maslin 2000
TRANSCRIPT
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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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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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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-
Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(4) 419434 (2000)
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-
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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-
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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,
Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(4) 419434 (2000)
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
0.610 6400 Globigerinoides trilobus
0.690 8182 Globigerinoides trilobus0.800 9351 Globigerinoides trilobus
0.800 9754 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
Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(4) 419434 (2000)
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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427AMAZON DISCHARGE IN PAST 12 000 14C YR
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).
Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(4) 419434 (2000)
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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429AMAZON DISCHARGE IN PAST 12 000 14C YR
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
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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-
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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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431AMAZON DISCHARGE IN PAST 12 000 14C YR
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
Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(4) 419434 (2000)
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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432 JOURNAL OF QUATERNARY SCIENCE
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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