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

Authors: EKREM BURSİN İŞLER, ALİ ENGİN AKSU, RICHARD NICHOLAS HISCOTT

Abstract: Aspects of the paleoclimatic and paleoceanographic evolution of the Aegean Sea since ~130 ka are revealed by quantitative variations in planktonic faunal assemblages, the Δ¹⁸O and Δ¹³C isotopic composition of benthic and planktonic foraminifera, and Mg/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 r² correlation coefficients of 0.92-0.95. Planktonic foraminiferal assemblages are similar, through time, across several deep basins, suggesting that major changes must have occurred in near synchroneity across the Aegean Sea. The data suggest that sapropels S3, S4, and S5 were deposited under a stratified water column during times of increased primary productivity and the development of a deep chlorophyll maximum layer. Under such conditions, oxygen advection via intermediate water flow must have been significantly reduced, in turn implying bottom water stagnation. Sapropel S1 lacks a deep phytoplankton assemblage; this faunal contrast between S1 and older sapropels indicates that S1 must have been deposited in the absence of a deep chlorophyll maximum layer. Cluster analysis shows a consistent coupling of Globigerina bulloides with Globigerinoides ruber during 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.

Keywords: Sapropel, planktonic foraminifera, SST, oxygen and carbon isotopes, Mg/Ca ratios, Quaternary, paleoclimate, paleoceanography, Aegean Sea

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