Constraining continental emergence using δ18O from Proterozoic seawater analyzed using high precision laser mass spectrometry

Abstract

The timing of widespread continental emergence above sea level in Earth's history is poorly constrained, with published estimates between 3.5 to 0.7 Ga. Establishing the timing of widespread emergence is critical in understanding the interconnected history of Earth's oceans, geosphere, and biosphere. Emergent continents can act as a source of nutrients to the biosphere (Large et al., 2018), can modulate climate via the carbonate-silicate weathering feedback (Walker et al., 1981), and may have been habitats for the origin and early evolution of life (Damer and Deamer, 2020).

A tool to further constrain continental emergence is the oxygen isotope composition of seawater (OISW). The OISW reflects the balance of two major fluxes: hydrothermal alteration of ocean crust and continental weathering. If continents are submerged, no weathering occurs, and the OISW will be relatively enriched in 18O, since weathering sequesters 18O relative to 16O. Previous work on Archean crust shows that hydrothermally altered oceanic crust can be a record of OISW, with limited emergence predicted at 3.2 Ga. We extend this work to sites between 2.68 to 1.72 Ga: the Noranda-Benoit volcanic complex, Snow Lake camp, and the Pecos greenstone belt. The first three sites have been characterized previously, and we present new O-isotope data from Pecos. Whole-rock δ18O data from Pecos range from 2.93 to 9.04‰, similar to modern crustal settings. Inversion of whole rock δ18O data, anchored by chlorite and quartz geothermometry at Pecos indicates OISW was -0.21 +/- 39‰ at 1.72 Ga. Our work suggests a two-step decrease in OISW, one between ~3.0 Ga to 2.5 Ga and a second between 1.5 to 0.7 Ga. This two-step evolution could reflect two major phases of supercontinent formation and the associated emergence of continental crust.