A high-resolution Sulphur Isotope Record of Neoarchean Atmospheric Chemistry: Deciphering the Driver of Atmospheric Bi-stability in the Prelude to the GOE.

Abstract:

Quantitative estimates of the composition of the Late Archean atmosphere are integral to deciphering the drivers of planetary oxygenation in the early Paleoproterozoic. However, the majority of geochemical proxies from marine sediments only inform on planetary redox via inference, making direct constraints on the ancient atmosphere difficult to place. Fortunately, the quadruple S-isotope (QSI) record (δ³⁴S, Δ³³S, and Δ³⁶S) is implicitly tied to the composition of the ancient atmosphere.

We have generated coupled QSI and δ¹³C_Org records from two cores through the Transvaal basin in South Africa (GKF01 and BH1-Sacha) and from three time-equivalent cores through the Hammersley basin in Western Australia. These records reveal similar, apparently synchronous, co-variation between deviations from “typical” Neoarchean Δ³⁶S/Δ³³S values and negative δ¹³C_Org values (C-S anomalies). We have interpreted these trends to reflect increased incorporation of ¹³C-depleted substrates (methane) into sedimentary matter following apparently episodically enhanced methanogenesis in the prelude to the GOE. These records suggest the terminal Neoarchean atmosphere operated in a bi-stable fashion, oscillating between a clear skies and organic-rich hazy state (Zerkle et al., 2010; Izon et al., in review).

We speculate that these C-S anomalies reflect enhanced nutrient availability, facilitating the proliferation of an oxygenic photosynthetic/methanogenic biosphere, concomitantly amplifying biogenic CH₄ and O₂ fluxes and altering atmospheric chemistry. Here we present a new S- and C-isotope data set, derived at unprecedented resolution, encompassing the youngest C-S anomaly identified in core GKF01. These analyses will be combined with Fe-speciation and trace-metal data to elucidate whether atmospheric methane levels were regulated by climatically driven changes in nutrient fluxes to the marine biosphere. Taken together, our research serves as an initial foray to determine how, and why, the early Earth redox balance shifted, initiating oxygenation and ultimately spawning a habitable planet capable of supporting macroscopic life.