organic carbon accumulation and preservation in surface sediments on the peru margin

Ž .Chemical Geology 152 1998 273–286

Organic carbon accumulation and preservation in surfacesediments on the Peru margin

Michael A. Arthur a,), Walter E. Dean b, Kirsten Laarkamp c

a Department of Geosciences, PennsylÕania State UniÕersity, UniÕersity Park, PA 16802, USAb U.S. Geological SurÕey, MS 980, Federal Center, DenÕer, CO 80225, USA

c Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Received 29 October 1997; accepted 30 June 1998

Abstract

Concentrations and characteristics of organic matter in surface sediments deposited under an intense oxygen-minimumzone on the Peru margin were studied in samples from deck-deployed box cores and push cores acquired by submersible ontwo transects spanning depths of 75 to 1000 m at 128 and 13.58S. The source of organic matter to the seafloor in these areasis almost entirely marine material as confirmed by the narrow range of d

13C of organic carbon obtained in the present studyŽ .y20.3 to y21.6‰; PDB and the lack of any relationship between pyrolysis hydrogen index and carbon isotope

Ž .composition. Organic carbon contents are highest up to 16% on the slope at depths between 75 and 350 m in sedimentsŽ .deposited under intermediate water masses with low dissolved oxygen concentrations -5 mmolrkg . Even at these low

concentrations of dissolved oxygen, however, the surface sediments that were recovered from these depths are dominantlyŽ .unlaminated. Strong currents up to 30 cmrs associated with the poleward-flowing Peru Undercurrent were measured at

depths between 160 and 300 m on both transects. The seafloor in this range of water depths is characterized by bedformsŽ .stabilized by bacterial mats, extensive authigenic mineral crusts, and or thick organic flocs. Constant advection of

dissolved oxygen, although in low concentrations, active resuspension of surficial organic matter, activity of organisms, andtransport of fine-grained sediment to and from more oxygenated zones all contribute to greater degradation and poorer initialpreservation of organic matter than might be expected under oxygen-deficient conditions. Dissolved-oxygen concentrationsultimately may be the dominant affect on organic matter characteristics, but reworking of fine-grained sediment and organicmatter by strong bottom currents and redeposition on the seafloor in areas of lower energy also exert important controls onorganic carbon concentration and degree of oxidation in this region. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: Organic carbon; Hydrogen index; Oxygen-minimum zone; Preservation; Slope currents

1. Introduction

Biological productivity in surface waters of theŽ y2 y1Peru margin is very high 100–400 g C m yr ;

) Corresponding author. Tel.: q1 814 863 6054; fax: q1 814863 7823; e-mail: [email protected]

.Suess et al., 1987 as the result of strong, wind-drivenŽ .upwelling e.g., Smith, 1981 . Decomposition of this

organic matter in intermediate waters is one of theimportant causes of a well-developed oxygen-mini-

Ž .mum zone OMZ; Fig. 1 that impinges on the outerŽshelf and upper slope e.g., Demaison and Moore,

.1980 . The composition of the pre-burial organic

0009-2541r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 98 00120-X

( )M.A. Arthur et al.rChemical Geology 152 1998 273–286274

Ž .Fig. 1. Lower Map showing the generalized bathymetry of the Peru margin, locations of the two submersible transects across the outerŽshelf and upper slope, locations of ODP drillsites and the general area of impingement of the most intense part of the OMZ O2

. Ž .concentration-5 mMrkg . Upper Panel of selected CTD-O profiles along the 13.58S transect.2

matter that accumulates at the sediment–water inter-face is important because it provides the baselinecharacteristics of organic matter that will be modi-fied during diagenesis and ultimate burial.

The characteristics of organic matter deposited onthe seafloor are thought to be primarily determined

Žby source e.g., terrestrial vs. marine, community.structure of primary producers, etc. , water depth,

extent of ‘processing’ by organisms in the watercolumn and at the sediment–water interface, concen-trations of dissolved oxygen in intermediate andbottom waters, and diagenetic processes at the sedi-

Žment–water interface e.g., Emerson and Hedges,

.1988 . Water depth determines, in part, the residencetime of organic matter in the water column, althoughthis is modified by organic ‘processing’, includinggrazing by zooplankton and fish, production of fecalpellets, bacterial decomposition, etc. Dissolved oxy-gen concentration is commonly thought to be themaster variable controlling the characteristics of or-ganic matter reaching the sediment–water interfaceŽ .e.g., Demaison et al., 1984 . However, several re-cent studies suggest that there may not be a differ-ence in rate and extent of degradation of organic

Ž . Žcarbon OC in oxic vs. anoxic settings e.g., Hen-richs and Reeburg, 1987; Jahnke, 1990; Pedersen

( )M.A. Arthur et al.rChemical Geology 152 1998 273–286 275

.and Calvert, 1990; Lee, 1992 , including marginswith well-developed OMZs such as the Gulf of

Ž .California Calvert et al., 1992 , the Oman marginŽ .Pederson et al., 1992 , and the California marginŽ .Reimers et al., 1992 . Processes that occur at thesediment–water interface include recycling of or-ganic matter by larger benthic organisms, bacterialremineralization, and reworking and resedimentationby currents and downslope transport.

The proportion of the organic matter that reachesthe sediment–water interface that is then ultimatelypreserved in the sediments during burial is called the‘burial efficiency’ and is primarily determined by the

Žsedimentation rate e.g., Muller and Suess, 1979;¨Henrichs and Reeburg, 1987; Ingall and Van Cap-

.pellen, 1990; Betts and Holland, 1991 . Burial effi-ciency may be partly or entirely independent of thedissolved oxygen concentration in bottom waters,

Žparticularly at high rates of sedimentation e.g.,.Tyson, 1995 . For example, Cowie and Hedges

Ž .1992 showed that burial efficiencies in oxic DabobŽ .Bay Washington and nearby anoxic Saanich Inlet

Ž .British Columbia are indistinguishable. Recentstudies also suggest that there is no strong differencebetween late Holocene OC accumulation rates in thedeep anoxic Black Sea and those in oxic open-oceansettings that have similar water depths and sedimen-tation rates when all sites are normalized to rates of

Žprimary production Canfield, 1989; Calvert et al.,.1991; Arthur et al., 1994 . However, there still re-

mains considerable disagreement on the role of bot-Žtom-water oxygenation on OC burial e.g., Canfield,

.1994 . A major obstacle to resolving the problem isthe great complexity of the natural environmentsinvolved, and, therefore, the interplay of many vari-ables. Although the distributions of surficial organicmatter on the Peru margin appear to be controlled bythe distributions of dissolved-oxygen concentrationsŽ .e.g., Demaison and Moore, 1980 , we will demon-strate that other processes produce these patterns andobscure any direct influences that dissolved oxygenmay have.

Although the characteristics of organic matter onŽthe Peru margin have been studied previously e.g.,

Reimers and Suess, 1983; Henrichs and Farrington,1984; Emeis et al., 1991; and various papers of

.Suess et al., 1990 , there has been no previousintensive, systematic sampling of surface sediments

in onshore–offshore transects across the water col-umn redox gradient. In the present paper we summa-rize results emphasizing the characteristics of or-

Žganic matter concentration, hydrogen richness, and.carbon isotopic composition in surface sediments

Ž .top 1 to 2 cm from the Peru outer shelf and upperŽ .slope water-depth range of 75 to 1000 m; Fig. 1 .

Detailed sampling of sediment for the present studyŽ Xwas undertaken along two transects 13830 S and

.128S; Fig. 1 in October and November, 1992, usingpush cores collected by the submersible JohnsonSea-Link II and box cores taken from the RrVSeward Johnson. The two transects in this study arelocated in regions characterized by the highest sur-face sediment OC concentrations on the Peru marginŽfrom 10 to more than 20%; Reimers and Suess,

.1983; Suess et al., 1987 . Our expedition took placeat the end of protracted 1991–1992 El Nino condi-˜tions, and, therefore, we cannot assert that the ob-served distribution of dissolved oxygen, temperature,fauna, and surface sediment characteristics are repre-sentative of ‘normal’ conditions for the margin. Ingeneral, however, the patterns that we observed donot differ greatly from those described by previous

Žworkers for non-El Nino conditions e.g., Suess et˜.al., 1987 . During El Nino events, dissolved-oxygen˜

concentrations in the water column increase, thestrength of the Peru Undercurrent increases signifi-cantly, and the flux of OC to the sediment–water

Ž .interface apparently increases Suess et al., 1987 .

2. Methods

A total of 36 box cores were collected from theRrV Seward Johnson, and 31, 3-h long dives withDSrV Johnson Sea-Link II were completed for thisinvestigation. Submersible dives resulted in high-res-olution video coverage of bottom operations, sedi-ment-type, and benthic and nektonic organisms, as

Žwell as recovery of 108 push cores, 32 large 25-. Žcm -diameter pore-water cores, and 25 small box 25.cm cores. For the purposes of this study, box cores

and submersible push cores were subsampled onboard ship within hours of recovery, sealed in plasticbags, and refrigerated. The samples analyzed in thisstudy represent the top 1 cm of all submersible pushcores and box cores that recovered the sediment–


water interface. For most cores, a sample also wascollected from 1–2 cm depth.

Bottom-current direction and magnitude weremeasured from the submersible at numerous stationsusing an external current meter. Water-column tem-perature and dissolved-oxygen profiles also weregenerated at key stations using a Sea-Bird Electron-

Ž . Ž .ics SBE19 conductivity–temperature–depth CTDsystem with an oxygen probe.

Ž .Total- and inorganic carbon TC and IC weredetermined using a UIC Coulometrics Coulometer

Ž .System 140 coulometer Engleman et al., 1985 onpowdered bulk-sediment samples. The wt.% CaCO3

is calculated as %IC divided by 0.12, the weightfraction of carbon in CaCO . The concentration of3

OC is calculated as the difference between TC andIC concentrations. The technique has a precision ofbetter than "0.5%.

ŽRock–Eval pyrolysis Tissot and Welte, 1984;.Peters, 1986 was used to provide measures of hy-

drogen and oxygen richness of organic matter thatcorrespond generally to the HrC and OrC elemental

Ž .ratios of organic matter Crossey et al., 1986 . ByŽ .this method, free and adsorbed hydrocarbons HC

released by programmed heating of a sample at arelatively low temperature are recorded as the area

Žunder the first peak of a pyrogram S1, in mg HCrg.sample . The second peak on a pyrogram is com-

posed of HC released by cracking of kerogen byŽheating the sample to 5508C S2, in mg HCrg

.sample . CO also is generated by kerogen degrada-2

tion and is analyzed as the third peak on a pyrogramŽ .S3, in mg CO rg sample . When normalized to the2

OC content, the S2 peak becomes the hydrogenŽ .index HI, in mg HCrg OC , and the S3 peak

Ž .becomes the oxygen index OI, in mg CO rg OC .2

The Rock–Eval pyrolysis technique has been usedfor many years as a rapid screening tool for thedetermination of the hydrocarbon source–rock poten-

Žtial of sedimentary rocks Tissot and Welte, 1984;.Peters, 1986 . With careful treatment, pyrolysis can

also be used to determine the hydrogen richness ofŽ‘protokerogen’ in modern sediments e.g., Liebezeit

and Wiesner, 1990; Calvert et al., 1992; Liebezeit,1992; Pederson et al., 1992; Arthur et al., 1994;

. ŽDean and Gardner, 1998 . ‘Protokerogen’ also.known as ‘humin’ consists largely of humic macro-

molecules as well as polysaccharides and proteins

Žnot yet completely degraded by heterotrophs Whelen.and Thompson-Rizer, 1993 . All Peru margin sam-

ples were washed several times in cold distilledwater, filtered, and dried at 508C to eliminate saltsprecipitated during drying of samples. Salt was foundto produce an interference on the flame ionization

Ž .detector FID at lower pyrolysis temperatures,thereby producing an apparent larger S1 peak or a‘double humped’ S2 peak. The programmable cycle3 on the Delsi Rock–Eval instrument was used for

Ž .analysis of samples as follows: 1 the sample washeated at a rate of 908Crmin to 1808C where it was

Ž .held for 2 min for measurement of S1; 2 thesample was then heated at a rate of 308Crmin to atemperature of maximum pyrolysis of 5508C formeasurement of S2. Reproducibility for duplicatesamples was, on average, "5% for values of S2 andHI and "3% for the temperature of maximum hy-

Ždrocarbon yield T ; range of 3638 to 4128C for themax.Peru margin samples .

Ž .Because Katz 1983 suggested that there may besome CO yield from breakdown of carbonate min-2

erals and this might affect values of S3, we analyzeda duplicate set of 12 samples after treatment with

Ž .buffered acetic acid pHs4 to remove carbonateminerals without significantly hydrolyzing the pro-tokerogen. The values of S3 obtained from the car-bonate-free samples were not significantly differentŽ ."4% from those obtained from the carbonate-bearing samples. Values of S2 were largely unaf-fected by this treatment, suggesting that a ‘matrix’

Ž .effect e.g., Peters, 1986; Crossey et al., 1986 didnot produce any differences among these samplesover the low range of CaCO contents observed3Ž .0–25% .

Stable carbon-isotope ratios of bulk OC weredetermined by standard techniques on decalcified

Žsamples Pratt and Threlkeld, 1984; Dean et al.,.1986 . A powdered sample was reacted with buffered

acetic acid for 24 h to dissolve carbonate minerals.The residue was then centrifuged, decanted, washedthree times, dried under flowing nitrogen at 508C,and combusted at 10008C with copper oxide in asealed quartz tube. The resulting CO was then2

dehydrated and purified in a high-vacuum gas-trans-fer system, and the isotope ratios determined using aFinnigan 252 isotope-ratio mass spectrometer. Re-sults are reported in the usual per mil d-notation


relative to the University of Chicago Peedee belem-Ž .nite PDB marine-carbonate standard:

d13C‰ PDB s R rR y1 =103,Ž . Ž .Ž .sample PDB

where R is the ratio of 13C to 12 C. Analyticalprecision of these analyses "0.1.

3. Organic carbon, pyrolysis and isotope data

Concentrations of OC in surface sediments showreasonably similar patterns with respect to depth for

Ž .both transects Figs. 2A and 3A , with values of 1 to5% at depths shallower than 100 m, increasing tomaximum values of 5 to 16% between 100 and 375m and decreasing to less than 5% at depths greaterthan about 375 m. The overall peak in OC alongeach transect corresponds to the impingement of themost intense oxygen deficits within the OMZ on the

Ž .upper slope Figs. 2D and 3D , and thus could beinterpreted as simply the result of better preservationof organic matter under oxygen-deficient conditions.However, both transects are characterized by a nar-row zone of lower OC concentrations between depthsof about 200 to 300 m, a zone well within the OMZ.The low OC values occur along an intermediate areaof the slope that is covered by laterally extensivephosphorite crusts and nodular phosphatic pavementsŽ .Figs. 2D and 3D and correspond to depths of

Ž .highest current velocities Figs. 2C and 3C . Theserelations will be discussed later.

Rock–Eval pyrolysis results for both transectsshow general patterns of increasing S2 and HI valueswith increasing OC concentrations. Values for S2and OC concentration display a strong positive linear

Ž .correlation Fig. 4 as is found in most data setsŽ .e.g., Langford and Blanc-Valleron, 1990 for mod-ern and ancient OC-rich strata of marine origin.Because the HI is derived from S2 by normalizing to

Ž . Ž . Ž . Ž .Fig. 2. Data for the 128S Peru margin transect showing: A percent organic carbon OC in surface sediments upper 1 cm ; B Rock–EvalŽ . Ž . Ž . Ž .pyrolysis hydrogen index in mg HCrg OC ; C current velocity in cmrs and general direction from north or south measured from

Ž . Ž .submersible; D dissolved oxygen concentration mMrkg measured in one CTD-O cast offshore. Also shown is the general lithology of2

surface sediment or outcrop on the transect.


Ž . Ž . Ž .Fig. 3. Data for the 13.58S transect on Peru margin showing: A percent organic carbon OC in surface sediments; B Rock–EvalŽ . Ž . Ž . Ž .pyrolysis hydrogen index in mg HCrg OC ; C current velocity in cmrs and general direction from north or south measured from

Ž . Ž .submersible; D dissolved oxygen concentration mMrkg measured in one CTD-O cast offshore. Also shown is the general lithology of2

surface sediment or outcrop on the transect.

w Ž . x%OC HI s S2r%OC = 100 , Langford andŽ .Blanc-Valleron 1990 suggested that the average

value of HI for a particular data set could be ob-tained by multiplying the slope of the regression

Ž .equation for a graph of %OC vs. S2 Fig. 4 by 100Ž .to convert to mg HCrg OC . The lines of regres-sion for data for samples from the 128 and 13.58S

Žtransects are very similar surface sediment in Fig..4A , with slope-derived average HI values of 382

and 393, respectively. These slopes are remarkably

similar to those for surface sediments deposited inŽ .OMZs on the Oman margin Pederson et al., 1992

Ž . Žand the Gulf of California Calvert et al., 1992 Fig..4C , although the surface sediments of the Peru

margin range to much higher values of both %OCŽ .and S2 Fig. 4C . The value of the positive intercept

on the %OC axis is an estimate of the amount ofŽ .refractory non-H-bearing organic matter present.

Based on this intercept, it would appear that theorganic matter on the Peru margin contains an aver-

Ž . Ž .Fig. 4. Scatter plots of %OC vs. Rock–Eval pyrolysis S2 values in mg HCrg sediment for: A Peru margin surface sediment samplesŽ .from the 128S and 13.58S transects. Linear regression equations and correlations coefficients r for the two transects also are given, and the

Ž .average regression line for the two transects is plotted as a solid line. B Sediments from Peru margin ODP Sites 679, 680, 681, 684, 686,687, and 688, and from Gulf of California DSDP Sites 479 and 480. Dashed line represents the regression equation for Hole 687A. Linear

Ž .regression line for surface sediments on the Peru margin A is shown as a heavy solid line. ODP Peru margin data are from Site Chapters inŽ . Ž . Ž .the work of Suess et al. 1990 and Emeis and Morse 1990 data indicated by E&M680A and E&M688A . DSDP Gulf of California data

Ž . Ž .are from Peters and Simoneit 1982 . C Data sets of sediments from other oxygen-depleted Holocene–Pleistocene settings: the CaliforniaŽ . Ž . Ž . Žmargin Dean and Gardner, 1998 ; Black Sea Unit I Arthur et al., 1994 ; Oman margin Pederson et al., 1992 ; Gulf of California Calvert

. Ž .et al., 1992 . Linear regression lines for surface sediments on the Peru margin A is shown as a heavy solid line.


Žage of about 1% OC from refractory i.e., highly.degraded or, possibly, terrestrial organic matter. This

is not a significant contribution for samples thatŽcontain more than 10% OC e.g., it would constitute

.10% of the total OC in such cases but is for sampleswith low concentrations of OC.

Data for sediments from ODP sites on the Perumargin generally plot above the regression line for

Ž .our surface sediments Fig. 4B , with average HIŽvalues of about 450–500 i.e., slope of about 4.5–5

.for regression equations in Fig. 4B . Most valuesplot close to the regression line for samples from

Ž .Hole 687A dashed line in Fig. 4B except data forsamples from Site 680 reported by Emeis and MorseŽ .1990 which exhibit a considerable amount of scat-ter.

As discussed above, both S2 and HI are measuresof the H-content of marine kerogen or protokerogenŽ .Tissot and Welte, 1984 . In ancient sedimentary

Ž .strata, high HI values more than 400 mg HCrg OCcommonly are interpreted as indicating enhancedpreservation of lipid-rich organic matter, implyingrelative lack of degradation of primary organic mat-

Ž .ter e.g., Pratt, 1994; Tyson, 1995 . By comparison,the slopes of the OC–S2 regressions for protokero-gen from the Peru margin, Gulf of California, and

Ž .Oman margin Fig. 4 suggest either that there hasŽ .been significant degradation oxidation of bulk or-

Ž .ganic matter that was initially H-rich and or thatthere is a significant fraction of admixed H-poorterrestrial organic matter in all samples. The organicmatter delivered to the sediments in all three of thesehigh upwelling areas should be overwhelmingly of

Žmarine origin Suess et al., 1987; Calvert et al.,.1992; Pederson et al., 1992; Luckge et al., 1996¨

because of the lack of major fluvial sources in theseregions, but a significant contribution from terrestrialsources, if present, should be reflected in the carbonisotopic signature of that organic matter.

Values of d13C of organic carbon in our surface

Ž .sediment samples from the Peru margin Fig. 5 havea very narrow range from y20.3 to y21.6‰. Thesevalues are well within the range of most values for

Ž .particulate organic matter POM collected by sur-face pumping during the 1992 Peru margin expedi-

Ž .tion y18.9 to y21.2‰; Pancost et al., in press . IfŽ .the large variation in H-richness S2 and HI values

observed in surface sediments resulted from a mix-

Fig. 5. Scatter plot of d13C of organic carbon in bulk organic

Žmatter vs. Rock–Eval pyrolysis hydrogen index in mg HCrg. ŽOC for Peru margin surface sediment samples only samples

.from 0–1 cm from both transects.

ture of organic matter from terrestrial and marinesources, we would also expect a large range invalues of d

13C and a positive correlation between HI13 Ž .and d C Dean et al., 1986 . The narrow range of

d13C in our samples, the lack of any relation be-

tween HI and d13C, and the similarity of the d

13Cvalues with those of marine plankton from the samearea suggest that the organic matter in Peru surfacesediments is likely derived from planktonic sourceswith little or no contamination by terrestrial organicmatter. Lignin abundances in a sample from 106 m

Ž .water depth on the eastern landward end of our13.58S transect indicate that terrestrial material con-tributes only a small, highly degraded fraction of the

Ž .organic material Bergamaschi et al., 1997 . OtherŽsources, such as bacterial mat components e.g.,

.Thioploca , are possible contributors, confined pri-Ž .marily to shallower areas of the shelf 75–150 m

where extensive mats were observed by submersibleŽ .Figs. 2 and 3 . Some of the high values of %OC and

Ž .HI observed between 100 and 150 m Figs. 2 and 3may be due to high surface abundances of Thioplocaon the upper slope, but we have no discriminatingdata that constrain the relative contributions ofplankton and Thioploca. If plankton are the predomi-nant sources of POM delivered to the sediment–waterinterface on the Peru margin, the relatively low andvariable S2 and HI values of the bulk OC in samplesfrom both transects suggest that planktonic organicmatter experienced greater oxidation than expected


Ževen under the essentially anoxic -5 mMrkg dis-.solved oxygen conditions that existed over most of

the upper slope during October and November, 1992Ž .Fig. 1 .

4. Organic matter sedimentation and accumula-tion

Despite the problems with discrimination of oxicvs. anoxic depositional environments using OC con-centrations or accumulation rates, one of the lessambiguous differences between these environments

Žis the apparent better preservation of lipid-rich H-. Žrich organic matter in anoxic settings e.g., Harvey

.et al., 1986 . Therefore, because of the prevailinglow dissolved-oxygen concentrations on the Perumargin, we expected to find consistently higher val-

Ž .ues of pyrolysis S2 and HI 400–600 mg HCrg OCin surface sediments there. The amount of hydrocar-

Ž .bons in the sediment S2 is highly correlated withŽ .amount of organic matter %OC; Fig. 4A , but why

is not the slope of the regression steeper, as it is forolder Quaternary sediments recovered at Ocean

Ž .Drilling Program ODP sites on the Peru marginŽ .Fig. 4B , and from the Black Sea, Gulf of Califor-

Ž .nia, and California margin Fig. 4C in which HIvalues of up to 900 do occur?

Ž .In sedimentary rocks, low values of HI -100usually indicate the presence of either highly oxi-dized organic matter of planktonic origin, or primar-ily higher plant material of terrestrial derivation.Consistently high HI values of )400 are foundmainly in rocks that were deposited in environments

Žinterpreted to have been anoxic e.g., as indicated bythe presence of fine laminations and absence of

.bethic fauna , and where there is no significantŽ .reworked material e.g., Pratt, 1994 . Mixing of ter-

Ž .restrial organic matter HI of -100 mg HCrg OCŽand well-preserved algal organic matter HI)600

.mg HCrg OC in various proportions is unlikely forthe Peru margin samples for reasons discussed above.The HI of organic matter deposited in the region oflow dissolved-oxygen concentration on the Peru slopedoes not correspond to values typical of organicmatter deposited in Quaternary and older ‘euxinic’settings. It is more likely that partial degradation ofauthocthonous marine organic matter produced the

dominance of HI values that are lower than 600Žaverage of about 400 for surface sediments and

.about 500 for older Quaternary sediments . The com-parison between OC-S2 slopes for different modern

Ženvironments appears to confirm this conclusion Fig..4B . In fact, we were surprised to find that the

Ž .California margin data of Dean and Gardner 1998indicate higher HI, implying better preservation oforganic matter on the California slope than for thePeru slope, even though the California margin proba-

Žbly has higher overall dissolved oxygen values usu-ally )10 mMrkg in the OMZ impinging on the

.slope and more significant inputs of terrigenousŽorganic matter than does the Peru margin e.g., Suess

.et al., 1987 .Thus, we are left with a paradox: the highest OC

concentrations occur on the Peru upper slope underprevailing anoxiardysoxia as might be expected, butthe degradation of organic matter, as indicated bypyrolysis S2 and HI, is about as extensive as insediments at shallower or greater water depths under-lying waters of greater dissolved oxygen concentra-tion; HI values on the upper slope are generallylower than those in many modern and ancient sedi-mentary deposits that accumulated underdysoxicranoxic conditions.

Dissolved oxygen concentrations were below 5mMrkg over much of the slope, and no macroben-thic organisms were observed between depths of 120

Ž .and 350 m Jahnke et al., 1993 . Abundant tests ofbenthic foraminifera were found in the upper fewcentimeters of many cores, although it is not clearwhether a significant proportion were alive at thetime of sampling. Euphausiid crustaceans were ob-served in abundance in the low-oxygen water col-

Ž .umn during daylight hours Fig. 6 , but migrated tosurface waters to feed at night. Despite the lowdissolved-oxygen concentration and paucity of infau-nal elements, most cores did not contain fine lamina-tions.

Sediment transport and reworking by strong bot-tom currents must exert a major control on OCpreservation on the Peru margin as previously sug-

Ž .gested by Suess et al. 1987 . We measured currentsas high as 30 cmrs at depths between 175 and 300

Ž .m within the core of the OMZ Fig. 2CFig. 3C . TheŽ .strongest southward-flowing poleward current is

Ž .the Peru Undercurrent Brockmann et al., 1980 ,

()

M.A

.Arthur

etal.r

Chem

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286282

Fig. 6. Sonogram from 33 kHz bottom profile along the 128S transect. Reflections in the water column are due to euphausiid crustaceans and particulates suspended by currentsthat are particularly abundant at about 240 m.


which is trapped over the upper slope and has awell-defined core at 50–300 m depth where it has

Žbeen studied in detail at 108S Shaffer, 1982; Huyer.et al., 1991 . The mean poleward velocity of the

Peru Undercurrent at 108S is 10 cmrs, but it com-monly attains sustained velocities of )25 cmrsover periods of 2 days to 2 weeks. Huyer et al.Ž .1991 indicate that the Peru Undercurrent remainsas a poleward-flowing current during strong El Nino˜events but that it is more variable and perhapssomewhat stronger over the outer shelf at thesetimes. On the two transects studied, the upper slopeis characterized by fields of ripples or small dunes inmud and sand stabilized, in part, by bacterial mats,as well as extensive authigenic mineral crusts and

Ž .other lithified strata, and or thick silty, sandy or-Ž .ganic flocs Figs. 2D and 3D containing moderately

abundant benthic foraminifera. Some of the gaps insampling for geochemical analyses that can be noted

Ž .in the profiles of OC and HI values Figs. 2 and 3Žreflect hard bottoms mostly phosphatic crusts and

.nodule horizons that could not be penetrated bypush cores or box cores; these gaps are greatestbetween 150–350 m water depth. Effects of currentwinnowing also are manifested in the distribution ofindurated, authigenic crusts and nodules exposed onthe margin and in high concentrations of particulate

Ž .matter in the water column Fig. 6 . Sediments in thecurrent-winnowed regions commonly are enriched insilt- and sand-sized phosphatic grains and benthicforaminifera. From the submersible, observers com-monly noted high particulate concentrations in thenear-bottom region, as well as traction transport oforganic flocs or fecal material and benthicforaminifera at the sediment–water interface atdepths characterized by strong currents.

Therefore, on much of the Peru upper slope,constant advection of dissolved oxygen in low con-centrations by bottom currents, resuspension of surfi-cial organic matter, activity of organisms, and trans-port to and from more oxygenated zones contributesto greater oxidation and poorer preservation of or-ganic matter than might be expected on the basis ofthe high carbon fluxes, shallow water depths, and thestrong OMZ that impinges on the upper slope. Thewinnowing and transport of finer-grained materialresults in OC-poor lag deposits of benthicforaminifera, terrestrial and phosphatic sand, and

other coarse-grained materials over much of the up-per slope. Furthermore, fine-grained, OC-rich mate-rial is concentrated in certain sheltered regions of the

Župper slope and transport to deeper regions Pak et.al., 1980; Suess et al., 1987 , as suggested by the

higher concentrations of OC below 600 m on bothŽ .transects Figs. 2A and 3A . However, because of

the longer residence time of organic matter at thesediment–water interface or in the water column as a

Ž .particle load e.g., Pak et al., 1980 resulting fromreworking by currents, it is exposed to more exten-sive degradation and, therefore, is more highly oxi-dized than would be anticipated. A similar situationoccurs on the Oman margin where low values of HIŽ .220–320 and high C:N ratios in organic matter

Ž .were attributed by Pederson et al. 1992 to degrada-Ž .tion linked to winnowing ‘hydrodynamic factors’

and attendant reworking of sediment. Suess et al.Ž . Ž . Ž .1987 estimate that 78% 200 m to 89% 400 m ofthe OC produced on the Peru margin is remineral-

Ž .ized in the water column. Of the 22% 200 m andŽ .11% 400 m preburial flux of OC to the sediment–

Žwater interface i.e., that measured in our surface.sediment samples , only 6% and 2%, respectively, is

ultimately buried.The preburial flux of OC to the sediment–water

interface probably is delivered mainly as fecal pelletsŽfrom anchovies and euphausiids Walsh, 1981;

.Staresnic et al., 1983 . As a result of the diel migra-tions of these organisms into and out of the photiczone, some of the fecal material is excreted at waterdepths of several hundred meters so that degradationof the organic matter probably is even lower than forfecal material excreted at the surface. According to

Ž . Ž .Walsh 1981 see also Suess et al., 1987 , thevirtual elimination of the phytoplankton grazing an-chovy population during El Nino conditions in-˜creases the preburial OC flux to as much as 70% ofprimary production and the net burial OC flux to asmuch as 59% of production. As a long-term indica-tion of the importance of anchovy grazing, it hasbeen argued that the OC content of sediments on thePeru margin has increased by an order of magnitudesince the collapse of anchovy populations in the

Ž .1970s Walsh, 1981 . Thus, the effect of grazing bynektonic and zooplanktonic organisms on export ofOC from the photic zone is 2-fold: increase inwater-column remineralization by primary consump-


tion, and decrease in water-column remineralizationby rapid fecal-pellet transport.

Despite the very low dissolved oxygen concentra-tions and high OC rain rates, OC preservation on thePeru margin appears to be highly sensitive to thedepth of deposition, sedimentation rate, and the in-tensity of flow in the Peru Undercurrent. Nearby

Ž .ODP drillsites 680 and 688 Fig. 1 indicate thatthere have been periods of deposition of laminated

Ž .sediment e.g., Kemp, 1990 characterized by moder-Ž .ate OC concentrations average 4–5% and HI val-

Žues of up to 940 mg HCrg OC average 400–500;.Emeis and Morse, 1990 . Although it is not entirely

certain that the water-column conditions that pre-vailed during the 1992 RrV Seward Johnson cruiseproduced the observed characteristics of surficialsediments at that time, it appears that conditions fordeposition of the laminated, high-HI sedimentarylayers, if widespread, were not like those at the timeof the 1992 cruise. The Peru margin is often cited asa modern analog for high-productivity, low-oxygenancient depositional environments, but we questionwhether the processes that are active in modernoxygen-deficient environments are well enough un-derstood to effectively apply these ‘analogues’ tointerpret paleoenvironments.

5. Conclusions

Concentrations of organic carbon in surface sedi-ments on the Peru margin at 128 and 13.58S rangefrom 4 to 16% on the outer shelf-upper slope, butmaximum pyrolysis hydrogen indices are only about400 mg HCrg OC. The source of organic matter tothe seafloor in these areas is primarily marine mate-rial, confirmed by the narrow range of OC isotope

Ž .values from y20.3 to y21.6‰ PDB , that resultsfrom high primary productivity associated with thePeru Current upwelling system. In general, OC con-tents were highest in sediments on the Peru upper

Ž .slope 150 to 350 m where intermediate watermasses with low dissolved oxygen concentrationsŽ .-5 mMrkg impinge on the slope. However, py-rolysis HI values, which indicate the initial extent ofdegradation of marine organic matter, are variable

Ž .and low mostly -400 mg HCrg OC . This sug-gests that even in the dominantly near-anoxic bottom

waters, initial organic carbon preservation is onlymoderate, probably because of intensive transportand reworking by strong bottom currents. In fact,selective winnowing by bottom currents may be thedominant mechanism for reducing the overall quan-

Ž . Ž .tity %OC and quality H-richness of organic mat-ter within the OMZ and for producing OC enrich-ments by deposition from waning currents in lower-slope mud belts adjacent to current-swept upper slopeintervals.

Acknowledgements

We gratefully acknowledge the captain and crewof the RrV Seward Johnson and the pilots and crewof the DSrV Johnson Sealink II for their outstand-

Žing support during the 1992 Peru expedition SJ.1092 . We thank Lisa Pratt and Tim Lyons for

thoughtful reviews of an earlier draft of this paper,and Tim Lyons and Kay-Christian Emeis for reviewsof this manuscript. This research was supported by

Ž .NOAA-NURP and NSF OCE 9014801 Arthur andŽ .by the USGS Global Change Program Dean . K.

Laarkamp participated in the research as an under-graduate at Penn State University.

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