Evidence for rapid trace element alteration of planktic foraminiferal shells from the Panama Basin: Manganese adsorption during vertical transport

Abstract

Abstract The trace elemental composition of planktic foraminifera shells serves as a proxy for understanding the physical environment in which foraminifera lived, but may also reflect chemical alterations that occur after death. Understanding how foraminiferal geochemistry varies across time, species, and after death is therefore essential for deconvolving ecological from diagenetic information and decoding the environmental signals contained within foraminiferal shells. We analyzed the trace elemental composition (Mg/Ca, Sr/Ca, Ba/Ca, Mn/Ca, and U/Ca) of four species of planktic foraminifera (Globigerina ruber, Neogloboquadrina dutertrei, Globorotalia menardii, and Globorotaloides hexagonus) from moored sediment traps (890, 2590, and 3560 m deep) in the Panama Basin collected over one calendar year (1979–1980). Shells recovered from the shallowest trap demonstrate moderate variability in trace elemental composition, both throughout the time series and between species. Most elemental ratios are not significantly influenced by depth, with the exception of Mn/Ca. Ratios of Mn/Ca are substantially higher in the 3560 m trap versus the 890 m trap, with increases with depth greater in the thin walled species G. ruber and G. hexagonus. Elevated Mn/Ca ratios are frequently noted in fossil foraminifera shells as the result of diagenesis within the sediment. Our results demonstrate the potential for Mn-enrichment of foraminiferal calcite, via Mn adsorption onto the surface of foraminiferal shells, within the water column during sinking and prior to deposition in the sediment.