Foraminiferal response to an active methane seep environment: A case study from the Adriatic Sea

It has been analysed the taxonomic structure and isotopic composition of the cytoplasm and tests of living (Rose Bengal stained, protoplasm-full) and dead (empty tests) benthic foraminifera (> 63 μm) in surface sediments from sites with and without methane seepage at water depths of about 25 m water in the northern Adriatic Sea, and compared these data to isotope analyses of carbonate rocks at the same locations. Foraminiferal density is higher in seep samples than in control ones, despite the low oxygen and high sulphide contents in sediments. The δ13C values in foraminiferal cytoplasm at the seep sites are lower than those in cytoplasm of the same species in control samples, suggesting that Beggiatoa may be a food source for the foraminifera, and explaining the higher foraminiferal density at the seep site. In contrast, the δ13C values of tests of living and dead benthic foraminifera at seep sites were not significantly different from those in tests of the same species collected at the control sites. Therefore, the carbon isotope values of foraminiferal tests in this area do not reflect the ones of the methane seepage, although the authigenic carbonate in calcareous sandstones of the methane seepage area is significantly negative and indicates the contribution of methane seepage. The similar carbon isotope values measured in the foraminiferal tests from shallow settings of the north-eastern Adriatic Sea seem therefore to be independent of methane seepages.