A CO ₂ greenhouse efficiently warmed the early Earth and decreased seawater ¹⁸ O/ ¹⁶ O before the onset of plate tectonics

Abstract

The low 18O/16O stable isotope ratios (δ18O) of ancient chemical sediments imply ∼70 °C Archean oceans if the oxygen isotopic composition of seawater (sw) was similar to modern values. Models suggesting lower δ18Osw of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Δ’17O) of Precambrian cherts. We show that high CO2 sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δ18Osw of seawater on the early Earth without affecting the Δ’17O. Hence, the controversial long-term trend of increasing δ18O in chemical sediments over Earth’s history partly reflects increasing δ18Osw due to decreasing atmospheric pCO2. We suggest that δ18Osw increased from about −5‰ at 3.2 Ga to a new steady-state value close to −2‰ at 2.6 Ga, coinciding with a profound drop in pCO2 that has been suggested for this time interval. Using the moderately low δ18Osw values, a warm but not hot climate can be inferred from the δ18O of the most pristine chemical sediments. Our results are most consistent with a model in which the “faint young Sun” was efficiently counterbalanced by a high-pCO2 greenhouse atmosphere before 3 Ga.

A mineral-based origin of Earth’s initial hydrogen peroxide and molecular oxygen

“…Although the isotope fractionation factors in the radical reactions at quartz-water interfaces could not be determined in this study, we highlight that such rapid oxygen transfer between water molecules and SBRs on abraded quartz surface is a hitherto unrecognized component that has broad implications and deserves further investigation in the future. One example of importance is the secular changes of oxygen isotope compositions in cherts and other sedimentary rocks (e.g., carbonate and shale) across the Earth’s history, which have been interpreted as a hot (up to 70°C) Archean Ocean, changes of seawater oxygen isotope compositions, progressive diagenetic alteration, or their combinations ( 66 , 71 – 73 ). It is preliminary to state that this is correct, but the experiments clearly suggest that modeling of these systems would be of high value.…”

A New Insight Into Seawater-Basalt Exchange Reactions Based on Combined δ18O—Δ′17O—87Sr/86Sr Values of Hydrothermal Fluids From the Axial Seamount Volcano, Pacific Ocean

“…Besides the utility of epidote measurements described in the Comparison With Mineral Data: Measured and Computed, we point out that many measurements of Precambrian cherts are shifted in Δ′ 17 O negatively compared to seawater-silica equilibrium curve [i.e. towards basaltic equilibrium (Sengupta et al, 2020;Zakharov et al, 2021b;Herwartz et al, 2021;Wostbrock and Sharp, 2021)]. Apart from changes in seawater temperature and diagenetic effects, the δ′ 18 O-Δ′ 17 O values of such cherts may indicate influences from hydrothermal solutions contributing to abiotic silica precipitation in the Archean (de Ronde et al, 1994;De Ronde et al, 1997;Hayles et al, 2019;Liljestrand et al, 2020;Zakharov et al, 2021b).…”

Was There Land on the Early Earth?

“…This geochemical approach is promising, though the interpretation of relevant data always involves geochemical reasoning or modeling, which may not be sufficiently robust. For example, the work of Johnson and Wing [129] is based on the oxygen isotopic composition of sea water, the interpretation of which is still controversial [198,199].…”