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

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 δ¹³C_DIC 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 δ¹³C_DIC of pore water are recorded by the δ¹³C difference (Δδ¹³C_Umed-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 δ¹³C_DIC. 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 (Δδ¹³C_Umed-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 Δδ¹³C_Umed-Epi values reveal a consistent relation to C_org 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, Δδ¹³C_Umed-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 C_org fluxes compared to the values based on recent primary production. Taking regional biases and uncertainties into account, we establish a first Δδ¹³C_Umed-Epi-based transfer function for C_org fluxes for the Mediterranean Sea.