2017 年 51 巻 1 号 p. 15-28
In this paper I review some key studies on environmental changes during the Mesozoic and Cenozoic Eras, based upon radiogenic isotopic records of heavy elements such as osmium (Os) and lead (Pb). The Earth’s surface has experienced extreme environment events like oceanic anoxic event and/or salinity crisis. Radiogenic isotopic records of marine sediments give us key clues for understanding changes in 1) fluxes of (un)radiogenic isotope species from the mantle or extraterrestrial sources, and 2) water exchange ratios between basins. In the first part, I show a line of evidence of Os and Pb isotopic records of Cretaceous marine sediments that suggest rapid and massive eruptions associated with emplacement of large igneous provinces (LIPs) occurred simultaneously with oceanic anoxic events (OAEs). This implies that a deep‒rooted mantle process drives oceanic environmental extremes. In the second part I show Os isotopic records of Mediterranean sediments of Miocene‒Pliocene time interval that show history of water exchange between Mediterranean basins and the Atlantic Ocean. Mediterranean sediments show a clear drop of Os isotopic ratios during the Messinian Salinity Crisis (MSC) at the end of Miocene, when a huge amount of evaporites was precipitated in the Mediterranean marginal and deep basins. The change in Os isotopic ratios supports the idea that water exchange rate between Atlantic and Mediterranean was decreased substantially, about one thirtieth during the MSC.