molecular weight distribution of dissolved organic carbon in marine sediment pore waters

20
Ž . Marine Chemistry 62 1998 45–64 Molecular weight distribution of dissolved organic carbon in marine sediment pore waters David J. Burdige ) , Kip G. Gardner Department of Ocean, Earth and Atmospheric Sciences, Old Dominion UniÕersity, Norfolk, VA 23529, USA Received 18 July 1997; revised 27 February 1998; accepted 6 March 1998 Abstract Ž . The molecular weight distribution of dissolved organic carbon DOC in pore waters from estuarine and continental Ž . margin sediments was examined using ultrafiltration techniques. The majority of this pore water DOC ;60–90% had a molecular weight less than 3 kDa. This percentage appeared to vary systematically among the different sediments studied Ž . and showed very slight changes with depth upper ;30 cm . The absolute concentration of this low molecular weight DOC Ž . LMW-DOC increased, along with total DOC, with depth in the sediments. LMW-DOC therefore represents the vast majority of the DOC that accumulated with depth in these sediment pore waters. These results have been examined in the context of a model which assumes that remineralization processes exert the primary influence on the molecular weight distribution of DOC in the upper portions of the sediments. This model, in conjunction, with other recent studies of DOC in sediment pore waters and in the water column, suggests that there was preferential accumulation of refractory LMW-DOC in Ž . sediment pore waters. Abiotic condensation reactions i.e., geopolymerization appear to have secondary effects on the observed molecular weight distributions of pore water DOC, at least in the upper portions of the sediments examined here. Using this model to explain differences in the molecular weight distributions in these sediments suggests that organic matter remineralization in continental margin sediments may be controlled more by hydrolytic processes than it is in estuarine sediments, where fermentative or perhaps respiratory processes may exert a greater overall control on carbon remineraliza- Ž tion. These observations provide further evidence that the extracellular hydrolysis of macromolecular i.e., high molecular . weight organic matter may not always be the rate limiting step in organic matter degradation. q 1998 Elsevier Science B.V. All rights reserved. Keywords: dissolved organic matter; DOC; marine sediments; early diagenesis; organic matter remineralization 1. Introduction As a class of compounds, dissolved organic car- Ž . bon DOC is thought to play an important role in both carbon remineralization and preservation in ma- rine sediments. During remineralization, sediment organic matter generally passes through one or more ) Corresponding author. Fax: q1-757-683-5303; e-mail: [email protected] DOC intermediates of increasingly smaller molecular Ž weights as it is oxidized to CO Henrichs, 1992; 2 . Deming and Baross, 1993; Alperin et al., 1994 . In anoxic sediments, where benthic macrofauna are es- sentially absent, bacteria mediate the remineraliza- tion process, and extracellular hydrolytic cleavage of particulate biopolymers to high molecular weight Ž . DOC HMW-DOC compounds is generally thought Ž to be the rate limiting step in this process King, . 1986; Hoppe, 1991; Meyer-Reil, 1991 . However, 0304-4203r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0304-4203 98 00035-8

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Page 1: Molecular weight distribution of dissolved organic carbon in marine sediment pore waters

Ž .Marine Chemistry 62 1998 45–64

Molecular weight distribution of dissolved organic carbon inmarine sediment pore waters

David J. Burdige ), Kip G. GardnerDepartment of Ocean, Earth and Atmospheric Sciences, Old Dominion UniÕersity, Norfolk, VA 23529, USA

Received 18 July 1997; revised 27 February 1998; accepted 6 March 1998

Abstract

Ž .The molecular weight distribution of dissolved organic carbon DOC in pore waters from estuarine and continentalŽ .margin sediments was examined using ultrafiltration techniques. The majority of this pore water DOC ;60–90% had a

molecular weight less than 3 kDa. This percentage appeared to vary systematically among the different sediments studiedŽ .and showed very slight changes with depth upper ;30 cm . The absolute concentration of this low molecular weight DOC

Ž .LMW-DOC increased, along with total DOC, with depth in the sediments. LMW-DOC therefore represents the vastmajority of the DOC that accumulated with depth in these sediment pore waters. These results have been examined in thecontext of a model which assumes that remineralization processes exert the primary influence on the molecular weightdistribution of DOC in the upper portions of the sediments. This model, in conjunction, with other recent studies of DOC insediment pore waters and in the water column, suggests that there was preferential accumulation of refractory LMW-DOC in

Ž .sediment pore waters. Abiotic condensation reactions i.e., geopolymerization appear to have secondary effects on theobserved molecular weight distributions of pore water DOC, at least in the upper portions of the sediments examined here.Using this model to explain differences in the molecular weight distributions in these sediments suggests that organic matterremineralization in continental margin sediments may be controlled more by hydrolytic processes than it is in estuarinesediments, where fermentative or perhaps respiratory processes may exert a greater overall control on carbon remineraliza-

Žtion. These observations provide further evidence that the extracellular hydrolysis of macromolecular i.e., high molecular.weight organic matter may not always be the rate limiting step in organic matter degradation. q 1998 Elsevier Science B.V.

All rights reserved.

Keywords: dissolved organic matter; DOC; marine sediments; early diagenesis; organic matter remineralization

1. Introduction

As a class of compounds, dissolved organic car-Ž .bon DOC is thought to play an important role in

both carbon remineralization and preservation in ma-rine sediments. During remineralization, sedimentorganic matter generally passes through one or more

) Corresponding author. Fax: q1-757-683-5303; e-mail:[email protected]

DOC intermediates of increasingly smaller molecularŽweights as it is oxidized to CO Henrichs, 1992;2

.Deming and Baross, 1993; Alperin et al., 1994 . Inanoxic sediments, where benthic macrofauna are es-sentially absent, bacteria mediate the remineraliza-tion process, and extracellular hydrolytic cleavage ofparticulate biopolymers to high molecular weight

Ž .DOC HMW-DOC compounds is generally thoughtŽto be the rate limiting step in this process King,

.1986; Hoppe, 1991; Meyer-Reil, 1991 . However,

0304-4203r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0304-4203 98 00035-8

Page 2: Molecular weight distribution of dissolved organic carbon in marine sediment pore waters

( )D.J. Burdige, K.G. GardnerrMarine Chemistry 62 1998 45–6446

Ž .recent studies by Arnosti et al. 1994 have sug-gested that this may not be the case in all sediments.At the same time, several proposed mechanisms forcarbon preservation in marine sediments, including

Žthe geopolymerization model Nissenbaum et al.,1971; Tissot and Welte, 1978; Krom and Westrich,

.1981; Hedges, 1988 and the mesoporeŽprotectionrsurface area adsorption model Mayer,

.1994a,b; Hedges and Kiel, 1995 , both suggest a rolefor DOC in sediment carbon preservation. However,the details of how this preservation occurs are not

Ž .well understood Hedges and Kiel, 1995 .One approach taken in examining the role of

DOC in sediment carbon cycling has involved deter-mining the molecular weight distribution of pore

Žwater DOC Krom and Sholkovitz, 1977; Orem andGaudette, 1984; Orem et al., 1986; Chin and

.Gschwend, 1991; Chin et al., 1994 . Such studieshave generally used this data to examine the possibleoccurrence of in situ geopolymerization reactions,whereby low molecular weight dissolved organiccompounds are thought to condense and form highermolecular weight dissolved humic substances. Thecontinued condensation of these dissolved humics isthought to eventually lead to the formation of partic-ulate material such as humin or kerogen, and thepreservation of this organic matter in sediments.

To further examine these concepts we have stud-ied the molecular weight distribution of pore waterDOC in contrasting estuarine and continental marginsediments. Our goal was to use this data to betterunderstand sediment carbon remineralization andpreservation, and the role of DOC as an intermediatein these processes. In particular, with these resultswe have developed a model for DOC cycling insediments that combines traditional models for car-bon cycling in anoxic sediments and the recentlyproposed size-reactivity continuum model for DOC

Žremineralization in the water column Amon and.Benner, 1996 .

2. Materials and methods

2.1. Study areas

The studies described here were carried out atthree contrasting estuarine sites in Chesapeake Bay,

three sites along the shelfrslope break of the mid-Ž .Atlantic continental margin see Fig. 1 and at a site

Žin Santa Monica Basin one of the low oxygenbasins in the southern California Borderland region;

.see the map in the work of Berelson et al., 1996 .The geochemical characteristics of these sedimentsare summarized below and in Table 1, and are alsodescribed in more detail in a number of previous

Žpublications Chesapeake Bay: Burdige and Hom-stead, 1994; Burdige et al., 1995; Cowan and Boyn-ton, 1996; Skrabal et al., 1997; Burdige and Zheng,1998; Marvin-DiPasquale and Capone, 1998; mid-Atlantic shelfrslope break: Ferdelman, 1994; SantaMonica Basin: Jahnke, 1990; Shaw et al., 1990;

.Berelson et al., 1996 .The sediments at site M3 in the Chesapeake Bay

and in the Santa Monica Basin are fine-grained,sulfidic sediments where sulfate reduction dominatessediment organic matter remineralization. Bioturba-tion is virtually absent in the Santa Monica Basinsediments, although at site M3 a few bivalve spatand polychaete worms inhabit the upper ;5 cm of

Ž .sediment in the early spring Kemp et al., 1990 . Thesediments at site S3 in Chesapeake Bay are silty

Žsands and extremely well-mixed bioturbated and.bio-irrigated by large tube worms and other benthic

Fig. 1. A map showing the Chesapeake Bay and mid-Atlanticshelfrslope break sites.

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D.J.B

urdige,K.G

.Gardnerr

Marine

Chem

istry62

199845

–64

47

Table 1Site characteristics

a aParameter Chesapeake Bay Mid-Atlantic shelfrslope break S. CaliforniaBorderland

M3 S3 N3 WC4 WC7 AI SM

Ž .Water depth m 15 12 10 390 775 740 900Ž .Bottom water temperature 8C ) 5–22 22 21 7 4 4 5

Ž . Ž . Ž . Ž .Bottom water salinity psu ) 15.6–20.5 10–20 28 20–30 9.9 -0.1–10 35 35 35 34.4Ž .Bottom water dissolved oxygen mM ) 9–420 190 260 220 290 290 ;9

b b c d d e d fŽ .Surface sediment TOC % )3 ;0.5 ;2–4 ;2 ;2 , 2.2 ;1.2 ;5–6g g h h,i h,i h,i jDepth-integrated sediment carbon oxidation rate 6.6"0.8 4.3"1.0 0.8"0.4 2.1"1.0 0.8"0.3 0.9"0.3 0.7"0.3

y2 y1Ž .C ; mol m yrox

)At the time of sampling. For the Chesapeake Bay sites, the salinity values in parentheses are general seasonal ranges.aSee Fig. 1 for the location of these sites.b Ž .From the work of Burdige and Homstead 1994 .c Ž .From the work of Burdige et al. 1995 .d Burdige, unpublished data.e Ž .From the work of Ferdelman 1994 , for a core collected at this site during an earlier study.f Ž . Ž .From the works of Jahnke 1990 and Shaw et al. 1990 .g Ž . Ž .From the works of Burdige and Zheng 1998 . These C values are integrated annual averages over the time period 3r95–10r96 determined from measured benthic ÝCOox 2

Ž Ž . .fluxes made during temporal studies at these sites see the work of Burdige and Homstead 1994 , for further details on the procedures for the benthic flux determinations .h Ž .Determined using measured ÝCO profiles as described in the work of Burdige and Homstead 1994 .2i Ž . Ž . Ž .Averages based on cores collected at these sites in 7r94 CH X , 7r95 CH XIV and 8r96 CH XVII .j Ž .Determined using in situ benthic landers as described in the work of Berelson et al. 1996 .

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( )D.J. Burdige, K.G. GardnerrMarine Chemistry 62 1998 45–6448

Ž .macrofauna Schaffner, 1990 . The sediments at siteN3 in the Bay are clay dominated and iron-rich, andcontain a diverse community of mixed polychaetes

Ž .and bivalves W. Boynton, personal communication .Organic matter in these sediments is largely terrestri-

Ž .ally-derived J. Cornwell, personal communication .The sediments on the mid-Atlantic shelfrslope breakare greyrgreen silty clays and, based on radiochemi-

Žcal measurements, some bioturbation D ;1.5–5B2 y1.cm yr occurs in the upper 20–30 cm of these

Ž .sediments Ferdelman, 1994 .

2.2. Sample collection

With the exception of the Santa Monica Basinsediments, all sediments were collected by box coreand sub-cored for further analysis. Santa MonicaBasin sediments were collected with an Ocean In-

Ž .struments Multi-Corer Barnett et al., 1984 . AllŽ .sub-cores with the exception of the site S3 cores

Ž .were processed cut into 0.5 to 2 cm sections underŽ .an inert N atmosphere, using a modified version2

of the glove bagrsectioning table described by ShawŽ .1989 . Inside this glove bag, the sectioned sedi-ments were placed in polycarbonate centrifuge tubesthat had been washed with HCl and rinsed severaltimes with distilled, deionized water before use.Control experiments showed that these tubes neitherscavenge DOC from, nor add it to, seawater. Sedi-ments were then centrifuged for 10–20 min at 5500

Ž .=g 7000 rpm at in situ temperatures.Pore waters were extracted from site S3 sediments

using a modified form of the pressurized core barrelŽ .technique Jahnke, 1988 . Most of these modification

are described in the work of Burdige and HomsteadŽ .1994 , although here pore waters were removedfrom the sediments by pulling a vacuum on thesampling syringe, rather than pressurizing the entirecore. This procedure was used with these sedimentsto avoid possible artifacts associated with the collec-tion by centrifugation of pore water samples forDOC analyses from heavily bioturbated sedimentsŽ .Martin and McCorkle, 1993; Alperin et al., 1998 .These workers have shown that measured pore wa-ters DOC concentrations from such centrifuge-col-lected samples may overestimate the true pore waterin situ DOC concentration, due to release of DOCfrom benthic macrofauna during core sectioning

andror centrifugation. In sediments that have lownumbers of benthic macrofauna, side-by-side com-parisons of these two pore water sampling tech-

Ž .niques centrifugation vs. pressurized core barrelsŽyield comparable DOC concentrations Burdige, un-

.published data .Regardless of the method of pore water collec-

tion, all samples were filtered through 0.45 mmGelman Nylon Acrodisc filters without exposure toambient air, and aliquots divided into different stor-age vessels for later analysis. Samples for total DOCanalyses were filtered into 8 ml cleaned glass vialssealed with Teflon-lined silicone septa, acidified topH 2 with 6 N HCl, and then quick frozen and stored

Žfrozen until analysis note that all glass and plas-ticware were cleaned as described in the work of

.Burdige and Homstead, 1994 . Samples for molecu-lar weight studies were filtered into small cleanedbrown glass bottles, acidified to pH 2 and storedrefrigerated under N until analysis. Processing and2

storage of samples for molecular weight studies un-der N is based on the results of Orem and Gaudette2Ž .1984 who showed that failure to take these precau-tions may lead to significant differences in the deter-mined DOC molecular weights in pore water sam-ples. Acidification does not change the molecularweight distribution of pore water DOC compoundsŽour results not shown here; also Chin, Y.-P., per-

.sonal communication , yet minimizes DOC loss fromŽrefrigerated samples due to biological activity Tupas

.et al., 1994 . A comparison of our frozen vs. refrig-erated samples for total DOC analyses indicated that

Žtheir concentrations differed by less than "3% ns.53 samples , similar to that observed by Tupas et al.

Ž .1994 .

2.3. Size fractionation studies

These analyses were carried out with ultrafiltra-tion techniques, using Amicon ‘Centricon’ micro-concentrators whose nominal molecular weight cut-offs are 3, 10, 30 and 100 kDa. Studies by Chin and

Ž .Gschwend 1991 showed that the 3 and 10 kDafilters yield molecular weights that agree well withvalues obtained by size exclusion chromatography.

In theory, the use of these filters should haveallowed us to separate DOC into the followingmolecular weight size classes: )100 kDa, 30–100

Page 5: Molecular weight distribution of dissolved organic carbon in marine sediment pore waters

( )D.J. Burdige, K.G. GardnerrMarine Chemistry 62 1998 45–64 49

kDa, 10–30 kDa, 3–10 kDa and -3 kDa. However,Ž .variable and non-reproducible results with the 30-

kDa filter made data obtained with these filtersdifficult to interpret. Similarly, because of the broad

Žmolecular weight cut-offs of these filters Aiken,1984; Chin and Gschwend, 1991; also see results

.below we were not able to use the 3 and 10 kDafilters to determine DOC molecular weights in the3–10 kDa range. In this paper then, we will onlyreport results obtained with the 3 and 100 kDafilters, allowing us to separate DOC into the follow-ing nominal molecular weight classes: )100 kDa,3–100 kDa, and -3 kDa. We will refer to theseDOC size classes as: DOC , DOC and DOC .100 3 – 100 3

Before using the filters we used a modification ofthe procedure described by Chin and GschwendŽ .1991 to clean the filters. The filters were firstsoaked in a 30:70 methanol: distilled, deionized wa-

Ž .ter DDW solution for 2–3 h, next soaked overnightin DDW, and finally flushed several times by repeat-edly centrifuging 2 ml of DDW through the filtersŽ .for a total volume of 12 ml per filter . The filterswere stored in DDW until use. Prior to ultrafiltration

Ž .of a sample, 2 ml of low DOC ;7–8 mM DDWwas passed through the filter by centrifugation, andthen analyzed for DOC. These filter blanks were 23Ž . Ž ."13 mM for the 100 kDa filters and 38 "16

Ž .mM for the 3 kDa filters ns65 for both filters .Given the variability in the filter blanks, all samplefiltrate concentrations were corrected with the blankvalue obtained with the particular filter used for thatultrafiltration. We observed no deterioration orbreakdown of the filters during this cleaning proce-dure and their subsequent use in the ultrafiltration ofa sample.

Pore water ultrafiltration was carried out by plac-ing 2 ml aliquots of the original sample into each ofthe upper reservoirs of 3 kDa and 100 kDa filterassemblies, which were then capped to minimizeevaporative sample loss during processing. The100 kDa filter assemblies were centrifuged at 1000

Ž .=g 3000 rpm for 20 min, while the 3 kDa filtersŽ .were centrifuged at 5500=g 7000 rpm for 2 h

Ž .both at 58C . The resulting filtrates and an aliquot ofthe unfiltered sample were analyzed for DOC by

Žhigh temperature catalytic oxidation HTCO; see.Section 4.2 . Concentrations of DOC in the different

size classes were determined by subtracting the ap-

propriate blank-corrected filtrate concentrations andunfiltered sample concentrations.

Replicate ultrafiltrations of polystyrene sulfonateŽ . Ž .PSS standards see Section 2.3.1 using the 100kDa filters yielded filtrate concentrations that agreed

Žto within "2% 5, 17.5 and 35 kDa PSS standards;.ns2 or 3 replicates per standard . Similar studies

using the 3 kDa filters resulted in replicate filtrateŽconcentrations that agreed to within "11% 1.43, 5,

. Ž .17.5 kDa PSS standards . Replicate ns4 analysesof one Chesapeake Bay site M3 pore water sampleyielded filtrate concentrations that agreed to within"4% for the 100 kDa filters and "6% for the 3kDa filter.

2.3.1. Calibration studiesBased on discussions in the literature about poten-

tial artifacts associated with the use of ultrafiltrationmembranes for the study of DOM in natural waters

Fig. 2. The results of calibration studies with the 3 and 100 kDamolecular weight cut-off Amicon ‘Centricon’ microconcentrators.The y-axis represents the relative concentration of the compoundrecovered in a particular size class, based on the ultrafiltration ofsolutions of each compound with the 3 and 100 kDa filters.‘Perfect’ behavior of these filters would result in 100% values forcompounds whose molecular weights were within the particularsize class, and 0% values for compounds with molecular weights

Ž .outside the size class. Symbols: v spolystyrene sulfonates PSS ;'snatural and synthetic proteins and peptides, and vitamin B .12

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( )D.J. Burdige, K.G. GardnerrMarine Chemistry 62 1998 45–6450

ŽAiken, 1984; Chin and Gschwend, 1991; Kildruff.and Weber, 1992 , we carried out the following

calibration studies. Solutions of organic compoundsof known molecular weights were prepared in 0.7 M

Ž .NaCl made up in low DOC DDW , and then sub-jected to ultrafiltration as described above. Prepara-tion of standards in an electrolyte that matches thatof seawater is based on results described by Kildruff

Ž .and Weber 1992 , who observed that the apparentmolecular weight cut-offs of these membranes wasdependent on the solution ionic strength below

Fig. 3. Depth profiles of: the absolute concentrations of totalDOC, and the relative concentrations of DOC in the -3 kDaŽ . Ž . Ž .DOC , 3–100 kDa DOC and )100 kDa DOC size3 3 – 100 100

classes, all in Chesapeake Bay site M3 sediments. In this figureand in Figs. 4 and 5, concentrations on the x-axis representbottom water values obtained from hydrocast samples. Symbols:

Ž . Ž .BsCH XII 3r95 ; `sCH XIV 7r95 ; 'sCH XVŽ . Ž10r95 . Aside from the general trends in the data discussed in

.the text , also note that the surface pore water sample in the CHXII profile contains a much higher total DOC concentration, avery low relative concentration of DOC and a much higher3

relative concentration of DOC . This observation is likely re-100

lated to the seasonal colonization of these sediments by benthicŽ .macrofauna see the discussion in Section 2.1 , and may be

artifactual, due to the presence of macrofauna in these sedimentsŽat this time of the year e.g., Martin and McCorkle, 1993; Alperin

.et al., 1998 .

Fig. 4. Depth profiles of: the absolute concentrations of totalDOC, and the relative concentrations of DOC , DOC , and3 3 – 100

Ž . Ž .DOC in Chesapeake Bay sites N3 ' and S3 ` sediments100Ž .both collected on cruise CH XV, 10r95 .

;0.05–0.1 M, yet was independent of ionic strengthŽat higher values also see discussions in the work of

Ž ..Chin and Gschwend 1991 . The ionic strengths ofall of our pore water samples were above 0.1 M, and

Žmost were effectively seawater values ;0.7; see.Table 1 .

The compounds used in these calibration studiesŽ .were: random coil PSS Aldrich Scientific of molec-

ular weights 1.43, 5, 17.5, 35 and 130 kDa; bovineŽ . Ž .serum albumin 69 kDa ; vitamin B 1.36 kDa ;12

the synthetic peptide Ala-D-isoglutaminyl-Lys-D-Ala-Ž .D-Ala 0.488 kDa; Sigma catalog aA1035 ; the syn-

Žthetic peptide Gly–Val–Leu–Ser–Asn– . . . 2.5.kDa; Sigma catalog aG9031 . The initial concentra-

tions of the protein, peptide and vitamin B solu-12

tions ranged from ;90–530 mmol C ly1 ; theseŽ .concentrations and those of ultrafiltered standards

Ž .were determined by HTCO see below . Concentra-tions of PSS standards were generally 500 mg PSSly1, with the exception of the 1.43 kDa PSS standardwhich was 2000 mg PSS ly1. Concentrations of PSS

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( )D.J. Burdige, K.G. GardnerrMarine Chemistry 62 1998 45–64 51

Fig. 5. Depth profiles of: the absolute concentrations of totalDOC, and the relative concentrations of DOC , DOC , and3 3 – 100

DOC in continental margin sediments. Symbols: Bssta. WC4100Ž . Ž .CH XIV, 7r95 ; `ssta. WC7 CH XIV, 7r95 ; 'ssta. AIŽ . Ž .CH XIV, 7r95 ; v ssta. SMB T95; 11r95 .

compounds were determined by UV absorption atŽ .280 nm Chin and Gschwend, 1991 , and corrected

Ž .as appropriate for absorption of the unfiltered andultrafiltered blank sodium chloride solution.

2.4. Analysis of DOC

Samples were analyzed for DOC by HTCO usinga Shimadzu TOC-5000 total carbon analyzer. Theprocedure used here was identical to that described

Ž .in the work of Burdige and Homstead 1994 withthe following modifications. Based on the results of

ŽBenner unpublished data cited in the work of SharpŽ ..et al. 1993 , a ;70 cm long piece of copper wire

was wound into a 0.5 cm thick plug and placed ontop of the Pt catalyst bed in the quartz combustiontube of the DOC analyzer; the combined length ofthe catalyst bed and Cu wire plug was 13 cm. Thismodification substantially improved sample oxida-

Ž .tion i.e., sample peak shape and also eliminated theoccasional occurrence of split peaks from a single

sample injection. Although Benner’s original sugges-tion was to replace a portion of the catalyst bed withpieces of muffled CuO wire, we found that saltbuild-up in the interstices of the pieces of CuO wireled to rapid clogging and breakage of quartz combus-tion tubes. Since the outer surface of the copper wirerapidly oxidizes to CuO when heated to 6808C in thepresence of the pure oxygen carrier gas, a plug ofwound Cu wire similarly improved sample combus-tion without these additional problems.

3. Results

3.1. Calibration of the ultrafiltration membranes

Calibration studies with these filters using knownorganic compounds are shown in Fig. 2. Small

Ž .amounts ;15% of compounds with molecularweights greater than 3 kDa appeared to pass through

Ž .the 3 kDa filters Fig. 2C , while compounds withmolecular weights as low as 35 kDa were alsoretained at similar low levels by the 100 kDa filtersŽ .Fig. 2A . Together, the combined use of the 3 and100 kDa filters led to an apparent recovery of only

Table 2Summary of DOC size fractionation studies

aSites DOC ) DOC ) DOC )3 3 – 100 100

Chesapeake BayM3

Ž .CH XII below 2 cm 87"4% 10"3% 4"2%CH XIV 87"5% 10"6% 3"3%CH XV 83"6% 15"4% 2"2%Overall average 86"3% 12"3% 3"1%Ž .S3 CH XV 92"3% 7"4% 0.5"1%Ž .N3 CH XV 88"6% 5"6% 8"7%

Mid-Atlantic shelfr slope breakŽ .AI CH XIV 62"10% 18"6% 21"4%

Ž .WC4 CH XIV 70"7% 13"9% 18"7%Ž .WC7 CH XIV 71"9% 14"7% 15"4%

S. California BorderlandŽ .SM T95 64"11% 18"10% 17"12%

a Ž .Cruises: CH XIIsChamps XII 3r95 ; CH XIVsChamps XIVŽ . Ž .7r95 ; CH XVs Champs XV 10r95 ; T95s Teflon 95Ž .11r95 .)As a percentage of the total DOC. These values are averagevalues for each of the cores shown in Figs. 3–5.

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( )D.J. Burdige, K.G. GardnerrMarine Chemistry 62 1998 45–6452

;80% for DOC compounds in the 3–100 kDaŽ .molecular weight range Fig. 2B .

Interpretation of this data is based on severalconsiderations. As discussed by Chin and GschwendŽ .1991 the pore sizes of these filters are based onmolecular weights of globular proteins, which maynot be appropriate model compounds for dissolvedorganic compounds found in sediment pore watersŽ Ž ..also see the work of Kildruff and Weber 1992 .Rather, it appears that random coil molecules such asPSS may be more representative of the ‘structure’ of

Žnatural DOC see discussions in the works of ChinŽ . Ž .and Gschwend 1991 and Kildruff and Weber 1992

.and references cited therein . At the same time how-Žever, at high ionic strengths such as those of seawa-

.ter the increased coiling of PSS molecules mayŽ .decrease their size e.g., Chin and Gschwend, 1991

allowing some ‘breakthrough’ of PSS compoundswhose molecular weights exceed those of a givenfilter.

Ž .Calibration studies by Chin and Gschwend 1991with PSS standards suggested that the same type of 3and 10 kDa ultrafiltration membranes used here actu-ally retain compounds whose molecular weights are5 times lower than those reported by the manufactur-

Žers i.e., the 3 kDa filter has an actual molecularweight cut-off of 0.6 kDa and the 10 kDa filter had 1

.of 2 kDa . Our calibration studies using these filterswith both PSS and other organic compounds areequivocal on this point, particularly since we wereunable to use the 3 and 10 kDa filters together toseparate DOC compounds into the 3–10 kDa molec-ular weight class. Alternately, we suggest that the

Ž .observations of Chin and Gschwend 1991 mayequally be explained by the broad transition from 0

Ž .to 100% recovery for each of these filters Fig. 2 . Inthe remainder of this paper, then, we will definemolecular weight size classes based on the nominalmolecular weight cut-offs reported by the manufac-turer, given the caveats of such a definition based onthe results discussed above.

Fig. 6. Absolute concentrations of DOC , DOC and DOC3 3 – 100 100Ž .vs. total DOC in estuarine Chesapeake Bay and continental

Ž .margin sediments. Also shown in these figures as a solid line isthe best-fit straight line through the data. In E. the dashed lineindicates that in these estuarine sediments there was a constant;30 mM concentration of DOC . Symbols: ^ssta.N3; %s100

Ž . Ž .sta. S3; (sst. M3 CH XII ; `ssta. M3 CH XIV ; v ssta.Ž .M3 CH XV ; Issta. SMB; essta. WC4; dotted lozenge

ssta. AI; lssta. WC7. By definition, concentration–concentra-tion plots such as these should be linear and pass through theorigin if the DOC concentration in a particular size class is aconstant fraction of the total DOC. This appears to be the case forDOC and DOC in the Chesapeake Bay sediments and3 3 – 100

DOC in the continental margin sediments, where the y-inter-3 – 100Žcepts are all within 2s of zero i.e., the origin; y6.7"15.4,

.1.0"16.6 and y33.7"28.0 mM, respectively . In contrast, thebest-fit y-intercepts in the plots of DOC and DOC in conti-3 100

nental margin sediments were significantly different than zeroŽ .50.3"18.6 and y50.0"10.0 mM . This implies that for each ofthese data sets the relative concentration of DOC in each size

Žclass which is actually the slope of a line between the origin and.a particular data point or a point on the best-fit line either

Ž . Ž .decreases DOC or increases DOC with increasing total3 100

DOC. Since DOC concentrations increase with depth in thesesediments, this then predicts that the relative concentration ofDOC should decrease with depth in the continental margin3

sediments, and that the relative concentration of DOC should100

increase with depth, consistent with the data in Fig. 5.

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3.2. Pore water results

Total DOC concentrations increased with sedi-Ž .ment depth at all of the sites Figs. 3–5 , and the

depth profiles shown here are similar to other re-Žported profiles in these sediments Burdige and

Homstead, 1994; Bauer et al., 1995; Burdige and.Zheng, 1998 and in similar continental margin sedi-

Ž .ments e.g., Martin and McCorkle, 1993 . The ma-jority of the DOC in the sediments we examined had

Ž .a molecular weight less than 3 kDa Table 2 . In theŽ .Chesapeake Bay estuarine sediments most of the

remaining DOC was found in the 3–100-kDa frac-tion, although in the continental margin sediments itwas more evenly distributed between the 3–100-kDaand )100-kDa fractions.

In the Chesapeake Bay sediments there was nosignificant change in DOC molecular weight distri-butions with depth, based on either: depth plots ofrelative DOC concentrations in the different size

Ž .classes Figs. 3 and 4 ; plots of DOC concentrationŽ .in a given size class vs. total DOC Fig. 6 . In the

continental margin sediments there may have been avery slight decrease with depth in the relative con-

Žcentration of low molecular weight DOC LMW-.DOC, i.e., DOC ; this appeared to be associated3

with the accumulation of HMW-DOC in the DOC100Ž .fraction Figs. 5 and 6 .

Plotting absolute DOC concentrations in each size

class against total DOC shows that the concentrationof DOC increased with total DOC, and that the two3

Ž .were strongly correlated Fig. 6A and B . The slopeof the line defined with the estuarine sediment datais different from that for the continental marginsediment data, although each is similar to the corre-sponding average in Table 2. Since total DOC gener-ally increased with depth in all of these sedimentsŽ .Figs. 3–5 these results imply that this LMW-DOCaccumulates with depth as a near-constant fraction oftotal DOC. Weaker positive relationships existedbetween the concentration of DOC and total3 – 100

Ž .DOC for both sediment types Fig. 6C and D ,although this material, too, accumulates with depth.In contrast, while DOC showed a strong positive100

correlation with total DOC in the continental marginŽ .sediments Fig. 6F , in the estuarine sediments the

concentration of DOC was apparently indepen-100Ž .dent of total DOC Fig. 6E . In these sediments, this

high molecular weight material appeared to have anear-constant concentration of ;30 mM. At facevalue this should imply that the relative concentra-tion of DOC should decrease with depth. How-100

ever, since this material is such a small percentage ofŽ .the total DOC in these sediments less than ;8% ,

and given the errors associated with these measure-ment, this depth trend is not observable in Figs. 3and 4.

ŽFinally, we also see that on average DOC as a3

Ž . Ž . Ž .Fig. 7. The relative concentration of DOC from Table 2 vs. sediment temperature left and sediment carbon oxidation rates C ; right3 ox

at the various sites studied. The solid line in each plot is the least-squares best-fit line through the data and indicates the positive relationshipbetween DOC and each of these two quantities. The positive relationship between DOC and sediment temperature was significant based3 3

2 Ž . Ž 2on both its r value s0.64 and the F-statistic which indicates the probability that this r value occurs by chance; F s12.3,2,6. Ž 2 .a-0.02 . In contrast, the relationship between DOC and C was much weaker r s0.32; F s3.28, 0.1-a-0.2 . Symbols:3 ox 2,6

Ž . Ž .Bsmid-Atlantic shelfrslope break sites sts. AI, WC4 and WC7 ; Issta. SMB; v ssts. S3 and N3 Chesapeake Bay ; `ssta. M3Ž .Chesapeake Bay .

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.percentage of total DOC was slightly higher in theŽ .estuarine Chesapeake Bay sediments than it was in

Žthe offshore, continental margin sediments ;85–.90% vs. ;60–70%; see Table 2 . These relative

DOC concentrations also showed a positive rela-3

tionship with sediment temperature and a weakerpositive relationship with sediment carbon oxidation

Ž .rates C ; Fig. 7 .ox

4. Discussion

4.1. Comparison with other studies

The molecular weight distribution of pore waterDOC has been examined in other coastal, marine andfreshwater sediments, and these results are summa-rized in Table 3 and compared with our results. In

Žsome of the sediments studied the organic-rich muds.of Loch Duich and some of the Boston Harbor sites ,

there is a near constant absolute concentration ofLMW-DOC with depth. The net accumulation ofHMW-DOC in the sediments therefore leads to adecrease with depth in the relative amounts of lowmolecular weight material. Such trends were not

Ž .observed in other sediments e.g., Great Bay , nor inŽ .our work Fig. 6 .

In general, we note that in many, but not all ofthese published studies, the relative concentration of

Ž .HMW-DOC increases with depth Table 3 . Suchtrends were seen at our continental margin sites, butnot in the Chesapeake Bay sediments. However, thechanges with depth in the relative amounts of HMW-DOC in the continental margin sediments we studiedwere not as dramatic as those seen in these other

Ž .published studies Table 3 .A more detailed comparison of these published

studies with our results is made difficult by the factthat most of these studies used filters with molecularweight cut-offs that are different from the ones weused. Therefore, definitions of ‘high’ and ‘low’molecular weight fractions in the different studies

Žare not unique and are sometimes overlapping also.see footnote 1 in Section 4.2 . Differences in the

characteristics of each of the filter membranes usedŽi.e., the molecular weight range over which thetransition from 0 to ;100% recovery occurs; e.g.,

.see Fig. 2 also makes it difficult to directly compare

our observations with these published results. At thesame time, some of the biogeochemical factors thatlead to the observed trends in molecular weightdistributions in the estuarine and continental marginsediments we studied may also explain the differ-ences in Table 3. These include differences in sourcesand reactivity of the organic matter undergoing rem-ineralization in the sediments, or perhaps differencesin the pathways by which this organic matter is

Ž .degraded see Section 4.2.2 for further details .

4.2. Controls on the DOC molecular weight distribu-tion in sediment pore waters

Previous studies examining the molecular weightdistribution of pore water DOC attributed the accu-mulation of HMW-DOC with depth to three possibleprocesses. The first, generally referred to as thegeopolymerization model, involves the formation ofhigh molecular weight ‘geopolymers’ from LMW-DOC compounds via abiotic polymerization reac-

Žtions Nissenbaum et al., 1971; Tissot and Welte,.1978; Krom and Westrich, 1981; Hedges, 1988 .

Increasing condensation of these proposed geopoly-Ž .mers or dissolved humic substances is eventually

thought to form particulate humics and kerogen, asthe dissolved humics eventually become insoluble.

The second explanation for the accumulation ofHMW-DOC with depth proposes that it results from

Žthe selective preservation and therefore accumula-.tion in pore waters of partially, degraded and more

Žrefractory components of the sediment POC Orem.et al., 1986 . Finally, it has been suggested that the

increase with depth in the molecular weight of porewater DOC results from changes in biotic or abioticprocesses in the sediments associated with the transi-tion between surficial, oxic sediments and deeper,

Žanoxic sediments Krom and Sholkovitz, 1977; Krom.and Westrich, 1981; Chin et al., 1994 .

The lack of significant depth variations in themolecular weight distribution of pore water DOC in

Ž .the sediments we studied Figs. 3–5 suggests thatprocesses such as geopolymerization or oxicranoxiceffects on DOC molecular weights are either not ofmajor significance in these sediments, or that theireffects cannot be detected by the particular molecu-lar weight cut-offs of the filters used in this studyŽi.e., geopolymerization reactions do occur but do

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Chem

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55

Table 3A comparison of our results with published DOC molecular weight depth trends in other sediments

Ž .Location Depth trend total DOC Molecular weight Depth trend for the high moleculara bŽ .cutoff HMW-DOC weight fraction

Marine sedimentscŽ .Loch Duich, Scotland fjord-type estuary

Ž . Ž . Ž .organic-rich mud sulfate reducing and methanogenic increase 0–80 cm 1 kDa increase with depth from ;40 to 80%Ž Ž . Ž .brown, sandy-mud ‘oxic’ sediments, no evidence of no change 0–55 cm 1 kDa constant with depth ;20–40%

.sulfate reductiondŽ . Ž . Ž .Great Bay, NH estuarine sediment decrease 0–15 cm , 1 kDa increase with depth from ;50 to 80%

Ž .increase 0–75 cmeŽ . Ž . Ž .Mangrove Lake anoxic, sapropelic sediment increase 0–200 cm 0.5 kDa decrease with depth from 95 to ;75%

f Ž . ŽBoston Harbor increase 0–30 cm 3 kDa increase with depth from ;10–20% to.;50–60% in two of the three sites;

Ž .constant with depth ;40–60% at the third sitegŽ . Ž . Ž .Chesapeake Bay estuarine sediments increase 0–25 cm 3 kDa constant with depth ;10–15%

g Ž . Ž .Continental margin sediments increase 0–25 cm 3 kDa increase with depth from ;20 to ;35%

Freshwater sedimentsf Ž . Ž .Upper Mystic Lake increase 0–50 cm 3 kDa increase with depth from ;5 to ;50%

h Ž .Lake Michigan not reported 0–10 cm number- and weight-average molecular weights increase with depthŽ .from 0.5–0.8 kDa at the surface to 0.7–1.1 kDa at 8.5 cm

a Ž .The molecular weights cutoffs listed here represent the nominal cut-off s of the filter used in each study to separate HMW-DOC and LMW-DOC.b‘Increase’ and ‘decrease’ are relative to the total DOC pool.c Ž .From the work of Krom and Sholkovitz 1977 .d Ž .From the work of Orem and Gaudette 1984 . The different total DOC depth trends are for cores collected at slightly different sites in Great Bay. All other observations are thecombined results from the two cores. The results discussed here are only for the cores processed under anoxic conditions.e Ž .From the work of Orem et al. 1986 .f Ž .From the work of Chin and Gschwend 1991 .g This study.h Ž .From the work of Chin et al. 1994 . Molecular weights were determined here by size exclusion chromatography.

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not produce new compounds whose molecular. 1weights exceed ;3 kDa . Arguments in the work

Ž .of Alperin et al. 1994 also support the suggestionthat geopolymerization reactions are not likely im-portant in marine sediments on early diagenetic timeand depth scales.

To explain our results, we have therefore devel-oped a model which assumes that the average molec-

Ž .ular weight distributions in surface upper ;30 cmsediments is primarily controlled by organic matterremineralization processes, and which also contains amodified form of the selective preservation explana-tion discussed above. This model is shown in Fig. 8.In the upper panel we visualize the degradation ofsediment POC to CO through DOC intermediates as2

a series of hydrolytic, fermentative and eventuallyrespiratory processes that produce and consume porewater DOC compounds with increasingly smaller

Žmolecular weights Laanbroek and Veldkamp, 1982;Henrichs, 1992; Deming and Baross, 1993; Alperinet al., 1994; Arnosti et al., 1994, and references

. Žtherein . While different POC starting materials e.g.,.proteins, carbohydrates are initially hydrolyzed to

Ždifferent DOC intermediates Colberg, 1988; McIn-.erney, 1988 , at least in anoxic or sub-oxic sedi-

ments, these hydrolytic and fermentative pathwaysappear to lead to a limited number of monomeric

Žlow molecular weight compounds or ‘biomono-.mers’ such as short chain organic acids and alcohols

and perhaps monomeric amino acids. Thesebiomonomers are then oxidized by the terminal res-

Žpiratory organisms in the sediments i.e., sulfate.reducing or denitrifying bacteria .

To further develop the model in Fig. 8A we haveincorporated into it the size-reactivity continuummodel for DOC remineralization in aqueous systemsŽ .Amon and Benner, 1996 . This latter model is basedon water column observations which show thatHMW-DOC represents a more reactive and less dia-genetically altered fraction of the total water column

ŽDOC than the more abundant LMW-DOC for addi-tional water column data in support of this model

1 Such factors might also explain the differences between ourresults and some of the published data in Table 3 if such geopoly-merization reactions produce compounds on early diagenetic timescales whose molecular weights are between ;1 and 3 kDa.

also see the works of Benner et al., 1992; Amon andBenner, 1994; Santschi et al., 1995; Guo et al., 1996;

.Guo and Santschi, 1997 .Ž .In the pore water sizerreactivity PWSR model

shown in Fig. 8B, sediment POC is initially hydro-lyzed to a class of HMW-DOC compounds, pre-dominantly composed of biological polymers such as

Ždissolved proteins or polysaccharides. Most but not.all of the HMW-DOC is degraded by hydrolytic and

fermentative processes to monomeric low molecularŽ .weight compounds mLMW-DOC that are then

rapidly oxidized by the terminal respiratory organ-isms in the sediments. At the same time though,some fraction of the HMW-DOC is only partiallyoxidized, leading to the production of what we refer

Ž .to here as polymeric LMW-DOC pLMW-DOC .This model does not exclude however, the possibilityof geopolymerization reactions forming pLMW-DOCfrom the mLMW-DOC pool, through reactions such

Žas the melanoidin or ‘browning’ reaction a sugar-amino acid condensation reaction; e.g., Hedges,

.1988 or through complexation reactions such asthose that have been described for short chain or-

Žganic acids such as acetate Christensen and Black-.burn, 1982; Michelson et al., 1989 . However, as

discussed above, if these reactions do occur, theirŽ .products on early diagenetic time scales still have

Žrelatively low molecular weights i.e., less than ;3.kDa .

This pLMW-DOC then becomes what is opera-Ž .tionally defined as humin Amon and Benner, 1996 ,

Žand humification i.e., production of soluble humic.and fulvic acids is now thought of as a process that

produces increasingly oxidized, LMW-DOCŽmolecules from sediment POC also see the work of

.Hatcher and Spiker, 1988 . As discussed in the workŽ .of Amon and Benner 1996 , pLMW-DOC appar-

ently escapes microbial remineralization because itŽ .‘no longer resemble s biomolecules.’ Essentially,

these models imply that selective preservation indeedleads to the accumulation of pore water DOC, withthe significant difference that it is refractory low,and not high, molecular weight DOC that is appar-ently preserved.

The PWSR model is somewhat analogous to themulti-G model used to describe the reactivity of

Ž .sedimentary POC Westrich and Berner, 1984 , al-though here we think of discrete DOC fractions with

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differing chemical compositions and reactivities.While the reactivity of both sediment POC and DOC

Žis likely a continuum Middelburg, 1989; Amon and.Benner, 1996 the conceptual approach of the multi-G

model has proven useful in understanding and study-ing POC decomposition in sediments. By analogythen, we suggest that the PWSR model will besimilarly useful in examining DOC cycling and reac-tivity in sediments.

The PWSR model explains several aspects of ourdata, in particular, the observation that DOC repre-3

Ž .sents the majority of the total DOC ;60–90% inŽ .these sediments Table 2 , and that this LMW-DOC

Ž .accumulates in an absolute sense with depth alongŽ .with total DOC in the sediments Fig. 6A and B .

Although DOC likely comprises both pLMW-DOC3

and mLMW-DOC, the latter is generally found atŽlower absolute concentrations Shaw et al., 1984;

.Burdige and Martens, 1990; Alperin et al., 1994 andis presumed to have a much faster turnover time.Therefore, we assume that the ‘properties’ of DOC3

are primarily controlled by pLMW-DOC. Possiblereasons for the differences in the relative concentra-tions of DOC in estuarine and continental margin3

sediments will be discussed below.The proposed rapid turnover of HMW-DOC in

the PWSR model is consistent with DOC waterŽcolumn studies Amon and Benner, 1994, 1996; Guo

.and Santschi, 1997 , and may explain why in thewater column and in the sediment pore waters westudied DOC is a small percentage of the total100

Ž .DOC -;20% in our samples; Table 2 . Again,differences in the behavior of this HMW-DOC inestuarine and continental margin sediment will bediscussed below.

4.2.1. Controls on DOC concentrations in sedimentpore waters

The PWSR model can also be used to explain theŽgeneral shape of DOC profiles in anoxic i.e., non-

. Ž .bioturbated or -bioirrigated sediments Fig. 9 . Theslight imbalance between DOC production and con-

Žsumption near the sediment surface due the produc-.tion of less reactive pLMW-DOC leads to the ob-

served accumulation of DOC with depth in mostsediments. Asymptotic DOC concentrations found at

Ž .depth in sediments may then result from: 1 aŽ .balance between DOC production from POC and

ŽDOC consumption primarily from the pLMW-DOC. Ž .pool; e.g., Alperin et al., 1994 ; 2 further changes

Ž .biotic or abiotic in pLMW-DOC that may decreasethe overall reactivity of this material, eventuallyleading to a situation in which the pLMW-DOCfound at depth is essentially nonreactive on early

Ždiagenetic time scales Hatcher and Spiker, 1988;.Amon and Benner, 1996 . While both suggestions

are consistent with the PWSR model, further studiesŽwill be needed to critically examine them e.g., see

discussions in the work of Burdige and ZhengŽ ..1998 . At the same time, recent studies have shownthat sorption of dissolved organic matter to sedimentparticles also plays a role in affecting pore water

ŽDOM concentrations Hedges and Kiel, 1995; Hen-.richs, 1995 , and that pore water DOC concentra-

tions may be ‘buffered’ by reversibly-sorbed DOC inŽequilibrium with the pore waters Thimsen and Keil,

.1998 . These processes may also affect DOC concen-trations at depth depending on: the intrinsic reactiv-ity of the pore water DOC and that which is ad-

Žsorbed to sediment particles Lee, 1994; Mayer,.1994b; Henrichs, 1995 ; the relative sizes of the pore

Žwater and sorbed DOC pools Thimsen and Keil,.1998 .

In addition to the DOC size fraction data pre-sented here, the PWSR model is consistent withother data on sediment DOC cycling and concentra-

Ž .tions. Alperin et al. 1994 quantified two DOCpools in the pore waters of the anoxic sediments of

Ž .Cape Lookout Bight CLB , NC, USA: an acid-Ž .volatile DOC pool AV-DOC which they suggested

contains volatile fatty acids and alcohols; a non-Ž .acid-volatile DOC pool NAV-DOC containing

larger molecular weight organic compounds andnon-volatile, small organic molecules such as dis-solved free amino acids. In general, AV-DOC was a

Ž .small fraction less than ;5% of the total DOC inCLB pore waters, and showed little depth variability.

ŽIn contrast, NAV-DOC increased continuously and.in most cases asymptotically to values that varied

seasonally between 3–5 mM, and therefore consti-tuted the majority of the total DOC in the porewaters. In a broad sense, AV-DOC should corre-spond to what we refer to here as mLMW-DOC,although some mLMW-DOC compounds are also

Ž .NAV e.g., dissolved free amino acids . However,their concentrations in sediment pore waters are quite

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Fig. 9. An explanation for how the PWSR model in Fig. 8B may explain DOC profiles commonly observed in marine sediments.

low in comparison to NAV-DOC or total DOCŽconcentrations in CLB sediments e.g., Henrichs et

al., 1984; Burdige and Martens, 1990; Burdige,.1991 . Similarly, with the exception of these small,

non-volatile organic compounds, NAV-DOC roughlycorresponds to the sum of HMW-DOC and pLMW-

ŽDOC with pLMW-DOC presumably constituting.the vast majority of the NAV-DOC .

The results of lignocellulose degradation studiesŽin salt marsh microcosms summarized in the work.of Hodson and Moran, 1995 also indicate that the

DOC derived from decaying marsh detritus has botha fast- and slow-decaying component, and that theoverall turnover rate of the DOC that accumulatesduring such degradation studies decreases with time.These observations demonstrate that the decomposi-

Fig. 8. A conceptual model for the role of DOC in the remineralization of POC in marine sediments. A. This figure illustrates the generalconcept that POC remineralization through DOC intermediates proceeds through a series of processes that lead to increasingly smaller DOCmolecules. Eventually, they produce a limited number of monomeric low molecular weight compounds that are used by the terminal

Ž .respiratory organisms in the sediment. B. This figure modified from the work of Alperin et al., 1994 incorporates the concepts shownŽ . Ž .above and the size-reactivity continuum model of Amon and Benner 1996 . As is described in the text and is also indicated here , the

reactivity of the pLMW-DOC pool is proposed to be much lower than that of either the HMW-DOC or the mLMW-DOC pools.

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tion of at least this type of sediment POC will lead toDOC components whose characteristics are consis-tent with the PWSR model.

4.2.2. Differences in the DOC molecular weightdistributions in estuarine and continental marginsediments

The results in Table 2 and Fig. 6 show thatabsolute and relative concentrations of DOC in dif-ferent size classes vary between estuarine and conti-nental margin sediments. The results in Fig. 7 sug-gest that temperature or perhaps overall rates ofsediment carbon oxidation may lead to these differ-ences. Temperature could be an important control-ling parameter if there are differences in the tempera-

Ž .ture dependences i.e., activation energy of the indi-vidual reactions in the overall remineralization ofsediment POC to CO through DOC intermediates.2

However, the critical examination of this suggestionis hampered in part by the lack of detailed knowl-edge about the specific pathways by which sedimentPOC is remineralized to CO and the temperature2

Ždependences of these pathways Reichardt, 1987;.Henrichs, 1992; Arnosti, 1997 . Changes in the over-

all rate of sediment carbon oxidation could explaindifferences in the distribution of the DOC intermedi-ates in POC remineralization depending on the kinet-ics and mechanisms of these reactions. Again, lackof information in this area precludes a further exami-nation of this possibility.

At the same time, differences in the intrinsicreactivity of the POC undergoing decomposition inthese sediments will also affect sediment carbonoxidation rates. These differences are likely relatedto the chemical structure, composition andror

Ž .sources e.g., terrestrial vs. marine of this organicŽmatter e.g., Whelan and Emeis, 1992; Henrichs,

.1993 . Given that the reactivities of specific com-pounds that comprise potential POC starting materi-

Žals are known to vary Cowie and Hedges, 1992;.Cowie et al., 1992 , and that there are differences in

Žsome of their initial ‘upstream’ DOC products see.references in Section 4.2 , considerations such as

these might explain the observations in Fig. 6 andTable 2. Differences in these upstream DOC inter-mediates might also affect the types of pLMW-DOCthat are produced in these sediments, therefore fur-ther affecting the reactivity and concentrations of

pLMW-DOC in estuarine vs. continental margin sed-iments. Additional work characterizing the microbialprocesses involved in POC remineralization, the sed-iment POC itself, and the DOC intermediates pro-duced during POC remineralization will be needed tofurther examine these suggestions.

Regardless of the exact causes of the observationsin Table 2 and Fig. 6, they suggest that there arelikely differences in the relative rates of the reactionsshown in Fig. 8B in the two sediment types weexamined. In particular, the lower absolute concen-tration of DOC in estuarine sediments and its lack100

of accumulation with depth along with total DOCŽ .compare Fig. 6E and F suggests that this materialmay turn over more rapidly in estuarine sedimentsthan it does in continental margin sediments. Incontrast, in continental margin sediments the hydro-lytic processes affecting this HMW-DOC may pro-vide an initial ‘upstream’ bottleneck in the overallremineralization process, allowing this material toaccumulate to some extent in both an absolute andrelative sense in these sediment pore waters. Thissuggests that remineralization processes in continen-tal margin sediments may be more controlled byhydrolytic processes than they are in estuarine sedi-ments, where ‘downstream’ fermentative or perhapsrespiratory processes may exert a greater overall

Žcontrol on carbon remineralization and may possi-bly also explain the enhanced accumulation of DOC3

.in estuarine sediments . Extracellular hydrolysis ofŽmacromolecular organic matter i.e., HMW-DOC or

.sediment POC has generally been thought to be theŽrate limiting step in organic matter degradation King,

.1986; Hoppe, 1991; Meyer-Reil, 1991 , although asŽ .Arnosti et al. 1994 have noted, this suggestion is

‘not well tested.’ Their polysaccharide degradationstudies support the suggestion that such hydrolyticprocesses may not be rate-limiting in all sediments,and our observations appear to be consistent withthese results.

4.3. Pore water DOC and sediment carbon preserÕa-tion

If pore water DOC plays a role in sedimentcarbon preservation, it must eventually be incorpo-rated into the ‘solid’ sediment matrix where it is thenburied and then becomes a part of the long-term

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Ž .global carbon cycle e.g., Hedges, 1992 . As dis-cussed above, geopolymerization reactions have beensuggested as one such mechanism for how this oc-

Ž .curs see references above , although this model isŽnot universally accepted e.g., de Leeuw and Largeau,

.1993 . The geopolymerization model also does notappear to be entirely consistent with the molecularweight data presented here.

Recently, it has been suggested that adsorption oforganic matter to mineral grain surfaces may actually

Žcontrol carbon preservation Mayer, 1994a,b; Hedges.and Kiel, 1995 . This process involves adsorption of

organic matter in small mesopores on the mineralsurfaces which both protect the organic molecules

Ž .from attack by microbial enzymes Mayer, 1994a,band may also possibly act as sites in which rates ofabiotic condensation reactions are enhanced by either

Žsteric- or concentration-related phenomena Mayer,.1994b; Collins et al., 1995; Hedges and Kiel, 1995 .

This sorption process appears to be at least partiallyŽ .reversible Thimsen and Keil, 1998 and some of the

organic matter sorbed to the sediments is still capa-ble of undergoing biological degradation once it is

Ž .desorbed Keil et al., 1994 .Ž .Since the size s of the mesopores may provide

some constraints on the upper limit of thesizermolecular weight of DOC molecules that can

Ž .be taken up in the mesopores Mayer, 1994b , theŽ .relatively small size of -3 kDa of most pore water

DOC would appear to aid in its possible interactionwith mesopore adsorption sites. Similarly, the pro-posed reactivity of this material in the PWSR modelmay also enhance its preservation, if the sorption

Žprocesses are reversible to any significant extent seediscussions in the works of Lee, 1994; Mayer, 1994b;

.and Henrichs, 1995 . Future studies in this area willrequire understanding how these proposed DOC poolsinteract with sediment particles, their intrinsic reac-tivities, and also better characterizing the differentprocesses affecting DOC once it is sorbed to sedi-ment particles.

5. Conclusions

Ž .1 The majority of the DOC in Chesapeake BayŽ .estuarine and several continental margin sediments

Ž .upper ;30 cm had a molecular weight less than 3kDa. The percentage of this LMW-DOC was slightlyhigher in estuarine sediments as compared to conti-

Žnental margin sediments ;85–90% vs. ;60–.70% . In these estuarine sediments most of the re-

maining DOC was found in the 3–100 kDa fraction,although in the continental margin sediments it wasmore evenly distributed between the 3–100 kDa and)100 kDa fractions.

Ž .2 The absolute concentration of DOC increased3

with total DOC, and both were strongly correlated.Since total DOC increased with depth in all of thesesediments, this implies that the majority of the DOCaccumulating in these sediments is of low molecularweight. In contrast, only in the continental marginsediments did DOC show a positive correlation100

with total DOC. In the estuarine sediments, theconcentration of DOC was apparently indepen-100

dent of total DOC, and was found at a near-constantconcentration of ;30 mM.

Ž .3 In the continental margin sediments there wasa very slight increase with depth in the relative

Ž .concentration of HMW-DOC MW)3 kDa . How-ever, these depth trends were much smaller thanthose observed in previously published studies. Incontrast, in the Chesapeake Bay sediments DOCmolecular weight distributions appeared to be con-stant with depth.

Ž .4 These results have been explained using amodel based on traditional views of carbon reminer-alization in anoxic sediments and the recently pro-posed size-reactivity continuum model for DOC cy-

Ž .cling in aqueous systems Amon and Benner, 1996 .In the PWSR model presented here, the remineraliza-tion of sediment POC leads to the small net produc-tion of what is referred to here as pLMW-DOC. ThispLMW-DOC is presumed to be much less reactive

Žthan other types of DOC HMW-DOC or mLMW-.DOC , and becomes what is operationally defined asŽ .humin i.e., soluble humic and fulvic acids . This

then implies that selective preservation of refractorylow, and not high, molecular weight DOC leads tothe accumulation of DOC with depth in sedimentpore water. This model is consistent with the molec-ular weight data presented here as well other pub-lished data on the biogeochemical properties of DOCin sediment pore waters and in the water column.The ability to incorporate concepts taken from the

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Ž .work of Amon and Benner 1996 size-reactivitycontinuum model for water column DOC cycling tosediment systems suggests that there may be somesimilarities in the ways that DOC molecules in thesedifferent environments are ‘protected’ from decom-position.

Ž .5 Differences between estuarine and continentalmargin sediments in the behavior of DOC in thethree size classes studied here may be explained bydifferences in the relative rates of the processesaffecting POC remineralization through DOC inter-mediates. These differences may be temperature re-

Žlated caused by differences in the activation ener-gies of the individual reactions in the remineraliza-

.tion process . They may also be related to differ-ences in the types of organic matter undergoingdecomposition in the different sediments. Neverthe-less, our observations suggest that remineralizationprocesses in continental margin sediments may bemore controlled by hydrolytic processes than theyare in estuarine sediments, where fermentative orperhaps respiratory processes may perhaps exert agreater overall control on carbon remineralization.

Ž Ž .This further demonstrates as Arnosti et al. 1994.have noted that extracellular hydrolysis of macro-

Žmolecular organic matter i.e., HMW-DOC or sedi-.ment POC is not always the overall rate limiting

step in organic matter degradation.

Acknowledgements

The majority of this manuscript was written whilethe senior author was on sabbatical at the Southamp-

Ž .ton Oceanography Centre UK , and he would like tothank John Thomson and the other staff members atSOC for their hospitality during this visit. We aregrateful to Fred Dobbs, Rodney Powell, Juli Hom-stead, Yu-Ping Chin and an anonymous reviewer forcritically reading earlier versions of this manuscript,and to Yu-Ping Chin for his advice and guidancewhen we began this work. Finally, we thank JohnHedges for his assistance as the Associate Editorhandling this manuscript. This work was supportedby a grant from the National Science FoundationŽ .OCE-930212 . Partial support for ship time was alsoprovided by the Office of Naval Research.

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