06.2.2021

A CO2 greenhouse efficiently warmed the early Earth and decreased seawater 18O/16O before the onset of plate tectonics

By Daniel Herwartz, Andreas Pack, and Thorsten J. Nagel

Significance

Due to the lower luminosity of the young Sun, climate modelers struggle to explain why the climate on early Earth was not freezing cold. This “faint young Sun paradox” is in conflict with apparently hot Archean ocean temperatures (∼70 °C) that can be estimated from the 18O/16O stable isotope ratio of chemical sediments. We show that the later temperatures had been overestimated because the 18O/16O of seawater also changed over time due to intense carbonatization and silicification of the oceanic crust, which consumes heavy 18O. Because these processes require high fluxes of CO2, greenhouse warming by a CO2-rich atmosphere appears most feasible to explain all observations.

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.

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