Stable oxygen and carbon isotope composition of benthic foraminifera from the western Mediterranean Sea

The influence of microhabitat, organic matter flux, and metabolism on the stable oxygen and carbon isotope composition of live (Rose Bengal stained) and dead (empty tests) deep-sea benthic foraminifera from the Gulf of Lions (western Mediterranean Sea) have been studied. The total range of observed foraminiferal isotope values exceeds 1.0 per mil for d18O and 2.2 per mil for d13C demonstrating a wide range of coexisting disequilibria relative to d18O of equilibrium calcite (d18OEQ) and d13C of bottom water dissolved inorganic carbon (d13CDIC). The mean d18O values reveal strongest disequilibria for the studied epifaunal to shallow infaunal species (Cibicidoides pachydermus, Uvigerina mediterranea, Uvigerina peregrina) while values approach equilibrium in deep infaunal species (Globobulimina affinis, Globobulimina pseudospinescens). The mean d13C values decrease with increasing average living depths of the different species, thus reflecting a dominant microhabitat (pore water) signal. At the axis of the Lacaze-Duthier Canyon a minimum d13CDIC pore water gradient of approximately -2.1 per mil is assessed for the upper 6 cm of the surface sediment. Although live individuals of U. mediterranea were found in different depth intervals their mean d13C values are consistent with calcification at an average living depth around 1 cm. The deep infaunal occurrence of U. mediterranea specimens suggests association with macrofaunal burrows creating a microenvironment with geochemical characteristics similar to the topmost centimeter. This also explains the excellent agreement between stable isotope signals of live and dead individuals. The ontogenetic enrichment in both d18O and d13C values of U. mediterranea suggests a slow-down of metabolic rates during test growth similar to that previously observed in planktic foraminifera. Enhanced organic carbon fluxes and higher proportion of resuspended terrestrial organic material at the canyon axis are reflected by d13C values of U. mediterranea on average 0.58 per mil lower than those from the open slope. These results demonstrate the general applicability of the d13C signal of this species for the reconstruction of past organic matter fluxes in the Mediterranean Sea. Further studies on live specimens are needed for a more quantitative paleoceanographic approach.