Provenance of sediments from the Niger Delta, Gulf of Guinea: Evidence from geochemistry

The Gulf of Guinea is surrounded by numerous waterways, where several rivers flow into the ocean, and the Niger River brings large amounts of land-derived debris into the Niger Delta every year. However, geochemical characteristics and sources of sediments in this region are still poorly constrained. Here, we collected two cores (GC09 and GC10) on the shelf and one core (GC05) on the shelf break of the Niger Delta, Gulf of Guinea to assess their provenance using total carbon (TC), total organic carbon (TOC), major elements, and trace elements. The results indicate that sediments on the Niger Delta are classified as iron-rich silty clay and silty clay based on their geochemical compositions. Surface sediments are enriched in quartz and feldspar, evidenced by relatively high levels of SiO₂, Al₂O₃, NaO₂, K₂O and CaO. Trace element ratios and TOC demonstrate organic matters were mainly derived from terrestrial inputs and were deposited under aerobic conditions. Geochemical index data (TOC, REEs, δCe, δEu) reflect that the redox conditions of deposition are more inclined to aerobic environment; the sea level gradually decreased and induced more oxic depositional environment over time. According to the location of the three sampling sites, the proportion of felsic in the provenance of sediments increased towards the mainland while the proportion of mafic decreased. With the increasing of water depth, the proportion of felsic source decreases from ~67% at GC10 site (Water depth is 1418 m) to ~50% at GC05 site (Water depth is 3245 m). In contrast, the proportion of mafic source increase from ~25% at GC05 site to ~50% at GC09 site (Water depth is 1377 m). Using chemical index of alteration (CIA), it is manifested that the sediments show high maturity and have experienced strong chemical weathering. Statistical analyses indicate terrigenous inputs contribute more than half materials for the Niger Delta sediments. The relatively stable depositional environment in this region favors the formation and development of gas hydrates. This study could be used to infer the sediment sources and paleoenvironment changes during sedimentation and also provide reliable information for a better understanding of sedimentary processes in global gas hydrate-related deltas.