Oxygen and hydrogen isotope compositions of seawater are controlled by volatile fluxes between mantle, lithospheric (oceanic and continental crust) and atmospheric reservoirs. Throughout geologic time oxygen was likely conserved within these Earth system reservoirs, but hydrogen was not, as it can escape to space . Hydrogen isotope ratios of serpentinites from the ~3.8Ga Isua Supracrustal Belt in West Greenland are between -53 and -99‰; the highest values are in antigorite ± lizardite serpentinites from a low-strain lithologic domain where hydrothermal reaction of Archaean seawater with oceanic crust at elevated temperatures was geochemically preserved, indicating that the δDSEAWATER was at most 25 ± 5‰ lower than modern VSMOW. We propose that the progressive increase in δDSEAWATER since this time is due to preferential uptake of hydrogen in continent-forming minerals and to hydrogen escape via biogenic methanogenesis . Mass balance considerations within the Earth system places a cumulative upper limit on elemental hydrogen loss to space of ~1.8x1022mol elemental hydrogen H, constraining maximum Archaean atmospheric methane levels at ~3.8Ga to <500ppmv (depending on the volume of continents present at that time), and the mass of Early Archaean oceans to ~109 to 126% of present day oceans. Oxygen isotope analyses from these Isua serpentinites (δ18O = +0.1 to 5.6‰ relative to VSMOW) indicate that early Archaean δ18OSEAWATER similar to modern oceans. Our observations suggest that the low-δ18O values of Precambrian sedimentary cherts and carbonates are not a consequence of isotope variability of seawater or extreme ocean temperatures [3,4], but rather are due to isotopic exchange with shallow hydrothermal fluids on the ocean floor or during diagenesis .  Lécuyer et al. (1998) Chem. Geol. 145, 249-261.  Catling et al. (2001) Science 293, 839-843.  Hren et al. (2009) Nature 462, 205-208.  Jaffrés et al. (2007) Earth-Science Reviews 83, 83-122.  Blake et al. (2010) Nature 464, 1029-1032.