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

Abstract The molecular weight distribution of dissolved organic carbon (DOC) in pore waters from estuarine and continental 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 and showed very slight changes with depth (upper ∼30 cm). The absolute concentration of this low molecular weight DOC (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 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 remineralization. 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.

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