[S-CG41] Hard-Rock Drilling Science: From Continental to Deep Sea Drilling, and Oman Project

[SCG41-P01] Oeanic lithosphere production and the Earth's surface environments

★Invited Papers

*Junichiro Kuroda¹, Kosuke T. Goto², Akira Ishikawa³, Azumi Kuroyanagi⁴ (1.Atmosphere and Ocean Research Institute, the University of Tokyo, 2.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 3.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 4.The Tohoku University Museum)

Keywords:Oceanic lithosphere, Greenhouse/Icehouse climate periods

It has been discussed that long-term changes in the production rates of oceanic plates have influenced on Earth's surface environments through Phanerozoic. For example, carbon cycles in the global oceans have been largely affected by CO₂ degassing along mid-ocean ridges and at hot spots, and precipitation of carbonate minerals through low-temperature hydrothermal alteration. Thus, the high/low rates of oceanic lithosphere production could be one of the major driving forces for the Greenhouse /Icehouse climate periods, which have occurred repeatedly on the scale of hundreds of millions of years. Because of the role of hydrothermal alteration as a removing process of Mg from seawater, large-scale changes in amplitude of hydrothermal activity associated with oceanic plate production have influenced on the secular variation of Mg concentrations in seawater, and thus also on carbonate biomineralization. Time periods in the late Cenozoic (Neogene and Quaternary) are classified as one of the "Icehouse" climatic episodes. The cooler climates may have been related to the relatively low rate of degassing associated with the reduced seafloor spreading rates for those time periods. In addition, the removal of Mg from seawater by hydrothermal alteration was relatively low, which resulted in relatively high concentrations of Mg remained in seawater in Neogene and Quaternary. On the other hand, Cretaceous represents one of the "Greenhouse" climate periods. Cretaceous is also characterized by relatively high rates of oceanic plate productions, and voluminous igneous province volcanisms, both of which may have maintained high degassing rates. The Mg removal rates were considered to be high, resulting in low Mg concentrations in seawater.

The likelihood of those classic hypotheses on the link between oceanic plate productions and surface environments have long been the subject of debate. Here, we introduce a new concept of wetness/dryness of oceanic lithospheres into the long-lasting discussions. "Dry" plates are less hydrated oceanic plates that were formed under conditions of higher production rates, as represented by the Pacific Plate, while "wet" plates are more hydrated ones formed under lower spreading rates. How those "dry" and "wet" oceanic plates have affected the surface environments throughout the Earth's history could become one of the major themes of the Earth sciences in the future. We discuss the validity and testability of the hypothesis that the differences between the "dry" and "wet" oceanic plates have caused variations in global seawater volumes and major (Ca, Mg etc.) and trace element (Sr, Os etc.) compositions of seawater, and also have affected the global climates and carbonate biomineralization.