| Research Abstract |
The proto-atmosphere just after the formation of the Earth would have been mainly composed Of CO_2 which is the second most abundant volatile species of the Earth's present surface environment. The question is how such a CO_2-rich atmosphere would have evolved to the present N_2-rich atmosphere. We studied this long-term evolution of the Earth's environment by using a simple geochemical cyde model of ten chemical elements (C, H, N, 0, S, Ca, Mg, Fe, K, Ar) between five reservois (the atmosphere, oceans, sea-floor, continents and mantle) coupled with tliernial evolution of Ciic mantle.
One of the main conclusions of this study is that a constant spreading rate of ocean floors over the history of the Earth is suggested to be consistent with the Ar degassing history contained by the atmospheric argon isotope data. Based on this degassing model we studied an evolution model of the atmosphere and oceans. We considered N_2, O_2, A_r, and CO_2 as the major components of the atmosphere, and H_2O, Ca^<2+>, Mg^<2+>, Fe^<2+>, SO^2-_, HCO^-_ and CO^2-_ as the components of oceans. The main results on the evolution of atmosphere and oceans are as follows : (1) The partial pressure of N_2 in the atmosphere is constant for the earliest history of the Earth. However, it decreases by 30% of the initial value with the beginning of biogeochemical carbon cycle. (2) The small imbalance between oxygen production and consumption through the biogeochemical cycles of C and S contributes to accumulation of O_2 in the atmosphere to the present level probably since 2.0 - 2.5Ga. (3) The ocean pH increases with decrease in partial pressure of CO_2 from 5 to the present value 8. The ocean pH was lower in the past than today as far as the atmospheric CO_2 level was higher in the past.
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