Evidence from massive siderite beds for a CO2-rich atmosphere before ~ 1.8 billion years ago

Abstract

It is generally thought that, in order to compensate for lower solar flux and maintain liquid oceans on the early Earth, methane must have been an important greenhouse gas before approximately 2.2 billion years (Gyr) ago. This is based upon a simple thermodynamic calculation that relates the absence of siderite (FeCO3) in some pre-2.2-Gyr palaeosols to atmospheric CO2 concentrations that would have been too low to have provided the necessary greenhouse effect. Using multi-dimensional thermodynamic analyses and geological evidence, we show here that the absence of siderite in palaeosols does not constrain atmospheric CO2 concentrations. Siderite is absent in many palaeosols (both pre- and post-2.2-Gyr in age) because the O2 concentrations and pH conditions in well-aerated soils have favoured the formation of ferric (Fe3+)-rich minerals, such as goethite, rather than siderite. Siderite, however, has formed throughout geological history in subsurface environments, such as euxinic seas, where anaerobic organisms created H2-rich conditions. The abundance of large, massive siderite-rich beds in pre-1.8-Gyr sedimentary sequences and their carbon isotope ratios indicate that the atmospheric CO2 concentration was more than 100 times greater than today, causing the rain and ocean waters to be more acidic than today. We therefore conclude that CO2 alone (without a significant contribution from methane) could have provided the necessary greenhouse effect to maintain liquid oceans on the early Earth.

The rise of oxygen and siderite oxidation during the Lomagundi Event

“…Siderite is a major constituent of Archean and Early Proterozoic sediments: It is extremely abundant in banded iron formations, often even more so than iron oxides (19). Siderite is also found in anomalously high concentrations in Proterozoic limestones and dolomites (20), where it arises from the replacement of Ca 2+ and Mg 2+ by Fe 2+ in the carbonate mineral lattice.…”

Low HesperianPCO2constrained from in situ mineralogical analysis at Gale Crater, Mars

“…Estimates of P CO2 on the Archean (>2.5 Ga) Earth, derived from paleosols and banded iron formations, also use the thermodynamic stability of clay minerals and magnetite (17)(18)(19), but are criticized because they rely on assumptions of mineral equilibrium and do not account for enhanced delivery of carbon and reducing power to sediments via biological activity (20,21). Metamorphic overprinting of original mineral assemblages in terrestrial rocks this old is ubiquitous and complicates interpretations further (22).…”

WatSen: searching for clues for water (and life) on Mars

“…Siderite forms when more reducing conditions occur (for example, in restricted basins), and were more prevalent prior to the oxygenation of the Earth's atmosphere circa 2.2 Gyr ago. The occurrence of siderite in Precambrian deposits has been taken as a measure of the CO 2 content of the terrestrial palaeoatmosphere (Ohmoto et al 2004). It is anticipated that similar inferences will be drawn from the analyses of the Martian regolith.…”