[S-RD33] 資源地質学

[SRD33-11] 地中海メッシニアン期塩分危機に形成された蒸発岩のマグネシウム同位体組成

*吉村 寿紘¹、荒岡 大輔²、黒田 潤一郎³、伊左治 雄太³、Jiménez-Espejo Francisco¹、Lugli Stefano⁴、Manzi Vinicio⁵、Roveri Marco⁵、大河内 直彦¹ (1.海洋研究開発機構・生物地球化学研究分野、2.産業技術総合研究所・地圏資源環境研究部門、3.東京大学・大気海洋研究所、4.Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia、5.Physics and Earth Science Department, University of Parma)

キーワード：メッシニアン期塩分危機、蒸発岩、マグネシウム同位体比

The Messinian salinity crisis (MSC) is a hydrological and biological crisis that occurred in the Mediterranean basin at the latest Miocene. The salt deposit reaches a maximum apparent thickness of ~3,000 m and are composed mainly of gypsum/anhydrite and halite. From a natural resources perspective, the MSC evaporates are important for such as potash salts and hydrocarbons resources. A large amount of the MSC salts originates from the evaporative concentration of seawater flowing in through the Strait of Atlantic gateway. The Mediterranean Basins have experienced repeated refilling of seawater and freshwater with different hydrological scenarios between western and eastern basins and between marginal and centers of basins. The Mg isotope ratios in seawater are homogeneous because of its long mean residence time (>10⁷ years) in the modern ocean. As evaporation proceeds, soluble potash salts start to accumulate in highly restricted depositional settings. Because Mg isotope fractionation is mainly related to low temperature processes at the surface environments, our aim is to use magnesium isotope ratios (δ²⁶Mg) of the Messinian evaporites to constrain evaporative concentration histories and the hydrochemical processes during the MSC. The δ²⁶Mg values of the MSC evaporates ranged from -3.18‰ to -1.22‰, which is highly varied and significantly lower than the value of modern seawater. Elucidation of the basin hydrochemistry from Mg isotopes is a new potential indicator of an extent of evaporative concentration events and dissolution/recycling of coeval marginal marine deposits.