Impact of climate and sea level changes on the ventilation of intermediate water and benthic foraminifer assemblages in the Gulf of Lions, off South France, during MIS 6 and 7

Benthic and planktic foraminifer assemblages together with oxygen stable isotope were studied in the PRGL1 borehole (Promess1), drilled on the upper slope of the Gulf of Lions, with a view to reconstruct the impact of climate changes on surface and bottom waters between MIS7 and MIS6. Foraminifer species were grouped by R-mode principal components analysis. We obtained 3 factors in benthic assemblages that were related to eutrophic, mesotrophic and oxygenated environments. In the planktic foraminifer assemblages four factors were considered that explained the changes of sea surface temperature, productivity and turbulence in the water column. Variations in the terrestrial organic carbon supply associated with eustatic sea level changes mainly controlled oxygen consumption and the benthic communities. During MIS7, the benthic communities were mainly mesotrophic owing to the low supply of terrestrial organic carbon because the coastline was distant from the slope. However, during MIS6 the sea level drop caused a rapid progradation of the River Rhone prodelta and dramatically increased the supply of terrestrial organic carbon, resulting in a pronounced increase in eutrophic foraminifers. Apart from the effect of the river discharge, which contributed to the reduced oxygen content, we recognized periods of intense bottom ventilation during MIS 6 and, to a lesser extent, in the substadial stages of MIS 7. The ventilation events are well matched by higher turbulence and convection in the water column during cold episodes associated with an intensification of the northwesterlies. In contrast, during the short, millennial scale warming events of MIS6 the stratification of the water column rapidly reduced bottom ventilation. These results are consistent with those reported in previous studies that point to enhanced dense-water formation in the Gulf of Lions during cold stages.

Research highlights
► We used benthic and planktic foraminifer to assess past changes in the Gulf of Lions.
► The changes of sea level control the supply of organic matter from the Rhône River.
► The intensity of northwesterly winds controls the oxygenation on seafloor sediments.