Fatty acid biomarkers as indicators of organic matter origin and processes in recent turbidites: The case of the terminal lobe complex of the Congo deep-sea fan

Abstract

The Congo River is connected to its submarine canyon and supplies large quantities of terrestrial organic carbon via powerful turbiditic currents down to the Congo deep-sea fan. We investigated sediment cores from the terminal lobe complex of the Congo deep-sea fan (∼750 km offshore, ∼5000 m water depth), in order to assess the value of fatty acid biomarkers as indicators of organic matter (OM) origin and processes affecting its distribution and preservation in recent turbidites. Sediments from the Congo deep-sea fan are enriched in fatty acids compared to the surrounding abyssal plains and their composition closely resembles that of sediments recovered in the Congo River. Long chain fatty acid (LCFA) biomarkers in conjunction with the branched vs isoprenoid tetraether index (BIT) show that OM mostly originates from soil erosion and continental higher plants. This material has undergone limited reprocessing during transit certainly due to tight interactions with mineral particles and rapid transfer. The presence of phytoplankton biomarkers at the entrance of the terminal lobe area highlights that inputs of fresh OM, albeit limited, can reach the lobe complex. By combining fatty acid profiles and geochemical proxies in a multivariate analysis, we highlight that OM degradation is mostly limited to the oxic layer, and that fine soil-derived particles and the coarser higher plant detritus display distinct depositional dynamics. Finally, LCFA are remarkably well preserved under anoxic conditions at different time scales, in recent turbidites deposited during the last century and those deposited several thousand years ago.