Emergence of Clay Mineral Formation on Early Earth as an Important Analogue for Exploring Extra-Terrestrial Life Forms

Abstract

Understanding the dynamic interaction of the Archean Earth’s surface processes, atmospheric conditions with biological activities on land and marine environments, are crucial for deciphering the biosphere's responses to extreme climatic conditions. The Archean Eon is widely accepted to have been dominated by anoxic conditions in the atmosphere-hydrosphere system although punctuated by episodes of whiffs of atmospheric oxygenation and oxygen oases at the surface oceans. Despite extensive research on the evolution of photosynthesis and biological productivity in Archean marine environments, limited attention has been paid to terrestrial biosphere which hinders a comprehensive understanding of Archean atmospheric conditions and coeval climates. In so far as the Archean world was exposed to weaker solar radiation compared to modern days, enhanced concentration of reducing gases (e.g., CO2, CH4, SO2, etc.) in the atmosphere, that resulted in greenhouse gas effects, likely served to mitigate the Faint Young Sun Paradox. However, sedimentary evidence of mid-Archean glacial deposits points to episodic cooling events, that further complicate efforts to reconstruct the evolution of the interplay between the Archean atmosphere, climate and terrestrial biosphere. To address these gaps, this study investigates clay mineralogy, trace metal, and stable isotope geochemistry of two well preserved Archean paleosols, including 1) the Mesoarchean paleosol of the Pongola Supergroup, South Africa (~2.98 Ga) which is associated with Earth's earliest major cooling event, and 2) the Late Neoarchean Lanseria paleosol (~2.6 Ga) from the Transvaal Supergroup, South Africa, which was followed by the Huronian glaciation. The data show high rate of pedogenic clay mineral production and diversification, i.e., smectite, berthierine and kaolinite, with a magnitude of kaolinite production much higher in the Lanseria profile compared to the older Pongola paleosol. The average C and N isotope data, at –28‰ and +4‰, respectively, of extracted clay minerals and bulk samples of both paleosols, combined with the mobilization of redox sensitive elements (e.g., U, Cr and Mo), suggest a soil biosphere involving aerobic and anaerobic metabolisms, consistent with CO2 fixation by photoautotrophs and possibly redox reactions in the nitrogen cycles involving oxic conditions on land. Our findings highlight for the first time a high rate of oxygenic photosynthesis and biologically mediated incongruent weathering during the formation of these Archean soil profiles. Our results imply an expanded weatherable land surface, during Mesoarchean thickening and stabilization of continental crust, which enhanced CO₂ uptake and decreased the coeval greenhouse gas effects. Decreasing atmospheric pCO2, coupled with elevated O2 production, would also have contributed to significantly remove atmospheric methane, further limiting the greenhouse effect, which ultimately failed to offset the weaker solar luminescence and explains the development of Mesoarchean glacial events. As a result of faster and larger growth of the continental crust and expanded weatherable land areas at the Archean-Proterozoic transition, the climatic responses from biologically mediated silicate weathering, as observed during the Mesoarchean, became even more pronounced. This might have paved the way to the Huronian glaciation and ultimately the great oxidation event (GOE).

Date of Award	6 Dec 2024
Original language	American English
Supervisor	Frantz Ossa Ossa (Supervisor)