Manganese incorporation in living (stained) benthic foraminiferal shells: a bathymetric and in-sediment study in the Gulf of Lions (NW Mediterranean)

Manganese geochemistry in deep-sea sedimentsis known to vary greatly over the first few centimeters,which overlaps the in-sediment depth habitats of severalbenthic foraminiferal species. Here we investigated manganeseincorporation in benthic foraminiferal shell carbonateacross a six-station depth transect in the Gulf of Lions,NW Mediterranean, to unravel the impacts of foraminiferalecology and Mn pore water geochemistry. Over this transectwater depth increases from 350 to 1987 m, while temperature(∼ 13°C) and salinity (∼38:5) remained relatively constant.Manganese concentrations in the tests of living (rosebengal stained) benthic foraminiferal specimens of Hoeglundina elegans, Melonis barleeanus, Uvigerina mediterranea, and Uvigerina peregrine were measured using laser ablation inductively coupled mass spectrometry (laser ablation ICPMS).Pore water manganese concentrations show a decreasefrom shallow to deeper waters, which corresponds to a generally decreasing organic-matter flux with water depth. Differencesin organic-matter loading at the sediment–water interfaceaffects oxygen penetration depth into the sediment andhence Mn pore water profiles. Mn= Ca values for the investigatedforaminiferal species reflect pore water geochemistryand species-specific microhabitat in the sediment. The observeddegree of variability within a single species is in linewith known ranges in depth habitat and gradients in redoxconditions. Both the Mn= Ca ratio and interspecific variabilityhence reflect past Mn cycling and related early diageneticprocesses within the sediment, making this a potential toolfor bottom-water oxygenation and organic-matter fluxes. Dynamicsof both in-sediment foraminiferal depth habitats andMn cycling, however, limit the application of such a proxy tosettings with relatively stable environmental conditions