EVIDENCE AND ARGUMENTS FOR EARLY METHANE AND AMMONIA IN EARTH'S ATMOSPHERE, AND AN ORGANIC-COMPOUND-RICH EARLY OCEAN

Most theoretical treatments of degassing of the Earth to provide an ocean and atmosphere assume an absence of iron during the degassing process. This is mainly based on the assumption that core separation began early and was continuously effective in removing iron from volatile-rich accreted matter. Straightforward consideration of the conditions of late accretion, which must necessarily be the source of Earth’s surface volatiles, shows that metallic iron must have been abundant under all scenarios expected for volatile release. Under these conditions, the gases present in the atmosphere, at least initially, must have been strongly reducing, e.g. methane and ammonia. The instability of these gases due to photolysis and atmospheric electrical discharges is well known, implying their reaction into higher molecular weight, more complex compounds that would condense into the global ocean. In other words, the Miller-Urey experiment is a sound model for early atmospheric processing. The high concentrations of organic compounds, whether dissolved, suspended, or deposited in sediment at the bottom of the ocean imply massive hydrothermal processing of organic-rich material in ocean thermal systems, with a constant regeneration of methane and ammonia into the atmosphere as part of a coherent abiotic carbon cycle. Carbon dioxide and carbonates become more important parts of the surface environment as a result of subduction of organic-rich sediment, where magmagenesis in the iron-depleted upper mantle naturally gives rise to carbon dioxide, which is emitted during volcanism over an extended period of time. The modern dominance of oxidized carbon (mostly as carbonates) is thus the result of a prolonged process, covering billions of years, rather than due to early degassing of carbon dioxide. This model of the early atmosphere and surface environment is more consistent with the geologic record than models of early degassing of carbon dioxide.