Late Quaternary paleoceanographic evolution of the Aegean Sea: planktonic foraminifera and stable isotopes

Abstract

Aspects of the paleoclimatic and paleoceanographic evolution of the Aegean Sea since ~130 ka are revealed by quantitativevariations in planktonic faunal assemblages, the δ 18 O and δ 13 C isotopic composition of benthic and planktonic foraminifera, andMg/Ca ratios in planktonic foraminifera extracted from five 6 10-m-long piston cores. Independent sea surface temperature (SST)estimates obtained using planktonic foraminiferal transfer functions and the Mg/Ca ratios show excellent agreement, with r 2correlationcoefficients of 0.92 0.95. Planktonic foraminiferal assemblages are similar, through time, across several deep basins, suggesting thatmajor changes must have occurred in near synchroneity across the Aegean Sea. The data suggest that sapropels S3, S4, and S5 weredeposited under a stratified water column during times of increased primary productivity and the development of a deep chlorophyllmaximum layer. Under such conditions, oxygen advection via intermediate water flow must have been significantly reduced, in turnimplying bottom water stagnation. Sapropel S1 lacks a deep phytoplankton assemblage; this faunal contrast between S1 and oldersapropels indicates that S1 must have been deposited in the absence of a deep chlorophyll maximum layer.Cluster analysis shows a consistent coupling ofGlobigerina bulloideswithGlobigerinoides ruberduring times of nonsapropel deposition,interpreted to require a stratified euphotic zone composed of a warm, nutrient-poor upper layer and a cooler, nutrient-rich lower layer.The covariation of these two species suggests increased river runoff that controlled the fertility and stratification of the surface waters.