Mesopelagic fish responses to Pleistocene climatic variability in the Eastern Mediterranean and implications for the biological pump

Mesopelagic fishes play a crucial role in the global carbon cycle through their diel vertical migration (DVM), but the impacts of neither natural nor anthropogenic climate change on DVM patterns are currently known. Studying the geological past can elucidate changes in DVM patterns under swelling climate pressure and allow estimating the impacts of the current climate crisis. We present a multi-proxy, ecosystem-level assessment of paleoenvironmental changes in the Eastern Mediterranean during the Middle Pleistocene (marine isotope stages MIS 23–18; 923–756 ka B.P.) and use the carbon and oxygen isotopic composition of fossil fish otoliths to assess the impacts of these changes on DVM and their possible implications on the biological pump. Temperature was the primary driver of ecosystem change during MIS 21 interglacial, whereas productivity became a dominant factor in MIS 19 interglacial. Responses of organisms throughout the water column varied. Our results indicate increased productivity across trophic levels during MIS 19, which affected foraminiferal biomasses, but did not inhibit fish DVM. In contrast, the early MIS 21 warming led to a reduction in DVM by mesopelagic fishes and consequently a drop in biological pump efficiency.