Decomposition Of Algal Organic Matter In Clay-enriched Marine Sediments Under Oxic And Anoxic Conditions

To understand the processes and pathways of organic matter decomposition in marine sediments is an important research subject as related to carbon cycling in the ocean. In this study, a series of laboratory incubation experiments were conducted to examine the decomposition of algal organic matter in clay-enriched marine sediments under oxic and anoxic conditions. During the 245 days incubation period, TOC, TO13C and concentration changes of the major algal fatty acid components (14:0, 16:0, 16:1, 18:1 and 20:5) and n-alkanes (C16-C23) were quantified in the samples. The results indicate that the organic matters were degraded more rapidly in oxic than anoxic conditions. Using a simple G model (-dGt / dt =∑kiGi), we calculated the decomposition rate constants for the fatty acids and n-alkanes. The values of fatty acids ranged from 0.011 to 0.069 d-1 and from 0.049 to 0.103 d-1 for the n-alkanes. The present of clay minerals montmorillonite and kaolinite showed rapid and strong adsorption of fatty acids thus affected their decomposition rate constants. In addition to the environmental condition and clay influence, it appears that the structure of the fatty acids and n-alkanes also have significant effects to the decomposition processes of the compounds. Fatty acids with more double bonds and n-alkanes with shorter carbon chain appeared to be decomposed more rapidly than the less double bonds and long chain compounds under both oxic and anoxic conditions.During the decomposition of algal organic matter, the values ofδ13C of TOC decreased due to the rapid decomposition of 12C rich compounds of sugars and protein. The decomposition processes also showed significant influence to the majot fatty acid compounds in the sediments: 14:0 and 16:0 turned depleted, 18:1 turned enriched and 16:0 and 20:5 exhibited no change. The results from this study suggest that when using organic compounds as special biomarkers and theirδ13C values for identifing organic matter sources and biogeochemical processes in sediments, the effects of early diagenesis on theirδ13C values must be examined and taken into account.