Reconstructing past planktic foraminiferal habitats using stable isotope data: a case history for Mediterranean sapropel S5

Abstract

A high-resolution stable O and C isotope study is undertaken on all planktic foraminiferal species that are reasonably continuous through an Eemian sapropel S5 from the western side of the eastern Mediterranean. The data are considered within a context of high-resolution isotope records for two further S5 sapropels from the central and easternmost sectors of the basin, alkenone-based sea surface temperature records for all three sapropels, and planktic foraminiferal abundance records for the same sample sets through all three sapropels. Results are compared with similar data for Holocene sapropel S1. The adopted approach allows distinction between species that are most suitable to assess overall changes in the climatic/hydrographic state of the basin, including depth-related differentiations and the main seasonal developments, and species that are most affected by variable biological controls or local/regional and transient physico–chemical forcings. It is found that a-priori assumptions about certain species’ palaeohabitats, based on modern habitat observations, may become biased when non-analogue conditions develop. In the case of Mediterranean sapropel S5, these consisted of enhanced freshwater dilution, elevated productivity, shoaling of the pycnocline between intermediate and surface waters, and stagnation of the subsurface circulation. Under these conditions, some species are found to ‘shift’ into habitat settings that differ considerably from those occupied today. The present multiple-species approach can identify such ‘anomalous responses’, and thus offers a sound background for further shell-chemistry investigations and quantitative interpretation of the isotopic profiles. We capitalise on the latter potential, and offer the first quantitative estimates of monsoon flooding into the Mediterranean during the deposition of Eemian sapropel S5.