Multiple oscillations in Neoarchaean atmospheric chemistry

Abstract

The Great Oxidation Event (GOE) represents a crucial juncture in Earth history, signifying the rise in atmospheric oxygen from parts per million to per cent levels at ~2.45-2.32 billion-years-ago (Ga). Although planetary oxygenation undoubtedly led to the inception of the contemporary Earth system, the trigger(s) and mechanism(s) controlling this chemical reorganisation remain elusive. Quantitative estimates of the atmosphere's composition in the prelude to the GOE are central to understanding this oxygenation event. Previous analyses of 2.65-2.5 Ga sediments from the Griqualand Basin (South Africa) invoke a tantalising picture of an unusual Earth environment, alluding to an atmosphere periodically dominated by a layer of organic particles ("haze") formed from methane photolysis. However, as yet this hypothesis has remained untested. Here we present four new coupled carbon and quadruple sulphur isotope records from distal, time equivalent (2.7-2.5 Ga), sedimentary successions from South Africa and Western Australia. These extended records reveal similar chemostratigraphic trends, supporting a dynamic terminal-Neoarchaean atmosphere, oscillating between a hazy state at elevated methane concentrations, and a haze-free anoxic background state. We suggest these atmospheric aberrations were related to heightened biogenic methane fluxes fuelled by enhanced nutrient delivery from climatically or weathering induced feedbacks. These data question the canonical view of a simple, unidirectional planetary oxygenation and signify that the overture to the GOE was governed by complex feedbacks within the Earth-biosphere system.