Mechanisms of Organic Matter Preservation in Continental Margin Sediments from the Gulf of Maine

Burial of organic carbon on marine shelves is an important process in the long-term sequestration of carbon from the marine carbon cycle. As organic matter in marine environments is consumed and resuspended by microorganisms, only a small portion is preserved. This organic material is preferentially associated with the fine-grained fraction of continental margin sediments. The strong correlation between organic matter content and fine-grained particles has led to speculation that organic matter may be adsorbed onto mineral surface and physically protected by incorporation into aggregates. Two methods were used to determine the role of aggregates in the physical preservation of organic carbon in marine sediments from the Gulf of Maine. First, carbon storage potential was measured through a range of particle and aggregate sizes. Silt-size (2-53 μm) and clay-size (\u3c2 \u3eμm) fractions were isolated using settling columns, and separated by density into particles (p\u3e2.3 g cm-1) and aggregates (p-1) using heavy liquid floatation. Systematic differences between preserved carbon content in silt and clay particle and aggregate fractions were determined by measuring organic carbon and specific surface area via a carbon analyzer and the nitrogen gas adsorption method. Second, differences in physical stability between silt and clay aggregates were measured. Relative size distributions of silt and clay aggregates were measured, via flow cytometry and x-ray disc centrifugation, before and after exposure to stepwise increasing levels of sonic energy. The highest levels of organic carbon were associated with clay-sized aggregates, but aggregates in the silt-size fraction around 3 cm below the sediment water interface appear to be the most physically stable. This suggests that although organic carbon is preferentially associated with aggregates, it may not be the sole mechanism controlling aggregate physical stability. Increased aggregate stability could be the result of physical process such as compaction, dewatering, lithification, and/or the quality of organic carbon beginning to occur around 3 cm of depth