The inception of the oceans and CO2-atmosphere in the early history of the Earth

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Abstract

After accretion and solidification of a “magma ocean”, the proto-atmosphere of the Earth is modeled to compose of 560 bar of H2O and 100 bar of CO2. The existence of a CO2-rich atmosphere in the early history of the Earth has been envisaged and supported by earlier studies. The results of the present study suggest that the oceans started to grow when the surface temperature of the Earth cooled to below approximately 300–450 °C. This study further suggests that CO2 was removed away from the early atmosphere as a dense supercritical H2O–CO2 mixture during condensation (or the growing ocean), and then formed carbonate rocks in early geological history. The scenario of early growth of the oceans is in line with the geochemical evidence revealed recently from detrital zircons 4.4 Gyr old. A large-scale hydrosphere probably never existed on Venus, and not for long, on Mars. Thus, the atmospheres of these latter planets are composed mainly of carbon dioxide.

Introduction

The origin and early evolution of the atmospheres and oceans of terrestrial planets are classic unsolved problems in the planetary sciences. These topics are also of great interest to science in general and have attracted attention from science-philosophers who have considered life in nature. Nearly all theories concerning these topics are model-dependent and constrained by various evidence, e.g., [1], [2]. H2O and CO2 are the two most abundant volatile species on the surfaces of Earth, Venus and Mars, e.g., [3], and are the most vital elements (C, H and O) for life on the Earth.

It has been suggested favorably that the surface of a growing planet may be covered entirely by a “magma ocean” in the late stage of accretion [4], [5]. Both H2O and CO2 in the proto-atmospheres of the terrestrial planets may be derived from the magma ocean upon solidification by outgassing, and the oceans were formed by condensation of the atmospheric H2O when the Earth's surface was cooled below the condensation temperature. Venus is deficient in H2O, relative to Earth, by a factor of 104 to 105. Although this deficiency may be accounted for by a speculative theory of hydrodynamic escape, Yung and DeMore [3] favored a theory of planetary evolution, which suggested that most water in the magma ocean is still trapped in the thick partial melting zone inside the present Venus, and only CO2 and a small amount of H2O were outgassed in the Cytherean atmosphere [6]. Both H2O and CO2 were outgassed into the proto-atmospheres of Earth and Mars after solidification of their magma oceans, but most water on the surface of Mars did not survive for long because of its small mass (a hydrogen-rich surface layer on Mars was confirmed by the Mars Odyssey mission [7], [8]).

The atmospheres of both Venus and Mars are composed of more than 95% CO2 [9], [10]. In comparison, one may suggest that a similar CO2-rich atmosphere (or as some more reduced carbonaceous gases, CO or CH4, which should soon be oxidized to CO2 by OH radicals produced from water vapor photolysis [11]) may have existed in the early history of the Earth. The CO2-rich atmosphere of the Earth has been envisaged and supported by earlier studies [2], [12].

The inception of oceans on the Earth by condensation has not been addressed in detail. CO2 in the early atmosphere of the Earth is generally believed to be removed mainly by photosynthetic organisms in the oceans, metabolising carbon from CO2 and releasing oxygen into the atmosphere, e.g., [13]. The abundance of oxygen in the Earth atmosphere may be explained by the occurrence of organisms, which, however, can probably account for only a very small amount of the missing CO2 in the early Earth atmosphere.

Section snippets

The model

The present atmosphere of Venus contains ca. 4.6×1020 kg CO2 and that of Mars contains at least ca. 2.1×1016 kg CO2. Thus, it is conveniently assumed that the early Earth atmosphere contained some 5.2×1020 kg CO2 (or 100 bar, see also [2]), in addition to H2O, if Venus, Earth and Mars were formed via a similar accretion process with similar infalling materials, because the Earth is more massive than Venus. Holland's estimate gives about 20 bar of CO2, which equates to ca. 9×1019 kg of CO2, in

Vaporization at high pressures

The vaporization temperature of H2O, CO2 and H2O–CO2 mixture at sufficiently high pressures can be inferred from data on specific volume and/or density of these materials at various temperatures and pressures determined by experimental studies and thermodynamic calculations [15], [16], [17]; and many references cited therein]. Although discrepancies exist among the available experimental data, Blencoe et al. [17] concluded that an excellent agreement was also found in some studies. As an

Inception of the oceans

The temperature for the onset of the “dense” supercritical H2O (the short-dashed curve in Fig. 2) appears insensitive to pressure variation above about 500 bar. If H2O and CO2 were to coexist as an ideal mixture (or inert to each other), according to Fig. 2, the first drop of “dense” supercritical H2O appeared (or the inception of the oceans) on the Earth when the surface temperature cooled to about 450±20 °C (“a” in Fig. 2). The atmospheric pressure must drop at high altitude. So, “dense”

Carbonization via the oceans

Plagioclase is the most abundant mineral species on the Earth's surface. Conversion of plagioclase by weathering processes produces Ca2+ and Na+ via the following reactions (see also, e.g., [20], [21]):

where 0≤x≤1;
where 0≤x≤0.333 (when 0.333<x≤1, opal on the right-hand side should be moved to the left-hand side and the amount of H2O should be adjusted accordingly), and the following reaction forms carbonates in a “dense” supercritical H2O–CO2 mixture,

It is most likely that these reactions took

Acknowledgements

The author is indebted to W. A. Bassett, I.-M. Chou, T. P. Mernagh, J. S. Owen and T. F. Yui for critical comments and discussion.

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