Evolution of Terrestrial Environments

地球形成后不久的原始大气将主要由二氧化碳组成，这是目前地球表面环境中第二丰富的挥发性物种。问题是，这样一个富含二氧化碳的大气层是如何演变成现在的富含氮气的大气层的。我们利用大气、海洋、海底、大陆和地幔五个储集层之间的十种化学元素(C、H、N、0、S、Ca、Mg、Fe、K、Ar)的简单地球化学循环模型，结合中国大陆地幔的历史演化，研究了地球环境的这种长期演化。本研究的主要结论之一是，地球历史上洋底的恒定扩张速率与大气Ar同位素数据所包含的Ar排气历史相一致。基于这个脱气模型，我们研究了大气和海洋的演化模型。认为N_2、O_2、A_r、CO_2是大气的主要成分，H_2O、Ca~(2+)、Mg~(2+)、Fe~(2+)、SO_2~(2-)、HCO~(-2)、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)地球早期历史上大气中N_2的分压是恒定的。然而，随着生物地球化学碳循环的开始，其降幅为初始值的30%。(2)通过C和S的生物地球化学循环，氧的产生和消耗之间的小的不平衡有助于大气中O_2的积累，可能是2.0-2.5Ga以来的水平。(3)海洋pH随CO_2分压从5降低到现在的值8而升高。过去的海洋pH比现在低，而大气中的CO_2水平过去较高。

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.