Manganese incorporation in living (stained) benthic foraminiferal shells: a bathymetric and in-sediment study in the Gulf of Lions (NW Mediterranean)
2018
Abstract. Manganese geochemistry in deep-sea sediments is known to vary
greatly over the first few centimeters, which overlaps the in-sediment
depth habitats of several benthic foraminiferal species. Here we investigated
manganese incorporation in benthic foraminiferal shell carbonate across a
six-station depth transect in the Gulf of Lions, NW Mediterranean, to unravel
the impacts of foraminiferal ecology and Mn pore water geochemistry. Over
this transect water 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 (rose bengal
stained) benthic foraminiferal specimens of Hoeglundina elegans ,
Melonis barleeanus , Uvigerina mediterranea , and
Uvigerina peregrina were measured using laser ablation inductively
coupled mass spectrometry (laser ablation ICP-MS). Pore water manganese
concentrations show a decrease from shallow to deeper waters, which
corresponds to a generally decreasing organic-matter flux with water depth.
Differences in organic-matter loading at the sediment–water interface affects
oxygen penetration depth into the sediment and hence Mn pore water profiles.
Mn ∕ Ca values for the investigated foraminiferal species reflect pore
water geochemistry and species-specific microhabitat in the sediment. The
observed degree of variability within a single species is in line with known
ranges in depth habitat and gradients in redox conditions. Both the Mn ∕ Ca
ratio and interspecific variability hence reflect past Mn cycling and
related early diagenetic processes within the sediment, making this a
potential tool for bottom-water oxygenation and organic-matter fluxes.
Dynamics of both in-sediment foraminiferal depth habitats and Mn cycling,
however, limit the application of such a proxy to settings with relatively
stable environmental conditions.
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