Geochemical evidence for variations in delivery and deposition of sediment in Pleistocene light–dark color cycles under the Benguela Current Upwelling System

Abstract

Distinctive light–dark color cycles in sediment beneath the Benguela Current Upwelling System indicate repetitive alternations in sediment delivery and deposition. Geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid-Pleistocene intervals from ODP Sites 1082 and 1084. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1%. Concentrations of CaCO 3 vary inversely to TOC and Al, which suggests that both carbonate dissolution and terrigenous dilution contribute to the light–dark cycles. Opal concentrations are independent of both TOC and CaCO 3, therefore eliminating diatom production and lateral transport of shelf material as causes of the light–dark cycles. δ 13C org and δ 15N tot values do not vary across light–dark sediment intervals, implying that the extent of relative nutrient utilization did not change. The stable δ 15N tot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the TOC-rich layers. Parallel changes in concentrations of indicator trace elements and TOC imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen. Cu, Ni, and Zn are enriched in the darker sediment as a consequence of greater organic matter delivery. Redox-sensitive metals vary due to loss (Mn and Ba) or enrichment (Mo) under reducing conditions created by TOC oxidation. Organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution, sulfate reduction and the concentration of metals. Thus, export production is considered ultimately responsible for the generation of the color cycles.