Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean

Abstract

The origin and evolution of Earth’s biosphere were shaped by the physical and chemical histories of the oceans. Marine chemical sediments and altered oceanic crust preserve a geochemical record of these histories. Marine chemical sediments, for example, exhibit an increase in their 18O/16O ratio through time. The implications of this signal are ambiguous but are typically cast in terms of two endmember (but not mutually exclusive) scenarios. The oceans may have been much warmer in the deep past if they had an oxygen isotope composition similar to that of today. Alternatively, the nature of fluid–rock interactions (including the weathering processes associated with continental emergence) may have been different in the past, leading to an evolving oceanic oxygen isotope composition. Here we examine approximately 3.24-billion-year-old hydrothermally altered oceanic crust from the Panorama district in the Pilbara Craton of Western Australia as an alternative oxygen isotope archive to marine chemical sediments. We find that, at that time, seawater at Panorama had an oxygen isotope composition enriched in 18O relative to the modern ocean with a δ18O of 3.3 ± 0.1‰ VSMOW. We suggest that seawater δ18O may have decreased through time, in contrast to the large increases seen in marine chemical sediments. To explain this possibility, we construct an oxygen isotope exchange model of the geologic water cycle, which suggests that the initiation of continental weathering in the late Archaean, between 3 and 2.5 billion years ago, would have drawn down an 18O-enriched early Archaean ocean to δ18O values similar to those of modern seawater. We conclude that Earth’s water cycle may have gone through two separate phases of steady-state behaviour, before and after the emergence of the continents. The water cycle was in two different steady states, before and after continental emergence, as recorded in the decreasing oxygen isotope values of seawater since the Archaean, according to an inverse geochemical model of the oceanic crustal record.