Post-depositional stability of long-chain alkenones under contrasting redox conditions

Abstract

PRYMNESIOPHYTE algae, which include the coccolithophorid species Emiliania huxleyi¹, are the recognized biological source of a series of long-chain (C₃₇, c₃₈, ₃₉, unsaturated methyl and ethyl ketones² widely observed in marine sediments³. Studies of E. huxleyi in culture have demonstrated that these biomarkers are attractive geochemical tools for palaeoceanographic study. No-tably, unsaturation patterns within the alkenone series change regularly with growth temperature^3–5 and the total alkenone abundance in the living plant cell is relatively constant, accounting for 5–10% of the total cellular organic carbon⁵. If these com-pounds are relatively well-preserved in sediments, profiles of alkenone unsaturation patterns and total alkenone concentration with depth in dated deep-sea cores^6–8 provide a temporal record of sea surface temperatures and the productivity of an important group of marine phytoplankton^3–5. Here we analyse the long-chain alkenone composition of sediment samples from above and below an oxidation front in an ungraded turbidite layer⁹ deposited 140 ±12 kyr BP in the Madeira Abyssal Plain¹⁰, to evaluate the post-depositional stability of these biomarkers under contrasting redox conditions. The results demonstrate that >85% of the total amount of these compounds is degraded over ∼8 kyr as a consequence of diffusion-controlled oxidation¹⁰. Remarkably, such extensive degradative loss has little effect on the unsaturation pattern of the residual biomarker series. Thus, we find that long-chain alkenones provide reliable indicators of sea surface temperature in the ocean. The total abundance of these biomarkers in sediments, however, is controlled not only by prymnesiophyte productivity, but also by their degree of exposure to oxidative degradation in the sedimentary process.