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

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Abstract. We have determined stable carbon isotope ratios of epifaunal and shallow infaunal benthic foraminifera in the Mediterranean Sea to relate the inferred gradient of pore water δ13CDIC to varying trophic conditions. This is a prerequisite for developing this difference into a potential 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). Within epifaunal taxa, the highest δ13C values are recorded for P. ariminensis, providing the best indicator for bottom water δ13CDIC. In contrast, C. pachydermus reveals minor pore water effects at the more eutrophic sites. Because of ontogenetic trends in the δ13C signal of U. mediterranea of up to 1.04 ‰, only tests larger than 600 µm were used for the development of the transfer function. The recorded differences in the δ13C values of U. mediterranea and epifaunal taxa (Δδ13CUmed-Epi) range from −0.46 to −2.13 ‰, with generally higher offsets at more eutrophic sites. The measured δ13C differences are related to site-specific differences in microhabitat, depth of the principal sedimentary redox boundary, and TOC content of the ambient sediment. 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 Lion suggest higher Corg fluxes compared to the values based on recent primary production. Taking regional biases and uncertainties into account, we establish a first Δδ13CUmed-Epi-based transfer function for Corg fluxes for the Mediterranean Sea.