Coccolithophore assemblages during the Holocene Climatic Optimum in the NE Mediterranean (Aegean and northern Levantine Seas, Greece): Paleoceanographic and paleoclimatic implications

Abstract

Recent paleoceanographic findings suggest that during the Holocene, NE Mediterranean climate was less stable than previously thought. Several recently generated Aegean paleoceanographic records of sapropel S1 suggest that during the Holocene short-term potentially global cooling episodes, reflecting the N. Atlantic climatic fluctuations and possibly related to solar variability, are superimposed on the underlying regional subtropical/tropical control on the hydrography and ecosystems. Detailed quantitative analyses of coccolithophores have been performed on the deep sea deposits of the NE Mediterranean Sea; four gravity core records from North Aegean Sea (SL- 152), SE Aegean (NS-14, NS-18) and South Cretan continental Margin/northern Levantine Sea (HCMR 2/22) respectively, with a focus on the regional expression of sapropel S1 deposition. In all sites, higher values of Florisphaera profunda, Helicosphaera carteri, Braarudosphaera bigelowii and the increase of stratification S-index within sapropel intervals can be interpreted as the result of elevated marine production in the deep photic zone and enhanced inflow of fresh water input during the S1 period. The stratification S-index is significantly higher in the North Aegean, in respect to SE Aegean and the South Cretan Margin (SCM), reflecting increased coccolithophore productivity at the lower photic zone, being associated with a remarkably stratified water column. The concomitant increased presence of B. bigelowii indicates the prominent riverine input in the North Aegean basins and possible influx of Black Sea waters after ∼9.0 ka BP. Consequently, the increased stratification of the water column is linked to the significant freshening of the North Aegean surface waters, particularly during the early stages of the Holocene Climatic Optimum (S1a depositional interval). A series of cooling events detected towards the end of sapropelic conditions (∼7.2 ka BP) mostly in the North Aegean site, may be linked to outbursts of cold northerly air masses and relevant pulses in the deep-intermediate water ventilation that caused the enhancement of surface marine productivity and the temporary cessation of S1 at ∼ 8.0 ka BP. In contrast, in the SE Aegean and the northern Levantine Sea, the water column remains still warm and stratified, influenced by moderate fresh water input, for at least 1 more ky.