Depositional History of the Namibian Mud Belt Reveals Late Holocene Weakening of the Benguela Coastal Current

Abstract

We investigate late Holocene changes in the northern Benguela Upwelling System using a multi‐proxy approach on shallow shelf deposits from the Namibian mud belt. Three sediment cores characterized by organic carbon‐rich, anoxic, and partly gas‐bearing sediments were retrieved along a sampling transect at 23°S in August 2019. A sub‐bottom profile recorded in a sampling campaign in February 2022 enabled stratigraphic correlation of the cores. As a novelty in this area to date, variations in terrigenous input patterns are reconstructed in one gravity core using major and trace element data, including rare earth elements, in a statistical provenance modeling approach. Results indicate a progressive decline in input of Orange River‐derived material over the past 3800 years, attributed to a weakening of the Benguela Coastal Current (BCC). This reduction in northward directed transport energy is reflected in increasing lamination frequency observed in Computed Tomography‐scan imagery and declining Zr/Al ratios toward the present. Sulfidic events observed along the Namibian coastline are interpreted as a direct consequence of the BCC weakening, since decreasing depositional energy facilitated organic matter deposition and degradation, the formation of a shallow sulfate–methane transition zone (SMTZ) and the accumulation of free sulfide and methane gas in the central mud belt area beginning around 1450 years BP. The establishment of sulfidic redox conditions in gas‐bearing sediments is recorded in sedimentary Mo/U ratios exceeding modern seawater Mo/U. This study highlights the strong coupling of oceanographic changes, sedimentation processes, and biogeochemical conditions in the Benguela Upwelling System.