A hot early Earth? Assessing Archean and Proterozoic temperature history under different geological constraints.

Abstract:

The temperature evolution of Archean and Proterozoic Earth remains controversial. The “Faint Young Sun” paradox has been a long-standing problem, attempting to resolve evidence for oceans back to at least 4 billion years ago with knowledge that the Sun was likely significantly dimmer. Further, geochemical reconstructions based on δ¹⁸O and δ³⁰Si data from cherts indicate warmer temperatures than today, over 30 °C throughout most of the Proterozoic, and exceeding 50 °C in the Archean and early Proterozoic. Isotopic analyses of phosphates point to hot Archean temperatures below 40 °C. These estimates are all higher than modern-day Earth’s average temperature of ~15 °C. Similarly, a lack of evidence for glaciation in the mid-Proterozoic seems to corroborate the narrative of a warmer early Earth.

Here, we use Atmos, a 1-D coupled photochemistry/climate model to study the possible range of surface temperatures for Archean and Proterozoic Earth based on a comprehensive set of literature constraints including atmospheric composition, pressure and surface albedo. We systematically assess which combinations of constraints - when applied simultaneously - are compatible with different temperature reconstruction approaches and which are incompatible.

Our results resolve the classic “Faint Young Sun” paradox - demonstrating average global surface temperatures permitting bands of ice-free oceans for all combinations of constraints considered in this study. However, they struggle to reconcile the geochemical evidence for high surface temperatures in the early Proterozoic with the reduced insolation from a faint young Sun if all geological and geochemical constraints suggested in the literature (e.g. CO₂ concentrations inferred from paleosols) are considered. They cannot explain the “hot early Earth” hypothesis from isotopic data unless other geological constraints are ignored.