Paleoenvironmental changes from stable isotopes in planktonic foraminifera from Eastern Mediterranean sapropels

Abstract

Parallel stable oxygen isotope analyses of six planktonic foraminiferal species from an Eastern Mediterranean deep-sea core present clear evidence for abnormal hydrographic regimes during deposition of sapropels S6 and S7. Samples from S7 are characterized by a strong depletion of 18O in epipelagic species, while meso- and bathypelagic forms are much less depleted. This indicates a shallow pycnocline. During deposition of S6 all species show a tendency towards lower values, pointing to a far more extended pycnocline. Density stratification of the watercolumn is suggested by the isotopic differences between the species Neogloboquadrina dutertrei, Globigerina bulloides and Globorotalia scitula. The latter represents the deepest element and shows the highest δ 18O values. Carbon isotopes show two species groups, each containing an epi-, meso-, and bathypelagic species. Under normal marine conditions group I ( Globigerinoides ruber, Neogloboquadrina dutertrei and Globorotalia truncatulinoides) has relatively high δ 13C values while species of group II ( Globigerina bulloides, Globorotalia scitula, and Globigerina quinqueloba) show relatively low δ 13C values, indicating a stronger “vital effect”. During sapropel deposition 12C isotopes are enriched, which we interpreted as the result of increased biological activity due to “low-oxygen” (stress) conditions. G. scitula shows different eco-phenotypes during times of sapropel deposition and the same may be true for the other members of group II. Failure to adapt to adverse environmental conditions may be the cause for the absence of the two group I members ( Globigerinoides ruber and Globorotalia truncatulinoides) in sapropel S6.