Limited continental emergence before 3.2 billion years recorded in oceanic crust oxygen isotopes
Abstract
Today, the Earth has bi-stable crust: oceanic and continental. Emergent continental crust (i.e., crust above sea level) drives a number of nutrient and elemental cycles in addition to climate feedbacks via weathering and albedo. When during Earth history crust became emergent, however, is poorly constrained. The oxygen isotope (δ18O) value of seawater can provide a record of continental emergence. Since this value is controlled by a balance of seafloor alteration and subaerial continental weathering, knowing its value through time can constrain when continents became emergent.
We built an inverse tracer-mass balance model to to estimate seawater δ18O using altered ocean crust. Specifically, we used data from a number of Phanerozoic settings and the 3.2 Ga Panorama District in the Pilbara, Australia, to estimate seawater δ18O through time. We find seawater δ18O to be +1‰ at 3.2 Ga, or 2‰ enriched compared to the modern, ice-free ocean. This value is well explained by a simple isotope exchange model that rapidly evolves from an initially high value of +7‰ to +1‰ given only oceanic crust alteration. Allowing continental emergence and subaerial weathering in our exchange model after 3.2 Ga predicts that seawater δ18O will drop from +1‰ to the modern value of -1‰, which matches existing constraints. That is, we suggest limited continental emergence before 3.2 Ga. We further explore a wide range of possible seawater δ18O values and histories via Monte Carlo simulations. We investigate how different isotope exchange parameters and fluxes affect seawater δ18O, and how this informs the history of emergent continents and their interaction with the atmosphere and hydrosphere.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V31G0133J
- Keywords:
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- 1020 Composition of the continental crust;
- GEOCHEMISTRY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY;
- 8110 Continental tectonics: general;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS