Isotope composition and volume of Earth’s early oceans

Abstract

Oxygen and hydrogen isotope compositions of Earth's seawater are controlled by volatile fluxes among mantle, lithospheric (oceanic and continental crust), and atmospheric reservoirs. Throughout geologic time the oxygen mass budget was likely conserved within these Earth system reservoirs, but hydrogen's was not, as it can escape to space. Isotopic properties of serpentine from the approximately 3.8 Ga Isua Supracrustal Belt in West Greenland are used to characterize hydrogen and oxygen isotope compositions of ancient seawater. Archaean oceans were depleted in deuterium [expressed as δD relative to Vienna standard mean ocean water (VSMOW)] by at most 25 AE 5‰, but oxygen isotope ratios were comparable to modern oceans. Mass balance of the global hydrogen budget constrains the contribution of continental growth and planetary hydrogen loss to the secular evolution of hydrogen isotope ratios in Earth's oceans. Our calculations predict that the oceans of early Earth were up to 26% more voluminous, and atmospheric CH 4 and CO 2 concentrations determined from limits on hydrogen escape to space are consistent with clement conditions on Archaean Earth. Both interpretations have been questioned based on the susceptibility of chemical sediments to diagenetic alteration over geologic time (2, 4), and for the latter, a lack of geologic evidence for the extreme greenhouse gas concentrations required to sustain such high temperatures under a less luminous young Sun (9).Minerals formed by metasomatic reactions between seawater and oceanic lithosphere provide an alternative proxy for early ocean chemistry (10-14). Here we report hydrogen and oxygen isotope compositions of 114 silicate mineral separates from the ca. 3.8 Ga Isua Supracrustal Belt (ISB) of West Greenland, including metamorphosed basalts, gabbros, and ultramafic rocks that represent fragments of Eoarchaean oceanic crust (see Fig. S1, Table S2). Minerals formed during heterogeneous late-stage CO 2 -or meteoric-dominated metamorphic overprinting are distinguished by characteristic trends in their coupled δD and δ 18 O values (Fig. S2). In contrast, serpentine minerals formed by reaction of seawater with ultramafic rocks (peridotites) preserved in a low-strain enclave of the ISB (15) define a different isotopic trend. Here we use these serpentines to constrain D∕H and 18 O∕ 16 O ratios of the Eoarchaean ocean. Serpentine Isotope GeochemistrySerpentine-group minerals are hydrous magnesian silicates commonly formed by hydrothermal infiltration of seawater into ultramafic oceanic crust, hydration of the mantle wedge by slabderived fluids, or fluid-rock interaction in continentally emplaced ultramafic lithologies (16)(17)(18). Serpentinized oceanic peridotites are dominantly composed of low-temperature (<300°C) polymorphs lizardite and chrysotile, and to a lesser extent antigorite, which forms at temperatures >250°C (17). In modern oceanic serpentinites, antigorites have δD between −46 and −30‰ (Figs. 1 and 2A, filled squares), values that indicate equilibrium with modern-...

Si isotope variability in Proterozoic cherts

“…While it is possible that O isotopes in Archean cherts reflect warm temperatures, the temperatures are warm because the ambient waters were hydrothermal, not seawater (e.g. Blake et al, 2010;Pope et al, 2012). To the extent that this is the case, Si and O isotopes might co-vary in Archean cherts not because they both record seawater T but because both reflect declining hydrothermal input through time.…”

WATER LOSS FROM TERRESTRIAL PLANETS WITH CO2-RICH ATMOSPHERES

“…Water is important on Earth most obviously because it is essential to all life, but major uncertainties remain regarding how it was delivered, how it is partitioned between the surface and mantle, and how much has escaped to space over time (Kasting & Pollack 1983;Hirschmann 2006;Pope et al 2012). Estimating the initial inventory is difficult because water delivery to planetesimals in the inner solar system during accretion was a stochastic process (Raymond et al 2006;O'Brien et al 2006).…”

Hydrogen-Nitrogen Greenhouse Warming in Earth's Early Atmosphere

“…3 we expressed the CO 2 carbonate-silicate weathering rate as 27. Modern ocean net primary productivity is concentrated in shallow regions close to continents (26), but in the early Archean, the continental crust volume was ∼ 3 times lower and the ocean volume may have been up to 25 percent greater than today (36,37). In the deep ocean away from submarine vents, rates of H 2 supply would be reduced by ∼ 10 3 compared to the mixed layer, implying a decrease of biological productivity there by a similar factor (26).…”