Evolution of Terrestrial Environments

地球形成后的原始大气主要由CO_2组成，CO_2是地球现今地表环境中第二丰富的挥发性物质。问题是这样一个富CO_2的大气是如何演变成现在的富N_2大气的。我们用一个简单的十种化学元素的地球化学循环模型研究了地球环境的长期演变（C，H，N，O，S，Ca，Mg，Fe，K，Ar）之间的五种元素（大气层、海洋、海底、大陆和地幔）本研究的主要结论之一是，在1970年代的历史上，地球与大气氩同位素数据所包含的Ar脱气历史是一致的。在此脱气模型的基础上，我们研究了大气和海洋的演化模型。我们认为N_2、O_2、Ar和CO_2是大气的主要成分，H_2O、Ca^<2+>、Mg^<2+>、Fe^<2+>、SO_2-_、HCO^-_和CO_2-_是海洋的主要成分。关于大气和海洋演化的主要结果如下：（1）在地球最早的历史时期，大气中的N_2分压是恒定的。但随着地球化学碳循环的开始，其值下降了约30%。(2)在2.0 - 2.5Ga以来，大气中O_2的积累可能是由于碳和硫的地球化学循环中氧的产生和消耗之间的微小不平衡所致。(3)随着CO_2分压的降低，海洋pH值从5上升到目前的8。过去海洋pH值比现在低，而过去大气CO_2浓度却比现在高。

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.