Warm Archean oceans reconstructed from oxygen isotope composition of early-life remnants

Deciphering the surface conditions on the Earth during Archean times (> 2.5 billion years ago – Ga) is crucial to constrain the conditions that promoted the development of life. The progressive shift through time of the oxygen isotopic compositions of Precambrian siliceous sediments – the so-called cherts – has been interpreted as indicating a secular decrease of seawater temperature by 50-80 °C from the early Archean to the present-day. However, this interpretation has been questioned, notably because it assumes that the seawater oxygen isotopic composition has remained globally constant since 3.5 Ga, though this has never been tested by direct isotopic measurements on Archean samples. Here we report measurements of the oxygen isotopic composition of carbonaceous matter indigenous to Precambrian cherts up to ca. 3.5 Ga. These new results demonstrate that the oxygen isotope composition of seawater during most of the Precambrian remained around 0 ± 5 ‰, which is consistent with the composition of present day seawater. Combined with the chert oxygen isotope composition record, this indicates that ca. 3.5 Ga ago ocean bottom-water temperatures were ~50-60 °C higher than today.