Pelagic ecosystem responses to changes in seawater conditions during the Middle Pleistocene Transition in the Eastern Mediterranean

We present a multiproxy, ecosystem-level assessment of paleoenvironmental change and its impacts on marine organisms in the Eastern Mediterranean during the Middle Pleistocene Transition, between 923 and 756 kyr B.P. (marine isotope stages MIS 23–18). This study combines analyses of organic biomarkers; organic matter content; carbon and oxygen stable isotopes on bulk sediment, surface-dwelling, deep-dwelling planktonic and benthic foraminifera, ostracods and fish otoliths; as well as foraminiferal, ostracod and sponge abundance estimates, with a statistical assessment of paleoenvironmental regime shifts and estimation of fish distribution depths in the past. Our results show that temperature and productivity played the most important role in driving ecosystem changes in the study area at different times: temperature was the primary driver during MIS 21 interglacial, whereas productivity became a dominant factor in MIS 19 interglacial. In addition, the responses of organisms throughout the water column varied. Both interglacials yielded relatively higher plankton and benthos biomasses. However, for fishes, the responses differed. The abrupt global warming that occurred in early MIS 21, which was also captured by our record, probably led to a reduction in diel vertical migration by mesopelagic fishes and consequently to the efficiency of the biological carbon pump. In contrast, increased productivity across trophic levels is attested for MIS 19 that subsequently dropped in MIS 18, affecting foraminifera, ostracod and sponge biomasses, but not inhibiting fish diel vertical migration. Therefore, we conclude that carbon sequestration during MIS 19 was likely enhanced.