Productivity modes in the Mediterranean Sea during Dansgaard–Oeschger (20,000–70,000 yr ago) oscillations

Abstract

The study of planktonic organisms during abrupt climatic variations of the last glacial period (Dansgaard–Oeschger oscillations, D–O) may reveal important insights on climatic, oceanographic and biological interactions. Here we present planktic foraminifera and coccolithophore data collected at the Ocean Drilling Program Site 963 (Sicily Channel), with a mean sampling resolution of respectively 43.5 and 98.9yr, over the interval between 70,000 and 20,000yr ago. The paleoenvironmental reconstruction suggests that three different scenarios can be seen across each D–O cycle: 1. oligotrophic surface water and a deep thermocline for the early Interstadials; 2. a Deep Chlorophyll Maximum and coccolithophore winter/spring blooming in the late Interstadials; 3. reduced productivity together with the shallowing of the nutricline depth during Stadials and Heinrich events. The unique mode of productivity dynamics is corroborated by comparing our paleoecological results with those published from high-resolution cores in the Alboran Sea clearly indicating reduced trophic levels during Stadials and Heinrich events. Finally, we argue that the density contrast between the Atlantic water inflow and subsurface water may have affected productivity dynamics in such a large area. The strong vertical density gradient may have hampered the vertical convection of the water column, producing a negative effect on biological productivity, especially during Stadial phases.