MOLECULAR-WEIGHT DISTRIBUTION OF DISSOLVED ORGANIC-CARBON IN MARINE SEDIMENT PORE WATERS

The molecular weight distribution of dissolved organic carbon (DOC) in pore waters from estuarine and continental margin sediments was exami ned using ultrafiltration techniques. The majority of this pore water DOC (similar to 60-90%) had a molecular weight less than 3 kDa. This p ercentage appeared to vary systematically among the different sediment s studied and showed very slight changes with depth (upper similar to 30 cm). The absolute concentration of this low molecular weight DOC (L MW-DOC) increased, along with total DOC, with depth in the sediments. LMW-DOC therefore represents the vast majority of the DOC that accumul ated with depth in these sediment pore waters. These results have been examined in the context of a model which assumes that remineralizatio n processes exert the primacy influence on the molecular weight distri bution of DOC in the upper portions of the sediments. This model, in c onjunction, with other recent studies of DOC in sediment pore waters a nd in the water column, suggests that there was preferential accumulat ion of refractory LMW-DOC in sediment pore waters. Abiotic condensatio n 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 cont inental margin sediments may be controlled more by hydrolytic processe s than it is in estuarine sediments, where fermentative or perhaps res piratory processes may exert a greater overall control on carbon remin eralization. These observations provide further evidence that the extr acellular hydrolysis of macromolecular (i.e., high molecular weight) o rganic matter may not always be the rate limiting step in organic matt er degradation. (C) 1998 Elsevier Science B.V. All rights reserved.