[M-IS32_30AM1] 地球掘削科学

[MIS32-07] 掘削試料からみる地中海の歴史

*黒田 潤一郎¹ (1.独立行政法人海洋研究開発機構)

キーワード：Mediterranean Sea, Messinian Salinity Crisis, osmium isotope

Mediterranean Sea has experienced an extreme event called Messinian Salinity Crisis (MSC) that represents a formation of gigantic evaporite deposits in deep basins. Although theis event has long been studied, a fundamental question whether the Mediterranean Sea was desiccated or not, still remains unsolved. In this presentation we review the recent achievements of the MSC. To understand hydrological conditions of the Mediterranean Sea during the Miocene-Pliocene, we report a series of Os isotopic record of marine sediment cores from four deep-sea drilling sites in the Balearic Basin, the Tyrrhenian Sea, the Ionian Basin and the Florence Rise, in comparison with the coeval sediments in North Atlantic. Osmium isotopic ratios of the pre-Messinian sediments in the western Mediterranean basin are almost identical to that of the coeval ocean water. In contrast, the pre-Messinian sediments in the eastern Mediterranean basin have significantly low ¹⁸⁷Os/¹⁸⁸Os values. This suggests that Os in the eastern Mediterranean was not fully mixed with western Mediterranean and North Atlantic, and that the basin isolation has already started much earlier than the MSC. The less radiogenic Os would have been supplied to the eastern Mediterranean by selective weathering of ultramafic rocks cropping out in the drainage areas, which contains high amount of non-radiogenic Os. The isotopic compositions of Os in gypsum and halite samples are significantly lower in eastern Mediterranean basins, compared with those of gypsum samples from the western Mediterranean basin, supporting the idea that limited exchange of seawater between eastern and western basins sustained also during the MSC. In all sites Pliocene sediments show more radiogenic Os isotopic ratios, which are close to the coeval oceanic values, indicating that Os started mixing with global seawater again.