REACTIVITY OF RECENTLY DEPOSITED ORGANIC-MATTER - DEGRADATION OF LIPID COMPOUNDS NEAR THE SEDIMENT-WATER INTERFACE

The usefulness of biomarker compounds buried in marine sediments depen ds upon a quantitative understanding of the effects of early diagenesi s on their distribution. To address this, a new experimental approach was utilized to determine rates of degradation in a coastal sediment. Rates of degradation for solvent-extractable lipid components were qua ntified in four sediment horizons composed of newly accumulated organi c matter (31-144 days since deposition). Sediment accumulation rate da ta derived from changes in the inventory of Be-7 (t(1/2) = 53.3 days) were combined with concentration data for lipid biomarker compounds, e nabling us to evaluate the reactivity of organic matter in the upper 8 cm of the rapidly accumulating sediments of Cape Lookout Eight, North Carolina, USA (CLB). Net rates of loss and rate constants were calcul ated for individual compounds belonging to three classes of lipids: fa tty acids, sterols, and n-alkanes. Individual components showed a rang e in reactivity, in some cases (fatty acids), attributable to differen ces in their biological sources. Rates and rate constants were consist ently highest in the surficial sediments (0-2.5 cm), indicating that t he reactivity of a given molecule(s) decreases over time, and beginnin g soon after deposition. Comparison with apparent rate constants (k') calculated over longer timescales (one and ten years) shows that stead y-state diagenetic models underestimate rates of degradation at or nea r the sediment-water interface by an order of magnitude.