These questions are being investigated through a multi-prong approach: analyses of biomarker distributions and stable carbon isotope compositions in age-dated sediment cores; degradation experiments using radiocarbon and stable-carbon labeled tracers in simulated environments; and degradation experiments using natural organic materials.

Studies of cores provide a basis for suggesting an order of relative reactivities of specific lipid biomarkers under the prevailing conditions in the sediment. For example, certain polyunsaturated fatty acids and highly branched isoprenoid alkenes have very short "lifetimes" in anoxic surface sediments of the Black Sea. Lipid half-lives in anoxic sediments of Long Island Sound were considerably longer than in sediments that experience oscillating oxic-anoxic conditions. Experiments with specific radiolabelled lipids (e.g., palmitic and oleic acid) in incubation chambers where bottom water was controlled to be oxic or anoxic showed clearly the importance of aerobic degradation on the fate of fatty acids.

Two important factors controlling carbon degradation are the quality of the organic input (related to its source) and the microbial system into which the organic matter is deposited (e.g., oxic, iron reducing, sulfate reducing, methanogenic, etc.). To determine how extensively carbon from varying sources could be remineralized under different microbial regimes, a degradation experiment was conducted in which either marine phytoplankton and estuarine seagrasses were mixed with anoxic sediment under sulfate-reducing or methanogenic conditions. Although organic matter is rapidly degraded under all conditions, seagrass carbon was much more resistant to degradation than phytoplankton carbon, and it does not matter what microbial regime was active. Consequently, when carbon is preserved in coastal sediment, it is more likely to be a result of the source of the organic matter rather than the type of microbial consortia breaking down the organic matter.

Selected References:

M. -Y. Sun and S.G. Wakeham. (1994) Molecular evidence for degradation and preservation of organic matter in the anoxic Black Sea basin. Geochim. Cosmochim. Acta 58: 3395-3406.

M.-Y. Sun, S.G. Wakeham and C. Lee. (1997) Rates and mechanisms of fatty acid degradation in oxic and anoxic coastal marine sediments of Long Island Sound, New York, USA. Geochim. Cosmochim. Acta 61: 341-355.

S.G. Wakeham, C. Lee, J.I. Hedges, P.J. Hernes and M.L. Peterson. (1997) Molecular indicators of diagenetic status in marine organic matter. Geochim. Cosmochim. Acta. 61: 5363-5369.

T.K. Pease, E.S. Van Vleet, J.S. Barre, and H.D. Dickins. (1998) Simulated degradation of glyceryl ethers by hydrous and flash pyrolysis. Org. Geochem. 29: 979-988.

T.K. Pease, S.G. Wakeham and C.S. Martens. (1998) Laboratory degradation of phytoplankton and seagrass in a closed vessel incubation I. Biogeochemical effects on lipid degradation. 1998 Ocean Sciences Meeting, American Geophysical Union/American Association of Limnology and Oceanography, San Diego, CA. EOS, Transactions. v.79, p.OS43.

M.-Y, Sun, S.G. Wakeham, R.C. Aller and C. Lee. (1998) Impact of seasonal hypoxia on diagenesis of phytol and its derivatives in Long Island Sound. Mar. Chem. 62: 157-173.

M.-Y, Sun, S.G. Wakeham. (1998) A study of oxic.anoxic effects on degradation of sterols at the simulated sediment-water interface of coastal sediments. Org. Geochem. 28: 773-784.

T.K. Pease, C.A. Hee, M.J. Alperin, and C.S. Martens. (1999) Biogeochemical effects on phytoplankton and seagrass carbon remineralization: A laboratory degradation experiment. 1999 Aquatic Sciences Meeting, American Society of Limnology and Oceanography, Santa Fe, NM.