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

Marine Chemistry 62 Ž1998. 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 The molecular weight distribution of dissolved organic carbon ŽDOC. in pore waters from estuarine and continental margin sediments was examined using ultrafiltration techniques. The majority of this pore water DOC Ž; 60–90%. had a molecular weight less than 3 kDa. This percentage appeared to vary systematically among the different sediments studied and showed very slight changes with depth Župper ; 30 cm.. The absolute concentration of this low molecular weight DOC ŽLMW-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 sediment pore waters. Abiotic condensation reactions Ži.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 remineralization. These observations provide further evidence that the extracellular hydrolysis of macromolecular Ži.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 carbon ŽDOC. is thought to play an important role in both carbon remineralization and preservation in marine 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 weights as it is oxidized to CO 2 ŽHenrichs, 1992; Deming and Baross, 1993; Alperin et al., 1994.. In anoxic sediments, where benthic macrofauna are essentially absent, bacteria mediate the remineralization process, and extracellular hydrolytic cleavage of particulate biopolymers to high molecular weight DOC ŽHMW-DOC. compounds is generally thought to be the rate limiting step in this process ŽKing, 1986; Hoppe, 1991; Meyer-Reil, 1991.. However,

0304-4203r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 4 2 0 3 Ž 9 8 . 0 0 0 3 5 - 8

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D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

recent studies by Arnosti et al. Ž1994. have suggested that this may not be the case in all sediments. At the same time, several proposed mechanisms for carbon preservation in marine sediments, including the geopolymerization model ŽNissenbaum et al., 1971; Tissot and Welte, 1978; Krom and Westrich, 1981; Hedges, 1988 . and the m esopore protectionrsurface area adsorption model ŽMayer, 1994a,b; Hedges and Kiel, 1995., both suggest a role for DOC in sediment carbon preservation. However, the details of how this preservation occurs are not well understood ŽHedges and Kiel, 1995.. One approach taken in examining the role of DOC in sediment carbon cycling has involved determining the molecular weight distribution of pore water DOC ŽKrom and Sholkovitz, 1977; Orem and Gaudette, 1984; Orem et al., 1986; Chin and Gschwend, 1991; Chin et al., 1994.. Such studies have generally used this data to examine the possible occurrence of in situ geopolymerization reactions, whereby low molecular weight dissolved organic compounds are thought to condense and form higher molecular weight dissolved humic substances. The continued condensation of these dissolved humics is thought to eventually lead to the formation of particulate material such as humin or kerogen, and the preservation of this organic matter in sediments. To further examine these concepts we have studied the molecular weight distribution of pore water DOC in contrasting estuarine and continental margin sediments. Our goal was to use this data to better understand sediment carbon remineralization and preservation, and the role of DOC as an intermediate in these processes. In particular, with these results we have developed a model for DOC cycling in sediments that combines traditional models for carbon cycling in anoxic sediments and the recently proposed size-reactivity continuum model for DOC remineralization in the water column ŽAmon and Benner, 1996..

three sites along the shelfrslope break of the midAtlantic continental margin Žsee Fig. 1. and at a site in Santa Monica Basin Žone of the low oxygen basins in the southern California Borderland region; see the map in the work of Berelson et al., 1996.. The geochemical characteristics of these sediments are summarized below and in Table 1, and are also described in more detail in a number of previous publications ŽChesapeake Bay: Burdige and Homstead, 1994; Burdige et al., 1995; Cowan and Boynton, 1996; Skrabal et al., 1997; Burdige and Zheng, 1998; Marvin-DiPasquale and Capone, 1998; midAtlantic shelfrslope break: Ferdelman, 1994; Santa Monica 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 dominates sediment organic matter remineralization. Bioturbation is virtually absent in the Santa Monica Basin sediments, although at site M3 a few bivalve spat and polychaete worms inhabit the upper ; 5 cm of sediment in the early spring ŽKemp et al., 1990.. The sediments at site S3 in Chesapeake Bay are silty sands and extremely well-mixed Žbioturbated and bio-irrigated. by large tube worms and other benthic

2. Materials and methods 2.1. Study areas The studies described here were carried out at three contrasting estuarine sites in Chesapeake Bay,

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

Table 1 Site characteristics

Water depth Žm. Bottom water temperature Ž8C.) Bottom water salinity Žpsu.) Bottom water dissolved oxygen Ž m M.) Surface sediment TOC Ž%. Depth-integrated sediment carbon oxidation rate ŽC ox ; mol my2 yry1 .

Chesapeake Bay a

Mid-Atlantic shelfrslope break a

S. California Borderland

M3

S3

N3

WC4

WC7

AI

SM

15 5–22 15.6–20.5 Ž10–20. 9–420 ) 3b 6.6"0.8 g

12 22 28 Ž20–30. 190 ; 0.5 b 4.3"1.0 g

10 21 9.9 Ž - 0.1–10. 260 ; 2–4 c 0.8"0.4 h

390 7 35 220 ; 2d 2.1"1.0 h,i

775 4 35 290 ; 2 d , 2.2 e 0.8"0.3 h,i

740 4 35 290 ;1.2 d 0.9"0.3 h,i

900 5 34.4 ;9 ; 5–6 f 0.7"0.3 j

)At the time of sampling. For the Chesapeake Bay sites, the salinity values in parentheses are general seasonal ranges. a See Fig. 1 for the location of these sites. b From the work of Burdige and Homstead Ž1994.. c From the work of Burdige et al. Ž1995.. d Burdige, unpublished data. e From the work of Ferdelman Ž1994., for a core collected at this site during an earlier study. f From the works of Jahnke Ž1990. and Shaw et al. Ž1990.. g From the works of Burdige and Zheng Ž1998.. These C ox values are integrated annual averages Žover the time period 3r95–10r96. determined from measured benthic ÝCO 2 fluxes made during temporal studies at these sites Žsee the work of Burdige and Homstead Ž1994., for further details on the procedures for the benthic flux determinations.. h Determined using measured ÝCO 2 profiles as described in the work of Burdige and Homstead Ž1994.. i Averages based on cores collected at these sites in 7r94 ŽCH X., 7r95 ŽCH XIV. and 8r96 ŽCH XVII.. j Determined using in situ benthic landers as described in the work of Berelson et al. Ž1996..

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

Parameter

47

48

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

macrofauna ŽSchaffner, 1990.. The sediments at site N3 in the Bay are clay dominated and iron-rich, and contain a diverse community of mixed polychaetes and bivalves ŽW. Boynton, personal communication.. Organic matter in these sediments is largely terrestrially-derived ŽJ. Cornwell, personal communication.. The sediments on the mid-Atlantic shelfrslope break are greyrgreen silty clays and, based on radiochemical measurements, some bioturbation Ž D B ; 1.5–5 cm2 yry1 . occurs in the upper 20–30 cm of these sediments ŽFerdelman, 1994.. 2.2. Sample collection With the exception of the Santa Monica Basin sediments, all sediments were collected by box core and sub-cored for further analysis. Santa Monica Basin sediments were collected with an Ocean Instruments Multi-Corer ŽBarnett et al., 1984.. All sub-cores Žwith the exception of the site S3 cores. were processed Žcut into 0.5 to 2 cm sections. under an inert ŽN2 . atmosphere, using a modified version of the glove bagrsectioning table described by Shaw Ž1989.. Inside this glove bag, the sectioned sediments were placed in polycarbonate centrifuge tubes that had been washed with HCl and rinsed several times with distilled, deionized water before use. Control experiments showed that these tubes neither scavenge DOC from, nor add it to, seawater. Sediments were then centrifuged for 10–20 min at 5500 = g Ž7000 rpm. at in situ temperatures. Pore waters were extracted from site S3 sediments using a modified form of the pressurized core barrel technique ŽJahnke, 1988.. Most of these modification are described in the work of Burdige and Homstead Ž1994., although here pore waters were removed from the sediments by pulling a vacuum on the sampling syringe, rather than pressurizing the entire core. This procedure was used with these sediments to avoid possible artifacts associated with the collection by centrifugation of pore water samples for DOC analyses from heavily bioturbated sediments ŽMartin and McCorkle, 1993; Alperin et al., 1998.. These workers have shown that measured pore waters DOC concentrations from such centrifuge-collected samples may overestimate the true pore water in situ DOC concentration, due to release of DOC from benthic macrofauna during core sectioning

andror centrifugation. In sediments that have low numbers of benthic macrofauna, side-by-side comparisons of these two pore water sampling techniques Žcentrifugation vs. pressurized core barrels. yield comparable DOC concentrations ŽBurdige, unpublished data.. Regardless of the method of pore water collection, all samples were filtered through 0.45 m m Gelman Nylon Acrodisc filters without exposure to ambient air, and aliquots divided into different storage vessels for later analysis. Samples for total DOC analyses were filtered into 8 ml cleaned glass vials sealed with Teflon-lined silicone septa, acidified to pH 2 with 6 N HCl, and then quick frozen and stored frozen until analysis Žnote that all glass and plasticware were cleaned as described in the work of Burdige and Homstead, 1994.. Samples for molecular weight studies were filtered into small cleaned brown glass bottles, acidified to pH 2 and stored refrigerated under N2 until analysis. Processing and storage of samples for molecular weight studies under N2 is based on the results of Orem and Gaudette Ž1984. who showed that failure to take these precautions may lead to significant differences in the determined DOC molecular weights in pore water samples. Acidification does not change the molecular weight distribution of pore water DOC compounds Žour results not shown here; also Chin, Y.-P., personal communication., yet minimizes DOC loss from refrigerated samples due to biological activity ŽTupas et al., 1994.. A comparison of our frozen vs. refrigerated samples for total DOC analyses indicated that their concentrations differed by less than "3% Ž n s 53 samples., similar to that observed by Tupas et al. Ž1994.. 2.3. Size fractionation studies These analyses were carried out with ultrafiltration techniques, using Amicon ‘Centricon’ microconcentrators whose nominal molecular weight cutoffs are 3, 10, 30 and 100 kDa. Studies by Chin and Gschwend Ž1991. showed that the 3 and 10 kDa filters yield molecular weights that agree well with values obtained by size exclusion chromatography. In theory, the use of these filters should have allowed us to separate DOC into the following molecular weight size classes: ) 100 kDa, 30–100

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

kDa, 10–30 kDa, 3–10 kDa and - 3 kDa. However, variable Žand non-reproducible. results with the 30kDa filter made data obtained with these filters difficult to interpret. Similarly, because of the broad molecular weight cut-offs of these filters ŽAiken, 1984; Chin and Gschwend, 1991; also see results below. we were not able to use the 3 and 10 kDa filters to determine DOC molecular weights in the 3–10 kDa range. In this paper then, we will only report results obtained with the 3 and 100 kDa filters, allowing us to separate DOC into the following nominal molecular weight classes: ) 100 kDa, 3–100 kDa, and - 3 kDa. We will refer to these DOC size classes as: DOC 100 , DOC 3 – 100 and DOC 3 . Before using the filters we used a modification of the procedure described by Chin and Gschwend Ž1991. to clean the filters. The filters were first soaked in a 30:70 methanol: distilled, deionized water ŽDDW. solution for 2–3 h, next soaked overnight in DDW, and finally flushed several times by repeatedly centrifuging 2 ml of DDW through the filters Žfor a total volume of 12 ml per filter.. The filters were stored in DDW until use. Prior to ultrafiltration of a sample, 2 ml of low DOC Ž; 7–8 m M. DDW was passed through the filter by centrifugation, and then analyzed for DOC. These filter blanks were 23 Ž"13. m M for the 100 kDa filters and 38 Ž"16. m M for the 3 kDa filters Ž n s 65 for both filters.. Given the variability in the filter blanks, all sample filtrate concentrations were corrected with the blank value obtained with the particular filter used for that ultrafiltration. We observed no deterioration or breakdown of the filters during this cleaning procedure and their subsequent use in the ultrafiltration of a sample. Pore water ultrafiltration was carried out by placing 2 ml aliquots of the original sample into each of the upper reservoirs of 3 kDa and 100 kDa filter assemblies, which were then capped to minimize evaporative sample loss during processing. The 100 kDa filter assemblies were centrifuged at 1000 = g Ž3000 rpm. for 20 min, while the 3 kDa filters were centrifuged at 5500 = g Ž7000 rpm. for 2 h Žboth at 58C.. The resulting filtrates and an aliquot of the unfiltered sample were analyzed for DOC by high temperature catalytic oxidation ŽHTCO; see Section 4.2.. Concentrations of DOC in the different size classes were determined by subtracting the ap-

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propriate blank-corrected filtrate concentrations and unfiltered sample concentrations. Replicate ultrafiltrations of polystyrene sulfonate ŽPSS. standards Žsee Section 2.3.1. using the 100 kDa filters yielded filtrate concentrations that agreed to within "2% Ž5, 17.5 and 35 kDa PSS standards; n s 2 or 3 replicates per standard.. Similar studies using the 3 kDa filters resulted in replicate filtrate concentrations that agreed to within "11% Ž1.43, 5, 17.5 kDa PSS standards.. Replicate Ž n s 4. analyses of one Chesapeake Bay site M3 pore water sample yielded filtrate concentrations that agreed to within "4% for the 100 kDa filters and "6% for the 3 kDa filter. 2.3.1. Calibration studies Based on discussions in the literature about potential artifacts associated with the use of ultrafiltration membranes for the study of DOM in natural waters

Fig. 2. The results of calibration studies with the 3 and 100 kDa molecular weight cut-off Amicon ‘Centricon’ microconcentrators. The y-axis represents the relative concentration of the compound recovered in a particular size class, based on the ultrafiltration of solutions of each compound with the 3 and 100 kDa filters. ‘Perfect’ behavior of these filters would result in 100% values for compounds whose molecular weights were within the particular size class, and 0% values for compounds with molecular weights outside the size class. Symbols: v s polystyrene sulfonates ŽPSS.; ' s natural and synthetic proteins and peptides, and vitamin B12 .

50

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

ŽAiken, 1984; Chin and Gschwend, 1991; Kildruff and Weber, 1992., we carried out the following calibration studies. Solutions of organic compounds of known molecular weights were prepared in 0.7 M NaCl Žmade up in low DOC DDW., and then subjected to ultrafiltration as described above. Preparation of standards in an electrolyte that matches that of seawater is based on results described by Kildruff and Weber Ž1992., who observed that the apparent molecular weight cut-offs of these membranes was dependent on the solution ionic strength below

Fig. 4. Depth profiles of: the absolute concentrations of total DOC, and the relative concentrations of DOC 3 , DOC 3 – 100 , and DOC 100 in Chesapeake Bay sites N3 Ž'. and S3 Ž`. sediments Žboth collected on cruise CH XV, 10r95..

Fig. 3. Depth profiles of: the absolute concentrations of total DOC, and the relative concentrations of DOC in the - 3 kDa ŽDOC 3 ., 3–100 kDa ŽDOC 3 – 100 . and )100 kDa ŽDOC 100 . size classes, all in Chesapeake Bay site M3 sediments. In this figure and in Figs. 4 and 5, concentrations on the x-axis represent bottom water values obtained from hydrocast samples. Symbols: B sCH XII Ž3r95.; `sCH XIV Ž7r95.; ' sCH XV Ž10r95.. Aside from the general trends in the data Ždiscussed in the text., also note that the surface pore water sample in the CH XII profile contains a much higher total DOC concentration, a very low relative concentration of DOC 3 and a much higher relative concentration of DOC 100 . This observation is likely related to the seasonal colonization of these sediments by benthic macrofauna Žsee the discussion in Section 2.1., and may be artifactual, due to the presence of macrofauna in these sediments at this time of the year Že.g., Martin and McCorkle, 1993; Alperin et al., 1998..

; 0.05–0.1 M, yet was independent of ionic strength at higher values Žalso see discussions in the work of Chin and Gschwend Ž1991... The ionic strengths of all of our pore water samples were above 0.1 M, and most were effectively seawater values Ž; 0.7; see Table 1.. The compounds used in these calibration studies were: random coil PSS ŽAldrich Scientific. of molecular weights 1.43, 5, 17.5, 35 and 130 kDa; bovine serum albumin Ž69 kDa.; vitamin B 12 Ž1.36 kDa.; the synthetic peptide Ala-D-isoglutaminyl-Lys-D-AlaD-Ala Ž0.488 kDa; Sigma catalog aA1035.; the synthetic peptide Gly–Val–Leu–Ser–Asn– . . . Ž2.5 kDa; Sigma catalog aG9031.. The initial concentrations of the protein, peptide and vitamin B12 solutions ranged from ; 90–530 m mol C ly1 ; these concentrations Žand those of ultrafiltered standards. were determined by HTCO Žsee below.. Concentrations of PSS standards were generally 500 mg PSS ly1 , with the exception of the 1.43 kDa PSS standard which was 2000 mg PSS ly1 . Concentrations of PSS

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

51

sample injection. Although Benner’s original suggestion was to replace a portion of the catalyst bed with pieces of muffled CuO wire, we found that salt build-up in the interstices of the pieces of CuO wire led to rapid clogging and breakage of quartz combustion tubes. Since the outer surface of the copper wire rapidly oxidizes to CuO when heated to 6808C in the presence of the pure oxygen carrier gas, a plug of wound Cu wire similarly improved sample combustion without these additional problems.

3. Results 3.1. Calibration of the ultrafiltration membranes

Fig. 5. Depth profiles of: the absolute concentrations of total DOC, and the relative concentrations of DOC 3 , DOC 3 – 100 , and DOC 100 in continental margin sediments. Symbols: B ssta. WC4 ŽCH XIV, 7r95.; `ssta. WC7 ŽCH XIV, 7r95.; ' ssta. AI ŽCH XIV, 7r95.; v ssta. SMB ŽT95; 11r95..

compounds were determined by UV absorption at 280 nm ŽChin and Gschwend, 1991., and corrected Žas appropriate. for absorption of the unfiltered and ultrafiltered blank sodium chloride solution. 2.4. Analysis of DOC Samples were analyzed for DOC by HTCO using a Shimadzu TOC-5000 total carbon analyzer. The procedure used here was identical to that described in the work of Burdige and Homstead Ž1994. with the following modifications. Based on the results of Benner Žunpublished data cited in the work of Sharp et al. Ž1993.., a ; 70 cm long piece of copper wire was wound into a 0.5 cm thick plug and placed on top of the Pt catalyst bed in the quartz combustion tube of the DOC analyzer; the combined length of the catalyst bed and Cu wire plug was 13 cm. This modification substantially improved sample oxidation Ži.e., sample peak shape. and also eliminated the occasional occurrence of split peaks from a single

Calibration studies with these filters using known organic compounds are shown in Fig. 2. Small amounts Ž; 15%. of compounds with molecular weights greater than 3 kDa appeared to pass through the 3 kDa filters ŽFig. 2C., while compounds with molecular weights as low as 35 kDa were also retained at similar low levels by the 100 kDa filters ŽFig. 2A.. Together, the combined use of the 3 and 100 kDa filters led to an apparent recovery of only

Table 2 Summary of DOC size fractionation studies Sites a

DOC 3 )

DOC 3 – 100 )

DOC 100 )

Chesapeake Bay M3 CH XII Žbelow 2 cm. CH XIV CH XV Overall average S3 ŽCH XV. N3 ŽCH XV.

87"4% 87"5% 83"6% 86"3% 92"3% 88"6%

10"3% 10"6% 15"4% 12"3% 7"4% 5"6%

4"2% 3"3% 2"2% 3"1% 0.5"1% 8"7%

Mid-Atlantic shelf r slope break AI ŽCH XIV. 62"10% WC4 ŽCH XIV. 70"7% WC7 ŽCH XIV. 71"9%

18"6% 13"9% 14"7%

21"4% 18"7% 15"4%

S. California Borderland SM ŽT95. 64"11%

18"10%

17"12%

a Cruises: CH XII sChamps XII Ž3r95.; CH XIVsChamps XIV Ž7r95.; CH XVs Champs XV Ž10r95.; T95s Teflon 95 Ž11r95.. )As a percentage of the total DOC. These values are average values for each of the cores shown in Figs. 3–5.

52

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

; 80% for DOC compounds in the 3–100 kDa molecular weight range ŽFig. 2B.. Interpretation of this data is based on several considerations. As discussed by Chin and Gschwend Ž1991. the pore sizes of these filters are based on molecular weights of globular proteins, which may not be appropriate model compounds for dissolved organic compounds found in sediment pore waters Žalso see the work of Kildruff and Weber Ž1992... Rather, it appears that random coil molecules such as PSS may be more representative of the ‘structure’ of natural DOC Žsee discussions in the works of Chin and Gschwend Ž1991. and Kildruff and Weber Ž1992. and references cited therein.. At the same time however, at high ionic strengths Žsuch as those of seawater. the increased coiling of PSS molecules may decrease their size Že.g., Chin and Gschwend, 1991. allowing some ‘breakthrough’ of PSS compounds whose molecular weights exceed those of a given filter.

Calibration studies by Chin and Gschwend Ž1991. with PSS standards suggested that the same type of 3 and 10 kDa ultrafiltration membranes used here actually retain compounds whose molecular weights are 5 times lower than those reported by the manufacturers Ži.e., the 3 kDa filter has an actual molecular weight cut-off of 0.6 kDa and the 10 kDa filter had 1 of 2 kDa.. Our calibration studies using these filters with both PSS and other organic compounds are equivocal on this point, particularly since we were unable to use the 3 and 10 kDa filters together to separate DOC compounds into the 3–10 kDa molecular weight class. Alternately, we suggest that the observations of Chin and Gschwend Ž1991. may equally be explained by the broad transition from 0 to 100% recovery for each of these filters ŽFig. 2.. In the remainder of this paper, then, we will define molecular weight size classes based on the nominal molecular weight cut-offs reported by the manufacturer, given the caveats of such a definition based on the results discussed above.

Fig. 6. Absolute concentrations of DOC 3 , DOC 3 – 100 and DOC 100 vs. total DOC in estuarine ŽChesapeake Bay. and continental margin sediments. Also shown in these figures Žas a solid line. is the best-fit straight line through the data. In E. the dashed line indicates that in these estuarine sediments there was a constant ; 30 m M concentration of DOC 100 . Symbols: ^ ssta.N3; % s sta. S3; (sst. M3 ŽCH XII.; `ssta. M3 ŽCH XIV.; v ssta. M3 ŽCH XV.; I ssta. SMB; essta. WC4; dotted lozenge ssta. AI; l ssta. WC7. By definition, concentration–concentration plots such as these should be linear and pass through the origin if the DOC concentration in a particular size class is a constant fraction of the total DOC. This appears to be the case for DOC 3 and DOC 3 – 100 in the Chesapeake Bay sediments and DOC 3 – 100 in the continental margin sediments, where the y-intercepts are all within 2 s of zero Ži.e., the origin; y6.7"15.4, 1.0"16.6 and y33.7"28.0 m M, respectively.. In contrast, the best-fit y-intercepts in the plots of DOC 3 and DOC 100 in continental margin sediments were significantly different than zero Ž50.3"18.6 and y50.0"10.0 m M.. This implies that for each of these data sets the relative concentration of DOC in each size class Žwhich is actually the slope of a line between the origin and a particular data point or a point on the best-fit line. either decreases ŽDOC 3 . or increases ŽDOC 100 . with increasing total DOC. Since DOC concentrations increase with depth in these sediments, this then predicts that the relative concentration of DOC 3 should decrease with depth in the continental margin sediments, and that the relative concentration of DOC 100 should increase with depth, consistent with the data in Fig. 5.

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

3.2. Pore water results Total DOC concentrations increased with sediment depth at all of the sites ŽFigs. 3–5., and the depth profiles shown here are similar to other reported profiles in these sediments ŽBurdige and Homstead, 1994; Bauer et al., 1995; Burdige and Zheng, 1998. and in similar continental margin sediments Že.g., Martin and McCorkle, 1993.. The majority of the DOC in the sediments we examined had a molecular weight less than 3 kDa ŽTable 2.. In the Chesapeake Bay Žestuarine. sediments most of the remaining DOC was found in the 3–100-kDa fraction, although in the continental margin sediments it was more evenly distributed between the 3–100-kDa and ) 100-kDa fractions. In the Chesapeake Bay sediments there was no significant change in DOC molecular weight distributions with depth, based on either: depth plots of relative DOC concentrations in the different size classes ŽFigs. 3 and 4.; plots of DOC concentration in a given size class vs. total DOC ŽFig. 6.. In the continental margin sediments there may have been a very slight decrease with depth in the relative concentration of low molecular weight DOC ŽLMWDOC, i.e., DOC 3 .; this appeared to be associated with the accumulation of HMW-DOC in the DOC 100 fraction ŽFigs. 5 and 6.. Plotting absolute DOC concentrations in each size

53

class against total DOC shows that the concentration of DOC 3 increased with total DOC, and that the two were strongly correlated ŽFig. 6A and B.. The slope of the line defined with the estuarine sediment data is different from that for the continental margin sediment data, although each is similar to the corresponding average in Table 2. Since total DOC generally increased with depth in all of these sediments ŽFigs. 3–5. these results imply that this LMW-DOC accumulates with depth as a near-constant fraction of total DOC. Weaker positive relationships existed between the concentration of DOC 3 – 100 and total DOC for both sediment types ŽFig. 6C and D., although this material, too, accumulates with depth. In contrast, while DOC 100 showed a strong positive correlation with total DOC in the continental margin sediments ŽFig. 6F., in the estuarine sediments the concentration of DOC 100 was apparently independent of total DOC ŽFig. 6E.. In these sediments, this high molecular weight material appeared to have a near-constant concentration of ; 30 m M. At face value this should imply that the relative concentration of DOC 100 should decrease with depth. However, since this material is such a small percentage of the total DOC in these sediments Žless than ; 8%., and given the errors associated with these measurement, this depth trend is not observable in Figs. 3 and 4. Finally, we also see that on average DOC 3 Žas a

Fig. 7. The relative concentration of DOC 3 Žfrom Table 2. vs. sediment temperature Žleft. and sediment carbon oxidation rates ŽC ox ; right. 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 relationship between DOC 3 and each of these two quantities. The positive relationship between DOC 3 and sediment temperature was significant based on both its r 2 value Žs 0.64. and the F-statistic Žwhich indicates the probability that this r 2 value occurs by chance; F2,6 s 12.3, a - 0.02.. In contrast, the relationship between DOC 3 and C ox was much weaker Ž r 2 s 0.32; F2,6 s 3.28, 0.1 - a - 0.2.. Symbols: B s mid-Atlantic shelfrslope break sites Žsts. AI, WC4 and WC7.; I s sta. SMB; v s sts. S3 and N3 ŽChesapeake Bay.; ` s sta. M3 ŽChesapeake Bay..

54

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

percentage of total DOC. was slightly higher in the estuarine ŽChesapeake Bay. sediments than it was in the offshore, continental margin sediments Ž; 85– 90% vs. ; 60–70%; see Table 2.. These relative DOC 3 concentrations also showed a positive relationship with sediment temperature and a weaker positive relationship with sediment carbon oxidation rates ŽC ox ; Fig. 7..

our observations with these published results. At the same time, some of the biogeochemical factors that lead to the observed trends in molecular weight distributions in the estuarine and continental margin sediments we studied may also explain the differences in Table 3. These include differences in sources and reactivity of the organic matter undergoing remineralization in the sediments, or perhaps differences in the pathways by which this organic matter is degraded Žsee Section 4.2.2 for further details..

4. Discussion 4.1. Comparison with other studies

4.2. Controls on the DOC molecular weight distribution in sediment pore waters

The molecular weight distribution of pore water DOC has been examined in other coastal, marine and freshwater sediments, and these results are summarized in Table 3 and compared with our results. In some of the sediments studied Žthe organic-rich muds of Loch Duich and some of the Boston Harbor sites., there is a near constant absolute concentration of LMW-DOC with depth. The net accumulation of HMW-DOC in the sediments therefore leads to a decrease with depth in the relative amounts of low molecular weight material. Such trends were not observed in other sediments Že.g., Great Bay., nor in our work ŽFig. 6.. In general, we note that in many, but not all of these published studies, the relative concentration of HMW-DOC increases with depth ŽTable 3.. Such trends were seen at our continental margin sites, but not in the Chesapeake Bay sediments. However, the changes with depth in the relative amounts of HMWDOC in the continental margin sediments we studied were not as dramatic as those seen in these other published studies ŽTable 3.. A more detailed comparison of these published studies with our results is made difficult by the fact that most of these studies used filters with molecular weight cut-offs that are different from the ones we used. Therefore, definitions of ‘high’ and ‘low’ molecular weight fractions in the different studies are not unique and are sometimes overlapping Žalso see footnote 1 in Section 4.2.. Differences in the characteristics of each of the filter membranes used Ži.e., the molecular weight range over which the transition from 0 to ; 100% recovery occurs; e.g., see Fig. 2. also makes it difficult to directly compare

Previous studies examining the molecular weight distribution of pore water DOC attributed the accumulation of HMW-DOC with depth to three possible processes. The first, generally referred to as the geopolymerization model, involves the formation of high molecular weight ‘geopolymers’ from LMWDOC compounds via abiotic polymerization reactions ŽNissenbaum et al., 1971; Tissot and Welte, 1978; Krom and Westrich, 1981; Hedges, 1988.. Increasing condensation of these proposed geopolymers Žor dissolved humic substances. is eventually thought to form particulate humics and kerogen, as the dissolved humics eventually become insoluble. The second explanation for the accumulation of HMW-DOC with depth proposes that it results from the selective preservation Žand therefore accumulation in pore waters. of partially, degraded and more refractory components of the sediment POC ŽOrem et al., 1986.. Finally, it has been suggested that the increase with depth in the molecular weight of pore water DOC results from changes in biotic or abiotic processes in the sediments associated with the transition between surficial, oxic sediments and deeper, anoxic sediments ŽKrom and Sholkovitz, 1977; Krom and Westrich, 1981; Chin et al., 1994.. The lack of significant depth variations in the molecular weight distribution of pore water DOC in the sediments we studied ŽFigs. 3–5. suggests that processes such as geopolymerization or oxicranoxic effects on DOC molecular weights are either not of major significance in these sediments, or that their effects cannot be detected by the particular molecular weight cut-offs of the filters used in this study Ži.e., geopolymerization reactions do occur but do

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

Molecular weight cutoff ŽHMW-DOC. a

Depth trend for the high molecular weight fractionb

Marine sediments Loch Duich, Scotland Žfjord-type estuary. c organic-rich mud Žsulfate reducing and methanogenic. brown, sandy-mud Ž‘oxic’ sediments, no evidence of sulfate reduction. Great Bay, NH Žestuarine sediment. d

increase Ž0–80 cm. no change Ž0–55 cm.

1 kDa 1 kDa

increase with depth Žfrom ; 40 to 80%. constant with depth Ž ; 20–40%.

1 kDa

increase with depth Žfrom ; 50 to 80%.

Mangrove Lake Žanoxic, sapropelic sediment. e Boston Harbor f

decrease Ž0–15 cm., increase Ž0–75 cm. increase Ž0–200 cm. increase Ž0–30 cm.

0.5 kDa 3 kDa

Chesapeake Bay Žestuarine sediments. g Continental margin sediments g

increase Ž0–25 cm. increase Ž0–25 cm.

3 kDa 3 kDa

decrease with depth Žfrom 95 to ; 75%. increase with depthŽfrom ;10–20% to ; 50–60%. in two of the three sites; constant with depth Ž ; 40–60%. at the third site constant with depth Ž ;10–15%. increase with depth Žfrom ; 20 to ; 35%.

Freshwater sediments Upper Mystic Lake f Lake Michiganh

increase Ž0–50 cm. not reported Ž0–10 cm.

3 kDa increase with depth Žfrom ; 5 to ; 50%. number- and weight-average molecular weights increase with depth Žfrom 0.5–0.8 kDa at the surface to 0.7–1.1 kDa at 8.5 cm.

a

The molecular weights cutoffs listed here represent the nominal cut-offŽs. of the filter used in each study to separate HMW-DOC and LMW-DOC. ‘Increase’ and ‘decrease’ are relative to the total DOC pool. c From the work of Krom and Sholkovitz Ž1977.. d From the work of Orem and Gaudette Ž1984.. The different total DOC depth trends are for cores collected at slightly different sites in Great Bay. All other observations are the combined results from the two cores. The results discussed here are only for the cores processed under anoxic conditions. e From the work of Orem et al. Ž1986.. f From the work of Chin and Gschwend Ž1991.. g This study. h From the work of Chin et al. Ž1994.. Molecular weights were determined here by size exclusion chromatography. b

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

Location

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D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

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not produce new compounds whose molecular weights exceed ; 3 kDa.. 1 Arguments in the work of Alperin et al. Ž1994. also support the suggestion that geopolymerization reactions are not likely important in marine sediments on early diagenetic time and depth scales. To explain our results, we have therefore developed a model which assumes that the average molecular weight distributions in surface Župper ; 30 cm. sediments is primarily controlled by organic matter remineralization processes, and which also contains a modified form of the selective preservation explanation discussed above. This model is shown in Fig. 8. In the upper panel we visualize the degradation of sediment POC to CO 2 through DOC intermediates as a series of hydrolytic, fermentative and eventually respiratory processes that produce and consume pore water DOC compounds with increasingly smaller molecular weights ŽLaanbroek and Veldkamp, 1982; Henrichs, 1992; Deming and Baross, 1993; Alperin et al., 1994; Arnosti et al., 1994, and references therein.. While different POC starting materials Že.g., proteins, carbohydrates. are initially hydrolyzed to different DOC intermediates ŽColberg, 1988; McInerney, 1988., at least in anoxic or sub-oxic sediments, these hydrolytic and fermentative pathways appear to lead to a limited number of monomeric low molecular weight compounds Žor ‘biomonomers’. such as short chain organic acids and alcohols and perhaps monomeric amino acids. These biomonomers are then oxidized by the terminal respiratory organisms in the sediments Ži.e., sulfate reducing or denitrifying bacteria.. To further develop the model in Fig. 8A we have incorporated into it the size-reactivity continuum model for DOC remineralization in aqueous systems ŽAmon and Benner, 1996.. This latter model is based on water column observations which show that HMW-DOC represents a more reactive and less diagenetically altered fraction of the total water column DOC than the more abundant LMW-DOC Žfor additional water column data in support of this model

1

Such factors might also explain the differences between our results and some of the published data in Table 3 if such geopolymerization reactions produce compounds on early diagenetic time scales whose molecular weights are between ;1 and 3 kDa.

also see the works of Benner et al., 1992; Amon and Benner, 1994; Santschi et al., 1995; Guo et al., 1996; Guo and Santschi, 1997.. In the pore water sizerreactivity ŽPWSR. model shown in Fig. 8B, sediment POC is initially hydrolyzed to a class of HMW-DOC compounds, predominantly composed of biological polymers such as dissolved proteins or polysaccharides. Most Žbut not all. of the HMW-DOC is degraded by hydrolytic and fermentative processes to monomeric low molecular weight compounds ŽmLMW-DOC. that are then rapidly oxidized by the terminal respiratory organisms in the sediments. At the same time though, some fraction of the HMW-DOC is only partially oxidized, leading to the production of what we refer to here as polymeric LMW-DOC ŽpLMW-DOC.. This model does not exclude however, the possibility of geopolymerization reactions forming pLMW-DOC from the mLMW-DOC pool, through reactions such as the melanoidin or ‘browning’ reaction Ža sugaramino acid condensation reaction; e.g., Hedges, 1988. or through complexation reactions such as those that have been described for short chain organic acids such as acetate ŽChristensen and Blackburn, 1982; Michelson et al., 1989.. However, as discussed above, if these reactions do occur, their products Žon early diagenetic time scales. still have relatively low molecular weights Ži.e., less than ; 3 kDa.. This pLMW-DOC then becomes what is operationally defined as humin ŽAmon and Benner, 1996., and humification Ži.e., production of soluble humic and fulvic acids. is now thought of as a process that produces increasingly oxidized, LMW-DOC molecules from sediment POC Žalso see the work of Hatcher and Spiker, 1988.. As discussed in the work of Amon and Benner Ž1996., pLMW-DOC apparently escapes microbial remineralization because it ‘no longer resembleŽs. biomolecules.’ Essentially, these models imply that selective preservation indeed leads to the accumulation of pore water DOC, with the significant difference that it is refractory low, and not high, molecular weight DOC that is apparently preserved. The PWSR model is somewhat analogous to the multi-G model used to describe the reactivity of sedimentary POC ŽWestrich and Berner, 1984., although here we think of discrete DOC fractions with

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

differing chemical compositions and reactivities. While the reactivity of both sediment POC and DOC is likely a continuum ŽMiddelburg, 1989; Amon and Benner, 1996. the conceptual approach of the multi-G model has proven useful in understanding and studying POC decomposition in sediments. By analogy then, we suggest that the PWSR model will be similarly useful in examining DOC cycling and reactivity in sediments. The PWSR model explains several aspects of our data, in particular, the observation that DOC 3 represents the majority of the total DOC Ž; 60–90%. in these sediments ŽTable 2., and that this LMW-DOC accumulates Žin an absolute sense. with depth along with total DOC in the sediments ŽFig. 6A and B.. Although DOC 3 likely comprises both pLMW-DOC and mLMW-DOC, the latter is generally found at lower absolute concentrations ŽShaw et al., 1984; Burdige and Martens, 1990; Alperin et al., 1994. and is presumed to have a much faster turnover time. Therefore, we assume that the ‘properties’ of DOC 3 are primarily controlled by pLMW-DOC. Possible reasons for the differences in the relative concentrations of DOC 3 in estuarine and continental margin sediments will be discussed below. The proposed rapid turnover of HMW-DOC in the PWSR model is consistent with DOC water column studies ŽAmon and Benner, 1994, 1996; Guo and Santschi, 1997., and may explain why in the water column and in the sediment pore waters we studied DOC 100 is a small percentage of the total DOC Ž-; 20% in our samples; Table 2.. Again, differences in the behavior of this HMW-DOC in estuarine and continental margin sediment will be discussed below. 4.2.1. Controls on DOC concentrations in sediment pore waters The PWSR model can also be used to explain the general shape of DOC profiles in anoxic Ži.e., nonbioturbated or -bioirrigated. sediments ŽFig. 9.. The slight imbalance between DOC production and consumption near the sediment surface Ždue the production of less reactive pLMW-DOC. leads to the observed accumulation of DOC with depth in most sediments. Asymptotic DOC concentrations found at depth in sediments may then result from: Ž1. a balance between DOC production Žfrom POC. and

57

DOC consumption Žprimarily from the pLMW-DOC pool; e.g., Alperin et al., 1994.; Ž2. further changes Žbiotic or abiotic. in pLMW-DOC that may decrease the overall reactivity of this material, eventually leading to a situation in which the pLMW-DOC found at depth is essentially nonreactive on early diagenetic time scales ŽHatcher and Spiker, 1988; Amon and Benner, 1996.. While both suggestions are consistent with the PWSR model, further studies will be needed to critically examine them Že.g., see discussions in the work of Burdige and Zheng Ž1998... At the same time, recent studies have shown that sorption of dissolved organic matter to sediment particles also plays a role in affecting pore water DOM concentrations ŽHedges and Kiel, 1995; Henrichs, 1995., and that pore water DOC concentrations may be ‘buffered’ by reversibly-sorbed DOC in equilibrium with the pore waters ŽThimsen and Keil, 1998.. These processes may also affect DOC concentrations at depth depending on: the intrinsic reactivity of the pore water DOC and that which is adsorbed to sediment particles ŽLee, 1994; Mayer, 1994b; Henrichs, 1995.; the relative sizes of the pore water and sorbed DOC pools ŽThimsen and Keil, 1998.. In addition to the DOC size fraction data presented here, the PWSR model is consistent with other data on sediment DOC cycling and concentrations. Alperin et al. Ž1994. quantified two DOC pools in the pore waters of the anoxic sediments of Cape Lookout Bight ŽCLB., NC, USA: an acidvolatile DOC pool ŽAV-DOC. which they suggested contains volatile fatty acids and alcohols; a nonacid-volatile DOC pool ŽNAV-DOC. containing larger molecular weight organic compounds and non-volatile, small organic molecules such as dissolved free amino acids. In general, AV-DOC was a small fraction Žless than ; 5%. of the total DOC in CLB pore waters, and showed little depth variability. In contrast, NAV-DOC increased continuously Žand in most cases asymptotically. to values that varied seasonally between 3–5 mM, and therefore constituted the majority of the total DOC in the pore waters. In a broad sense, AV-DOC should correspond to what we refer to here as mLMW-DOC, although some mLMW-DOC compounds are also NAV Že.g., dissolved free amino acids.. However, their concentrations in sediment pore waters are quite

58

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D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

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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 DOC concentrations in CLB sediments Že.g., Henrichs et al., 1984; Burdige and Martens, 1990; Burdige, 1991.. Similarly, with the exception of these small, non-volatile organic compounds, NAV-DOC roughly corresponds to the sum of HMW-DOC and pLMWDOC Žwith pLMW-DOC presumably constituting the vast majority of the NAV-DOC..

The results of lignocellulose degradation studies in salt marsh microcosms Žsummarized in the work of Hodson and Moran, 1995. also indicate that the DOC derived from decaying marsh detritus has both a fast- and slow-decaying component, and that the overall turnover rate of the DOC that accumulates during 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 general concept that POC remineralization through DOC intermediates proceeds through a series of processes that lead to increasingly smaller DOC molecules. Eventually, they produce a limited number of monomeric low molecular weight compounds that are used by the terminal respiratory organisms in the sediment. B. This figure Žmodified from the work of Alperin et al., 1994. incorporates the concepts shown above and the size-reactivity continuum model of Amon and Benner Ž1996.. As is described in the text Žand 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 to DOC components whose characteristics are consistent with the PWSR model. 4.2.2. Differences in the DOC molecular weight distributions in estuarine and continental margin sediments The results in Table 2 and Fig. 6 show that absolute and relative concentrations of DOC in different size classes vary between estuarine and continental margin sediments. The results in Fig. 7 suggest that temperature or perhaps overall rates of sediment carbon oxidation may lead to these differences. Temperature could be an important controlling parameter if there are differences in the temperature dependences Ži.e., activation energy. of the individual reactions in the overall remineralization of sediment POC to CO 2 through DOC intermediates. However, the critical examination of this suggestion is hampered in part by the lack of detailed knowledge about the specific pathways by which sediment POC is remineralized to CO 2 and the temperature dependences of these pathways ŽReichardt, 1987; Henrichs, 1992; Arnosti, 1997.. Changes in the overall rate of sediment carbon oxidation could explain differences in the distribution of the DOC intermediates in POC remineralization depending on the kinetics and mechanisms of these reactions. Again, lack of information in this area precludes a further examination of this possibility. At the same time, differences in the intrinsic reactivity of the POC undergoing decomposition in these sediments will also affect sediment carbon oxidation rates. These differences are likely related to the chemical structure, composition andror sources Že.g., terrestrial vs. marine. of this organic matter Že.g., Whelan and Emeis, 1992; Henrichs, 1993.. Given that the reactivities of specific compounds that comprise potential POC starting materials are known to vary ŽCowie and Hedges, 1992; Cowie et al., 1992., and that there are differences in some of their initial ‘upstream’ DOC products Žsee references in Section 4.2., considerations such as these might explain the observations in Fig. 6 and Table 2. Differences in these upstream DOC intermediates might also affect the types of pLMW-DOC that are produced in these sediments, therefore further affecting the reactivity and concentrations of

pLMW-DOC in estuarine vs. continental margin sediments. Additional work characterizing the microbial processes involved in POC remineralization, the sediment POC itself, and the DOC intermediates produced during POC remineralization will be needed to further examine these suggestions. Regardless of the exact causes of the observations in Table 2 and Fig. 6, they suggest that there are likely differences in the relative rates of the reactions shown in Fig. 8B in the two sediment types we examined. In particular, the lower absolute concentration of DOC 100 in estuarine sediments and its lack of accumulation with depth along with total DOC Žcompare Fig. 6E and F. suggests that this material may turn over more rapidly in estuarine sediments than it does in continental margin sediments. In contrast, in continental margin sediments the hydrolytic processes affecting this HMW-DOC may provide an initial ‘upstream’ bottleneck in the overall remineralization process, allowing this material to accumulate to some extent in both an absolute and relative sense in these sediment pore waters. This suggests that remineralization processes in continental margin sediments may be more controlled by hydrolytic processes than they are in estuarine sediments, where ‘downstream’ fermentative or perhaps respiratory processes may exert a greater overall control on carbon remineralization Žand may possibly also explain the enhanced accumulation of DOC 3 in estuarine sediments.. Extracellular hydrolysis of macromolecular organic matter Ži.e., HMW-DOC or sediment POC. has generally been thought to be the rate limiting step in organic matter degradation ŽKing, 1986; Hoppe, 1991; Meyer-Reil, 1991., although as Arnosti et al. Ž1994. have noted, this suggestion is ‘not well tested.’ Their polysaccharide degradation studies support the suggestion that such hydrolytic processes may not be rate-limiting in all sediments, and our observations appear to be consistent with these results. 4.3. Pore water DOC and sediment carbon preserÕation If pore water DOC plays a role in sediment carbon preservation, it must eventually be incorporated into the ‘solid’ sediment matrix where it is then buried and then becomes a part of the long-term

D.J. Burdige, K.G. Gardnerr Marine Chemistry 62 (1998) 45–64

global carbon cycle Že.g., Hedges, 1992.. As discussed above, geopolymerization reactions have been suggested as one such mechanism for how this occurs Žsee references above., although this model is not universally accepted Že.g., de Leeuw and Largeau, 1993.. The geopolymerization model also does not appear to be entirely consistent with the molecular weight data presented here. Recently, it has been suggested that adsorption of organic matter to mineral grain surfaces may actually control carbon preservation ŽMayer, 1994a,b; Hedges and Kiel, 1995.. This process involves adsorption of organic matter in small mesopores on the mineral surfaces which both protect the organic molecules from attack by microbial enzymes ŽMayer, 1994a,b. and may also possibly act as sites in which rates of abiotic condensation reactions are enhanced by either steric- or concentration-related phenomena ŽMayer, 1994b; Collins et al., 1995; Hedges and Kiel, 1995.. This sorption process appears to be at least partially reversible ŽThimsen and Keil, 1998. and some of the organic matter sorbed to the sediments is still capable of undergoing biological degradation once it is desorbed ŽKeil et al., 1994.. Since the sizeŽs. of the mesopores may provide some constraints on the upper limit of the sizermolecular weight of DOC molecules that can be taken up in the mesopores ŽMayer, 1994b., the relatively small size of Ž- 3 kDa. of most pore water DOC would appear to aid in its possible interaction with mesopore adsorption sites. Similarly, the proposed reactivity of this material in the PWSR model may also enhance its preservation, if the sorption processes are reversible to any significant extent Žsee discussions in the works of Lee, 1994; Mayer, 1994b; and Henrichs, 1995.. Future studies in this area will require understanding how these proposed DOC pools interact with sediment particles, their intrinsic reactivities, and also better characterizing the different processes affecting DOC once it is sorbed to sediment particles.

5. Conclusions Ž1. The majority of the DOC in Chesapeake Bay Žestuarine. and several continental margin sediments

61

Župper ; 30 cm. had a molecular weight less than 3 kDa. The percentage of this LMW-DOC was slightly higher in estuarine sediments as compared to continental margin sediments Ž; 85–90% vs. ; 60– 70%.. In these estuarine sediments most of the remaining DOC was found in the 3–100 kDa fraction, although in the continental margin sediments it was more evenly distributed between the 3–100 kDa and ) 100 kDa fractions. Ž2. The absolute concentration of DOC 3 increased with total DOC, and both were strongly correlated. Since total DOC increased with depth in all of these sediments, this implies that the majority of the DOC accumulating in these sediments is of low molecular weight. In contrast, only in the continental margin sediments did DOC 100 show a positive correlation with total DOC. In the estuarine sediments, the concentration of DOC 100 was apparently independent of total DOC, and was found at a near-constant concentration of ; 30 m M. Ž3. In the continental margin sediments there was a very slight increase with depth in the relative concentration of HMW-DOC ŽMW ) 3 kDa.. However, these depth trends were much smaller than those observed in previously published studies. In contrast, in the Chesapeake Bay sediments DOC molecular weight distributions appeared to be constant with depth. Ž4. These results have been explained using a model based on traditional views of carbon remineralization in anoxic sediments and the recently proposed size-reactivity continuum model for DOC cycling in aqueous systems ŽAmon and Benner, 1996.. In the PWSR model presented here, the remineralization of sediment POC leads to the small net production of what is referred to here as pLMW-DOC. This pLMW-DOC is presumed to be much less reactive than other types of DOC ŽHMW-DOC or mLMWDOC., and becomes what is operationally defined as humin Ži.e., soluble humic and fulvic acids.. This then implies that selective preservation of refractory low, and not high, molecular weight DOC leads to the accumulation of DOC with depth in sediment pore water. This model is consistent with the molecular weight data presented here as well other published data on the biogeochemical properties of DOC in sediment pore waters and in the water column. The ability to incorporate concepts taken from the

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work of Amon and Benner Ž1996. size-reactivity continuum model for water column DOC cycling to sediment systems suggests that there may be some similarities in the ways that DOC molecules in these different environments are ‘protected’ from decomposition. Ž5. Differences between estuarine and continental margin sediments in the behavior of DOC in the three size classes studied here may be explained by differences in the relative rates of the processes affecting POC remineralization through DOC intermediates. These differences may be temperature related Žcaused by differences in the activation energies of the individual reactions in the remineralization process.. They may also be related to differences in the types of organic matter undergoing decomposition in the different sediments. Nevertheless, our observations suggest that remineralization processes in continental margin sediments may be more controlled by hydrolytic processes than they are in estuarine sediments, where fermentative or perhaps respiratory processes may perhaps exert a greater overall control on carbon remineralization. This further demonstrates Žas Arnosti et al. Ž1994. have noted. that extracellular hydrolysis of macromolecular organic matter Ži.e., HMW-DOC or sediment POC. is not always the overall rate limiting step in organic matter degradation.

Acknowledgements The majority of this manuscript was written while the senior author was on sabbatical at the Southampton Oceanography Centre ŽUK., and he would like to thank John Thomson and the other staff members at SOC for their hospitality during this visit. We are grateful to Fred Dobbs, Rodney Powell, Juli Homstead, Yu-Ping Chin and an anonymous reviewer for critically reading earlier versions of this manuscript, and to Yu-Ping Chin for his advice and guidance when we began this work. Finally, we thank John Hedges for his assistance as the Associate Editor handling this manuscript. This work was supported by a grant from the National Science Foundation ŽOCE-930212.. Partial support for ship time was also provided by the Office of Naval Research.

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