WHY SO MANY ARCHEAN GYPSIC PALEOSOLS?

No calcic paleosols are known on either Earth or Mars before 2.4 Ga, when known pedogenic salts in organized salic (By) horizons of nodules and desert roses were largely sulfates: kieserite, gypsum, bassanite, and barite. The most common pedogenic salts after 2.4 Ga are carbonates such as calcite and dolomite. Archean gypsum in paleosols is puzzling because sulfate is the most oxidized sulfur compound, and there is evidence from paleosols and other proxies that Archean air and soil were dysoxic to anoxic. Gypsid paleosols are locally abundant in 3.7 Ga rocks of Candor Chasma and Gale Crater on Mars, 3.7 Ga Isua Supracrustals of Greenland, 3.3 Ga Strelley Pool Formation of Western Australia, 3.2 Ga Moodies Group of South Africa, 3.0 Ga Farrel Quartzite of Western Ausralia, 2.2 Ga Sugarloaf Quartzite of Wyoming, 1.9 Ga Stirling Range Formation of Western Australia. Gypsic paleosols remain locally common in Mesoproterozoic to Pleistocene sediments and still form today in hyperarid regions such as the Atacama Desert of Chile. The lateral transition from gypsic to calcic soils in the Atacama desert is an ecological divide from gypsic proteobacterial-actinobacterial to calcic cyanobacterial-algal-fungal communities. Transition from dysoxic purple sulfur proteobacteria to oxic algal-cyanobacterial communities on land may explain the paradox of abundant Archean pedogenic sulfate, because there are microfossils with the carbon isotopic composition and morphology of purple sulfur bacteria in Archean paleosols of the Farrel Quartzite. Whether a similar biotic explanation applies to Martian gypsic paleosols is a major unanswered question.