Stable carbon isotope deviations in benthic foraminifera as proxy for organic carbon fluxes in the Mediterranean Sea

We have determined stable carbon isotope ratios of epifaunal and shallow infaunal benthic foraminifera to relate the inferred gradient of pore water δ13CDIC to varying trophic conditions, and to test the potential of developing a transfer function for organic matter flux rates. The data set is based on samples retrieved from a well-defined bathymetric range (400–1500 m water depth) of sub-basins in the western, central and eastern Mediterranean Sea. Regional contrasts in organic matter fluxes and associated δ13CDIC of pore water are recorded by the δ13C difference (Δδ13CUmed-Epi) between the shallow infaunal Uvigerina mediterranea and epifaunal species (Planulina ariminensis, Cibicidoides pachydermus, Cibicides lobatulus). The Δδ13CUmed-Epi values range from −0.46 to −2.13 ‰, with generally higher offsets at more eutrophic sites. Because of ontogenetic shifts in the δ13C signal of U. mediterranea of up to 1.04 ‰, only tests larger than 600 µm were used for the quantitative environmental evaluation. The measured δ13C deviations are related to site-specific differences in microhabitat, depth of the principal redox boundary, and TOC content. The Δδ13CUmed-Epi values reveal a consistent relation to Corg fluxes estimated from satellite-derived surface water primary production in open-marine settings of the Alboran Sea, Mallorca Channel, Strait of Sicily and southern Aegean Sea. In contrast, Δδ13CUmed-Epi values in areas affected by intense resuspension and riverine organic matter sources of the northern to central Aegean Sea and the canyon systems of the Gulf of Lions suggest higher Corg fluxes compared to the values based on recent surface primary production. Considering the regional biases and uncertainties, a first Δδ13CUmed-Epi based transfer function for Corg fluxes could be established for the Mediterranean Sea.