Anoxia and possible export production spikes in the Red Sea during Termination II: evidence from U-decay series and organic C concentrations

Abstract:

The late Quaternary history of the Red Sea is characterized by sharp increases in sea surface salinity during glacial maxima in response to global sea level drop. These imposed an extremely weak current exchange between the Red Sea and the Indian Ocean through the Bab-el-Mandeb Straits that resulted in temporal changes in stratification, productivity and subsurface oxygenation of the Red Sea. The combined effect of these perturbations has been interpreted to impose extended aplanktonic zones in Red Sea sediments centered at glacial maxima. Yet the dynamics of the transition between glacial and interglacial stages in the Red Sea are still not well understood.

Here, we present evidence for the occurrence of a strong anoxic episode during the penultimate transition from Marine Isotope Stage (MIS) 6 to MIS5 (Termination II) based on uranium concentrations, isotopic ratios, and organic carbon concentrations (Corg) studied in core KL23 taken by R/V Meteor from northern Red Sea (24^o44.88’N 35^o03.28’E) at 702 m water depth.

(²³⁴U/²³⁸U) activity ratios and U concentrations start rising from their MIS6 levels (~1 and 1.9 ppm, respectively) ca. 140 ka, and peak at 135-132 ka (1.08 and 3.1 ppm). These patterns are matched by changes in Corg, and the onset of all slightly precedes sea level rise patterns. Thereafter, uranium and Corg decrease sharply, reaching minima that characterizes the rest of MIS5e, ca. 130 ka. Uranium activity ratios however, decrease gradually from their deglacial peak to a minimum value (~0.94) at 122 ka. In view of the redox sensitive behavior of U, the buildup of U concentrations would support anoxic conditions, rather than a rise in export production, as the most likely explanation for the preservation of Corg in the sediments. Yet, the high (²³⁴U/²³⁸U) values that imply a dominance of open seawater U in the samples, together with d¹³C values of foraminifera bracketing the studied interval that display a depletion trend indicating an increase in primary production, both suggest that an increase in export production cannot be ruled out as the source of high Corg in the sediments. We will evaluate the possibility of an increase in export production that is coeval with the aplanktonic zone in this core in the context of ²³⁰Th excess data, sea level changes and paleo-circulation of the Red Sea.