LINKING THE STABLE ISOTOPE EVOLUTION OF SEAWATER TO MODES OF CRUSTAL EVOLUTION

The evolution of the ocean is intimately tied to the evolution of the crust. Comparative planetology suggests the existence of Hadean oceans, formed rapidly as a result of planetary accretion and differentiation. The temperature dependence of ¹⁸O/¹⁶O fractionation dictates that the primordial oceans would have been ¹⁸O-enriched relative to the mantle and modern ocean. The primordial ocean hydrogen isotope composition is expected to have been D-depleted with respect to the modern ocean. Seafloor hydrothermal alteration at mid-ocean ridges is efficient enough that oxygen isotopic composition is buffered near its present day value within a few hundred million years of the onset of plate tectonics. Growing oxygen isotope data sets on Archean greenstone belts indicate that the oceans had achieved steady state by the beginning of the Archean pushing the origin of the ocean and plate tectonics well into the Hadean. The volume of the present day ocean is large enough so that in the absence of continents, the Earth would be a water world. The depth of the ocean basins is tied to both the seafloor spreading rates and the volume and thickness of the continental crust. Deep ocean basins as well as subaerial continents require the development of thick buoyant continental crust. Erosion ensures that mean continental elevation tracks sea level. The major processes affecting the hydrogen isotope composition of the ocean involve D-enrichment by either hydrogen loss to space (very large fractionation, ~200 per mil) or to the crust and mantle in the form of hydrous minerals (ten’s of per mil); both processes decrease the oceanic volume. However, the D/H ratio of the fully-formed ocean is difficult to shift given the normal fluxes of water associated with the rock cycle (time constants on the order of billions of years); this is confirmed by the limited and constant range of data from hydrothermally-altered submarine rocks. The difference between the hydrosphere plus lithospheric water and possible juvenile water derived from partial melts of the mantle may be a measure of cumulative water loss (<20%). Alternatively, the coincidence in the ranges of D/H ratios in sedimentary, metamorphic and primary igneous rocks may suggest that the water sampled by mantle-derived melts is predominantly water recycled into the lithosphere at subduction zones.