Reactivity of recently deposited organic matter: Degradation of lipid compounds near the sediment-water interface

Abstract

The usefulness of biomarker compounds buried in marine sediments depends upon a quantita tive understanding of the effects of early diagenesis 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 quantified in four sediment horizons composed of newly accumulated organic matter (31–144 days since deposition). Sediment accumulation rate data derived from changes in the inventory of Be-7 ( t 1/2 = 53.3 days) were combined with concentration data for lipid biomarker compounds, enabling us to evaluate the reactivity of organic matter in the upper 8 cm of the rapidly accumulating sediments of Cape Lookout Bight, North Carolina, USA (CLB). Net rates of loss and rate constants were calculated for individual compounds belonging to three classes of lipids: fatty acids, sterols, and n-alkanes. Individual components showed a range in reactivity, in some cases (fatty acids), attributable to differences in their biological sources. Rates and rate constants were consistently highest in the surficial sediments (0–2.5 cm), indicating that the reactivity of a given molecule(s) decreases over time, and beginning soon after deposition. Comparison with apparent rate constants ( k′) calculated over longer timescales (one and ten years) shows that steady-state diagenetic models underestimate rates of degradation at or near the sediment-water interface by an order of magnitude.