The early hydrosphere The gases released from the Earth during its early

The early hydrosphere the gases released from the

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The early hydrosphere The gases released from the Earth during its early history, including water vapour, have been called excess volatiles because their masses cannot be accounted for simply by rock weathering. These volatiles are thought to have formed the early atmosphere of the Earth. At an initial crustal temperature of about 600° C, almost all of these compounds, including H 2 O, would have been in the atmosphere. The sequence of events that occurred as the crust cooled is difficult to reconstruct. Below 100° C all of the water would have condensed, and the acid gases would have reacted with the original igneous crustal minerals to form sediments and an initial hydrosphere that was dominated by a salty ocean. If the reaction rates are assumed to have been slow relative to cooling, an atmosphere of 600° C would
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have contained, together with other compounds, water vapour, carbon dioxide, and hydrogen chloride (HCl) in a ratio of 20:3:1 and cooled to the critical temperature of water ( i.e., 374° C). The water therefore would have condensed into an early hot ocean. At this stage, the hydrogen chloride would have dissolved in the ocean (about one mole per litre), but most of the carbon dioxide would have remained in the atmosphere, with only about 0.5 mole per litre in the ocean water. This early acid ocean would have reacted vigorously with crustal minerals, dissolving out silica and cations and creating a residue composed principally of aluminous clay minerals that would form the sediments of the early ocean basins. This is one of several possible pathways for the early surface of the Earth. Whatever the actual case, after the Earth’s surface had cooled to 100° C, it would have taken only a short time for the remaining acid gases to be consumed in reactions involving igneous rock minerals. The presence of cyanobacteria ( e.g., blue-green algae) in the fossil record of rocks older than three billion years attests to the fact that the Earth’s surface had cooled to temperatures lower than 100° C by this time, and neutralization of the original acid volatiles had taken place. It is possible, however, that, because of increased greenhouse gas concentrations (see below) in the Early Archean era (about 3.8 to 3.4 billion years ago), the Earth’s surface could still have been warmer than today. If most of the degassing of primary volatile substances from the Earth’s interior occurred early, the chloride released by the reaction of hydrochloric acid with rock minerals would be found in the oceans or in evaporite deposits, and the oceans would have a salinity and volume comparable to that of today. This conclusion is
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based on the assumption that there has been no drastic change in the ratios of volatiles released through geologic time. The overall generalized reaction indicative of the chemistry leading to the formation of the early oceans can be written in the form: (Primary igneous rock minerals + acid volatiles + H 2 O ) (Sedimentary rocks + oceans + atmosphere).
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