Northern Levantine and Adriatic Quaternary planktic foraminifera; Reconstruction of paleoenvironmental gradients

Abstract

A detailed study of the planktic foraminiferal records of three cores, recovered near Crete, shows very consistent changes. Unfortunately, the record of T87/2/13G, our shallowest core, is bioturbated too much for detailed comparisons, but it can still be used to discuss the general trends. The mutual record of the other two cores, T87/2/20G and T87/2/27G, extends back to about 125,000 yr B.P. Principal Components Analyses performed on the combined records of T87/2/20G and T87/2/27G yield two axes that can be interpreted paleoecologically, namely a temperature-axis and an "annual stability" axis. The latter is a measure of stratification within the euphotic layer. In this study, we consider T87/2/20G and T87/2/27G to be reference cores for the eastern Mediterranean. Subsequently, we interprete the faunal variation in two cores from the Adriatic Sea (IN 68-5 and IN 68-9), containing records extending back to 15,000 and 18,000 yr B.P., in the context of these reference-cores. This is done by calculating scores of the Adriatic records along the two relevant PCA axes derived from the northern Levantine records. Also, cluster analyses of the Adriatic and northern Levantine records are compared. On the average, the Adriatic Sea appears to have been considerably colder than the region around Crete, during the past 18,000 years. This temperature difference was largest during the pleniglacial and only small during the Holocene. We found no distinct expression of a Younger Dryas cooling event in the Adriatic records, neither on our PCA axis, nor in the oxygen isotope record. This contradicts previous paleoclimatic reconstructions, based mainly on paleobotanical evidence, which suggests that the Younger Dryas event greatly influenced circum-Mediterranean areas. As yet, we have no ready explanation for this discrepancy. Low annual stability values dominate both the pleniglacial and the Holocene intervals of the Adriatic records. This suggests that homothermal conditions were as prevalent in the Adriatic euphotic layer at pleniglacial times as they are at present. In the present-day Adriatic, homothermal conditions are associated with deep convection and subsequent Eastern Mediterranean Deep Water (EMDW) formation, and therefore our data suggest that EMDW formation was as effective at pleniglacial times as it is at present. High annual stability values in the transitional interval, from 12,700 to 9600 yr B.P., in the Adriatic records are suggestive of low EMDW formation-rates at that time. In combination with high productivity throughout the eastern Mediterranean, which has previously been inferred from the planktic foraminiferal record, these low EMDW formation rates may have preconditioned the eastern Mediterranean for sapropel formation. Show more