Lipid biomarker record in surface sediments at three sites of contrasting productivity in the tropical North Eastern Atlantic

摘要

Selected lipid biomarkers were analyzed in modern aerobic surface sediments from the tropical NE Atlantic off the Mauritanian coast, in the frame of the JGOFS EUMELI program. This paper explores how sedimentary molecular proxies record productivity and vascular plant inputs. Dry weight-normalized concentrations and TOC-normalized concentrations of biomarkers poorly matched the gradient of higher-plant inputs and of primary productivity. In contrast, mass accumulation rates of long-chain n-alkanols and n-fatty acids (80-710 and 210-1750 μg m~(-2) yr~(-1), respectively) showed good agreement with dust inputs transported between 15 and 24°N by NE trade winds, whereas long-chain n-alkanes showed a distinct pattern. At the coastal site, n-alkanols and n-fatty acids predominated over long-chain n-alkanes. Cross-shelf changes in proportions of n-fatty acids and n-alkanols relative to n-alkanes point to an increased degradation of terrigeneous waxes when going offshore. The cross-shelf C/N ratio poorly registered vascular plant inputs, most probably because denitrification influenced C/N values at the eutrophic site.Mass accumulation rates of phytoplanktonic biomarkers declined from the eutrophic to the oligotrophic site, reflecting the primary productivity variation. Mass accumulation rates of highly branched isoprenoid hydrocarbons, C37 n-alkenones, n-alkyl diols and dinosterol varied from 3 to 410, 9 to 1600, 12 to 360 and 7 to 320 μg m~(-2)yr~(-1), respectively. They target the productivity of Haslea-type diatoms, coccolithophorids, eustigmatophytes and dinoflagellates. While the results encourage the development of molecular proxies of palaeoproductivity and of palaeophytocommunities, progress is still needed to deconvolute the impact of degradation on mass accumulation rates and to move towards quantitative calibrations.