Ocean mixing timescale through time and implications for the origin of iron formations

Abstract

(Figure Presented) Our study examines whether the ocean mixing timescale has remained constant throughout Earth’s history. If varied, it could have affected the distribution of geochemical tracers in ancient seawater, complicating interpretations of sedimentary archives. For example, the modern ocean mixing timescale is similar to the neodymium (Nd) residence time, allowing distinct Nd isotopic compositions (ε<inf>Nd</inf>) to coexist in different oceanic basins. However, it is unknown whether the Archean ocean was more or less isotopically heterogeneous, and how this was recorded by banded iron formations (BIFs). We use an Earth system model to investigate the sensitivity of ocean mixing dynamics to variations in day length, surface pressure, continental configuration, and tidal dissipation. Our experiments indicate that the ocean mixing timescale fluctuated between a few hundred and a couple of thousand years since the Archean. Coupling our mixing model with a Nd cycling model in the Archean ocean, our simulations suggest that hydrothermal fluids could have mixed with other water masses carrying Nd from sediments and rivers before reaching the continental shelf. The large range of ε<inf>Nd</inf> in some BIFs might therefore reflect the weathering of exposed juvenile and ancient igneous rocks, challenging prevailing views on the hydrothermal source of iron in BIFs.