Pope, Emily Catherine3; Rosing, Minik Thorleif3; Bird, Dennis K.2
1 Natural History Museum of Denmark, Natural History Museum of Denmark, Faculty of Science, Københavns Universitet2 Stanford University3 Natural History Museum of Denmark, Natural History Museum of Denmark, Faculty of Science, Københavns Universitet
Oceanic serpentinites and hydrous silicate minerals that are formed in subduction-related volcanic and hydrothermal environments obtain their hydrogen isotope composition (δD) from seawater-derived fluids, and thus may be used to calculate secular variation in δDSEAWATER. Hydrogen isotope compositions of serpentine and fuchsite from the ca. 3.8 Ga Isua supracrustal belt in West Greenland range from -99 to -53‰, and -115 to -61‰, respectively. The highest values indicate that Eoarchean seawater had a δD that was at most 25 ± 5‰ lower than modern oceans. Deuterium-poor water is potentially sequestered from oceans over geologic time by continental growth, large-scale glaciation events, biologically mediated hydrogen escape to space, and subduction of water that is chemically bound in alteration minerals of the ocean crust. The extent to which any of these fluxes have occurred since the Eoarchean is constrained by the hydrogen isotope composition of the minerals at Isua. We developed a first-order mass balance model of δDSEAWATER evolution delimited by δD of Isua serpentine and fuchsite and that of modern seawater. The ca. 25‰ change in δDSEAWATER can be accounted for by the development of the modern cryosphere (9‰), continental growth (as much as 10‰ if continents grew continuously from 0% to 100% of their modern volume since 3.8 Ga) and hydrogen escape to space before the rise of an oxygen-rich atmosphere. ~1.0 ± 0.8 x 1022 mol of elemental hydrogen released to space via biogenic methanogenesis would account for the remainder of the observed isotopic shift in seawater. This estimate is consistent with independent approximations of atmospheric methane concentrations in the early Archean, and is within an order of magnitude of the amount of hydrogen escape required to oxidize the continents before the rise of atmospheric oxygen. Volatile ingassing to the mantle at subduction zones and outgassing in arcs and mid-ocean ridges are apparently equivocal on modern Earth, suggesting there is currently no net flux of water into the mantle. However, estimates that the mass equivalent of Earth’s modern oceans have been sequestered into the deep mantle during subduction over Earth history would significantly change the factors controlling global hydrogen budget as we have proposed. Incorporating this additional outgoing flux of deuterium-poor water from oceans (ΔDMANTLE-SEAWATER = -60 ± 20‰) since the formation of Isua serpentines would require that either continental growth must have happened primarily before 3.8 Ga, and/or methane was not a significant atmospheric gas in the Eoarchean. Developing a more robust geological record of δDSEAWATER using other well-preserved vestiges of hydrated ocean crust or arc-related hydrous minerals is critical to resolving the relationship between these controls on the global water budget.
The Faculty of Science; composition of the hydrosphere; stable isotope geochemistry; early environment of Earth; evolution of Earth