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JOURNAL OF QUATERNARY SCIENCE (2000) 15 (4) 419434Copyright 2000 John Wiley & Sons, Ltd.

Palaeoreconstruction of the Amazon Riverfreshwater and sediment discharge using

sediments recovered at Site 942 on theAmazon FanM. A. MASLIN1,2*, E. DURHAM1, S. J. BURNS3, E. PLATZMAN4, P. GROOTES5, S. E. J. GREIG1, M-J. NADEAU5,MARKUS SCHLEICHER5, U. PFLAUMANN2, B. LOMAX1 and N. RIMINGTON61Environmental Change Research Centre, Department of Geography, University College, 26, Bedford Way, London, WC1H

0AP, England2Geologisch-Palaontologisches Institut, Universitat Kiel, Olshausenstrasse 40, 24098 Kiel, Germany3Stable Isotope Laboratory, Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland4Department of Geology, University College London, Gower Street, London, England5Leibniz Labor fur Altersbestimmung und Isotopenforschung, Max-Eyth-Str. 1113, D-24118 Kiel, Kiel University, Germany

6Department of Earth Sciences, Cardiff University, PO Box 914, Park Place, Cardiff, CF10 3YE, Wales

Maslin, M. A., Durham, E., Burns, S. J., Platzman, E., Grootes, P., Greig, S. E. J., Nadeau, M.-J., Schleicher, M., Plfaumann, U., Lomax, B. and Rimington, N.

2000. Palaeo-reconstruction of the River Amazon freshwater and sediment discharge using sediments recovered at Site 942 on the Amazon Fan. J. Quaternary

Sci., Vol. 15, pp. 419434. ISSN 0267-8179.

Received 29 October 1999; Revised 17 January 2000; Accepted 17 January 2000

ABSTRACT: A continuous reconstruction of palaeoclimate has been generated for the last

12 000 14C yr BP, from ODP site 942 (545N, 496W at a water depth of 3346 m), drilled tothe west of the Amazon Fan. Records from the site suggest that during the Younger Dryas, the

Amazon Basin was extremely dry, and Amazon River discharge was low. This increased aridity

is hypothesised to be due predominantly to reduced precipitation. In addition, there is evidencefor a discharge event at the end of the Younger Dryas, coeval with Termination 1B, that

produced an estimated Amazon River outflow equivalent to present-day values, and an increase

in sediments derived from the Andes. The timing of this event, coincident with the warming ofthe Andean Ice Sheet, suggests that it was at least partly driven by meltwater produced by the

retreat of Andean glaciers, but also required an increase in regional rainfall resulting from

changes in climate. Site 942 also demonstrates that the sediment input to the western part ofthe Amazon Fan from the river ceased between 9900 and 9500 14C yr BP, at a time when sea

level was 4050 m below the present level. If this value is truly indicative of the sea levels at

which the sediment supply to the fan switched off, then it is far greater than the 30 m belowcurrent sea-level suggested previously. Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: stable isotope ratios (oxygen and carbon); sea-surface temperature estimates; planktonic

foraminiferal assemblages; magnetic parameters.

Introduction

In 1542 Francisco de Orellana led the first European voyagedown the Amazon River (Smith, 1990). Not only did this

* Correspondence to: M. A. Maslin, Environmental Change Research Centre,

Department of Geography, University College, 26 Bedford Way, London

WC1H 0AP, UK.

E-mail: [email protected]

Present address: Open University, Milton Keynes, MK7 6AA, UK.

Present address: Department of Animal and Plant Science, University of

Sheffield, Alfred Denny Building, Western Bank, Sheffield, S10 2TN, UK.

Contract grant sponsor: The Deutsche Forschungsgemeinschaft

Contract grant sponsor: Natural Environment Research Council (UK)

Contract grant number: GR9/03526

intrepid voyage give the Amazon River its name (Smith,

1990), but it started an almost mystical wonder of the

greatest river in the world, something we still feel today.The Amazon River discharges approximately 20% of all

freshwater carried to the oceans and its associated basin is

the largest in the world, covering an area of 7 050 000 km2

(Franzinelli and Potter, 1983). The Amazon River freshwater

discharge is over 6300 km3 yr1 (ca. 0.2 Sv) and carries withit nearly 1 Gt of sediment per year, over 80% of whichoriginates in the Andes (Milliman and Meade, 1983; Meade,

1994). This massive output of sediment to the Atlantic Oceanis the primary reason for the long extended continental shelfand has generated the Amazon deep-sea fan complex. The

sediments deposited in the Atlantic Ocean via the AmazonRiver have been shown to provide a unique insight into

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420 JOURNAL OF QUATERNARY SCIENCE

variation in the climate of the Amazon Basin and, in parti-cular, have been used to reconstruct changes in temperature

and aridity during the last glacial period (e.g. Arz et al.,

1998; Haberle and Maslin, 1999; Harris and Mix, 1999).These reconstructions are essential if we are to test the

Pleistocene tropical rainforest-refuge hypothesis (Haffer,

1969) and thus understand the reasons for the immense

diversity and species endemism of the Amazon Basin

(Colinvaux, 1989; Colinvaux et al., 1996; Cowling et al.,in press).

Amazon fan

The Amazon Fan is located off the Brazilian continentalmargin in the equatorial Atlantic and is the third largestmodern mud-rich deep-sea fan. The fan extends 700 km

downslope of the shelf break with an average gradient of0.4 on to the Demerara Abyssal Plain at a water depth of4800 m (Flood et al., 1995). The fan exhibits an elongated

radial pattern covering 330 000 km2 and has a maximumthickness of 45 km. The total volume of sediment has beenestimated to be in excess of 700 000 km3 (Damuth and

Flood, 1985; Flood et al., 1995). The Amazon Fan began todevelop in the early Miocene, following the final phase ofthe tectonic uplift of the Andes, which caused a huge

increase in erosion and therefore sediment transport of theAmazon River (Castro et al., 1978; Curry et al., 1995; Hoornet al., 1995). The Amazon Fan is unique and unusually well

structured because of its onoff supply of sediment, whichis controlled by fluctuations in sea-level (Milliman et al.,

1975). During interglacial periods, when sea-level is high,

sediment supply to the Amazon Fan is switched off, and the

sediment load from the river is transported in longshorecurrents to the northwest and deposited on the continental

shelf, inshore of the shelf break (see Fig. 1). Consequently,much of the interglacial sedimentation on and surrounding

the Amazon Fan is pelagic and accumulates at rates of

between 5 and 10 cm ka1 (Mikkelsen et al., 1997). When

sea-level is low during glacial periods, the terrigenous sedi-

ment load within the river is transported directly to the fan(Damuth and Fairbridge, 1970; Damuth and Kumar, 1975)

resulting in very high sedimentation rates ranging from 100

to over 5000 cm ka1 (Mikkelsen et al., 1997).Seventeen sites were drilled on the Amazon Fan during

Ocean Drilling Project Leg 155 (see Fig. 2). The palaeocli-

matic aim of this leg was to use the extremely high sedimen-

tation rates to obtain continental and oceanic climate recordsthat could be compared in resolution with the Greenland

ice-cores (Dansgaard et al., 1993). This provides a means ofinvestigating rapid changes of climate, hydrology and veg-

etation of the Amazon Basin during the last glacial stage

(e.g. Haberle and Maslin, 1999). These records also enablethe investigation of the regional oceanography, in particular

the circulation changes of the Western Equatorial Atlantic

(e.g. Fig. 2, adapted from Maslin, 1998). Of particular interestis the path of the North Brazilian Coastal Current (NBCC),

the only surface water current to across the Equator (Picaut,1985; Philander and Pacanowski, 1986).

The high sediment supply to the Amazon Fan makes it

very dynamic in nature and susceptible to reworking (Piperet al., 1997; Maslin et al., 1997; Maslin, 1998). Major con-cerns include: local slumping, flows, turbidites and erosion

as well as deposition of older material from higher up thefan complex and from the continental shelf and slope. Great

Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(4) 419434 (2000)

care therefore is required when choosing site locations andwhen interpreting the results. Despite these complications,

it has been shown that, good climatic records can be

obtained from the Amazon Fan (e.g. Damuth, 1975; 1977;Showers and Bevis, 1993).

ODP Site 942

Site 942 (545N, 496W at a water depth of 3346 m) wasdrilled to the west of the Amazon Fan to provide continuous

high-resolution palaeoclimate records. Site 942 lies adjacentto the main Amazon Fan complex and therefore benefitsfrom the enhanced glacial sedimentation rates and the reten-

tion of a continuous record with ample pelagic input forclimate reconstructions. The material from this site was visu-ally scanned in detail for any reworking, including microtur-

bidites, the youngest disturbed section found was at approxi-mately 24 m b.s.f. (below sea-floor), which has been datedat about 40 ka (see Fig. 3).

The location of Site 942 is critical because it monitorsthe meeting and mixing of the North Brazil Coastal Current(NBCC) and the Amazon River freshwater discharge (Fig. 2).

The North Brazil Coastal Current (NBCC) is a key componentof the Atlantic Ocean heat and salinity budget, because itis the only surface water current to cross the Equator,

exporting both heat and salinity to the North Atlantic(Metcalf and Stalcup, 1967; Richardson and Walsh, 1986).From December to June, when wind stress variation causes

an increase in the NBCC transport, the NBCC may extendinto the Guyana Current, which links in with the Caribbean

Current (Picaut, 1985; Philander and Pacanowski, 1986).

The NBCC therefore can influence the temperature and

salinity of both the Caribbean and the Gulf of Mexico,which are the sources of the Florida Current and the Gulf

Stream (Levitus, 1982). The knock-on effect of this is thatthe cross-equatorial transport of the NBCC ultimately can

affect the characteristics of the surface waters reaching the

Nordic Seas, and thus the deep water formed there. Between July and November, the NBCC turns eastward (retroflects)

into the eastward flowing North Equatorial Counter-current

(NECC), switching off this cross-equatorial transport (Fig. 2).It has been speculated that during the last glacial period,

the enhanced zonal winds would have increased the retro-

flection of the NBCC (Fig. 2) and decreased its cross-equa-torial heat transport (Showers and Bevis, 1993; Flood et al.,

1995; Maslin, 1998).

Methods

Stable isotopes

High sedimentation rates at Site 942 mean that very fewbenthic foraminifers were retrieved and therefore oxygen

and carbon isotopes were measured on only planktonicforaminifers. The samples were freeze dried and then wetsieved through a 63 m mesh sieve, dried in a 60C oven

and weighed. The samples were then dry sieved between250 and 350 m from which up to 30, and no less than10, individual planktonic foraminiferal tests were picked

for each species per sample. Each sample of planktonicforaminifers was reacted with 100% phosphoric acid at

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421AMAZON DISCHARGE IN PAST 12 000 14C YR

Figure 1 Changes in sea-level over past 80 ka (calendar years) as compiled by McGuire et al. (1997) from the sea-level reconstructionsbased on Barbados (Fairbanks, 1989) and Pacific cores (Shackleton, 1987) compared with (i) marine transgression over the continental shelf

described by Millimann et al. (1975) (see AD) and (ii) Amazon River sediment influx reconstructed using detailed 14C dating of ODP Site

942C, which shows the Amazon Fan onoff switch occurring between the shaded regions.

90C in an on-line automated preparation system. Theresulting CO2 was analysed on a VG Prism II ratio mass

spectrometer at the Stable Isotope Laboratory, GeologicalInstitute, University of Berne. Corrections for the reaction

between phosphoric acid and carbonate at 90C were

applied to the results. Repeated analyses of standard materialshow a reproducibility of better than 0.1 for 18O and

0.05 for 13C. All results were calibrated to peedee belem-

nite (PDB) scale using the National Bureau of Standards(NBS) 19 standard. Six planktonic foraminifer species were

analysed: Globigerinoides ruber, G. trilobus, G. sacculifer,

Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and

Globorotalia truncatulinoides. These six species were chosenfor their relatively high abundance in the Amazon Fan

sediments (Flood et al., 1995) and because they represent arange of water depths from the near-surface dwellers down

to thermocline dwellers. It should be noted that variations

in both the number of planktonic foraminifer per sampleand low numbers of specimens per sample can be a large

source of error (Trauth, 1995). The enhanced sedimentation

rates of the Amazon Fan, however, mean that the usualbioturbation-induced error is greatly reduced.

Planktonic foraminifer assemblagesOverall stratigraphy of the sediments recovered by ODP

Leg 155 from the Amazon Fan sediments was based onbiostratigraphy (calcareous nanofossils and planktonic fora-


minifers; Mikkelsen et al., 1997), palaeomagnetism (Floodet al., 1995; Cisowski and Hall, 1997), stable isotope records

(Maslin et al., 1997; Showers et al., 1997), amino acid

racemisation (Wehmiller and Hall, 1997), and seismicinterpretations (Pirmez and Flood, 1995; Flood et al., 1995;

Piper et al., 1997a, b). At Site 942 the stratigraphy wasbased on the Ericson Zones (Ericson and Wollin, 1956;

Ericson et al., 1961) and palaeomagntic events (Cisowski and

Hall, 1997) (see Fig. 3). The Ericson zones are based on theappearance of the menardii-complex (G. tumida and G.

menardii) in interglacial deposits and the absence of it in

the glacial deposits. In addition, one planktonic foraminifers

abundance shift datum was used, the disappearance Pulleni-atina obliquiloculata (Yp. obliq.) at approximately 40 ka (e.g.,

Prell and Damuth, 1978). These dates provide the overallstratigraphy of Site 942 shown in Fig. 3.

Sea-surface temperature estimates

Sea-surface temperature (SST) estimates were calculated fromthe relative abundance of planktonic foraminifer species. Toobtain samples for relative counting of planktonic foraminifer

species, the dried samples 63 m were sieved at 150 mand then split using Soiltest CL-242A as many times asrequired to obtain a subsample of approximately 300 whole

planktonic foraminifers. The final split was placed on amicropalaeontological picking tray. All whole, or nearly

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Figure 2 The modern-day seasonal variation in the flow of the NBCC (Picaut, 1985; Philander and Pacanowski, 1986) and the postulatedglacial circulation reconstructed from isotopic studies of Leg 155 sediments (Maslin et al., 1997; Maslin, 1998). Squares are the locations of

the 17 sites that were drilled as part of Leg 155. NBCC, North Brazil Coastal Current; NEC, North Equatorial Current; NECC, North

Equatorial Counter-current; NBCC Retro, retroflection of the NBCC.

whole, planktonic foraminifers, were identified and counted

using the CLIMAP group taxonomy (Kipp, 1976).A major assumption when estimating environmental para-

meters from abundance data is that species distributions arerelated systematically to only one parameter of the environ-ment in which they live (Imbrie and Kipp, 1971; Birks

et al., 1990). This contains two assumptions, a systematicrelationship and dominant control by a single parameter,which are conceptually different. In reality, every assemblage

is jointly influenced linearly and/or non-linearly by manyfactors, such as nutrient availability, light intensity, inter-


specific competition, temperature, salinity, etc. (e.g. Be,

1977). Different combinations of these controlling factorsmay lead to the same faunal composition.

In addition the fossil assemblage may not necessarilyreflect the true living floating foraminiferal assemblage.Changes from the true assemblage may be caused by

factors such as: differential transportation to the sediment,differential dissolution of species, bioturbation, sample split-ting errors, and errors in species identification. The final

problem, the fixed sum problem, associated with themethods of estimating SSTs quantitatively is that the data

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Figure 3 Hole 942A, comparison between stable isotopic records of Globigerinoides Sacculifer (Showers et al., 1997), the relativeabundance of key biostratigraphical species and SIMMAX estimated SST and palaeoproductivity. Note that (b) denotes a biostratigraphical

age (Mikkelsen et al., 1997), (pm) denotes a palaeomagnetic event (Cisowski and Hall, 1997) and MT is an abbreviation of micro-turbidite

as found by Flood et al. (1995).


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are usually in percentage values and not in absolute abun-dance values. This means that common species covary

inversely, even without any inverse relationship in their

absolute abundances.These problems were monitored using the fragmentation

ratio and the planktonic to benthic foraminifers ratio and

were considered when interpreting the SSTs estimates. In

this study the SIMMAX modern analogue technique (MAT)

was used to estimate SST (Pflaumann et al., 1996). This isbecause this technique also can be used to reconstruct

surface water productivity. Reviews of this and othermethods of estimating SST are given in Prell (1985) and

Pflaumann et al., (1996).

The SIMMAX MAT calculates the similarity coefficientsbetween a down-core sample and each of the core-top

samples (e.g., Prell, 1985; Pflaumann et al., 1996). From

among all the core-top samples, it then selects a subset ofsamples that are most similar to that down-core sample as

modern analogues. The SST and productivity estimate of the

core sample is obtained by averaging the values of itsmodern analogues by either a weighted or unweightedapproach. In the weighted solution, a sample with a larger

similarity coefficient contributes more to the SST or pro-ductivity estimate. Moreover it must be remembered that theerrors associated with the SIMMAX SST estimates are at least

1.2C at the 80% confidence limit. The errors for theproductivity estimates are less easily quantified but are large.The methodological errors, however, do not take account

of the continuing debate on the reliability of foraminiferalSST estimates in the tropics, which seem to constantly under-estimate the glacialinterglacial temperature change (e.g.,

Hostetler and Mix, 1999; Mix et al., 1999).

Magnetic parameters

Samples from 942C were separated into the fine- ( 63 m)and coarse-grained fractions. The fine-grained fraction was

then treated with ascetic acid to remove the calcium carbon-

ate (CaCO3), and hydrogen peroxide for removal of organic

material. The samples were then freeze-dried overnight and

weighed, before grain-size and magnetic characterisationanalyses were carried out.

Magnetic characterisation studies measure variations in

the major mineral magnetic properties of natural materials,resulting from changes in composition, concentration and

particle size. These properties have been found to be sensi-

tive indicators of environmental processes (King and Chan-

nel, 1991). Magnetic characterisation studies were carriedout on approximately 100 samples from site 942C. A small

amount of each sample of known weight was packed tightlyinto a plastic bag to prevent the material from moving about,

and the bag was placed in a small plastic vial. A blank vial

was also prepared, containing only an empty plastic bag,in order to allow all measurements to be corrected for

the containers.

Magnetic susceptibility

Magnetic susceptibility measurements were made on the

Kappabridge KLY-2 Magnetic Susceptibility system at theDepartment of Geology, University College London. Mag-netic susceptibility is a measure of the ease with which a

material can be magnetised, and often reflects the concen-tration of magnetic minerals within a sample. Volume mag-


netic susceptibility measurements were recorded in 106

dimensionless SI units and corrected by subtracting the sus-

ceptibility of the container. The measurement was then con-

verted to a mass specific magnetic susceptibility, by dividingthe volume susceptibility by the density, of each sample, in

order to obtain a measurement in 106m3 kg1. The volume

of each sample was taken as 10 cm3, the volume of the

plastic containers assumed by the magnetic susceptibility sys-

tem.

Anhysteritic remanent magnetisation

Anhysteritic remanent magnetisation (ARM) is sensitive to

both the concentration of magnetic minerals within a sample,and also the grain size of the magnetic material. It is

imparted by subjecting the sample to a strong alternating

magnetic field, which is smoothly decreased to zero inthe presence of a weak DC bias field. The samples were

demagnetised initially in a peak AF field at 200 mT, which

was then decreased to zero with a steady decay rate of

0.005 mT half-cycle using a D-2000 A.F. Demagnetiser. AnARM was subsequently applied to the sample by subjecting

it to a peak AF field of 100 mT with a biasing DC field of0.05 mT, then letting the AF field decay at a rate of

0.005 mT. The samples were demagnetised along one axis

(the z-axis) only, because experimentation on severalsamples suggested that the difference in measurement when

demagnetising along all three axes (x, y and z) was negli-

gible. The intensity of magnetisation or the value of ARMin amps per metre (A m1) was then measured by placing

the sample in the Geofyzika JR-5A high sensitivity spinner

magnetometer. These measurements were corrected by sub-tracting the value of the container. The readings were then

altered to A m2 kg1, which adjusts the measurement

according to the mass of each sample, by dividing themeasurement by the density, with a standard volume for

each sample assumed by the system to be 11.15 cm3.

Isothermal remanent magnetisation

Isothermal remanant magnetisation (IRM) is the magnetis-

ation acquired by a sample exposed to a (strong) DC mag-

netic field. As the intensity of the magnetic field increases,the acquired magnetisation increases until the sample

reaches a saturation point, beyond which it cannot increase.

This value is a measure of the saturated isothermal remanent

magnetisation (SIRM). Two isothermal remanent magnetis-ations were imparted on the samples. The first magnetisation

involved the application of a pulse of a magnetic field of 2tesla (T) and resulted in the acquisition of the SIRM, a

measure of the concentration of magnetic minerals within

the sample. The samples were placed in a holder andinserted into the ASC IM1030 Impulse Magnetiser, which

produced a short duration high field pulse when the trigger

was pushed. The value of the SIRM was then measured byinserting the sample into the Geophyzika JR-5A spinner mag-

netometer.Following measurement of the SIRM, an IRM of 0.3 T was

applied to the samples, in the reverse direction to the field

of 2 T. The samples were then measured in the spinnermagnetometer. The values for the SIRM and IRM

0.3T werecorrected for the container, and recalculated to units of

A m2 kg1 to account for the weights of the samples, bydividing by the density, assuming a volume of 11.15 cm3.

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These two IRM measurements were used to obtain theHIRM (hard isothermal remnant magnetisation), defined as

HIRM = (IRM0.3T + SIRM)/2

and the S ratio, which is defined as

S ratio = IRM0.3T/SIRM

The HIRM reflects the concentration of high coercivity min-

erals, such as haematite and goethite within the material(Thompson and Oldfield, 1986). The S ratio is a quantitativemeasure of the degree of saturation, or a measure of the

proportion of lower coercivity magnetic minerals to highercoercivity magnetic minerals (King and Channel, 1991), andis sometimes considered as the proportion of magnetite/

haematite.

Grain size

Grain-size analyses were made on approximately 100

samples using the Sedigraph 5100, which incorporates abuilt-in computer interface for storage and analysis of thedata. The Sedigraph uses a collimated beam of X-rays to

sense the changing concentration of fine particles settling in

a suspension through time (Coakley and Syvitski, 1991) toprovide precise measurements of the grain-size diameter

spectra within the samples. Approximately 2 g of each sam-

ple was mixed in a beaker with 40 ml of calgon. The samplewas stirred and then placed in an ultrasonic bath for 2 min

before being poured and loaded into the Sedigraph machine.

After each analysis the Sedigraph machine was rinsed anddrained.

The Sedigraph provides mass per cent measurements, bothcumulatively and independently for various size fractions

between 1 and 63 m and a record of the total amount ofmaterial finer than 0.98m. It was also able to provide amode and median grain-size diameter for each sample.

Age model

The age model for coarse resolution study of Hole 942A isbased on biostratigraphy, palaeomagentic events and oxygen

isotope stratigraphy as described in Piper et al. (1997b) and

Maslin et al. (1998) (see Fig. 3). The high-resolution age

model for Hole 942C is based on 14 AMS radiocarbondating of planktonic foraminifers (Table 1). All the AMS

radiocarbon dates were measured at the Leibniz Labor furAltersbestimmung und Isotopenforschung, Kiel University.

There are two major problems with the 942C age model.

The first is that there is a coring gap of 4 m in the core,therefore material between 12 000 and 20 000 14C yr has

been lost. In this study only the last 12 ka are studied in

detail, results of 2050 ka are presented in Durham (1997)and Grieg (1998). The second problem associated with the

age model is caused by four dated samples that lie withinthe 14C plateau between 9500 and 10 000 yr. The adoptedage model through this region therefore is based on sedimen-

tation rates. A large increase in terrigenous material occursbelow a depth of 95 cm, and this is assumed to be theinflection point of the age model (see Fig. 4). Sedimentation

rates, based on calendrical years, after calibration, usingcalib 3 (Stuiver and Reimar, 1993), are given in Fig. 5,


Table 1 AMS radiocarbon dates on wood fragments and planktonicforaminifers picked from sediment recovered from Hole 942C

Depth Wood Planktonic Species

(m) fragments foraminifers

0.190 1222 Globigerinoides trilobus

0.350 2614 Globigerinoides trilobus0.510 4614 Globigerinoides trilobus


0.690 8182 Globigerinoides trilobus0.800 9351 Globigerinoides trilobus


0.970 9270 Mixed1.050 9600 Mixed

1.470 11910

1.800 10110 Mixed2.040 11580 10290 Mixed

2.930 12210 10220 Mixed

3.540 12390 11390 Mixed4.140 12440 12000 Mixed

Figure 4 Age versus depth plot for Hole 942C based on AMS14C dates shown in Table 1. Note that the planktonic foraminifershave been calibrated with a standard ocean reservoir 400 yr

correction. The age model chosen is based on a control point

selected due to sedimentation rate constraints, see text. Note that

wood fragments were also dated and their ages ranged from

12 390 to 11 580.

which shows the decrease in sedimentation rate after9750 yr.

Discussion

Sediment discharge into the Amazon Fan

Controls on sediment discharge

The type and amount of sediment discharge by the AmazonRiver is controlled by many factors, including underlying

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Figure 5 Age versus depth plot for Hole 942C based on the age model shown in Fig. 4, sedimentation rates in calendrical years, aftercalibration using calib 3 (Stuiver and Reimar, 1993). Filled squares are the AMS 14C dates and open squares are the sediment samples in

this study.

geology, climate, relief and vegetation type, which all influ-

ence weathering patterns and rates. The geology of the

Amazon Basin is complex, consisting of the Andean cordil-lera in the west, the Brazilian and Guianan Precambrian

shields to the north and south of the river, respectively, and

Palaeozoic to Quaternary sedimentary strata exposed in 30

50 km-wide belts parallel to the main stem of the river. Inaddition, nearly 90% of the length of the Amazon is below

200 m in elevation and runs near the Equator through wet,hot jungle (Sioli, 1975). The remaining 10% runs through

the Andes Mountains where elevations are as great as

4000 m (Gibbs, 1967). Despite the small proportion of theriver that runs through the Andes it has been estimated that

at present approximately 84% of the Amazon River sus-

pended solid load is derived from this region (Gibbs, 1967).Extensive study of the sands recovered from the Amazon

Fan by ODP Leg 155 confirms that glacial sediments on thefan are dominated by an Andean source (Rimington, 1999;Rimington et al., in press). It has been suggested that there

was a significant increase in non-Andean sediment duringthe last glacial stage as a result of the incision of the AmazonRiver caused by lower global sea-level (e.g. Irion et al.,

1995). Rimington (1999), from the known increase in theAmazon River gradient during the last glacial stage, has


calculated the total possible river erosion and has found the

volume to be equal to 30 Gt, about 30 yr of the current

Amazon River suspended sediment output. This is insignifi-cant when dealing with sedimentation on the Amazon Fan

over a glacial period lasting 60 000 yr.

Sediment storage within the Amazon Basin

Estimates of the length of time that sediment is stored in the

Amazon Basin before being deposited on the Amazon Fan

are difficult to quantify, as most of the material depositedcannot be dated. Microscopic wood fragments have, how-

ever, been found in the sediment at Site 942 during periods

of high terrestrial input. The wood fragments occur over adepth of 3.5 m, representing the timespan between 12 000

and 10 000 yr BP. Five wood-fragment samples were pickedand cleaned and AMS 14C dated (Fig. 4). The dates rangedfrom 12 390 to 11 580 yr (Fig. 4), suggesting that over a

period of 2000 yr a store of wood that accumulated within800 yr was transported to the fan. It has been implied thatvalleys flooded and large lakes formed from 14 ka onwards,

owing to rapidly rising sea-level during Termination IA,which acted as a barrier to river outflow (Irion et al., 1995).

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Figure 6 Comparison of the Hole 942C magnetic susceptibility, ARM, SIRM, HIRM and S-ratio records against age. Shaded bar representsthe Younger Dryas period.

Figure 7 Comparison of the Hole 942C, total sediment % 63 m, lithogenic sediment % 63 m, biogenic sediment % 63 m,lithogenic/biogenic ratio 63 m and % grain size 2 m records against age. Shaded bar represents the Younger Dryas period.


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Hence eroded wood and other sediment may have beentrapped and stored in these flooded valley and lakes and

released only slowly. Alternatively, the wood fragments may

have been transported directly into the Amazon Fan but therewas a significant delay in the re-distribution of sediment from

the main fan to Site 942. Dating of wood fragments is one

of the few means of dating the storage of sediment within

the Amazon Basin and Fan, but many more samples need

to be dated before anything more conclusive can be stated.

Quantity and type of sediment discharge recorded at Site

942

Magnetic characterisation records can be used to monitor

fluxes of terrigenous sediments and to investigate palaeocli-

matic relationships resulting from differences in the concen-trations, accumulation rates, grain sizes and compositions of

magnetic material. The magnetic characterisation records

from site 942C provide information on the timing of terrigen-ous sediment input to the Amazon Fan and on possible

Figure 8 (A) 942C oxygen isotope records of the planktonic foraminifers Globigerinoides trilobus trilobus, G. ruber and Neogloboquadrinadutertrei plotted against depth. (B) 942C carbon isotope records of the planktonic foraminifers G. trilobus trilobus, G. ruber and N.

dutertrei plotted against depth. Note that at 4.25 m deep in the core there is a 4 m gap, which covers 1220 14C yr (Durham, 1997; Grieg,

1998).


sediment sources. The magnetic susceptibility and SIRMrecords show responses to changes in magnetic mineral

concentration, whereas ARM values reflect both magnetic

mineral concentrations and grain size, with ARM increasingexponentially with decreasing grain size. In addition, HIRM

is assumed to represent haematite concentrations, and the S

ratio to represent the magnetite/haematite ratio (see Fig. 6).

The magnetic mineral concentration signal reaching the

Amazon Fan is believed to be predominantly driven bymagnetite, which is a component of the Andesitic material

of the Andes. Magnetite is frequently the primary influenceon magnetic mineral concentration and grain-size records,

because it is more strongly magnetic than haematite. The

high S-ratio values indicate higher proportions of magnetitecompared with haematite, and suggest that the Amazon Fan

generally received large concentrations of magnetite, but

that at times this was relatively depleted by increased con-centrations of high coercivity haematite.

The records of magnetic mineral concentration show a

characteristic decrease within the period representing thetransition from the last glacial period into the Holocene,

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Figure 9 Oxygen isotope records of the planktonic foraminifers Globigerinoides trilobus trilobus, G. ruber and Neogloboquadrina dutertreiplotted against the age model shown in Fig. 4.

implying that the supply of terrigenous material to the fanat this time was considerably reduced, and the increasing

ARM values at this time indicate that the grain-size of themagnetic material reaching the fan was also greatly reduced

(Fig. 6). Holocene magnetic characterisation records indicate

low magnetic mineral concentrations and grain-sizes, sug-gesting that magnetic material, indicative of a terrigenous

input, was being diluted by the relative increase of biogenic

material to the site, as a result of the switch in sedimentsource from terrigenous to pelagic associated with the rise

in sea-level.This transition in sediment type is supported by grain-size

evidence (Fig. 7). The record of the total 63 m sediment

fraction shows a decrease associated with the transition from

the last glacial to the Holocene, which also is observed inthe 63 lithogenic fraction. This suggests a change in

sediment supply from fine-grained terrigenous material tomore coarse-grained pelagic debris, which consists predomi-

nantly of fairly large foraminifer tests. This transition is most

clearly observed in the lithogenic/biogenic ( 63 m) ratio,which shows a decrease in values after Termination IB.

More importantly, evidence from the magnetic parameters

and grain-size data can be used to consider the exact timingat which the Amazon Fan switched off, which can also be

used to determine the sea-level at which this occurred. Itshould be noted that the Younger Dryas and TerminationIA (Durham, 1997; Showers et al., 1997) have very little

influence sedimentation rates and thus on the overall terres-trial sediment input compared with the massive change thatoccurred at the time of Termination IB. A major decrease

in the lithogenic/biogenic ratio in the 63 m fraction,magnetic parameters and the grain-size data is observed


between 10 200 and 950014

C yr BP (see Fig. 10). Duringthis transition the sedimentation rates drop from between 16

and 34 cm ka1 to less than 8 cm ka1 (see Fig. 5), i.e., fromsignificant terrestrial deposition to pelagic dominated sedi-

mentation. Additionally, at about 10 000 14C yr BP the lastwood fragments are found in the samples (see Fig. 4 and

10). The reduction in terrestrial sediment deposition at Site

942 occurred when sea-levels were between 40 and 50 mbelow current levels (Fairbanks, 1989; Fig. 1). This suggests

that sediment supply to the fan from the river occurred only

when sea-levels were lower than this. If this value is trulyindicative of the sea levels at which the sediment supply to

the fan switched off, then it is far greater than the 30 m

below current sea-level suggested by Milliman et al. (1975).

It also suggests that glacial sediment older than 7580 kashould be hard to find in the Amazon Fan because sea-

level at this time had not dropped enough to switch theriver sediment supply into the fan.

Freshwater discharge from the Amazon River

Although there is general acceptance that temperatures dur-

ing the last glacial period were up to 6C cooler in theAmazon Basin (e.g., Rind and Peteet, 1985; Colinvaux, 1989;Colinvaux, 1996), the magnitude of increased aridity is dis-

puted. Haberle and Maslin (1999) showed from pollendeposited in the Amazon Fan that the temperature reductionallowed cold-adapted taxa from montane vegetation to

expand into areas usually dominated by tropical rain forest.Yet, no evidence was found of a dramatic increase in aridity

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Figure 10 Comparison of Hole 942C N. dutertrei 18O (see text), N. dutertrei 13C, SIMMAX estimated sea-surface temperature (SST)(Pflaumann et al., 1996) and the ratio of lithogenic to biogenic material ( 63 m).

nor the proposed massive expansion of the savannahrequired by the refuge hypothesis (Colinvaux et al., 1996;Haberle and Maslin, 1999). Thompson et al., (1995) inferred

increased aridity from the dust records in the Peruvian ice-cores, this is supported by the estimation of the availablemoisture from oxygen isotope records of Lake Juno in Peru

(Seltzer et al., 2000). In addition, Damuth and Fairbridge(1970) inferred increased glacial aridity from increased feld-


spar, chlorite and other coarse clastic components withinthe deep-sea sediments.

More recently Morton and Hallsworth (1994; 1999) used

the apatitetourmaline index (ATi) as a proxy for weathering,because apatite is susceptible to loss through weatheringand tourmaline is not. In the Amazon Basin it appears

that the ATi is more influenced by changes in source area(Rimington, 1999). Another weathering proxy Rb/Sr, which

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is not affect by source, varies very little in the Amazonsediments and thus may suggest little change in the aridity

of the region (Rimington, 1999).

Showers and Bevis (1988) first used the Amazon Fansediments to reconstruct Amazon River palaeodischarge

based on the oxygen isotopic difference between planktonic

and benthic foraminifers from Amazon Fan sediments. This

reconstruction, however, did not take account of the large

and not necessarily coeval temperature changes in both thesurface and deep water above the Amazon Fan, which have

a large effect on both planktonic (Arz et al. 1998; 1999)and benthic foraminifer stable isotope records (Burns and

Maslin, 1999).

From Site 942 detailed oxygen isotope records were pro-duced for six species. The oxygen isotope records of G.

trilobus trilobus and G. trilobus sacculifer and N. dutertrei

and P. obliquiloculata. are virtually identical, therefore Figs 8and 9 illustrate only the oxygen isotope records for G.

trilobus trilobus, G. ruber and N. dutertrei plotted against

depth and age. Most notable is the similarity of the threerecords despite the large range of water depth covered, forexample G. ruber is a surface dweller, whereas N. dutertrei

prefers cooler nutrient-rich waters and lives at greater depthclose to the tropical seasonal thermocline.

The N. dutertrei oxygen isotope record was chosen for

further detailed investigation because N. dutertrei lives atdepth and therefore provides a record that is unaffected byrapid shifts in surface water salinity, which frequently is

influenced by the numerous lenses of freshwater that breakoff from the Amazon River outflow plume (Flood et al.,1995). Instead it records the longer term mixed signal

between the NBCC and the Amazon River freshwater dis-charge. This is evident in the smoothness of the N. dutertrei

record compared with that of the near-surface dwelling G.

ruber. The planktonic foraminifer oxygen isotope records, in

addition to the river signal, contain a record of global icevolume, surface water temperature and local changes in

evaporation and precipitation (e.g., Duplessy et al., 1991;Maslin et al., 1995). In addition the planktonic oxygen iso-

tope record also may be monitoring salinity and positional

changes in the NBCC as a result of variations in the intensityof its retroflection (Maslin et al., 1997). To investigate the

local changes in more detail the N. dutertrei was normalised

to zero and corrected for the ice volume effect (see Fig. 10).The best estimate of the changes in the volume of the

continental ice sheets is derived from the sea-level curve

based on dated coral reefs drilled off-shore of Barbados(Fairbanks, 1989; Bard et al., 1990a, b), with 14C AMS dates

going back to 18 200 400 yr BP with a resolution of up

to a 100 yr. Estimates for the conversion of sea-level to 18Ochanges in the surface waters have ranged from 0.011 m1

to 0.013 m1 (Labeyrie et al., 1987; Shackleton, 1987;Fairbanks, 1989). Duplessy et al. (1991) and Maslin et al.

(1995) adopted a mean value 0.012 m1, which is used

in this study. The N. dutertrei 18O is shown in Fig. 10and represents changes in the local surface water tempera-

ture and salinity.

Arz et al., (1998, 1999) have shown from planktonic oxy-gen isotope records from core sites below the NBCC, but

south of the Amazon River mouth, significant shifts in SSTover the last 80 ka, including an approximate 34C shiftbetween the last glacial period and the Holocene and 2C

shift between the Younger Dryas and the Holocene.Although SST estimates from planktonic foraminifer assem-blages are unreliable in the tropics (e.g. CLIMAP, 1976), the

SIMMAX SST reconstruction at Site 942C also suggests alocal 2C drop during the Younger Dryas (Fig. 10). This


temperature affect of approximately 2C influences the oxy-gen isotope record by ca. 0.5 (ONeil et al., 1969; Shackle-

ton, 1974). If this is taken from the N. dutertrei 18O

record of the Younger Dryas this still leaves approximately1 that can be attributed to changes in freshwater input. If

it is assumed that the modern mixing ratio between the

NBCC and the Amazon River outflow, of 5:1 (Levitus, 1982;

Schmitz, 1995), and the isotopic difference between the

Amazon River water (5; Grootes, 1993) and the NBCC(+1; Arz et al., 1998, 1999; Maslin, 1998) has remained

similar during the last 12 ka, then the local 0.75 representsa drop in Amazon River discharge of approximately 40%

during the Younger Dryas.

This suggestion of an arid Younger Dryas correspondswith both the glaciological and Amazon pollen evidence

(e.g. Clapperton, 1998; Haberle and Maslin, 1999) and is

also supported by carbon isotope evidence. Planktonic fora-minifers (Maslin et al., 1997; Fig. 10) and organic carbon

13C (Schnieder et al., 1997) both, in part, monitor the input

of dissolved plant 12C-rich organic matter by the AmazonRiver discharge (Bird et al., 1992). During the Younger Dryasboth 13C records are extremely positive, indicating a dra-

matic reduction in outflow of plant 12C-rich organic matter,and therefore implying a reduction of Amazon River dis-charge.

At about 10 200 14C yr (ca. 11 400 cal. yr), at the sametime as Termination IB, there is a massive short-lived peakof Amazon River discharge (Fig. 10), which is found in

all the planktonic foraminifer records. This discharge eventcorresponds to a peak in the lithogenic/biogenic ratio (Figs. 7and 10) and the S ratio (Figure 6), suggesting both an

increase in sediment discharge and, because of the increasein magnetite, a greater contribution from Andean sources.

This discharge event is also coeval with the first major

melting of the Andean ice sheet (Thompson et al., 1995).

Hence the increased Andean sediment source and freshwaterdischarge indicate that at least part of the discharge event

was caused by the melting of the Andean ice sheet. It isunlikely, however, that this produced sufficient meltwater to

account for the huge volume of water associated with the

discharge event. Therefore this event is hypothesised to bethe result of a combination of tropical glacier meltwater and

an increase in precipitation. This is reasonable because the

amelioration of the regional climate during Termination IBwould have led to an increase in precipitation. In addition

the step-like increase in the oxygen isotope records suggest

this increase in precipitation is maintained after the dischargeevent (see Fig. 10). After Termination IB there is a gradual

return to modern-day Amazon River discharge values, which

appear to have slowed briefly from 6 to 8 ka, indicating aslight reverse to more arid conditions. Discharge then gradu-

ally increases until it reaches the present-day levels at about

34 ka (Fig. 10).

Conclusion

The Amazon Fan and its adjacent area provides an excellentsource of material containing information concerning theinfluence of glacialinterglacial changes on the climate of

the Amazon Basin. Site 942 has been used to reconstructthe amount and timing of both Amazon River freshwaterand sediment discharge over the last 12 000 yr BP. The

reconstructions suggest that during the Younger Dryas periodthe Amazon Basin was extremely dry, and hence the Ama-

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zon River discharge was very low, mainly as a result ofreduced precipitation. There is evidence for an Amazon

River discharge event at the end of the Younger Dryas,

coeval with Termination IB, during which the estimatedAmazon River outflow was equivalent to present-day levels.

During this discharge event there was an increase in depo-

sition of sediment originating from the Andes and it was

coeval with the warming of the Andean Ice Sheet (Thompson

et al., 1995), suggesting that it was, at least in part, the resultof meltwater produced by the retreating Andean glaciers.

However, an enhanced level of regional rainfall, due to theamelioration of climate, is also required, and this level

persisted after the end of the discharge event. Site 942 also

provides evidence that the sediment input to the westernpart of the Amazon Fan ceased between 10 200 and 950014C yr BP, when sea-level was between 40 and 50 m below

the present level. This suggests that sediment supply to thefan from the river occurred only when sea-levels were belowthis. If this value is truly indicative of the sea-levels at which

the sediment supply to the fan switched off, then it is fargreater than the 30 m below current sea-level suggested byMilliman et al., (1975).

Acknowledgements The authors are grateful to Catherine Pyke and

Nick Mann of the Department of Geography, UCL, Drawing Office.

The Deutsche Forschungsgemeinschaft and NERC (grant GR9/03526)

for supporting this study. We would like to thank Drs Kroon and

Ganssen whose extensive suggestions greatly improved this paper.

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Documents

Maslin 2000