Millimeter-scale profiling of abyssal marine sediments: role of bioturbation in early sterol diagenesis

Near surface sediments from an abyssal northeast Atlantic site were sectioned using a newly designed Precision Core Extruder (PCE). Downcore profiles of excess 210Pb and sterols showed pronounced millimeter-scale variability reflecting specific degradation and bioturbation processes. Excess 210Pb and sterol concentrations decreased sharply in the uppermost 5 mm, and pronounced subsurface maxima were present. Stonol/stenol ratios, and the relative abundance of cholesterol, increased below 10 mm, indicating biodegradation involving Δ5 double bond hydrogenation, C-24 dealkylation and/or de novo synthesis. Sterol profiles indicate that early diagenesis of labile compounds is dominated by processes operating near the sediment/water interface, and that the penetration of labile material into oxic sediments is largely controlled by advective processes, associated with infaunal feeding rather than diffusive-like mixing.