Mediterranean sea surface temperatures and planktonic foraminifera palaeoecology during short-term climate oscillations of the Late Pleistocene

Abstract

The Mediterranean is a semi-enclosed sea, ideally located for recording and amplifying global climate signals. Heinrich Stadial 1 (HS1), the Bølling-Allerød (BA) and Younger Dryas (YD) are periods of extreme climatic change that occurred during the last deglacial. Planktonic foraminifera were utilised to address the extent the Mediterranean Sea responded to these events, and how they impacted sea surface temperatures (SSTs), hydrology and distribution of planktonic foraminifera on a basin-wide scale. This was further expanded upon in the Gulf of Lion, in order to determine the palaeoenvironmental impacts and main drivers in this region during the deglacial.

A database of planktonic foraminiferal counts from 67 cores located across the Mediterranean Sea were compiled. SSTs were reconstructed using Artificial Neural Networks (ANN) and mapped for each chronozone, along with key planktonic foraminifera and palaeoenvironmental proxies. Mean annual SSTs ranged from 13.57°C and 14.19°C during HS1 and the YD, both cooler than the Last Glacial Maximum (LGM). SSTs during the BA increased to 15.03°C. The western basin, Adriatic and Aegean Seas remained cool and strongly eutrophic, dominated by Neogloboquadrina species, G. bulloides, T. quinqueloba and G. inflata since the BA. The eastern basin was more productive than today, with a mix of eutrophic species and G. ruber plexus, though it became increasingly seasonal, oligotrophic and stratified as SSTs warmed during the BA and YD. These reconstructions highlight the complexity of these chronozones across the Mediterranean Sea. They also illustrated how global drivers such as the southerly position of the polar front, North Atlantic atmospheric circulation, precipitation and wind strength, sea level, summer insolation and position of the ITCZ governed these signals. River discharge and meltwater pulses were regionally important along northern margins.

Analysis of the high-resolution Gulf of Lion core M40/4 82-2SL focused the late HS1 to mid Holocene (~15.5 to 7.4 kyr). SSTs were predicted using ANN, and a combination of faunal abundance, multivariate statistics (PCA) and palaeoenvironmental proxies were utilised to reconstruct the palaeoenvironment. Predicted SSTs showed strong correlation with Greenland δ18O ice cores, especially during the BA (15.19°C). SSTs during the late HS1 (10.18°C) and YD (8.3°C) were cooler than the LGM. The assemblage was strongly eutrophic throughout, with a more diverse, seasonal assemblage restricted to the early Bølling and Holocene. Holocene SSTs were cooler than expected (15.06 to 15.76°C), as the early Holocene was overprinted by a strong cyclical river/meltwater signal. This freshening of surface waters impacted the depth of the pycnocline, which had significant impacts on N. incompta. Variations in wind-strength, precipitation, river discharge and Alpine meltwaters were the main factors governing the Gulf of Lion during the Late Pleistocene to mid Holocene transition.