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U.S. Department of the Interior

U.S. Geological Survey

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Trace-Element Deposition in the Cariaco 

Basin, Venezuela Shelf, under SulfateReducing Conditions—a History of the 

Local Hydrography and Global Climate, 

20 ka to the Present

Professional Paper 1670
Trace-Element Deposition in the Cariaco 

Basin, Venezuela Shelf, under SulfateReducing Conditions—a History of the 

Local Hydrography and Global Climate, 

20 ka to the Present

By David Z. Piper and Walter E. Dean

Professional Paper 1670

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

United States Government Printing Office: 2002

For additional copies please contact:

USGS Information Services

Box 25286

Denver, CO 80225

This report and any updates to it are available online at

http://geopubs.wr.usgs.gov/prof-paper/pp1670/

Additional USGS publications can be found online at

http://geology.usgs.gov/products.html

For more information about the USGS and its products:

Telephone: 1–888–ASK–USGS (1–888–275–8747)

World Wide Web: http://www.usgs.gov/

Text edited by James W. Hendley II

Layout and design by Sara Boore

Manuscript approved for publication, October 22, 2002

 

Cataloging-in Publication data is on file with the Library of Congress
Contents

Abstract.........................................................................................................................................................................................

Introduction...................................................................................................................................................................................

Analytical Techniques....................................................................................................................................................................

The Age Model...............................................................................................................................................................................

Partitioning Trace Elements between Terrigenous-Source and Seawater-Source Fractions—A Trace-Element Model...................

Trace-Element Depositional Rates in the Surface Sediment—A Test of the Model....................................................................

Trace-Element Accumulation Rates during the Past 20,000 Years—An Application of the Model................................................

22 to 14.8 ka (bioturbated sediment).........................................................................................................................................

14.8 to 10.4 ka (distinctly laminated sediment)......................................................................................................................

10.4 to 0 ka (faintly laminated sediment)................................................................................................................................

Elemental Ratios and Concentrations—An Alternative Model of Environmental Conditions.......................................................

Summary and Conclusions............................................................................................................................................................

References.....................................................................................................................................................................................

Figures

 1. Bathymetry of the northwest continental shelf of Venezuela showing location of core PL07-39PC...............................

2. Concentrations of Al2O3, Cd, Mo, and Zn with depth in core PL07-39PC.......................................................................

3. Age model for Cariaco Basin sediment............................................................................................................................

4. Relation between the concentrations of Al2O3 and CaO in core PL07-39PC..................................................................

5. Relations between the concentration of the terrigenous fraction, and Al2O3, and the concentrations 

of major-element oxides in core PL07-39PC....................................................................................................................

6. Relations between the concentration of the terrigenous fraction and the concentrations of trace elements 

that exhibit approximate single trends with the terrigenous fraction in core PL07-39PC................................................

7. Relation between the concentrations of the terrigenous fraction and manganese (Mn) in core PL07-39PC....................

8. Relations between the concentration of the terrigenous fraction and the concentrations of Mo and Cd 

in core PL07-39PC............................................................................................................................................................

9. Relations between the concentration of the terrigenous fraction and the concentrations of trace elements in core 

PL07-39PC that have strong factor loadings on the same factor as Al2O3, but have concentrations that are significantly greater than can be attributed solely to a terrigenous contribution.........................................................................

10. Percentage of Cd, Cu, Ni, and Zn and Mo and V removed from SO4

2- -reducing bottom waters of modern marine basins.......

11. Schematic for the Cariaco Basin, showing “model” accumulation rates of the marine and 

terrigenous fractions of Cd and Mo...................................................................................................................................

12. Measured accumulation rates of the terrigenous and marine fractions of trace elements versus depth 

in core PL07-39PC......................................................................................................................................................................

13. Relations between the concentrations of Mo and Zn and Mo and Cd in the seawater-derived fractions 

of sediment in core PL07-39PC........................................................................................................................................

14. Relation between the concentrations of organic carbon and sediment depth in piston core PL07-39PC.........................

Tables

1. Concentrations in sediment of core Pl07-39PC of major-element oxides........................................................................

2. Concentrations of major-element oxides and trace elements (1) in the world shale average (WSA) and the 

terrigenous fraction of the Cariaco Basin, (2) in marine biogenic debris, and (3) in seawater.........................................

3. Major components in sediment samples from the Cariaco Basin, in percent, calculated from 

major-oxide concentrations (table 1).................................................................................................................................

4. Correlation coefficients for major-element oxides in sediment from the Cariaco Basin..................................................

5. Varimax orthogonal transformation solution, principal components factor extraction method, of the log10 

of trace-element analyses and major sediment components in sediment core PL07-39PC, calculated from 

major-element-oxide analyses, using Statviewtm II software............................................................................................

6. Concentrations of trace elements in seawater and organic matter in the Cariaco Basin and their 

rates of deposition in the surface sediment of core PL07-39PC.......................................................................................

7. Calculated rates of deposition for the Cariaco Basin of marine Mo and Cd (1) as particulate 

organic matter settling out of the photic zone (the biogenic fraction) and (2) from bottom water 

under conditions of SO4

2- reduction (the hydrogenous fraction).......................................................................................

01

01

04

04

04

28

30

34

35

36

37

38

39

02

03

04

13

21

22

23

24

25

27

29

33

37

38

05

15

16

20

23

26

31
1

Abstract

A sediment core from the Cariaco Basin on the Venezuelan continental shelf, which recovered sediment that has been 

dated back to 20 ka (thousand years ago), was examined for 

its major-element-oxide and trace-element composition. Cadmium (Cd), chromium (Cr), copper (Cu), molybdenum (Mo), 

nickel (Ni), vanadium (V), and zinc (Zn) can be partitioned 

between a siliciclastic, terrigenous-derived fraction and two 

seawater-derived fractions. The two marine fractions are (1) 

a biogenic fraction represented by nutrient trace elements 

taken up mostly in the photic zone by phytoplankton, and (2) 

a hydrogenous fraction that has been derived from bottom 

water via adsorption and precipitation reactions. This suite 

of trace elements contrasts with a second suite of trace elements—barium (Ba), cobalt (Co), gallium (Ga), lithium (Li), 

the rare-earth elements, thorium (Th), yttrium (Y), and several 

of the major-element oxides—that has had solely a terrigenous 

source. The partitioning scheme, coupled with bulk sediment 

accumulation rates measured by others, allows us to determine 

the accumulation rate of trace elements in each of the three 

sediment fractions and of the fractions themselves.

The current export of organic matter from the photic zone, 

redox conditions and advection of bottom water, and flux 

of terrigenous debris into the basin can be used to calculate 

independently trace-element depositional rates. The calculated rates show excellent agreement with the measured rates 

of the surface sediment. This agreement supports a model of 

trace-element accumulation rates in the subsurface sediment 

that gives a 20-kyr history of upwelling into the photic zone 

(that is, primary productivity), bottom-water advection and 

redox, and provenance. Correspondence of extrema in the geochemical signals with global changes in sea level and climate 

demonstrates the high degree to which the basin hydrography 

and provenance have responded to the paleoceanographic and 

paleoclimatic regimes of the last 20 kyr.

The accumulation rate of the marine fraction of Mo 

increased abruptly at about 14.8 ka (calendar years), from less 

than 0.5 μg cm-2 yr-1 to greater than 4 μg cm-2 yr-1. Its accumulation rate remained high but variable until 8.6 ka, when 

it decreased sharply to 1 μg cm-2 yr-1. It continued to decrease 

to 4.0 ka, to its lowest value for the past 15 kyr, before gradually increasing to the present. Between 14.8 ka and 8.6 ka, its 

accumulation rate exhibited strong maxima at 14.4, 13.0, and 

9.9 ka. The oldest maximum corresponds to melt-water pulse 

IA into the Gulf of Mexico. A relative minimum, centered at 

about 11.1 ka, corresponds to melt-water pulse IB; a strong 

maximum occurs in the immediately overlying sediment. 

The maximum at 13.0 ka corresponds to onset of the Younger 

Dryas cold event. This pattern to the accumulation rate of 

Mo (and V) can be interpreted in terms of its deposition from 

bottom water of the basin, the hydrogenous fraction, under 

SO4

2- -reducing conditions, during times of intense bottomwater advection 14.8 ka to 11.1 ka and significantly less 

intense bottom-water advection 11 ka to the present.

The accumulation rate of Cd shows a pattern that is only 

slightly different from that of Mo, although its deposition was 

determined largely by the rain rate of organic matter into the 

bottom water, a biogenic fraction whose deposition was driven 

by upwelling of nutrient-enriched water into the photic zone. 

Its accumulation exhibits only moderately high rates, on average, during both melt-water pulses. Its highest rate, and that 

of upwelling, occurred during the Younger Dryas, and again 

following melt-water pulse IB. The marine fractions of Cu, Ni, 

and Zn also have a strong biogenic signal. The siliciclastic terrigenous debris, however, represents the dominant source, and 

host, of Cu, Ni, and Zn. All four trace elements have a considerably weaker hydrogenous signal than biogenic signal.

Accumulation rates of the terrigenous fraction, as 

reflected by accumulation rates of Th and Ga, show strong 

maxima at 16.2 and 12.7 ka and minima at 14.1 and 11.1 ka. 

Co, Li, REE, and Y have a similar distribution. The minima 

occurred during melt-water pulses IA and IB, the maxima 

during the Younger Dryas and the rise in sea level following 

the last glacial maximum.

Introduction

The Cariaco Basin is located on the continental shelf of 

Venezuela (fig. 1). It consists of two subbasins separated by 

Trace-Element Deposition in the Cariaco Basin, Venezuela 

Shelf, under Sulfate-Reducing Conditions—A History of the 

Local Hydrography and Global Climate, 20 ka to the Present

By David Z. Piper and Walter E. Dean

Introduction
2 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

an 900-m deep saddle. The basin is more than 1400 m deep 

and isolated from deep water of the Caribbean Sea by a sill 

that has a maximum water depth of 146 m. The sill limits 

exchange of water between the Cariaco Basin and Caribbean 

Sea, imposing a residence time of bottom water in the basin of 

approximately 100 yr (Deuser, 1973). As a result of the slow 

renewal of bottom water, oxidation by bacterial respiration of 

organic matter settling out of the photic zone has completely 

utilized O2 and NO3

-

 as electron acceptors and established 

SO4

2- -reducing conditions below approximately 250-m to 

300-m depth (Richards and Vaccaro, 1956; Richards, 1975). 

Oxygen respiration occurs in the uppermost approximately 

200 m of the water column and denitrifying conditions at the 

intermediate depth. This redox profile controls the removal of 

several trace elements from the bottom water by adsorption/

precipitation reactions (for example, Cd, Cu, Mo and V) and 

their deposition on the sea floor (Jacobs and others, 1987; 

Dean and others, 1999) as a hydrogenous sediment fraction.

Organic matter and organic-matter-hosted trace elements, 

or trace nutrients, represent a second sediment fraction, the 

biogenic fraction. Their deposition is governed largely by primary productivity, which averages 1.5 g C m-2 d-1 (Varela and 

others, 1997). Approximately six percent of the organic matter 

fixed by algae in the photic zone annually is exported to depth 

(Müller-Karger and others, 2001), but less than two percent 

actually settles onto the sea floor. The four-percent difference 

represents the organic matter that is oxidized at depth by SO4

2-

reduction, as it settles through the water column (Thunell and 

others, 2000). However, primary productivity is highly seasonal (Peterson and others, 1991). During winter and spring 

when the Intertropical Convergence Zone (ITCZ) is located 

south of the equator, strong northeast trade winds produce 

intense upwelling and elevated primary productivity. Remains 

of the plankton production are preserved in the sediment as a 

light-colored lamina. During summer and autumn, the ITCZ 

shifts north of the equator; trade winds weaken and upwelling 

is reduced.

The summer/autumn shift of the ITCZ produces an 

increase of rainfall along the northern coast of South America, 

resulting in an increase in the flux into the basin of a third 

sediment fraction, the terrigenous fraction and its hosted 

trace elements. The seasonally elevated accumulation of this 

fraction is deposited as a dark lamina that, together with the 

lighter-colored lamina of the winter/spring period of more 

intense upwelling, forms an annual couplet, or varve (Peterson 

and others, 1991; Hughen and others, 1996a).

During the last glacial maximum (LGM), the basin was 

nearly isolated from the Caribbean Sea. A rising sea level, 

beginning about 18 ka (Fairbanks, 1989), imposed a complex 

signal of temporal changes on rates of upwelling, bottomwater advection, and the flux and composition of terrigenous 

debris into the basin (Peterson and others, 1991; Yarincik and 

others, 2000). These changes are now reflected in the lithology 

of the sediment. In piston cores from the deepest part of the 

basin (Peterson and others, 1991, 1995), the sediment is bioturbated below approximately 500 to 1,000 cmbsf (centimeters 

below the sea floor). It is distinctly laminated above this unit 

to about 300 cmbsf. From 300 cmbsf to the surface, the sediment continues to be laminated, but laminae are thinner and 

weakly defined (fig. 2). Peterson and others (1991) interpreted 

the lithologic change occurring between the bioturbated unit 

and the distinctly laminated unit, dated at about 14.8 ka (Lin 

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Figure 1. Bathymetry of the northwest continental shelf of Venezuela, in meters, showing location of core PL07-39PC on the ridge, or 

saddle, between the two subbasins. The figure is modified from Peterson and others (1995).
3

and others, 1997; Hughen and others, 1998), as recording the 

onset of elevated primary productivity in the photic zone and 

of SO4

2- -reducing conditions in the bottom water. Such reducing conditions have continued to the present (Richards, 1975).

The accumulation rates of trace elements should have 

recorded these changes in the hydrography, primary productivity, and redox of the water column. As each source of 

elements—terrigenous, marine-biogenic, and marine-hydrogenous—contributes a unique inventory of several elements, 

partitioning of major-element oxides and trace elements 

into the three fractions provides a check of the calculations 

of the depositional rate of each fraction and the interpretation of the dynamics of the environment that controlled that 

rate. Trace elements in the terrigenous fraction should have 

been controlled merely by the flux of terrigenous siliciclastic 

debris into the basin, but ultimately by provenance. The rate 

of accumulation of the trace-element inventory in excess of 

the terrigenous contribution should have been governed by (1) 

the export of organic-matter-bound trace elements, or trace 

nutrients, out of the photic zone and onto the sea floor and 

(2) redox conditions in the bottom water. The flux of organic 

matter settling into the bottom water contributed to both of 

these factors. Organic matter carries trace nutrients directly 

to the sea floor, for example, Cd, Cu, Ni, and Zn, and it is 

the dominant reductant whose oxidation establishes the level 

of redox conditions—O2 respiration, denitrification, or SO4

2-

reduction—in the water column. The flux, into the bottom 

water, of the three oxidants—O2, NO3

-

, and SO4

2-—and of 

trace elements that make up the hydrogenous fraction can be 

expressed in terms of the composition of the source water and 

the bottom-water residence time, or advection. Similarly, the 

flux of organic matter settling through the water column is 

determined by primary productivity, which is limited by the 

concentrations of nutrients in the source water and the rate of 

upwelling of that water into the photic zone. These relations 

demonstrate the primary role of the basin hydrography to the 

deposition of trace elements.

This study examined the distributions of several majorelement oxides and trace elements in piston core PL07-39PC 

(fig. 1). The current hydrography, marine chemistry, and 

biology of the basin have been used to develop a model of 

elemental depositional rates. The model rates for the trace ele-

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Figure 2. Concentrations of Al2O3, Cd, Mo, and Zn with depth in core PL07-39PC, in centimeters below the sea floor (cmbsf). 

Depth has been adjusted by deletion of a turbidite unit at 170 cmbsf. Sediment is faintly laminated to 300 cmbsf, distinctly laminated between 300 and 600 cmbsf, and bioturbated below 600 cmbsf, with a massive, fine-grained gray layer at 700 cmbsf (Peterson and others, 1991; Lin and others, 1997). Radiocarbon ages are reservoir-corrected AMS 14C ages as reported by Lin and others 

(1997). Calendar ages are given at the far left, using the age model of Hughen and others (1998). Sections of the core indicated by 

shading correspond to the Younger Dryas cold event and melt-water pulses IA and IB (Fairbanks, 1989).

Introduction
4 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

ments agree closely with their measured accumulation rates 

in the uppermost surface sediment. Reversing the problem, 

accumulation rates of trace elements during the last 15 to 20 

ka are used to determine past rates of upwelling into the photic 

zone and bottom-water advection. Unfortunately, modeling 

temporal changes in the hydrography, biology, and chemistry 

of the basin from the accumulation rates of trace elements is 

unavoidably limited by possible changes in the trace-element 

composition of terrigenous debris, plankton (biogenic debris), 

and source seawater from their current composition. On the 

plus side, the occurrence of laminated sediment in the uppermost approximately 600 cm of core PL07-39PC (fig. 2) suggests that the trace-element record for the most recent 14.8 kyr 

has not been made more complex by biological and physical 

reworking of the sediment.

Acknowledgments—Comments and suggestions by colleagues at the U.S. Geological Survey—George Breit, Randolph Koski, Victor Mossotti, and Michele Tuttle—are greatly 

appreciated.

Analytical Techniques

Sediment samples, 1-cm thick, were taken from piston 

core PL07-39PC at a 5-cm interval (table 1). Samples were 

dried at 90 ̊C, then ground to pass a 100-mesh sieve. Elemental concentrations were determined by inductively coupled, 

argon-plasma, emission spectrometry (ICP-AES) in the analytical laboratories of the U.S. Geological Survey (USGS) 

(Baedecker, 1987). Concentrations of major-element oxides 

and trace elements in most samples exceed their detection 

limit by more than tenfold, which allows for a precision of less 

than 5 per cent. Cd is the sole major exception, for which most 

samples from the laminated section of the core have a concentration in the range of twofold to fivefold the detection limit, 

which gives a precision of approximately 10 percent. The precision has been established by comparing analyses of selected 

samples analyzed by ICP-AES, neutron activation analysis, 

and X-ray fluorescence, carried out in three different laboratories (Piper and Isaacs, 1994, 1996). Additional discussions of 

the analytical techniques, precision, and accuracy are given in 

Baedecker (1987).

The Age Model

The age model for the core, developed by Hughen and 

others (1998), plus dry bulk density measured every 5 cm 

permit calculations of bulk sediment and component accumulation rates. Their age model (fig. 3) is based on 29 age 

determinations, by AMS 14C, of hand picked samples of the 

planktonic foraminifera Globigerina bulloides (Lin and others, 

1997). Ages were corrected (fig. 2) for a 420-year marine-14C 

reservoir. Hughen and others (1998) used varve chronology to calibrate the corrected radiocarbon ages between 

12.6 and 7.6 14C kyr, tree rings (Komer and Becker, 1993) 

to calibrate radiocarbon ages after 7.6 14C kyr, and correlation of gray-scale measurements in cores from this basin 

with accumulation rates in the GISP2 Greenland ice core 

to calibrate radiocarbon ages greater than 13 14C kyr. Lin 

and others (1997) further subtracted out a turbidite unit that 

occurs between 170 and 200 cmbsf. The corrected 14C ages, 

as reported by Lin and others (1997), are given in figure 2. 

Throughout the rest of the paper, ages are given in calendar 

years B.P. and designated as ka (kiloannum).

Partitioning of Elements Between 

Terrigenous- and Seawater-Source 

Fractions—a Trace-Element Model

Identifying the hydrography, marine chemistry, and biology of deposition of a sedimentary unit from its marine fraction of trace elements begins by determining the terrigenous 

contribution in each sample. The terrigenous trace-element 

contribution is subtracted from the bulk trace-element composition to give the marine, or seawater, contribution. The 

concentration of Al2O3 is commonly used as a measure of 

the concentration of the terrigenous fraction (Leinen, 1977; 

Isaacs, 1980; Dymond, 1981; Medrano and Piper, 1995). For 

the Cariaco Basin, the Al2O3 concentration in the terrigenous 

fraction itself is determined from the relation between Al2O3

and CaO. The highest values in a plot of the two element 

oxides (fig. 4) represent a mixing curve for calcite and siliciclastic debris. Calcite is derived largely from the water column 

(Werne and others, 2000) and the siliciclastic debris is ter-

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Figure 3. Age model for Cariaco Basin sediment taken from Hughen 

and others (1998). Sediment depth is in centimeters below the sea 

floor (cbsf).
5

Table 1. Concentrations in sediment of core PL07-39PC of major-element oxides.

[All concentrations in percent, except for MnO and trace elements, which are in parts per million. Elemental analyses are by inductively coupled, argonplasma, emission spectrometry (ICP-AES). Ages are in calendar years B.P.]

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Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
12 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

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Table 1. Concentrations in sediment of core PL07-39PC of major-element oxides—Continued.
13

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Figure 4. Relation between the concentrations of Al2O3 and CaO 

in core PL07-39PC. The curve represents a mixing curve for CaCO3

(CaO content of 50.7 percent) and the terrigenous fraction (Al2O3

content of 18.1 percent), shifted up scale to intersect the y-axis at 

the CaO concentration of calcite. The amount that a sample is displaced below the curve reflects its content of biogenic SiO2, organic 

matter, and dolomite.

Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
14 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

rigenous in origin (Yarincik and others, 2000). A curve drawn 

through these values is shifted up scale to intersect the CaO 

axis at 56 percent, the concentration of CaO in pure calcite. 

X-ray diffraction analyses of four of the samples (James Hein, 

personal commun.) have shown that the calcite contains a 

small amount of Mg, requiring a slight downward adjustment 

of the curve. The curve extrapolates to an Al2O3 content in the 

aluminosilicate fraction of 18.1 percent at a CaO content in the 

terrigenous fraction of 3.6 percent, the CaO concentration for 

the world shale average, or WSA (table 2; Wedepohl, 1969-

1978). The Al2O3 concentration is greater than the mean of 

15.5 percent for the WSA, but it is within the range of values 

used to establish the mean (Wedepohl, 1969-1978; Clarke, 

1924). Thus, the conversion factor for the terrigenous fraction 

is 5.5, or (100/18.1) × Al2O3.

Offset of samples from the adjusted curve of Al2O3 and 

CaO largely reflects their content of organic matter and biogenic SiO2 (opal-A). Biogenic SiO2 accounts for most of the 

offset of samples from the distinctly laminated unit (table 

3; fig. 4), having a relatively high concentration only within 

this unit of the core (Peterson and others, 1995). Dolomite 

has a very limited distribution (table 3). The concentration 

of organic carbon in the laminated sections is approximately 

5 percent (Parker and Sackett, 1976; D. W. Murray, unpub. 

data; table 3), but exhibiting a minimum between 500 and 350 

cmbsf. It is approximately 3 percent in the bioturbated section, 

except for several samples immediately below the distinctly 

laminated section, which have less than 1 percent organic 

carbon. Of course, organic matter, rather than merely organic 

carbon, determines the offset due to this component of sediment. The relation between organic carbon and organic matter 

in particulate matter within the water column can approximate 

the stoichiometry of plankton (Copin-Montegut and CopinMontegut, 1983), but the relation between the two within the 

sediment is likely strongly variable. For the Cariaco Basin, it 

has not been ascertained. A factor of 2 is arbitrarily used here 

to convert organic carbon to organic matter. It can be as little 

as 1.5 in ancient marine deposits (Powell and others, 1975).

The composition of the terrigenous fraction, as opposed 

to its concentration, is evaluated by examining the concentrations of TiO2, K2O, and Fe2O3 (fig. 5). Their concentrations in 

this fraction are determined from plots of their concentrations 

versus the concentration of Al2O3 × 5.5, or the bulk terrigenous 

fraction (fig. 4). All three exhibit positive correlations with the 

terrigenous fraction (table 4). The similarity of their concentrations with their concentrations in WSA (table 2), obtained 

by extrapolation of the regression curve to 100 percent terrigenous fraction and to the origin (fig. 5), supports a solely 

terrigenous source for these major oxides.

The concentrations of TiO2 and K2O, nonetheless, show 

subtle differences within the terrigenous fraction of the three 

separate lithologic sections. Their ratios to Al2O3 in core 

PL07-39PC show extremes in the bioturbated section and the 

faintly laminated section. Yarincik and others (2000) attribute the differences to different sources for the terrigenous 

debris—(1) dominantly local rivers for the bioturbated section 

and (2) a mix of debris from local rivers, the Orinoco River 

and Amazon River systems, and eolian material from the 

Sahara Desert for the faintly laminated section. The Orinoco 

River and Amazon River systems now swamp the contribution 

of the other sources (Clayton and others, 1999) and have done 

so throughout deposition of much of the faintly laminated 

section of the core. An unstated assumption made by Yarincik 

and others (2000), and one made here also, is that this fraction 

of sediment is nonreactive. It has maintained its composition during its rain onto the sea floor (deposition) and burial 

(accumulation). Clayton and others (1999) report a similar 

trend in the clay mineralogy of the sediment. Such differences 

in both major-element-oxide concentrations and clay-mineral 

abundances, between lithologic units, contribute to the scatter 

in the data. The distribution of samples along the regression 

curves (fig. 5), however, reflects simple mixing of the terrigenous debris with the major biogenic phases—biogenic SiO2, 

calcite, and organic matter.

The plot of Fe2O3 concentrations along a single curve (fig. 

5C) indicates a constant Fe2O3 concentration in the terrigenous 

fraction. A seawater contribution of Fe, precipitating from pore 

water possibly as pyrite (Lyons, 1996) or by settling organic 

matter, is masked by Fe contributed by the siliciclastic debris. 

The coarseness of the normative calculation simply does not 

allow identification of a marine Fe fraction. Also, it does not 

preclude that some portion of the Fe, originally contributed by 

terrigenous debris, might now be present as pyrite.

The relation between a first group of trace elements and 

the terrigenous fraction is similar to the relations between 

TiO2, K2O, and Fe2O3 and the terrigenous fraction. The group 

includes Ba, Ce, Co, Ga, La, Li, and Th (fig. 6). Their concentrations are established by mixing of the terrigenous fraction 

with marine fractions. That is, the curves extrapolate to the 

origin and to WSA values at 100 percent terrigenous debris. 

Li and La resemble K2O; Ga and Th resemble Fe2O3; and Co 

resembles TiO2. Ba is enriched above a solely terrigenous 

contribution in several samples from the faintly laminated unit, 

between 175 and 200 cmbsf. Otherwise, its distribution resembles that of K2O. R-mode factor analysis further shows these 

trace elements have high loadings on the same factor as terrigenous debris (table 5), defined by the Al2O3 concentrations.

Manganese exhibits a slight enrichment in several samples 

from two core sections (fig. 7), between 530 and 690 cmbsf 

and greater than 790 cmbsf. Several of these samples also 

have Co contents above the value that can be assigned solely 

to the terrigenous fraction (fig. 6). These enrichments might 

be attributable to a hydrogenous fraction that accumulated 

under oxic bottom-water conditions and was redistributed after 

deposition of the overlying sediment under SO4

2- -reducing 

conditions (see below). However, the variation in composition 

of the terrigenous fraction, noted above, and the weakness of 

these enrichments above the terrigenous contribution impose a 

major uncertainty on interpretations of the enrichments above 

a terrigenous contribution.

Mo and Cd constitute a second group of trace elements 

(fig. 8). They are strongly enriched above a terrigenous contribution throughout the laminated sections of the core. A 

relatively large error in assessing the terrigenous contribution 
15

Table 2. Concentrations of major-element oxides and trace elements (1) in the world shale average (WSA) and the terrigenous fraction of the Cariaco Basin, (2) in marine biogenic debris, and (3) in seawater.

[Values in parentheses give concentrations in the terrigenous fraction of Cariaco Basin sediment (column 2), concentrations in suspended organic 

matter calculated from trace-element depth profiles in the Caribbean Sea (column5), and concentrations in seawater of the Caribbean Basin at 1,000-m 

depth. Seawater concentrations not in parentheses are of open-ocean seawater from 2000-m depth in the Pacific Ocean. Dashes indicate that data are 

not available or are not considered in this study]

Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model

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16 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

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Table 3. Major components in sediment samples from the Cariaco Basin, in percent, calculated from major-oxide concentrations (table 1). 

[Normative calculations are discussed in detail in Piper and Isaacs (1994, 1996). Biogenic SiO2, not listed, can only be estimated from 100 percent minus 

the sum. Blanks indicate no data are available]
17

Table 3. Major components in sediment samples from the Cariaco Basin, in percent, calculated from major-oxide concentrations 

(table 1)—Continued.

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Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
18 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

Table 3. Major components in sediment samples from the Cariaco Basin, in percent, calculated from major-oxide concentrations 

(table 1)—Continued.

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19

Table 3. Major components in sediment samples from the Cariaco Basin, in percent, calculated from major-oxide concentrations 

(table 1)—Continued.

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Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
20 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

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Table 3. Major components in sediment samples from the Cariaco Basin, in percent, calculated from major-oxide concentrations 

(table 1)—Continued.

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Table 4. Correlation coefficients for major-element oxides in sediment from the Cariaco Basin.

[Org. C, organic carbon; and sed. rate, sedimentation rate]
21

will little affect the calculation of their marine contribution. 

A Mo concentration in the terrigenous fraction, nonetheless, 

might be obtained by a curve drawn through those samples 

with the lowest measured concentrations and its extrapolation 

to 100 percent terrigenous material. The curve is subsequently 

referred to as the terrigenous maximum. The curve extrapolates to the WSA value for Mo of 2.0 ppm (table 2), supporting 

the interpretation that the curve defines the terrigenous contribution of Mo. The concentration of Cd in samples from the 

bioturbated section of the core is commonly below the detection limit of 2 ppm (table 1). A Cd concentration of 0.2 ppm in 

the terrigenous fraction, its established concentration in WSA, 

is assumed and assigned to samples with measured Cd values 

below the detection limit. Clearly, some of these samples may 

have a Cd concentration between a terrigenous contribution 

and its limit of detection.

A large marine source for Cd and Mo is supported by 

their high factor loading on the factor dominated by organic 

matter (table 5). Such loadings, or correlations, may be further 

indicative of the phase within the sediment that hosts a trace 

element.

Trace elements that constitute a third group include Cr, 

Cu, Ni, Pb, V, and Zn (fig. 9). Similar to the trace elements 

in the first group, they have high factor loadings on the factor 

for which the terrigenous fraction has a high factor loading 

(table 5). Unlike the first group, a single regression curve 

for each trace element extrapolates to a positive value on the 

y-axis and to a value greater than the WSA value at 100 per 

cent terrigenous fraction (table 2). One method of estimating 

the terrigenous contribution of this group is to assume WSA 

concentrations. The terrigenous maximum for Mo (fig. 8A) 

and regression curve for trace elements interpreted as having 

only a terrigenous contribution (for example, Co, Ga, the rareearth elements, and Th) support this procedure. However, even 

the distributions of these elements (fig. 6) suggested slightly 

different concentrations in the terrigenous fraction of the different lithologic units, similar to K2O and TiO2 (Yarincik and 

others, 2000). As variations in contributions by the several 

sources of the terrigenous fraction during the last 20 kyr were 

quite prominent (Yarinick and others, 2000), a terrigenous 

fraction of variable composition might have been expected.

These apparent variations in composition of the terrigenous fraction clearly place an uncertainty on the calculations of the seawater contribution of trace elements made in 

this study, particularly for this third group, as the terrigenous 

contribution dominants the trace-element inventory. For 

example, Pb values in the bioturbated samples define rather 

poorly a composition equal to a WSA value that allows for 

only a minor marine fraction. Cr, Ni, and V values in samples 

from the bioturbated section also seem to define a terrigenous maximum, but the curves extrapolate to concentrations 

at 100 percent terrigenous debris that are greater than their 

concentrations in WSA (table 2). These trends suggest that 

the composition of the bioturbated sediment section might be 

representative of the terrigenous fraction, except for Cu. Cu 

has its highest concentration in samples from the bioturbated 

section. Zn shows a single trend, which gives an extrapolated 

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Figure 5. Relations between the concentration of the terrigenous 

fraction (and Al2O3 taken from figure 4) and the concentrations of 

selected major-element oxides in sediment from core PL07-39PC. The 

broken curves give the linear regression curves for each element 

oxide for the three different lithologies of the core (fig. 2). The solid 

curves give the relations for the world shale average (WSA) (Wedepohl, 1969-1978; Piper and Isaacs, 1996), mixed with diluting phases, 

for example biogenic SiO2, calcite-aragonite, and organic matter. The 

regression curves for a single major-element oxide commonly cannot 

be distinguished; they also may be masked by the curve representing 

the relation in WSA.

Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
22 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

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Figure 6. Relations between the concentration of the terrigenous fraction and the concentrations of trace elements that exhibit approximate 

single trends with the terrigenous fraction from the three sediment lithologies of core PL07-39PC. Symbols and abbreviations are the same as 

in figure 5. The curves for each trace element extrapolate to the origin and to approximate concentrations in the world shale average (WSA) at 

100 percent terrigenous debris. The Ba concentration in samples between 165 and 200-cmbsf (table 1) is above a solely terrigenous contribution. The relation between terrigenous debris and Al2O3 is taken from figure 4.
23

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Table 5. Varimax orthogonal transformation solution, principal components factor extraction method, of the log10 of trace-element analyses and major sediment components in sediment core PL07-39PC, calculated from major-element-oxide analyses, 

using Statviewtm II software. 

[Values greater than 0.5 are in bold. Org. mat., organic matter]

Figure 7. The relation between the concentrations 

of the terrigenous fraction and manganese (Mn) in 

sediment of core PL07-39PC. Samples within the 

shaded area are from the depth intervals 530 to 590 

cmbsf (centimeters below the sea floor) and 760 to 

900 cmbsf. Symbols representing the samples are 

the same as in figure 5. The relation between Al2O3

and terrigenous debris is taken from figure 4.

Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
24 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

value that is significantly greater than its concentration in 

WSA (fig. 9).

Two different criteria are used to estimate the terrigenous 

contribution of this third group of trace elements, recognizing that any procedure employed might introduce possible 

errors. For Cr, Ni, Pb, and V, the terrigenous contribution is 

defined by samples from the bioturbated section of the core 

with lowest concentrations. For Cu and Zn, the terrigenous 

contribution is based on concentrations of Cu and Zn in WSA 

(fig. 9; table 2). The procedure gives a major seawater contribution of Cu and Zn to the bioturbated section of the core. 

This contribution may represent an artifact of an incorrectly 

adjusted siliciclastic contribution of Cu and Zn and not a high 

seawater input. However, using the same procedure that was 

used to determine the terrigenous contributions of Cr, Ni, Pb, 

and V would give negative concentrations of marine Cu and 

Zn in many samples from the laminated sections of the core. 

Thus, trace-element distributions below 730 cmbsf, or prior 

to 17 ka, are considered only very briefly. The trace-element 

compositions of these different terrigenous sources need to be 

ascertained by some alternative method, possibly through a 

leaching experiment, before a more complete discussion can 

be presented.

The trace-element fraction in each sample above the terrigenous contribution is partitioned into the two marine source 

fractions—(1) the biogenic fraction, composed mostly of 

planktonic debris derived from the photic zone (Schuffert and 

others, 1994; Thunell and others, 2000) and (2) the hydrogenous fraction, derived from seawater by precipitation and 

adsorption reactions (Goldberg, 1963). Similar to the terrigenous fraction, the two marine sources may not have had 

constant compositions throughout the depositional history of 

the core. Based on an analysis of cores collected by the Ocean 

Drilling Program, the actual species composition of the algal 

community shows a change in the main primary producers at 

about 9.5 ka, from dominantly diatoms to dominantly nannoplankton (Werne and others, 2000). The same change is seen 

in core PL07-39PC at about 270 cmbsf (10 ka; fig. 2). The 

estimated concentration of biogenic SiO2 decreases from as 

great as 30 percent to less than 5 percent (table 3; Peterson and 

others, 1995). The trace-nutrient demand of the two algal communities may have been quite different. Unfortunately, it is not 

currently possible to evaluate this uncertainty. In the calculations presented here, their overall trace-nutrient demands are 

assumed to have been the same.

The depositional rates of Cd, Cu, Ni, and Zn offer perhaps the best opportunity to ascertain the trace-element contribution by organic matter (table 6). All four are taken up in 

the photic zone by plankton (Martin and Knauer, 1973; Boyle 

and others, 1976, Boyle and others, 1977; Sclater and others, 

1976; Collier and Edmond, 1984; Brumsack, 1986). The stoichiometry for organic matter, that is, its elemental composition (table 2), can be taken from recent analyses of plankton 

and suspended organic matter collected from the Pacific 

Ocean (Martin and Knauer, 1973; Collier and Edmonds, 

1984; Sherrell, 1989). Although these measurements exhibit 

considerable scatter (Piper, 1994), the best estimate from 

the measurements for individual trace elements has been 

examined by comparing the measured value with the value 

calculated from its seawater profile (Piper and Isaacs, 1994, 

1996). This procedure is similar to evaluating the nitrogen 

and phosphorus stoichiometry of organic matter (Redfield and 

others, 1963). For the open ocean, agreement is quite good, 

particularly for Cd and Cu.

This scheme is used to calculate the Cd, Cu, and Ni concentrations in plankton within the area of the Cariaco Basin. 

The seawater-depth profiles of the three trace elements in the 

Venezuela Basin of the Caribbean Sea (Jacobs and others, 

1987) are used rather than their seawater profiles in the open 

ocean. The method relies on the profiles being determined by 

the uptake of trace elements in the photic zone by algae, oxidation of the host organic matter by bacteria and zooplankton 

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Figure 8. The relations between the concentration of the terrigenous 

fraction and the concentrations of Mo (A) and Cd (B) in sediment of 

core PL07-39PC. In A, a single curve defines the minimum values and 

the Mo concentration in the world shale average (WSA). In B, the 

curve for WSA represents a Cd concentration of 0.2 ppm, although 

the detection limit for Cd by ICP-AES is 2 ppm. Samples having a concentration less than 2 ppm are plotted as having a concentration of 1 

ppm, but they could have an even lower concentration. Symbols and 

abbreviations are the same as in figure 5. The relation between Al2O3

and terrigenous debris is taken from figure 4.
25

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Figure 9. The relations between the concentration of the terrigenous fraction and the concentrations of trace elements in sediment of core 

PL07-39PC that have strong factor loadings on the same factor as Al2O3 (table 5) but have concentrations (table 1) that are significantly greater 

than can be attributed solely to a terrigenous contribution (table 2). Least-square best-fit curves for this group of elements commonly extrapolate to positive values on the y-axis. The curves labeled T.M. (terrigenous maximum) give the concentrations of Cr, Ni, Pb, and V in the terrigenous fraction, as determined from their minimum values, similar to the determination of Mo in the terrigenous fraction (fig. 8A). Symbols and 

abbreviations are the same as in figure 5. The relation between Al2O3 and the terrigenous fraction is taken from figure 4.

Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
26 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

at depth, and advection. In support of this assumption, the 

Cd:PO4

3- ratio in the water column is constant in the Venezuela Basin (Jacobs and others, 1987), although its value of 

0.6 × 10-3 is slightly less than half the stoichiometric value 

of organic matter from the Pacific Ocean (Brumsack, 1986). 

The strongly varying absolute concentrations of both Cd and 

PO4

3-, but constant Cd:PO43- ratio, further suggests that the 

trace-element stoichiometry of particulate organic matter is 

maintained throughout the water column and onto the sea 

floor. The Cu value also is significantly less than its value 

in organic debris and plankton from the Pacific Ocean, but 

its profile is rather complex; the Ni value is 2.4-fold greater 

than its measured value in plankton (table 2). The value for 

Ni is likely too high as few analyses of plankton and organic 

detritus have recorded such a high value (18 ppm). However, 

this value is used as a measure of the export of Ni from the 

photic zone into the bottom water, recognizing that the actual 

uptake of Ni by plankton as a trace nutrient is likely less than 

this value.

The contribution of trace elements by organic matter can 

be calculated from the stoichiometry of the organic matter 

(table 6) and the rain rate of organic matter into the bottom 

water. Sediment traps suspended in the water column (MüllerKarger and others, 2001) have shown that only about 1.7 

percent of the organic matter produced in the photic zone 

survives to the sea floor (Thunell and others, 2000; MüllerKarger and others, 2001). Thunell and others (2000) have 

demonstrated that oxidation of organic matter occurs largely 

above 250 m, but it continues throughout the deeper part of 

the water column. Because of the method used here of calculating the contribution of trace elements by the bottom water 

(the hydrogenous fraction), the trace-element fraction within 

the organic matter that is oxidized within the SO4

2- -reducing 

region of the water column must be added to the fraction that 

actually settles onto the seafloor as particulate organic matter. 

For example, the profile of Cd in the Cariaco Basin (Jacobs 

and others, 1987) suggests that 90 percent of the Cd advecting 

into the bottom water is removed by precipitation/adsorption 

reactions, the hydrogenous fraction. This amount is in addition to the amount released in the water column via oxidation 

of settling organic matter. Thus, the biogenic input of trace 

elements to the sediment is defined by the organic matter settling into the uppermost layer of the SO4

2- -reducing region 

of the water column, not merely by the much smaller fraction 

of organic matter that settles onto the sea floor. The organic 

matter collected by the sediment trap suspended in the water 

column at 275-m depth (Müller-Karger and others, 2001) is 

assumed to have measured the flux of organic matter settling 

into the bottom water.

The hydrogenous fraction that accumulates under each of 

the different levels of bacterial respiration in bottom water is 

characterized by a unique suite of trace elements (Piper and 

Isaacs, 1994, 1996). As examples, Co and Mn accumulate 

within the hydrogenous fraction under bottom-water condi-

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Table 6. Table 6. Concentrations of trace elements in seawater and organic matter in the Cariaco Basin and their rates of deposition in the surface sediment of core PL07-39PC.

[Calculated rates are based on trace-element concentrations in organic matter (the biogenic contribution) and terrigenous debris (the terrigenous contribution) 

and the flux of each through the water column and on the bottom-water residence time and difference in elemental concentrations in source water and bottom 

water (the hydrogenous contribution). Measured rates represent accumulation rates based on the concentrations of trace elements in the surface sediment and 

the bulk sediment accumulation rate]
27

tions of O2 respiration. The rare-earth elements accumulate 

under conditions of O2 respiration and denitrification. Cr 

and V accumulate under conditions of denitrification and 

SO4

2- reduction. Mo accumulates only under the more negative redox conditions of SO4

2- reduction. The marine fraction of each of these trace elements should be dominated by 

their hydrogenous fraction, as all have low concentrations in 

organic debris (table 2).

Mo also has a low concentration in terrigenous debris 

(table 2), making it an ideal trace element for identifying conditions of SO4

2- reduction in the bottom water. Cd, Cu, V, and 

Zn also accumulate from the water column under SO4

2- -reducing redox conditions. Their rates of deposition, and that of Mo, 

are controlled by the level of their removal from the bottom 

water, relative to their concentrations in the source water, and 

the residence time of bottom water. The initial composition 

of the bottom water for the entire period of deposition of the 

sediment core is taken as equal to the current composition of 

water between 100 and 250-m depth in the basin (tables 2 and 

6). Water at this approximate depth was shown by Jacobs and 

others (1987) to be the source of water currently fluxing into 

the deeper region of the basin. It is also considered the source 

of water upwelling into the photic zone (Müller-Karger and 

others, 2001). As noted above, the residence time of bottom 

water is about 100 yr (Deuser, 1973).

Based on analyses of the water column of this and additional modern basins, the degree of removal of Cd and Cu 

from the source water, within the deep part of the basin, is 

essentially independent of the concentration of H2S (fig. 10A). 

By contrast, the removal of Mo in the bottom water from the 

source water is strongly dependent upon the concentration 

of H2S (fig. 10B). The removal of V from the source water 

versus H2S is inexplicably complex (fig. 10B). In the Cariaco 

Basin, ∆V is 65 percent (fig. 10B). Ni profiles of other basins 

suggest that Ni is not removed from the bottom water under 

SO4

2- -reducing conditions. Zn has not been measured in the 

Cariaco Basin, to our knowledge, but its profile in the other 

basins that exhibit SO4

2- reduction in the bottom water parallels the profiles of Cd and Cu (fig. 10A). The same is considered to apply to the Cariaco Basin. Its concentration in the 

source water is assumed to equal its concentration in the open 

ocean of the North Atlantic (table 6). Emerson and others 

(1979) and Murray and others (1983) have shown that Cr is 

reduced to the insoluble 3+ valence state under denitrifying 

to SO4

2- –reducing conditions, but its distribution has been 

reported only for Saanich Inlet and the O2 minimum zone of 

the eastern Pacific Ocean. Still, the depositional rates of Cd, 

Cu, Ni, V, and Zn offer a check on our interpretation of the 

deposition and eventual accumulation of Mo as defining the 

level of SO4

2- reduction in the water column and bottom-water 

advection.

In summary, the depositional rate of each trace element 

in the surface sediment is calculated from the sum of its rain 

rate onto the sea floor within the three source fractions. The 

calculations are based on (1) the current depositional rate of 

terrigenous debris and its composition, (2) the flux of organic 

matter into the bottom water of the basin and its trace-element 

stoichiometry, and (3) the residence time of bottom water and 

its depletion of trace elements relative to the trace-element 

concentrations of the water advecting into the deep region of 

the basin. The model defines the relationship of the deposition 

of trace elements to upwelling of seawater into the photic zone 

and advection of seawater into the deeper region of the basin.

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Figure 10. The percentage of Cd, Cu, Ni, and Zn (A) and Mo and 

V (B) removed from SO4

2- -reducing bottom waters of modern 

marine basins (Brewer and Spencer, 1974; Kremling, 1983; Jacobs 

and others, 1985, 1987; Emerson and Huested, 1991; Landing and 

Lewis, 1991), relative to their mean concentrations in the sources of 

bottom water, versus the mean concentration of H2S in the bottom 

waters. The curves represent regressions of best fit, of the form 

y = C1 + C2log [x], for which R = 0.75 (Cd), 0.31 (Cu), 0.00 (Ni), 0.70 

(Zn). The equation for curve (1) for ∆Mo versus H2S (B), R = 0.96, 

is used in the calculations in table 7. It does not include the datum 

from Framvaren Fjord. Curve (2), R = 0.66, includes all Mo data. 

Depth profiles of Cd, Cu, and Zn in the Baltic Sea, Black Sea, and 

Framvaren Fjord exhibit strong depletions in the bottom water; from 

which all three may precipitate as sulfides (Jacobs and others, 

1985). In the Cariaco Basin, only Cd and Cu have been measured, 

where they also are strongly depleted in the bottom water (Jacobs 

and others, 1987). Zn has not been reported, but its depletion in the 

other basins suggests that its depletion in the bottom water of the 

Cariaco Basin is similar to that of Cd and Cu. The concentration of 

Ni is essentially constant with depth in all of the basins for which it 

has been reported. The relation between ∆V and H2S clearly does 

not exhibit a simple regression.

Partitioning of Elements Between Terrigenous- and Seawater-Source Fractions—a Trace-Element Model
28 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

Trace-Element Depositional Rates in the 

Surface Sediment—A Test of the Model

Marine trace-element sources are (1) organic matter that 

escapes oxidation in the oxic layers of the water column and 

settles into the bottom water and onto the sea floor and (2) 

the dissolved load of trace elements in bottom water that are 

removed by precipitation/adsorption reactions in the water 

column (Goldberg, 1963; Brewer and Spencer, 1974; Jacobs 

and Emerson, 1982; Jacobs and others, 1985, 1987; Helz and 

others, 1996) and at the benthic boundary (Francois, 1988; 

Emerson and Huested, 1991; Crusius and others, 1996). The 

biogenic contribution of Cd is calculated from the export of 

organic matter from the photic zone into the SO4

2- -reducing 

region of the water column and its calculated concentration 

in plankton for the Venezuela Basin of 5.5 ppm (table 2). The 

flux of organic carbon measured by Thunell and others (2000) 

and Müller-Karger and others (2001) is converted to the flux 

of organic matter (table 6), based on the stoichiometry of 

plankton (Redfield and others, 1963). The mean rain rate of 

organic matter at 255 m is 8.89 mg cm-2 yr-1 (Müller-Karger 

and others, 2000), giving a depositional rate for Cd in the 

organic fraction of 0.049 μg cm-2 yr-1. Compared to the measured bulk accumulation rate for the surface sediment in the 

core of 0.061 μg cm-2 yr-1, the marine biogenic fraction alone 

contributes 80 percent to the bulk sediment value (fig. 11).

A lower Cd concentration in the SO4

2- -reducing part of 

the water column (Jacobs and others, 1987) than in the overlying, denitrifying part of the water column reflects Cd precipitation from the bottom water by a direct, albeit complex, 

reaction with S2- (Jacobs and others, 1985). Its deposition (fig. 

11; table 6) is calculated from (1) the difference between its 

concentration in the SO4

2- -reducing part of the water column 

and the above region of the water column, (2) the thickness of 

the SO4

2- -reducing portion of the water column above the core 

site of about 600 m, or 60 L cm-2, and (3) the residence time of 

bottom water of 100 yr (Deuser, 1973). Cd removal from the 

bottom water gives a depositional rate of 0.006 μg cm-2 yr-1.

The depositional rate of the terrigenous fraction currently 

is 7.36 mg cm-2 yr-1, requiring approximately 30 percent of the 

total of the sediment-trap material collected at 950 m, of 25 mg 

cm-2 yr-1 (Thunell and others, 2000), to be terrigenous in origin. 

Although the 30 percent seems to be a reasonable value, Al2O3, 

biogenic SiO2, and CaCO3, unfortunately, were not reported. 

The Cd accumulating as a terrigenous fraction, 0.002 μg cm-2

yr-1, reflects its concentration in and deposition of the terrigenous debris (table 2, fig. 11). The Cd accumulation-rate sum 

for the three fractions is 0.057 μg cm-2 yr-1, compared to a measured rate of 0.061 μg Cd cm-2 yr-1 in the uppermost sample of 

sediment. The agreement of the two values is extremely good, 

considering the possible uncertainties discussed previously. Of 

course, uncertainties will become greater as the calculations 

are applied to sediment at depth in the core. The agreement 

also suggests that the sediment largely settled through the 

water column rather than having been derived from the sides 

of the basin by resuspension of previously deposited material. 

Even so, the texture of some individual laminae at depth in the 

core clearly reflects mass movement of sediment down slope 

(Hughen and others, 1996a).

The measured Zn accumulation rate in the surface sediment is 1.187 μg cm-2 yr-1; the calculated depositional-rate sum 

is 1.763 μg cm-2 yr-1 (table 6). Its rate calculated within the 

biogenic fraction may account for most of this discrepancy. 

Whereas the concentration of Zn in plankton estimated from 

actual measurements is 110 ppm, its seawater profile with PO4

3-

in the North Atlantic Ocean gives a value of 30 ppm (Bruland 

and Franks, 1983). Using this stoichiometric value reduces 

the calculated biogenic contribution and improves agreement 

between the calculated and measured rates. The measured 

accumulation rate of Cu is 0.318 μg cm-2 yr-1 and the calculated sum is 0.339 μg cm-2 yr-1. Ni shows a level of agreement 

between the calculated and measured rates that is only slightly 

poorer (table 6). Thus, the biogenic contribution of Cd, Cu, Ni, 

and Zn dominates their marine fraction; it dominates the bulk 

accumulation rate only of Cd.

A minimum rate of upwelling can be estimated from the 

export of these four trace elements from the oxic region of 

the water column, that is, export of organic matter, or new 

production (Eppley and Peterson, 1979), and their concentrations in upwelled water. For Cd, the upwelling rate equals 

(0.049 μg cm-2 yr-1 ÷ 0.011 μg L-1) × 1,000 cm3 L-1 = 45 m yr-1. 

The upwelling rate is higher by the loss of Cd laterally across 

the shelf into the Venezuela Basin, in the Ekman layer. The 

maximum upwelling rate will be the total uptake rate of Cd in 

the photic zone divided by its concentration in the upwelling 

water, or approximately 760 m yr-1.

The accumulation rate of Mo is dominated by the hydrogenous fraction. Its water-column depth profile (Jacobs and 

others, 1987; Emerson and Huested, 1991) reflects a mean 

removal in the SO4

2- -reducing part of the water column of 15 

percent of its concentration in the uppermost 250 m of the 

water column. Its depositional rate as a hydrogenous fraction 

is 0.963 μg cm-2 yr-1 (table 6, fig. 11), as a terrigenous fraction 

0.015 μg cm-2 yr-1, and as a biogenic fraction 0.018 μg cm-2 yr-1. 

The latter two represent only 3 percent of the bulk Mo accumulation rate. Again, the agreement between the measured and 

calculated accumulation rates is excellent.

The closeness of the two Mo accumulation rates is somewhat surprising, considering decadal changes in the concentration of H2S within the water column of as much as 30 percent 

(Richards, 1975; Scranton and others, 1987) and the relation 

between the removal of Mo from the water column and the 

concentration of H2S (fig. 10B). As an individual sample represents a minimum of approximately 25 years under our sediment-sampling scheme, each sample averages over time scales 

of several decades. A finer sampling scheme of cores of undisturbed sediment is required to evaluate decadal and shorter 

trends in bottom-water chemistry.

The mean rate of advection of water into the bottom of 

the basin is much less intense than the upwelling rate. It is 

simply the total thickness of the O2-depleted region of the 

water column divided by the residence time of bottom water, 

or approximately 1,200 m ÷ 100 yr = 12 m yr-1.
29

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Figure 11. Schematic for the Cariaco Basin, showing the relation of “model” accumulation rates of the marine and terrigenous 

fractions of Cd and Mo (in parenthesis) to current conditions of primary productivity, water-column redox, bottom-water residence 

time, and terrigenous input. Depth ranges are not to scale. The two curves in the left side of the figure represent approximate seawater concentration versus depth profiles for Mo (× 10-3) and Cd. Model concentrations of Cd and Mo in the sediment, given at the 

bottom of the figure, compare closely with their measured concentrations in the surface sediment (table 1).

Trace-Element Depositional Rates in the Surface Sediment—A Test of the Model
30 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

Removal of trace elements from the bottom water (precipitation or adsorption onto settling particles or diffusion 

across the benthic boundary) is likely different for the different trace elements (Jacobs and others, 1985). Adsorption and 

diffusion reactions at the benthic boundary layer have been 

advocated as the mechanisms for the deposition of Mo in this 

and similar sedimentary environments (Francois, 1988; Pedersen and others, 1989; Emerson and Huested, 1991; Crusius 

and others, 1996), whereas Cd, Cu, and Zn are deposited as 

sulfides (Jacobs and others, 1985). Whichever mechanism(s) 

might account for Cd, Cu, Mo, Ni, V, and Zn removals from 

bottom water, it appears that their rates of deposition reflect 

the chemistry and hydrography of bottom water rather than 

the chemistry of sediment pore water. The agreement between 

model depositional rates of the different trace elements with 

their measured accumulation rates further supports the values 

used for their concentrations in planktonic debris, siliciclastic 

debris, and seawater. Baring temporal changes in these values, 

which are not possible to ascertain, and any diagenetic redistribution of the elements, it should be possible to ascertain 

historical changes in primary productivity and bottom-water 

residence times within approximately 25 percent.

Trace-Element Accumulation Rates 

During The Last 20,000 Years—An 

Application of the Model

The excellent agreement between the calculated and measured accumulation rates of Cd, Cu, Mo, Ni, and Zn in the 

surface sediment allows changes in the measured trace-element 

accumulation rates at depth in the sediment core to be interpreted in terms of past changes in (1) provenance, (2) primary 

productivity, and (3) bottom-water residence time and H2S concentration. Bottom water contributes less to the accumulations 

of Cd, Cu, Ni, and Zn than the rain rate of organic matter and 

should do so throughout the full range of biological and geochemical conditions. Thus, the distributions of marine Cd, Cu, 

Ni, and Zn may have recorded historical levels of primary productivity, that is, of the export of organic matter from the photic 

zone to the bottom water and seafloor. We focus on Cd because 

of its minor contribution by bottom water (table 6) and its low 

concentration in the terrigenous siliciclastic debris (table 2).

In relating the marine Cd and other trace-element accumulation rates to the hydrography of the basin, several 

assumptions must be made about the geochemistry of the 

source fractions: (1) phytoplankton maintained a constant 

stoichiometry of 5.5 ppm Cd, (2) Cd removal from bottom 

water under conditions of SO4

2- reduction was constant at 90 

percent over the full range of H2S concentrations considered 

(fig. 10A), (3) water fluxing into the bottom water had a constant Cd concentration of 0.011 ppb, the concentration of Cd 

in the water column currently at 100 to 250 m (Jacobs and 

others, 1987), (4) the Cd concentration in terrigenous debris 

fluxing into the basin was 0.2 ppm and constant, and (5) 

organic matter exported from the photic zone increased as a 

percentage of primary productivity. Any historical variations 

in the trace-element composition of plankton and seawater that 

may have occurred over the past 20,000 yr represent possibly 

sources of error that can be merely acknowledged.

The fifth assumption requires some explanation. The 

export of organic matter from the photic zone possibly does 

not bear a simple linear relationship with primary productivity. 

Baines and others (1994) showed that, of the organic matter 

produced in the photic zone, the percentage exported from 

the photic zone increases as primary productivity increases. 

Müller-Karger and others (2001) concluded that, for the 

Cariaco Basin, the percentage of organic matter settling out 

of the photic zone was constant throughout the range of primary productivity. They calculated regression curves from 

their measurements of primary productivity and the flux of 

organic-matter at four depths in the water column (their fig. 

11). The curve representing the sediment-trap data at a water 

depth of 275 m, alone, predicted a flux that remained a constant 

percentage of primary productivity throughout the full range 

of primary productivity. Sediment traps at three other depths 

recorded a percentage increase in export with increasing primary 

productivity. At a primary productivity of 500 mg C m-2 d-1, traps 

at 455 m, 930 m, and 1,225 m collected organic matter equaling 

one percent of primary productivity. At a primary productivity 

of 2,500 mg C m-2 d-1, the percentage increased to 4.6 percent 

at 433 m, 2.6 percent at 930 m, and 1.7 percent at 1,225 m. 

An increase in the percentage flux at 275 m as well as at the 

other three depths, from approximately 1 percent to 6.8 percent 

over the same range of primary productivity, is obtained if the 

datum at a primary productivity of 475 mg C m-2 d-1 is deleted. 

Deletion of this single measurement is similar to the authors’ 

deletion of two measurements that they interpreted as being 

outliers. Thus, we interpret their figure 11 as showing that all 

four sediment traps recorded percentage increases in the flux 

of organic matter with increasing primary productivity. This 

interpretation of a percentage increase in the export of organic 

matter out of the photic zone under the condition of increasing 

primary productivity is supported by the clogging of sediment 

traps at all depths, immediately following their deployment in 

May of 1996. Müller-Karger and others (2001) attributed the 

event to a plankton bloom observed in that month. They did not 

consider the event in their model. Such events, how ever brief 

and few in number, might provide for an even greater percentage of primary productivity to be exported out of the photic 

zone than our calculations suggest (table 7). Even if that is not 

fully the case, the large flux of organic matter settling into the 

bottom water from this single event suggests that the flux of 

organic matter settling into the bottom water versus primary 

productivity is far more complex than a simple linear relation 

between the two.

This interpretation is used to model the flux of organic matter 

and trace nutrients (Cd) out of the photic zone and into the 

bottom water (table 7). Doubling primary productivity, for example, from 1.5 g C m-2 d-1 (550 g C m-2 yr-1) to 3 g C m-2 d-1 (1100 g 

C m-2 yr-1), increases the export of organic carbon from the photic 

zone from 5.8 percent to 6.7 percent of primary productivity, or 
31

from 32 g C m-2 yr-1 to 74 g C m-2 yr-1 (table 7). Export of the 

nutrient trace elements will increase proportionally.

 The model calculations of H2S in the bottom water begin 

by taking the current conditions of primary productivity (550 

g C m-2 d-1), bottom-water residence time (100 yr) and H2S 

concentration in the bottom water (30 μmol L-1). The concentration of H2S in the bottom water is then allowed to change by 

(1) the rate of bacterial respiration, which is established by the 

flux of labile organic matter into the bottom water and, ultimately, by primary productivity, and (2) bottom-water advection, or residence time (Codispoti, 1980). As the production of 

H2S is inextricably coupled both to primary productivity and 

bottom-water advection, its concentration cannot be considered solely as a function of one or the other. The removal of 

trace elements from bottom water, likewise, will be dependent 

upon both factors. Conceptually, the response of H2S in the 

bottom water to a change in each can be considered separately. 

The concentration of H2S should change one-to-one with a 

change in bottom-water residence time. Doubling the residence time of bottom water simply doubles the integrated flux 

of organic matter settling through a parcel of water and the 

concentration of H2S (table 7), assuming that the rate of bacterial respiration is independent of the concentration of H2S. At 

the low concentrations of H2S in the Cariaco Basin of 30 μmol 

L-1 (Emerson and Huested, 1991), this is a reasonable assumption. Holding residence time constant, a change in the concentration of H2S will equal the change in the flux of organic 

matter into the bottom water, as driven by a change in primary 

productivity. Doubling primary productivity will increase 

somewhat more than twofold the flux of settling organic 

matter, bacterial respiration, and the concentration of H2S.

The percent removal of Cd, Cu, and Zn, from bottom 

water, their hydrogenous fraction, responds weakly to a 

change in the concentration of H2S (fig. 10A). Their removal 

Table 7. Table 7. Calculated rates of deposition for the Cariaco Basin of marine Mo and Cd (1) as particulate organic matter settling out of the 

photic zone (the biogenic fraction) and (2) from bottom water under conditions of SO4

2- reduction (the hydrogenous fraction).

[Calculations are made for four different rates of primary productivity in the photic zone and several different residence times of bottom water for each level of 

primary productivity. H2S concentrations are based on its current concentration in the Cariaco Basin of approximately 30 μmol L-1 (Emerson and Huested, 1991), 

at the residence time of bottom water of 100 yr and primary productivity of 550 g carbon m-2 yr-1. H2S concentration is allowed to change 1-to-1 with a change 

in residence time. It is allowed to change with primary productivity by the percentage change in export of organic matter from the photic zone to 255-m depth 

(see text). ∆Mo values are taken from the curve given in figure 10B. Mo depositional rate equals [∆Mo × 60 L cm-2 × 10.7 μg Mo L-1] ÷ [στ yr], for which στ is the 

bottom-water residence time. ∆Cd (not listed) is held constant at 90 percent, its current removal from the Cariaco Basin and roughly that for the full range of H2S 

concentrations as observed in other basins (fig. 10A)]

Trace-Element Accumulation Rates During The Last 20,000 Years—An Application of the Model

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32 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

is about 70 to 90 percent throughout the full range of H2S concentrations, from the low concentration of H2S in the Baltic 

Sea (Kremling, 1983) to the highest concentration of H2S 

measured for the bottom water of Framvaren Fjord (Jacobs 

and others, 1985). Ni is not removed from the bottom water by 

reaction with S2- (Jacobs and others, 1985), above the contribution of Ni added directly to the water column via the oxidation of settling organic matter, even at the high levels of H2S 

in the water column of Framvaren Fjord. Thus, a change in the 

concentration of H2S in bottom water, alone, does not change 

significantly the accumulation rates of any of these four trace 

elements.

The Mo removal from bottom water is complicated by 

the relation between ∆Mo and the concentration of H2S (fig. 

10B). The percentage depletion of Mo in the bottom water 

of modern basins exhibiting O2 depletion versus H2S is nonlinear over the full range of H2S concentrations. As a result, 

the Mo accumulation rate can exceed an increase in H2S, in 

response to an increase in primary productivity. For example, 

a threefold increase in H2S, from 15 μmol L-1 to 45 μmol L-1, 

increases ∆Mo from 3.6 to 35 percent (fig. 10B). Therefore, a 

threefold increase in primary productivity can increase the Mo 

accumulation rate as much as tenfold, at low H2S concentrations. At H2S concentrations above about 150 μmol L-1, ∆Mo 

increases only slightly with an increase in H2S (fig. 10B, table 

7). The increase in removal of Mo from bottom water under 

higher H2S concentrations (fig. 10B) is compensated increasingly at long residence times by the reduced flux of bottom 

water, that is, of dissolved Mo and other trace elements, into 

the basin. Under a rate of primary productivity of 1,100 g C m-2

yr-1, an increase in residence time from 50 yr to 250 yr (table 7) 

increases by fivefold the concentration of H2S in bottom water 

and the removal of Mo from the water column by about 2.5-

fold (fig. 10B). However, the accumulation rate of Mo actually 

decreases by twofold at the longer bottom-water residence 

time, owing to the fivefold reduction in the flux of Mo into the 

bottom water.

Increasing the bottom-water residence time reduces the 

accumulation rates of Cd, Cu, and Zn from bottom water, 

approximately one-to-one throughout the range of H2S concentrations (table 7). That is, doubling the residence time of 

bottom water halves their rate of accumulation within the 

hydrogenous fraction by halving their flux into the basin. 

Changing the H2S concentration by changing the residence 

time should not alter the accumulation rate of Ni.

Modeling the Mo accumulation from the hydrography of 

a basin that exhibits SO4

2- reduction in the bottom water (table 

7) shows several features that are critical to our interpretation of the sediment record: (1) Large temporal variations in 

the Mo accumulation rate are more easily achieved for brief 

bottom-water residence times than for bottom-water residence 

times of long duration. For example, at a primary productivity 

of 825 g C m-2 yr-1, a decrease in residence time, from 1000 

yr to 500 yr, increases the accumulation rate of Mo 0.4 μg 

cm-2 yr-1 (table 7). Increasing the residence time from 40 yr 

to 50 yr, similarly increases the Mo accumulation rate about 

0.4 μg cm-2 yr-1. Therefore, a Mo accumulation-rate pattern 

that exhibits sharp temporal changes over a period of several 

thousand years is indicative of a bottom-water residence time 

that varied over a few tens of years, whereas a pattern that 

shows a gradual change in the Mo accumulation rate suggests 

a bottom-water residence time that varied between one hundred and several hundred years. (2) The accumulation rate of 

Cd is practically independent of bottom water residence time, 

owing to its low accumulation rate within the hydrogenous 

fraction. (3) A Mo:Cd ratio less than one can be achieved 

only at bottom-water residence times less than a few years or 

greater than several thousand years. The former was probably 

never sustained for a time long enough to have been detected 

by our sampling scheme; the latter is simply unlikely to have 

occurred. (4) Mo exhibits a maximum accumulation rate at a 

decreasing bottom-water residence time as primary productivity increases.

Several of the temporal changes in trace-element accumulation rates in the Cariaco Basin (fig. 12) are approximately 

synchronous with global climatic events, although accumulation of the trace elements within any single event exhibits 

strong variability. Thus, our results must be taken as preliminary until a more detailed examination of the geochemistry of 

this and similar sedimentary basins more firmly constrains the 

relations between trace-element accumulation and chemical, 

biological, and hydrographic properties of the water column.

In the following discussion, the accumulation rates of the 

trace elements from 20 ka to the present are considered. Prior 

to about 17 ka, the entire water column was oxic (Peterson and 

others, 1991). Additionally, much of the shelf was exposed 

during this earlier period. The source of the terrigenous fraction of sediment could have been quite different from its current source. Whereas the Orinoco/Amazon River systems now 

contribute substantially to the terrigenous fraction, they may 

not have done so prior to about 15 ka (Clayton and others, 

1999). Owing to this and other factors, the geochemistry of 

the sediment may not have retained an accurate record of the 

dynamics and chemistry of the immediately overlying water 

column, nor recorded conditions that can be related to the current oceanographic regime. Following this time, the sediment 

record of the Cariaco Basin can provide a relatively high-resolution record of the hydrography of water-column because of 

the high bulk-sediment accumulation rate, between 10 and 85 

mg cm-2 yr-1. Of course, the adverse effect of a higher accumulation rate is increased dilution of the marine trace-element 

record by terrigenous debris, CaCO3, and biogenic SiO2.

Calculations of the accumulation rates for Cr and V (fig. 

12) are included, but are not discussed in any depth. The concentration of Cr in the water column is not known to have been 

measured. This is rather unfortunate as it and V as well are 

removed from the water column by precipitation and adsorption 

reactions under conditions of denitrification and continue to be 

deposited under conditions of SO4

2- reduction (Piper, 1994). It 

should be noted that the mean concentration of Cr in the laminated sections of the core, relative to Mo (table 2), is similar to 

their relative concentrations in open-ocean seawater, Mo greater 

than Cr, but the Cr concentration in the water column must be 

measured before considering this element further.
33

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Figure 12. Measured accumulation rates of the terrigenous and marine fractions of trace elements versus depth in core PL07-39PC. Note 

the different y-axis scales. Shaded areas represent the Younger Dryas event and melt-water pulses IA and IB (Fairbanks, 1989). Model 

ages are taken from Lin and others (1997) and Hughen and others (1998) (fig. 3). Boxed letters in A and E are keyed to model calculations 

and footnotes in table 7. Crosses give actual calculated values.

Trace-Element Accumulation Rates During The Last 20,000 Years—An Application of the Model
34 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

The removal of V from SO4

2- -reducing bottom water 

is complex (fig. 10B), such that a change in its accumulation rate from bottom water, in response to a change in H2S 

concentration, is not possible to predict. ∆V is higher in the 

Cariaco Basin than in Lake Nitinat and Framvaren Fjord, both 

of which have significantly higher H2S concentrations in the 

water column at depth (fig. 10B). Even so, the current 65 percent removal of V from the bottom water in the Cariaco Basin 

(fig. 10B) contributes to a calculated accumulation-rate sum 

for the surface sediment that agrees to within 80 percent of the 

measured rate (table 6).

22 to 14.8 ka (bioturbated sediment)

Interpretation of the geochemistry of the sediment record 

between 22 and 17 ka is complicated by the shelf having been 

exposed, or entirely within the photic zone, making the basin 

environment far different from what it is today and from what 

it was like over the past approximately 17 ka. Water from the 

Caribbean Sea now enters the Cariaco Basin across two sills, 

one at about 120-m depth in the northern part of the basin, east 

of Tortuga Island, and one at about 146-m depth at the western 

end of the basin (fig. 1). The latter provided only a shallow 

connection with the Caribbean Sea at the maximum glacial 

low stand between 25 and 20 ka, when sea level was 120 m 

below its current level (Fairbanks, 1990). The foraminiferal 

record (Peterson and others, 1991; Lin and others, 1997) 

shows that the basin was not totally isolated. Nonetheless, near 

isolation of the basin could have had a significant impact on 

the flux of trace elements into the basin and their accumulation 

onto the sea floor. One of several possible impacts could have 

been that the chemical composition of surface sea water and 

fresh water entering the basin had a dissolved trace-element 

concentration that was quite different from seawater likely 

entering at depth currently. This condition makes somewhat 

tenuous any interpretation of the geochemistry of the bioturbated section of the sediment, prior to about 17 ka.

Following the LGM, eastern Mexico, Central America, 

and Venezuela were cool and arid due to a shift in the ITCZ, 

perhaps to its present winter position (Bradbury, and others, 

1981; Bradbury, 1997; Fritz and others, 2001). Much of the 

region was covered by dry grasslands (Markgraf, 1989, 1993). 

Expansion of neotropical savannas, such as the Orinoco 

Llanas of Venezuela, reflected the dryer conditions (Behling 

and Hooghiemstra, 2001). Cool conditions may have resulted 

from low sea-surface temperatures, a result of large volumes 

of glacial melt water flowing down the Mississippi River (for 

example, Overpeck and others, 1989). Lin and others (1997) 

measured the oxygen-isotope record of foraminifera that suggested sea-surface temperatures were as much as 4 ̊C cooler, 

but interpreted the record as reflecting intense upwelling, 

rather than elevated glacial melt water, that may, nonetheless, 

not have supported high primary productivity. The dominance 

of the foraminifera Globigerina ruber over G. bulloides in the 

bioturbated sediment lead Peterson and others (1991) to conclude that primary productivity, indeed, was lower. Accumulation rates of Cd (fig. 12E) also favor low primary productivity, 

but the problem of the interpretation of its accumulation prior 

to 15 ka must be recognized.

The rise in sea level, beginning about 20 ka, coincided 

with an increase in the accumulation of Th and Ga beginning 

at the end of the LGM and continuing to 16.2 ka (fig. 12). This 

increase possibly reflected reworking of previously deposited 

sediment on the shelf by a transgressive ocean. The occurrence 

at 700 cmbsf of a fine-grained layer and of δ18O-depleted 

foraminiferal calcite led Lin and others (1997) to conclude 

there was a “freshwater event” at this time. A water column of 

low salinity suggests that, even at this relatively recent time, 

connection with the Caribbean Sea was quite different from 

its character of today. It also suggests elevated erosion in the 

terrestrial environment and elevated accumulation of terrigenous debris. However, arid conditions apparently prevailed 

in central and northern South America (Lin and others, 1997), 

although the period corresponds to extremely wet conditions 

in the western United States. As examples, maximum lake 

levels in the Bonneville and Lahontan lake systems (Benson, 

1991; Thompson and others, 1993) and increased runoff to the 

Pacific Ocean along central California (Gardner and others, 

1997) were achieved at approximately this time.

The oxygen isotope data of Lin and others (1997) also 

show a convergence of δ18O values in the surface-dwelling G. 

ruber and the deeper-dwelling Neogloboquadrina dutertrei

during this time, from which they concluded that the watercolumn was well mixed. The low accumulation rate of marine 

Mo between 610 and 700 cmbsf (figs. 2 and 12) and the bioturbated texture of the sediment further indicate that advection 

of oxic water into the deeper part of the basin prevented the 

establishment of SO4

2- reduction in the bottom water.

Mn is mildly enriched in the sediment from the bioturbated section below 760-cm depth (fig. 7). As Mn deposition 

from seawater is greatly enhanced under oxic conditions 

(Landing and Bruland, 1987), its distribution supports the 

interpretation of deposition of this sediment under oxic 

bottom-water conditions. A Mn enrichment is also seen in 

the basal sediment of the distinctly laminated unit. One possible source of this enrichment is the bioturbated sediment, 

via diagenetic migration of Mn up core at some time following its deposition and reprecipitation possible as a carbonate. 

Thomson and others (1996) have described in some detail the 

diagenetic behavior of Mn in the pelagic environment of the 

Atlantic Ocean. Oxidizing, Mn-enriched deep-sea sediment 

in otherwise reducing, Mn-depleted sediment also has been 

described from the Bering Sea (Gardner and others, 1982) and 

the North Pacific Ocean (Dean and others, 1989).

Oxic, bottom-water conditions might also account for 

the slight enrichment of Co (fig. 6; table 1) in these samples, 

an element that accumulates with Mn under oxic conditions. 

Some fraction of Cu, Ni, and Zn in the bioturbated unit might 

have had a similar origin as they too can accumulate under 

oxic conditions, albeit at a very slow rate, as in the pelagic 

environment. The accumulation rates of the marine fraction of 

Mo and of the bulk Cd are below detection at the close of this 

time period. Their absence in this sediment reflects their lack of 
35

precipitation/adsorption under oxic conditions and possible loss 

of biogenic debris and its trace-element inventory upon oxidation of the organic matter at and near the surface of the sea floor.

14.8 to 10.4 ka (distinctly laminated sediment)

The cool, dry conditions that characterized terrestrial 

environments around the Caribbean Sea at the close of the 

LGM continued through the Younger Dryas (Hodell and 

others, 1991). However, dramatic changes in marine conditions began abruptly at 14.8 ka with sea-level rise that accompanied melt-water pulse (mwp) IA. The rise to dominance of 

the foraminifera Globigerina bulloides over G. ruber (Peterson 

and others, 1991) and the increase in the Cd accumulation rate 

(fig. 12) indicate increased primary productivity. The increase 

probably reflected the penetration of nutrient-enriched water 

from the Caribbean Sea across the shelf into the photic zone 

of the Cariaco Basin. The occurrence of distinctly laminated 

sediment and increase in the accumulation rate of Mo (fig. 12) 

further indicate the introduction of H2S in the bottom water.

The accumulation rate of marine Mo was high throughout this period, exhibiting prominent maxima of 4 μg cm-2

yr-1 centered at 580 cmbsf and 480 cmbsf. Its rate was highly 

variable throughout the time period, but with its mean accumulation rate decreasing more or less uniformly from the 

maximum at 480 cmbsf to 1.2 μg cm-2 yr-1 near the end of the 

period. Bottom water, clearly, was SO4

2- reducing throughout 

this time.

The Mo maxima correspond approximately to major 

hydrographic and climatic events during this glacial-Holocene 

transition. The marine Mo maximum at 580 cmbsf (14.4 ka) 

occurs within mwp-IA (Fairbanks, 1990), whereas the maximum at 480 cmbsf (13.0 ka) corresponds to the onset of the 

Younger Dryas event. One result of the melt-water pulse was 

the lowering of surface-water salinity in the Gulf of Mexico, 

probably leading to an increase in water-column stability. 

If this same condition extended into the Caribbean Sea, the 

increased stability of the water column, that is, decrease in 

bottom-water advection, in conjunction with the increase in 

primary productivity (Peterson and others, 1991), could have 

contributed to the establishment of SO4

2- -reducing conditions in the bottom water. The increase in primary productivity is reflected by the increase in the Cd accumulation rate 

(fig. 12E).

The minimum Mo accumulation rate at the end of this 

period occurs during mwp-IB, in sharp contrast to its maximum accumulation rate during mwp-IA. Yet, the Cd accumulation rate was similar for the two melt-water pulses. The 

dissimilar responses of the two elements during the meltwater pulses suggests a certain level of independence between 

the rates of bottom-water advection and upwelling into the 

photic zone. The greater rise of sea level up to the time of 

mwp-IB than up to the time of mwp-IA may have diminished 

the effect that a melt-water pulse had on bottom-water advection in the Cariaco Basin. The residence time of bottom 

water simply continued to increase into mwp-IB, to as much 

as 250 yr (table 7), following a trend begun toward the end of 

the Younger Dryas. Upwelling, on the other hand, achieved 

an intermediate level of approximately 825 g C m-2 yr-1 during 

both events.

The marine Cd accumulation rate was highly variable 

throughout this time, although much higher, on average, than 

the current level (fig. 12E). During the Younger Dryas, its 

mean accumulation rate was approximately twice as great as 

during either melt-water pulse. The zooplankton composition 

(Peterson and others, 1995) suggests a level of primary productivity during mwp-IB that was comparable to the current 

level, higher than during mwp-IA, but lower than during the 

Younger Dryas. These relative differences roughly approximate the observed differences in the mean accumulation rate 

of Cd (fig. 12E).

The model (table 7) underestimates the accumulation 

rate of Cd (fig. 12E), when primary productivity was high, for 

example, at the onset of the younger Dryas. The interpretation used here of figure 11 from Müller-Karger and others 

(2001) apparently underestimates the percentage of organic 

matter exported into the bottom water at times of high primary productivity. Alternatively, the Cd concentration in the 

bottom-water source, the Cd concentration currently in the 

water between 100- and 250-m depth, may have been underestimated. Both explanations are plausible. The concentrations of Cd and other nutrient elements in the water column at 

100- to 250-m depth are determined largely by the oxidation 

of organic matter settling through this section of the water 

column. The elevated level of primary productivity would 

have supplied an equally elevated load of nutrients out of the 

photic zone to this intermediate depth. Eventual advection of 

this water into the bottom water would have supplied a higher 

flux of Cd to precipitate as a sulfide (hydrogenous fraction). 

However, export of a greater percentage of primary productivity out of the photic zone and into the bottom water (biogenic 

fraction), at the higher level of primary productivity, has the 

potential of having had a much greater impact (table 6). One 

possible mechanism of achieving a higher percentage of primary productivity settling into the bottom water, than given by 

figure 11 of Müller-Karger and others (2001), may have been 

the occurrence of a greater frequency of pulses in primary 

productivity than has been observed during the past few years, 

of an intensity that roughly equaled the event that clogged all 

sediment traps throughout the water column in May of 1996.

The accumulation rate of terrigenous debris, as recorded 

by the accumulation rates of Ga and Th (fig. 12D), also shows 

strong variation. The two melt-water pulses are marked by 

minima in the accumulation rate of the terrigenous fraction. 

The entrapment of sediment on the shelf apparently dominated 

any resuspension of shelf sediment by the transgressive sea, 

as occurred during the initial rise in sea level at the end of the 

LGM. The flux of the terrigenous fraction shows a maximum 

during the Younger Dryas event. An increased flux of sediment from local rivers, or of sediment from the Orinoco and 

Amazon Rivers, may have accounted for this major pulse in 

the terrigenous accumulation rate. The lake Valencia sediment 

record suggests a wetter climate at about 10 14C ka in northern 

Trace-Element Accumulation Rates During The Last 20,000 Years—An Application of the Model
36 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

Venezuela than the period following (Bradbury and others, 

1981), but the record, unfortunately, does not extend back to 

the period prior to the Younger Dryas. A summary by Leyden 

(1995) of data from three sites in Guatemala and Costa Rica 

suggests that precipitation increased throughout deglaciation, 

due to strong monsoon conditions.

In summary, the high and strongly variable accumulation 

rate of Cd during this time indicates a robust rate of upwelling 

that exhibited sharp temporal variations. A similar pattern for 

Mo likely reflects a mean residence time of bottom water that 

was a few tens of years, rather than hundreds of years. A possibly colder and less stable global climate than currently could 

have contributed to the accumulation-rate patterns of the two 

trace elements.

10.4 to 0 ka (faintly laminated sediment)

Sea level rose steadily by 25 m during the Holocene 

(Fairbanks, 1989). Marine conditions within the area might 

have been relatively stable, compared to the preceding 4.4 

kyr. However, both Mo and Cd show strong maxima at 9.9 

ka, sharp decreases between 9 ka and 8.6 ka, and gradual 

decreases after 8.4 ka to their lowest accumulation rates of the 

last 15 kyr, at 4.0 ka. Both elements exhibit a smooth increase 

in accumulation rate to about 0.70 ka, before decreasing 

slightly to current levels.

The high Cd accumulation rate near the very beginning of 

this period, centered at 9.9 ka (fig. 12E), required an increase 

in primary productivity, following its lower accumulation 

rate during the early part of mwp-IB (table 7). The increase 

in flux of organic matter into the bottom water alone would 

have increased the concentration of H2S, thus increasing the 

removal of Mo from the bottom water. The lower amplitude 

variation in the accumulation rate of Mo across this high, for 

approximately 2.0 kyr, than was characteristic of its accumulation during the preceding approximately 4.4 kyr (fig. 12A), 

suggests a longer bottom water residence time than characterized the preceding period of time, during which it was a few 

tens of years (table 7). The stable low accumulation rate of Mo 

after 8.0 ka suggests that the bottom-water residence time continued to increase, in the range of several hundred years. Thus, 

an increase in bottom-water residence time, coupled with a 

decreasing level of primary productivity, led to the decreasing 

accumulation rates of the marine fraction of most trace elements at 8.4 ka, following their high rates at 9.9 ka.

The change of primary producers from siliceous diatoms 

to calcareous nannoplankton at approximately 11 ka, possibly driven by a reduction in primary productivity (Werne and 

others, 2000), that is, upwelling and nutrient availability, corresponded to the change in lithology from distinctly laminated 

to faintly laminated, not to the decrease in the Cd accumulation rate between 8.6 and 8.4 ka. Somewhat contrary to the 

change in primary producers, but in agreement with the accumulation rate of Cd, Peterson and others (1991) interpreted 

the distribution of N. dutertrei as indicating sustained high primary productivity into the upper part of the faintly laminated 

sediment. The foraminiferal record and Cd accumulation both 

suggest a slightly lower level of primary productivity during 

the maximum that preceded the sharp decrease than during 

the Younger Dryas period, although the Cd record during the 

Younger Dryas is characterized by large variations, between 

0.3 and 0.08 μg cm-2 yr-1 Cd, a seemingly unstable time.

The pattern of trace-element accumulation rates since 8.4 

ka could have been driven by a wetter climate pattern in early 

to mid-Holocene time in the Caribbean Sea area (Hodell and 

others, 1991) and increased water-column stability. Lake Valencia, northern Venezuela, was shallow and saline prior to 11 ka 

(Bradbury and others, 1981; Bradbury, 1997). The lake began 

to fill by 11 ka and was full and overflowing by about 8.8 ka. 

Thereafter, the lake remained fresh. During the early Holocene, 

dry scrub vegetation in the lake Valencia basin was replaced by 

forests (Leyden, 1985). By 8 ka lake sites in eastern Mexico, 

Central America, Haiti, and elsewhere in Venezuela also were 

wetter than today (Fritz and others, 2001; Hodell and others, 

1991). Hodell and others (1991) concluded from 18O/16O ratios 

in ostracod shells from Lake Miragoane, Haiti, that the lake 

level began to increase about 8 ka and remained high until 

about 3.2 ka. They observed that the inferred lake-level record 

resembled changes in the annual precipitation cycle as estimated by the seasonal insulation difference at the top of the 

atmosphere at 10 ̊ N between August and February. Lake levels 

in the circum-Caribbean region remained high until about 3.5 

ka, marking the beginning of a late Holocene dry phase.

Wetter conditions during the early to middle Holocene also 

should have been accompanied by a higher influx of terrigenous 

material. Haug and others (2001) found increases in concentrations of Fe and Ti in sediment deposited in the Cariaco Basin 

between 10 ka and 5 ka, in a core from Ocean Drilling Program 

(ODP Site 1002 at a water depth of 893 m). They attributed these 

increases to be due to a middle Holocene period of increased 

precipitation and riverine discharge of terrigenous debris. Our 

data for Fe and Ti also show maxima in lower to mid-Holocene 

sediments, at 9 ka, as do the data for Ga and Th, our preferred 

indicators of terrigenous detritus. However, accumulation rates 

of Ga and Th (fig. 12D) indicate that the terrigenous influx was 

not nearly as great as during the LGM and Younger Dryas. They 

attributed decreases in concentrations of Fe and Ti after about 

5 ka as due to a southward migration of the ITCZ and attendant 

lower precipitation over the Cariaco Basin.

The early-to-middle Holocene wet interval in the tropics 

contrasted with the extratropical regions of North America 

where mid-Holocene aridity (the so-called altithermal) was 

widespread with strong westerly zonal winds (for example, 

Dean and others, 1996; Fritz and others, 2001). An abrupt 

change to dry conditions at 8.2 ka, with zonal winds over 

North America, was attributed by Dean and others (2002) to 

an abrupt northward migration of the polar front in response to 

changes in the relative proportions of land, water, and, especially, glacial ice on North America during the early Holocene. 

This change in atmospheric circulation over North America 

also may have been coupled to the atmospheric circulation 

over the Atlantic Ocean and (or) thermalhaline circulation 

within the Atlantic Ocean, as suggested by Shuman and others 
37

(2002). A wetter mid-Holocene indicates that the ITCZ had 

moved north of the geographical equator to its present summer 

position, moving the easterly trade winds farther north and 

bringing more precipitation to the Caribbean region and 

reduced upswelling to the Cariaco basin. The reduced upwelling and attendant lower productivity are indeed suggested by 

the extremely low Cd and Mo accumulation rates in the midHolocene sediments at about 4.0 ka (figs. 12A, C). The interpretation of wetter conditions, however, seems to conflict with 

the interpretation here of the minimum Ga snd Th accumulation rates as reflecting day conditions.

A 15 percent increase in primary productivity, following 

the mid-Holocene minimum, can account for the maxima in 

Mo and Cd accumulation rates at 0.70 ka. Such a high would 

have increased the organic-bound Cd flux by about 20 per 

cent, as well as increasing by 20 percent the H2S concentration in the bottom water. At a constant bottom-water residence 

time, the accumulation rate of Mo would have increased even 

more. This maximum may be related to the “Little Ice Age” 

that occurred in the last millennium, but the interpretation 

clearly requires confirmation from the examination of the sediment record from other basins and a more detailed examination of surface and near-surface sediment from the Cariaco 

Basin than presented here.

The accumulation rate of Ni following mwp-IB is somewhat different from the other trace elements (fig. 12C). The 

maximum occurred at a more recent time than the maxima 

for Cd, Mo, and V at 9.9 ka. The results presented here seem 

to offer no explanation for this shift, to well after the maxima 

of these other trace elements. Ba, which shows a high rate of 

accumulation at 7.4 ka also needs to be examined in greater 

detail to ascertain the factors that controlled its accumulation. 

The accumulation rates of marine Cu and Zn show only slight, 

relatively smooth variations for this entire period of time.

Elemental Ratios and Concentrations—

An Alternative Model of Environmental 

Conditions

Changes in trace-element ratios in the marine fraction (fig. 

13A) can be interpreted in terms of changes in the intensity of 

upwelling, that is, primary productivity in the photic zone, relative to changes in the intensity of bottom-water advection, but 

only in a very general way. The Zn-Mo ratio in the bioturbated 

sediment closely approaches the curve represented by their 

concentration ratio in plankton (and terrigenous debris). A 

shift in a value along the curve reflects the amount of organic 

matter oxidized and/or diluted by calcite, biogenic SiO2, or 

terrigenous debris. The rain rate of organic matter into the 

bottom water, that is, primary productivity, must have been the 

dominant factor controlling the accumulation of trace-element 

nutrients from seawater. As bottom-water advection was high 

enough to maintain bacterial O2 respiration, metals were not 

reduced to relatively insoluble valence states, the major exception being Mn. Concentrations of Mo and Zn in samples corresponding to the two melt-water pulses occur within a field that 

is displaced farthest from the plankton curve and toward the 

seawater curve. Bottom-water advection and primary productivity combined to give a maximum Mo:Zn ratio of approximately 4. Mo and Zn accumulation as organic matter at these 

times was less important than accumulation from bottom water 

under SO4

2- -reducing conditions. However, the accumulation 

rates of Zn and Cd (fig. 12) clearly show that primary productivity was quite high during part of each melt-water pulse. 

Thus, the ratio does not identify the level of primary productivity. It does suggest that the relation between the upwelling 

rate and bottom-water advection contributed similarly to traceelement accumulation rates for these two periods.

Sediment that accumulated during the Younger Dryas 

event has an intermediate Mo:Zn concentration ratio. The bio-

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Figure 13. Relations between the concentrations of Mo and Zn (A) 

and Mo and Cd (B) in the seawater-derived fractions of sediment in 

core PL07-39PC. Only samples younger than 17 kyr are shown. The 

broken curve in A represents the Zn:Mo ratio for seawater and the 

solid curve the ratio for plankton, both extrapolated into the sediment field. Sample symbols are the following: bioturbated unit, blue 

squares; melt-water pulse IA, green open diamond; melt-water pulse 

IB green filled diamond; Younger Dryas, green crosses; uppermost 

250 cm of the faintly laminated unit, red circles. The patterned areas 

are of sediment-core intervals identified in figure 2, the Younger Dryas 

and melt-water pulses IA and IB.

Elemental Ratios and Concentrations—An Alternative Model of Environmental Conditions
38 Trace-Element Deposition in the Cariaco Basin, Venezuela Shelf, under Sulfate-Reducing Conditions 

logical and bottom-water-redox signals were both important. 

The low absolute marine Cd and Zn concentrations (fig. 2) 

suggest that primary productivity was lower than at present, but 

their accumulation rates (fig. 12) clearly required primary productivity to have been much higher during the Younger Dryas.

The Mo:Zn ratios in samples from the faintly laminated 

sediment occupy the same field as samples from the Younger 

Dryas. Primary productivity was lower during the period of 

deposition of this sediment than during the Younger Dryas (fig. 

12), yet the concentrations of Zn and Cd (figs. 2 and 13) suggest the opposite. This relation demonstrates the difficulty of 

interpreting the sediment record from elemental concentrations 

alone. Perhaps most important for this study, the distribution 

of concentration ratios of the marine fraction of trace elements 

into separate fields defined by lithology lends support to our 

overall partitioning scheme of trace elements into a terrigenous fraction and the two marine fractions (tables 2 and 6).

The relation between Mo and Cd exhibits a somewhat 

similar distribution, in that the different lithologic units fall 

into separate fields, mwp-IB being the exception (fig. 13B). 

The few samples from this unit occur in the field defined by 

the faintly laminated unit. However, more samples of this unit 

need to be analyzed to determine its geochemistry accurately, 

relative to that of the other units.

An often used alternative procedure to interpret the 

marine fraction of trace elements is through normalization to 

Al2O3, or another measure of the terrigenous fraction of sediment. The use of this procedure can reveal interesting aspects 

of provenance (Yarincik and others, 2000), when applied to 

the major-element oxides and trace elements within the terrigenuos fraction alone, but the procedure also identifies the 

marine fraction. Unfortunately, it obscures interelement relations within the marine fraction of the sediment. For example, 

the Mo:Al2O3 concentration ratio can vary independently of 

any change in the marine environment, as it is a function of 

the accumulation rate of the terrigenous fraction, as well as a 

function of the hydrography of the water column. Also, a Mo:

Al2O3 ratio greater than that of WSA has been considered by 

others as evidence enough that a sedimentary deposit accumulated under SO4

2- -reducing conditions. If the Mo:Zn, or Mo:

Cd, ratio of the marine fraction of sediment closely approaches 

the ratio of plankton, as it does in several organic-carbonenriched sedimentary deposits (Piper, 1994) and in the bioturbated section of this deposit, the bottom water of the basin was 

unlikely to have been SO4

2- reducing, despite Mo concentrations that are elevated above a terrigenous contribution. Such 

deposits can have exceptionally high Mo:Al2O3 ratios. The 

procedure advocated here, of examining interelement relations 

in the marine fraction of sediment alone, allows for unequivocal identification of the bottom water geochemistry. However, 

it cannot identify either the level of upwelling nor of bottomwater advection in absolute terms.

The distribution of organic carbon also demonstrates the 

difficulty of identifying the past hydrography of the basin 

from elemental concentrations. The concentration of organic 

carbon, or organic matter (table 3), in the sediment that accumulated during the Younger Dryas was at its very lowest for 

the most recent 15 kyr (fig. 14), during the time when primary 

productivity was at its very highest (fig. 12E). These low concentrations reflect the relatively high accumulation rates of 

terrigenous debris (fig. 12D) and biogenic SiO2 (Peterson and 

others, 1995) during the Younger Dryas, that is, dilution by 

inorganic phases of the organic matter that settled to the sea 

floor. This interpretation is further supported by the negative 

relation that characterizes the entire core, between the concentration of organic matter and bulk sediment accumulation rate 

(table 4). The overall decrease in organic carbon with depth in 

the core, for which the regression has a standard deviation of 

0.65 and is most strongly represented by curves drawn through 

the concentration extrema (fig. 14), suggests, however, that 

a combination of dilution and preservation contributed to its 

distribution. And finally, the roughly parallel relation in the 

uppermost 300 cm of the core, between the distribution of 

organic carbon and accumulation rate of Cd (fig. 12E), shows 

that primary productivity also was important to the ultimate 

concentration of organic carbon in this deposit.

Summary and Conclusions

(1) The terrigenous sediment fraction in the Cariaco 

Basin, as measured by the distributions of Ga, Th, and Al2O3, 

recorded high accumulation rates that corresponded to the rise 

in sea level at the end of the last glacial maximum (LGM), 

beginning at approximately 21 to 18 ka. A second high corresponded to the Younger Dryas cold event. The first set 

of high values possibly reflects resuspension of previously 

exposed shelf sediment and its re-deposition within the basin, 

in response to the transgression of the ocean across the shelf 

following a long period of exposure. The second may reflect 

an increase in the rate of erosion in the source areas. The terrigenous sediment record for the Holocene seems to agree 

only moderately with lake records of the Caribbean region, 

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Figure 14. Relation between the concentrations of organic carbon 

(0.5 × organic matter) and sediment depth in piston core PL07-39PC, in 

centimeters below the sea floor (cmbsf).
39

of wet conditions in early Holocene becoming dry in the mid 

Holocene.

(2) The massive texture of sediment deposited immediately prior to 14.8 ka indicates that the sediment was bioturbated and the bottom water oxic. A slight marine Mn and Co 

fraction in this sediment and low values for marine Mo and 

Cd at the very top of the section support such redox conditions. The upper most part of this sediment unit may also have 

provided a small fraction of several trace elements to the very 

basal part of the overlying laminated unit.

(3) The accumulation rates of seawater-derived Mo and 

Cd, beginning 14.8 ka, show that primary productivity in the 

photic zone increased sharply and bottom-water chemistry 

changed from oxic to SO4

2- reducing. Major peaks in their 

accumulation rates correspond approximately to melt-water 

pulse IA and the onset of the Younger Dryas cold event. Mean 

accumulation rates of the two metals were high for the entire 

period from 14.8 ka to 8.6 ka, but both exhibited large, shortterm variations. The temporal variations in accumulation 

rates of these two elements were driven by a combination of 

changes in upwelling into the photic zone (primary productivity) and advection of bottom water (bottom-water residence 

time). On average, primary productivity was high and the residence time of bottom water was a few tens of years.

(4) By 8.4 ka, primary productivity had decreased sharply 

by as much as a factor of 2 over a period of 1 kyr; bottom 

water residence times increased from a few tens of years, 

typical of the period 14.8 kyr to 11 ka, to as much as several 

hundred years. Primary productivity then exhibited a gradual 

decline to 4.0 ka, followed by an equally gradual reversal 

to the present. Diminished primary productivity provided a 

greatly reduced flux of organic matter to the bottom water 

from the preceding 4.4 kyr. Nonetheless, bacterial respiration was able to maintain SO4

2- -reducing conditions in the 

bottom water up to the present time, owing to the relatively 

long bottom-water residence times. The relation of the trend 

in water column advection for this period (8 ka to the present) 

with the trend in climate over North America and the North 

Atlantic Ocean suggests that the hydrography of the Cariaco 

Basin was strongly linked to the global climate.

(5) Accumulation rates of Cd, Mo, and the other trace 

elements examined (Cr, Cu, Ni, V., and Zn) show a slight 

maximum centered at about 0.70 ka, possibly resulting from 

a moderately elevated level of primary productivity, that is, 

upwelling rate, that may have reflected cooling during the 

“Little Ice Age” of the last millennium.

(6) Variations in the Zn:Mo and Cd:Mo concentration 

ratios provide support for the partitioning scheme used in this 

study, whereby a nonreactive terrigenous fraction of trace elements is distinguished from a marine fraction that had both 

a biogenic (photic-zone) source and a hydrogenous (bottomwater) source. Trace-element ratios of the marine fraction 

alone, within this and ancient deposits, such as black shales, 

also can identify the type of bacterial respiration of the bottom 

water under which the deposits accumulated. Unfortunately, 

the results of this study demonstrate that neither elemental 

ratios nor absolute concentrations offer help in ascertaining the 

intensity of upwelling, that is, primary productivity, of these 

deposits. The overall distribution of organic carbon similarly 

fails to reflect solely the changes in primary productivity. In 

order to identify this property of deposition, it is necessary to 

have measured or, at the very least, to be able to estimate sediment accumulation rates.

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