bonnetia-dominated shrub savanna and

dwarf forest was registered. Radiocarbon dates are

lacking in these sandy sediments extremely poor in

organic carbon, but most probably this record repre-sents Late Holocene time Hooghiemstra, unpub-

.lished data .

( X X )4.3.4. Site Lake Pata 0816 N; 66841 W

Based on 12 radiocarbon dates, the sediments of .the upper 160 cm of core Lake Pata Fig. 3 repre-

.sent the last 42,000 years Colinvaux et al., 1996b .

The pollen diagram includes some 45 pollen spectra

and represent an ice-age record like the Carajas

diagram. Reduced precipitation is evidenced from

roughly 30,000 to 14,000 BP. The pollen record

shows, however, that sufficient precipitation re-

mained to support rain forest. Increased representa-

.tion during the last glacial maximum LGM ofcool montane trees, such as Podocarpus, Hedyos-

mum and Ilex, may indicate that temperatures had

lowered. These results are in many respects in agree-

ment with other information. Lake Pata is located in .a predicted forest refugium Fig. 3 and the pollen

record supports the concept published by Van der .Hammen and Absy 1994 . Also the more dominant

role of Ilex in the last glacial Amazonian forests isin agreement with the records from Araracuara Van

.der Hammen, in press, a; Behling et al., in press .

The dry period from roughly 30,000 to 14,000 BP

corresponds to the period of dry conditions regis- .tered in Carajas Fig. 2 , but also in the Colombian

Andes e.g., Lake Fuquene; Van Geel and Van der.Hammen, 1973 and is most possibly related to the

precession cycle.

( )5. The last glacial maximum LGM in Amazonas

First CLIMAP estimates indicated for the LGM

sea surface temperatures only a small temperaturedecrease of ca. 128C CLIMAP Project Members,

.1976, 1981 . However, palynological data from alti-

tudes between 2500 and 4000 m in the Colombian

and Venezuelan Andes showed during the LGM a

temperature decrease of 798C as compared to mod-

ern conditions. The glacial lapse rate may have been

slightly higher than today because of the drier air: .0.7 instead of 0.68C per 100 m Bakker, 1990 .

. .Colinvaux 1989c and Bush et al. 1990 inferred

from Amazonian pollen records a LGM temperature

decrease of maximally 68C. This estimate has been

based on increased percentages of pollen grains of

montane forest taxa, mainly Podocarpus and

Hedyosmum. The question how much temperature

lowered at sea level during the LGM is still in .debate. The observations by Gentry 1986a, 1986b

that montane trees do occur at low elevation under

very high precipitation regimes, such as in Choco,seems of crucial importance. But at this moment we

do not have a calibration tool to use this information

for quantitative temperature estimates. The lowering

of temperature in the tropical lowlands during glacial

time is still in debate. There might be an uncertainty

of "28C, but for the time being a value of 48" 28C

for the LGM temperature depression in the Amazonbasin seems to be a safe figure and interval CLIMAP

Project Members, 1976, 1981; Rind and Peteet, 1985;Colinvaux, 1989c, 1996; Bush et al., 1990; Anderson

and Webb, 1994; Guilderson et al., 1994; Van der

Hammen and Absy, 1994; Broecker, 1995; Stute et

al., 1995; Colinvaux et al., 1996a,b; De Oliveira and.Colinvaux, 1996; Van der Hammen, in press, a .

5.1. Reduced LGM precipitation and the geographi-

cal location of forest refugia

In view of the fact that the climate was much

. drier north Lake Valencia, Venezuela , west Co-. .lombian Andes and south Brazil of Amazonia and

inside parts of Amazonia Rondonia, Carajas, Lake.Pata during the LGM, we may as a first approxima-

tion, and on the basis of the available data, estimate

the effect of the reduction of precipitation in the

entire area of Amazonia. For that aim we may use

the modern precipitation map of Amazonia and the .sites of Katira Van der Hammen and Absy, 1994 ,

Georgetown Wijmstra and Van der Hammen, 1966;. Van der Hammen, 1974 and Carajas Absy et al.,

.1991 , where the forest disappeared, and Araracuara .Van der Hammen et al., 1992a and Lake Pata .Colinvaux et al., 1996b , where the forest did not

.disappear Fig. 3 . Moreover there is the fact that

today we find savannas, and dry forest or cerrado,

where the annual rainfall is less than 1500 mm. If the

rainfall in the area was reduced by 500 mm, Carajas

would be in the savanna zone, and if it were reduced


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11/37


1000 mm, Georgetown and Katira could also be

under savanna, whereas Araracuara and Lake Pata .would remain under forest Fig. 3 . Notorious is the

permanent existence of rain forest in a large area in

western to northwestern Amazonia, besides other

relatively large areas in the centre and northwest of

Amazonia. This reconstruction is, of course, only a

first approximation that should be corrected on the

basis of new and more detailed precipitation maps

and new palynological data.

This hypothesis of a reduced modern precipita-

tion pattern to explain the geographical position of

the LGM forest refugia only makes use of a globally

lower precipitation level during the LGM and the

geography of the Andes. The earlier mentioned as-

pects of precessional climate forcing and the

glacialrinterglacial temperature oscillations will

make this assumption more complex. We assume

that, depending on the geographical area, the vegeta-tion in the Amazon basin during the Late Quaternary

was either permanently rain forest, possibly partly

with a different forest composition compared to pre-

sent-day situation, or experienced periods with

semi-deciduous tropical forest, or even was replaced

by different types of savanna-like vegetation, all

depending changes in annual precipitation, seasonal-

ity in precipitation, temperature changes, and geo-

graphical setting. But in fact the hypothesis of a

reduced modern precipitation pattern is elegant in the

sense that it provides good possibilities to be testedwhen evidence from new sites comes available. In

this respect the results of the newly published42,000-years-long pollen record of Lake Pata Colin-

.vaux, 1996 are interesting: there is evidence of a

drier climate but forest was not replaced by

savanna-like vegetation during the LGM. This site is

located in the area indicated by Van der Hammen .and Absy 1994 as a potential forest refugium.

Therefore, the record of Lake Pata is unable to

invalidate the forest refugium hypothesis and is in

agreement with the pattern predicted.

5.2. Eidence of high precipitation in the Andes and

Colombian Amazonas

During the Middle Pleniglacial cold and wet pe-

riod, in the Andes considerable quantities of gravels

and sands, partly of fluvioglacial origin, were de-posited in the valleys Van der Hammen et al.,

.1980r81 . In the same period terrace gravels, sands

and silts were deposited in the Middle Caqueta area,and probably also by other west Amazonian rivers .Van der Hammen et al., 1992b . After ca. 30,000

BP during the cold and dry Late Pleniglacial, rivers

incised in these sediments and only started sedimen-

tation again during the Late Glacial and Holocene,viz. after approximately 12,500 BP Van der Ham-

.men et al., 1992b . In lower Amazonia, rivers incised

also in earlier terrace sediments, in this case because

of the low sea-level stand during glacial age. During

the following Late Glacial and Holocene sea-levelrise, the lower Amazon valley became a large

estuary, until it was filled up with sediments, and

the present-day broad zone with varzea vegetation,

characterized by seasonal inundation, was formed. It

is clear, therefore, that not only the vegetation suf-

fered the effect of the glacial climate change, but

also the river systems.

6. The ice-age Amazon as reflected in the offshore

marine sediments

Pollen analysis of offshore marine sediment cores

have at several locations proven to be a valuable

source of information concerning environmental and

climatic change on the adjacent continent. Therefore,

a palynological study of cores from the Amazon fan

was expected to be a source of information of the

environmental conditions of the ice-age Amazon.

However, there are several reasons to doubt the

value of the pollen signal from Amazon fan sedi-

ments.

.Fig. 3. Maps showing the modern distribution of rain forest in relation to the present-day 2500, 2000, and 1500 mm precipitation isoline a . . . .After a reduction of precipitation of 25% b and 40% c new 1500 mm iso-contours show the potential area with rain forest. Scenario c

meets the available palynological data for the glacial Amazon: presence of rain forest in Mera, Araracuara and Pata, and replacement of rain .forest by savanna-like vegetation in Katira Rondonia , Carajas, and Georgetown. The situation might also have been intermediate between

. . . .scenarios b and c , forest areas being larger than indicated in c . Modified after Van der Hammen and Absy 1994 .


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First, many different environments supply pollen

to the Amazonian drainage system, of which the

interpretation may be unclear: graminaceous pollen

grains may reflect open savanna-like vegetation, but

also floating grass-rich meadows on the Amazonian .rivers and in the varzeas e.g., Hoorn, 1994c , which

makes the interpretation of such records unsure.

Secondly, glacial periods with sea-level stands

some 120 m below the present-day level caused

massive erosion in the lower parts of the Amazon

drainage system. These sediments have been trans-

ported to the offshore Amazon fan and have been

re-deposited there again. This process must have

caused a mixing of pollen grains from different

source areas and of different ages, possibly including

sediments from earlier glacial-interglacial cycles.

Both aspects should be taken into account when the

pollen signal of sediments from the Amazon fan are

used to infer glacial environmental conditions of theinterior of the Amazon basin.

A third aspect relates to the gallery forests that

often accompany the river valleys in savanna areas.

Such forests prevent pollen grains from savanna

vegetation to reach the river system and we expect

savanna vegetation is poorly represented in the

river-transported pollen spectrum.

The pollen record from core Leg 155 shows a

very monotonous representation of pollen taxa from

all possible source areas: montane forest, lowland

rain forest, gallery forest along rivers, swamps, opensavanna vegetation, aquatic vegetation and fern

.spores Haberle, 1997; Hoorn, 1997 . Moreover, the

interpretation of the pollen record of Gramineae is

ambiguous, as grasses may represent open savanna-

like vegetation, as well as floating meadows rich in

grasses, that occur frequently on Amazonian riverssee also the discussion in Hoorn, 1994c and Hoorn

. . et al., 1995 . Haberle 1997 and Colinvaux 1996,.1997 interpret the monotonous records and low

representation in the Amazon fan sediments of pollen

grains from open vegetation types as evidence of low

representation of savanna-like vegetation and a sta-

ble rain forest cover since the last interglacial. For

the three reasons mentioned above, we think such

conclusions are not justified and Amazon fan pollen

records are very difficult to use as a document to

infer environmental change in the huge drainage

basin.

7. Conclusions

.1 The upheaval of the northern Andes since the

Middle Miocene led to a separation between the rain

forest of Amazonia at one hand, and those of the

Choco and the Magdalena Valley at the other hand. .2 In the intracontinental basin between the old

shields and the newly formed Andes, estuarine

lacustrine phases occurred when sea level was low,

and coastal environments with mangrove vegetation

occurred during high sea-level stands. These tempo-

ral and spatial alternations between salt and fresh

water ecosystems caused a dynamic and diverse

history for different geographical areas, possibly

stimulating floral evolution and biodiversity in some

areas and extinction in others. .3 Although data are scarce, it may be possible

that extinction of plant taxa was a more common

phenomenon in the Quaternary than previouslythought. MiocenerPliocene plant diversity may have

been larger that at present-day. Huge modern phyto-

diversity perhaps should be better regarded as a

legacy from the Tertiary rather than as a product of

the Quaternary. .4 Precession-related changes in the geographical

.position of the caloric equator equatorial rain belt ,

and river dynamics as a result of small tectonic

movements, have possibly been constant factors of

stress on the Amazonian rain forest ecosystem. In the

Quaternary significant changes in temperature, re-lated to the series of ice-ages, increased environmen-

tal stress. .5 The forest refugia hypothesis and the opin-

ion that Amazonian rain forest was not replaced by

savanna-like vegetation during the last ice-age under

climatic dry conditions are not necessarily conflict-

ing. According to our view both scenarios did occur

and represent the extreme scenarios under dry and

wet climatic conditions. .6 Four aspects are important when the forest

.history of the Amazon basin is considered: a the .annual migration of the caloric equator ITCZ be-

tween about 88N and 38S causing an annual latitudi-

nal shift of the equatorial rain belt, leading to sea- .sonal variations in precipitation; b the precession

cycle of orbital forcing causing an oscillation of the

equatorial rain belt with a period of about 20,000 .years; c the temperature oscillations at sea-level of


13/37


some 58C during Quaternary glacial cycles causing

the composition of the Amazonian lowland forest .changed all over the Quaternary of composition; d

the concave shape of the eastern slopes of the Andes

between 58N and 158S act as a trap for humid

Atlantic air masses causing continuous convective

rains in northwestern Amazonas, irrespective of pre-

cession forcing. This explains continuous forest cover .forest refugium in northwestern Amazonas.

.7 The modern composition of Amazonian rain

forests should be seen as the situation of the present

time-slice, rather than a constant characteristic of the

Quaternary. .8 Using a simple reduced modern precipitation

pattern for the Amazon basin the geographical posi-

tion of the LGM forest refugia can be estimated. A

reduction of precipitation between 25 and 40% ex-

plains the available pollen evidence, including sites

with an uninterrupted forest cover Mera, Araracuara,.Pata and sites where rain forest was replaced by

.savanna Katira, Carajas, Georgetown . .9 The monotonous pollen records for all ecolog-

ical groups from sediment cores of the Amazon fan

evidence a thoroughly mixed pollen association orig-

inating from many different vegetation types in the

huge Amazonian drainage basin. The monotonous

grass record cannot be regarded as evidence for the

absence or existence in the Amazon basin of periods

with much savanna. We postulate savanna is poorly

represented in the river-borne pollen signal becauseof the gallery forests along the rivers. Pollen records

from Amazon fan sediments are unsuitable to infer

with any detail vegetational and environmental

changes in the remote hinterland.

Acknowledgements

The first author would like to thank the Nether- .lands Foundation for Scientific Research NWO for

the invitation to participate in the annual Huygens

Lecture, which was the immediate motive to prepare

this paper. Paul Colinvaux is thanked for his gra-

cious contribution in this lecture evening between

opponents. Hermann Behling is thanked for provid-

ing additional references. An anonymous reviewer is

thanked for critical comment and improvement of

the English. This paper is dedicated to the scientific

community of the PoleEquatorPole transect

.through the Americas PEP-I , which forms an exit-

ing forum for interdisciplinary cooperation.

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