A 10-fold decline in the deep Eastern Mediterranean thermohaline overturning circulation during the last interglacial period

Present-day Mediterranean deep-waters are well oxygenated, but the episodic formation of organic rich sediments (sapropels) indicates that this pattern was frequently perturbed in the past. Both high export productivity and disruption of the thermohaline circulation, leading to reduced deep-water ventilation, have been proposed to account for sapropel deposition and anoxia. The last interglacial sapropel S5 is considered one of the most strongly developed. Here, we apply the redox-sensitive Mo and U (elemental and isotope) systems to quantify the intensity of anoxic deep-water conditions in the Eastern Mediterranean Sea from ODP core 967 (2550 mbsl). Both U and Mo show strong authigenic enrichment, coupled to progressive increase in delta(MOauth)-M-98 (+1.2-1.8 parts per thousand to +2.0-2.3 parts per thousand) and decrease in delta U-238(auth) (+0.10 parts per thousand to -0.15 parts per thousand) from the beginning to the end of S5, suggesting increasing water column euxinia and removal fluxes of Mo and U. Based on modern euxinic basins, we show that sedimentary delta U-238(auth) can be used to derive estimates of water column U depletion and, ultimately, deep-water renewal rates. These principles are first tested on the modern Black Sea, which yields calculated deep water renewal times of 830(+690)/(-500) yrs, in good agreement with independent estimates. Applying these principles to the end of S5 suggests bottom-water U depletion of similar to 50% and deep-water renewal times of 1030(+820)/(-520) yrs. The significantly slower deep-water renewal rates in the Eastern Mediterranean Sea compared to today (similar to 100 yrs) would have played an important role in the formation of sapropel S5 and are consistent with the proposed suppression of overturning during the last interglacial, due to increased stratification resulting from higher riverine freshwater input under enhanced monsoon forcing. (C) 2018 Elsevier B.V. All rights reserved.