Modern sedimentation and geochemical imprints in sediments from the NW Madagascar margin

Abstract

The NW Madagascar continental margin receives high loads of terrigenous particulate organic matter during the wet season and especially linked to extreme events, originating from two major rivers, the Betsiboka and the Mahavavy Rivers. This particulate matter contains a high content of iron minerals from the weathering of red ferruginous/ferralitic soils of Madagascar. The presence of pockmarks, i.e. gas or fluid expulsion features on the continental slope, testifies to past/present methane migration through the sedimentary column, associated with early diagenetic processes. This study globally aims at deciphering the interactions between episodic sedimentation and geochemical processes influenced by fluids upward migration, using a sediment trap mooring and interface sediment cores at two sites on the continental slope. The present-day sedimentation along this margin undergoes two patterns. During the wet season, high continental fluxes generally result in an increase in particle fluxes on the slope. Longshore currents may deflect river plumes alongslope resulting in some periods of low particle flux on the slope during the wet season. During the dry season, the particles collected in the water column are probably originated from sediment remobilization from the shelf and slope. The observed progradation of the Mahavavy River delta and prodelta between 1984 and 2016 argues for an increase in temporary connections of the river with the canyon head during extreme events, inducing pulsed sedimentation offshore. This pulsed sedimentation could be responsible for enhancing pyritization on surface sediments, due to higher inputs of terrigenous organic matter and iron oxides. Finally, methane upward migration also influences the pyritization process through anaerobic oxidation of methane using sulfate as an electron acceptor and methanotrophs associated with this reaction significantly impact the δ13C of organic carbon towards more negative values.