Decomposition of 14C-labeled organic substances in marine sediments’

The depth variation of total organic carbon (TOC), organic matter composition, and porewater composition in marine sediments suggests that different components of the organic matter undergo decomposition at widely different rates. The decomposition of 14C-labeled organic substances was followed in sediment microcosms in the laboratory. The substances used were chosen to simulate a portion of material settling to the sediment-water interface (a marine diatom) or hypothesized components of refractory sediment organic matter (melanoidins and a bacterial polymer). The microcosms were found to be good models of the sediment-water interface in terms of how well they mimicked sediment decomposition rates and processes. The decomposition of the labeled material and the natural sediment TOC were monitored over 1 month: the water overlying the sediment remained oxic, and net consumption of nitrate was small. There was no detectable sulfate reduction. The algae and the bacterial polymer were decomposed on average 9 x faster than the melanoidins and 90 x faster than the natural sediment TOC. The soluble fraction of the algae was decomposed more rapidly than the particulate material. Berner (1980) proposed a model for the multiple “pools” of organic matter in sediments being decomposed at different rates. This “multiple-G” model is based in part on depth profiles of total organic carbon (TOC) and porewater-dissolved sulfate in anoxic marine sediments, which show a marked decrease in the rate of sulfate reduction with depth even though a substantial fraction of the surface TOC concentration remains. Others (e.g. Jorgensen 1978; Murray et al. 1978; Henrichs and Farrington 1984; Martens and Klump 1984) have found that the multiple-G model, with firstorder decomposition kinetics for a labile organic fraction, fits porewater and sediment data from a variety of sediments. Additional evidence for the multiple-G model has been presented by Westrich and Berner (1984), who found that plankton subjected to a previous interval of oxic decomposition decomposed more slowly when added to anoxic sediments than did fresh plankton. In several studies of plankton decomposition (Garber 1984 and refcrenccs cited therein) initially rapid remineralization rates ’ This research was supported by NSF grants OCE 8214537 and OCE 8415557. Additional support was provided by the donors of the Petroleum Research fund, administered by the American Chemical Society. Contribution no. 582 from the Institute of Marine Science. decreased 10 x or more after l-2 weeks, indicating that the residual organic matter was relatively refractory. Very few data are available, however, on the precise nature of the labile and refractory components of sediment organic matter, and this hampers our understanding of sediment decomposition processes. By analogy to complex polymeric substances in soils, this uncharacterized fraction is often referred to as “humic material,” although humic substances in marine sediment differ markedly in composition from those in soils (Nissenbaum 1974; Ertel and Hedges 1983). There are several hypotheses concerning the nature of sediment organic matter. First, different classes of biochemical compounds have different decomposition rates based on their structures and susceptibility to biological attack. For example, Hanson and Tenore (198 1) found the rate of decomposition of a marsh grass (Spartina) to be lower than those of microand macroalgae and attributed this result to the greater content of refractory polycellulose and lignin in the Spartina and the greater soluble organic content of the algae. Second, melanoidins (disordered polycondensates of sugars and amino acids) have been suggested as a component of refractory marine sediment humic materials (Nissenbaum et al. 1972; Krom and Sholkovitz 1977; Ertel and Hedges 1983). Third, microbial cell walls