| 研究実績の概要 |
We have built a model for the long-term evolution of the hydrogen isotope composition of the surface water, water in the mantle and in crustal rocks (Kurokawa, Foriel et al, 2018). Processes affecting the abundance of water and its H isotopic composition are hydrogen escape (in anoxic atmosphere conditions), water degassing during volcanism and ingassing during subduction, water-rock exchanges during alteration of the seafloor and weathering of the continents.
The first output of this modeling is that the modern H isotope composition of seawater requires that in the past, the water cycle was significantly different form the present-day cycle. Either hydrogen escape, regassing from the surface to the mantle, or faster plate tectonics must have occurred in the Archean era and perhaps after. In any case, the amount of water in the early ocean should have been greater than it is now, as was hypothesized in the proposal for this project.
However, in order to differentiate which process drove the loss of water from the surface, isotopic composition of the seawater and of the mantle water need to be estimated. This requires analysis of ancient rocks for H isotopes as well as O isotopes as oxygen data can help identify the isotopic signature of the seawater. A method for analyzing O isotopes in silicates was adapted in the lab (Ueno Lab, ELSI/Tokyo Institute of Technology). These measurements could not be completed before the termination of the project, but the method is established and analysis will continue in the Ueno Lab.
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