Oxygen isotopes in hydrothermally altered ocean crust record seawater δ18O ... - Benjamin Johnson

Oxygen isotopes in hydrothermally altered ocean crust record seawater δ18O evolving with continental growth and emergence
Benjamin Johnson
Iowa State University
Emergent continental crust interacts with the Earth system in a number of ways, modulating climate and providing nutrients through weathering, and providing physical habitat space for organisms. The timing of widespread continental emergence above sea level in Earth's history is poorly constrained, with estimates between 3.5 to 0.7 Ga.
Here, we use the oxygen isotope composition of seawater (OISW) as a proxy for widespread continental emergence. The OISW is buffered by two major water-rock interactions: hydrothermal alteration of ocean crust and continental weathering, and is thus sensitive to tectonic evolution through time. Here we quantitatively enforce isotopic and fluid mass-balance over multiple dimensions in hydrothermally altered oceanic crust, we exploit the well-developed framework of geophysical inverse theory to estimate fluid δ18O values (i.e., seawater) with an associated uncertainty. Given independent estimates of temperature from, fluid inclusions, mineral-specific isotopic compositions, and/or chlorite geothermometry, we applied this approach to a series of hydrothermally altered oceanic-crustal rock sections ranging in age from 0.2 Ma to 3.2 Ga to produce an expanded record of seawater δ18O values through Earth history. We suggest that seawater δ18O was high (≈3‰) until the Mesoarchean Era, after which it dropped in two discrete steps of ≈2‰ each, between ≈3 and 2.5 Ga and ≈2 and 1.5 Ga, reaching modern, ice-free values of -1‰ between 1.5 and 1 Ga. These steps coincide with peaks in the number of preserved zircons, and may reflect a pulsed onset of widespread, continental crust generation at subduction zones and Wilson cycling.