Temperature and salinity variations of Mediterranean Sea surface waters over the last 16,000 years from records of planktonic stable oxygen isotopes and alkenone unsaturation ratios

Alkenone unsaturation ratios and planktonic δ18O records from sediment cores of the Alboran, Ionian and Levantine basins in the Mediterranean Sea show pronounced variations in paleo-temperatures and -salinities of surface waters over the last 16,000 years. Average sea surface temperatures (SSTs) are low during the last glacial (averages prior to 13,000 years: 11–15°C), vary rapidly at the beginning of the Holocene, and increase to 17–18°C at all sites during S1 formation (dated between 9500 and 6600 calendar years). The modern temperature gradient (2–3°C) between the Mediterranean sub-basins is maintained during formation of sapropel S1 in the Eastern Mediterranean Sea. After S1, SSTs have remained uniform in the Alboran Sea at 18°C and have fluctuated around 20°C in the Ionian and Levantine Basin sites. The δ18O of planktonic foraminifer calcite decreases by 2‰ from the late glacial to S1 sediments in the Ionian Basin and by 2.8‰ in the Levantine Basin. In the Alboran Sea, the decrease is 1.7‰. Of the 2.8‰ decrease in the Levantine Basin, the effect of global ice volume accounts for a maximum of 1.05‰ and the temperature increase explains only a maximum of 1.3‰. The remainder is attributed to salinity changes. We use the temperature and salinity estimates to calculate seawater density changes. They indicate that a reversal of water mass circulation is not a likely explanation for increased carbon burial during S1 time. Instead, it appears that intermediate and deep water formation may have shifted to the Ionian Sea approximately 2000 years before onset of S1 deposition, because surface waters were as cold, but saltier than surface water in the Levantine Basin during the Younger Dryas. Sapropel S1 began to form at the same time, when a significant density decrease also occurred in the Ionian Sea.