The Origin of the Ocean, How Early, How Big: Evidence from the Paleoarchean
Abstract
Hydrogen and oxygen isotope ratios on Archean rocks provide constraints on the origin of the ocean. The invariance in the distribution of oxygen isotopes of greenstones and ophiolites of any age suggest that the mechanism for buffering of the oxygen isotope ratio of seawater was operative pre-4.0 Gyr ago. In the plate tectonic regime, the thicknesses of the volcanic and oceanic (gabbro) layers in the oceanic crust set up the correct temperature and permeability gradients for hydrothermal alteration over a broad temperature range that results in the buffering of the oxygen isotopic composition of the ocean and as such the oxygen isotopic data suggest the presence of early pre-Archean plate tectonics . Given the ranges of modern tectonic rates, the time constants (100 million year scale) for the plate tectonic buffering of seawater suggest that the ocean would move within a few per mil of its present day value within a few hundred million years of its formation. In contrast to oxygen isotopes, there is no consensus on the origin and isotopic composition of the Earth's initial hydrogen reservoir and there is no obvious temperature driven buffering mechanism for deuterium in seawater. There is also a remarkable overlap between the D-isotopic compositions of sedimentary, metamorphic and primary igneous rocks which are all depleted in D relative to modern seawater. This suggests that there is much recycling that occurs as a result of subduction or that by coincidence the D fractionation between the hydrosphere and the low temperature products of chemical weathering is close to the difference between modern seawater and high temperature fractionation associated with primary water derived from igneous magma. The major processes that affect the D isotopic composition of the ocean are hydrogen loss at the top of the atmosphere or through rehydration of the mantle by subduction (both of these enrich the remaining ocean in D) and the D-depletion of the ocean relative to its present-day value by outgassing of magmatic or metamorphic water. Only the outgassing of presumptive D-depleted juvenile water depletes the oceans in D relative to their present day value. Remelting of metasediments or dehydration of metamorphic rocks only reverses the D-enrichment associated with near surface processes. Evaluation of the potential hydrogen loss depends upon a knowledge of the juvenile D content of the Earth, the fractionation associated with the outgassing and the bulk value of the lithosphere and the hydrosphere. New hydrogen isotope data on the Paleoarchean at 3.5 Gyr does not support the idea of large amounts of hydrogen loss or gain to oceans suggesting that the oceans achieved their present day volume very early in Earth history. Since freeboard of the continents is tied to global spreading rates and the thickness of the continental crust, ocean volume and Archean sedimentary associations provide insights on the evolution of the early crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.U13A0031G
- Keywords:
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- 1022 GEOCHEMISTRY / Composition of the hydrosphere;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry