Eastern Mediterranean Sea circulation inferred from the conditions of S1 sapropel deposition

摘要

Holocene eastern Mediterranean Sea sediments contain an organic-rich sapropelS1 layer that was formed in oxygen-depleted waters. The spatial distributionof this layer revealed that during S1 deposition, deep waters were anoxicbelow a depth of 1800 m. However, whether this boundary permanently existedfrom the early to the mid-Holocene has not been examined yet. To answer thisquestion, a multi-proxy approach was applied to a core retrieved close to the1800 m boundary (at 1780 m). We measured the bulk sediment elementalcomposition, the stable isotopic composition of the planktonic foraminiferGlobigerinoides ruber and the abundance of benthic foraminiferasince the last deglaciation. The result indicates that authigenic U and Moaccumulation began around 13–12 cal ka BP, in concert with surface waterfreshening estimated from the G. ruber δ18O record. Theonset of bottom and pore water oxygen depletion occurred prior to S1deposition inferred from barium enrichment. In the middle of the S1deposition period, reduced authigenic V, Fe and As contents and theBr/Cl ratio indicated short-term bottom-water re-oxygenation. A sharpMn peak and maximal abundance for benthic foraminifera marked a totalrecovery for circulation at approximately 7 cal ka BP. Based on ourresults and existing data, we suggest that S1 formation within the upper1780 m of the eastern Mediterranean Sea was preconditioned by reducedventilation, resulting from excess freshwater inputs due to insolationchanges under deglacial conditions that initiated between 15 and12 cal ka BP within the upper1780 m. Short-term re-oxygenation in the Levantine Basin is estimated tohave affected bottom water at least as deep as 1780 m in response to coolingand/or the reduction of freshwater inputs. We tentatively propose thatcomplete ventilation recovery at the S1 termination was depth-dependent, withearlier oxygenation within the upper 1780 m. Our results provide newconstraints on vertical water column structure in the eastern MediterraneanSea since the last deglaciation.