Climate control on banded iron formations linked to orbital eccentricity.

Astronomical forcing associated with Earth's orbital and inclination parameters ("Milankovitch" forcing) exerts a major control on climate as recorded in the sedimentary rock record, but its influence in deep time is largely unknown. Banded iron formations, iron-rich marine sediments older than 1.8 billion years, offer unique insight into the early Earth's environment. Their origin and distinctive layering have been explained by various mechanisms, including hydrothermal plume activity, the redox evolution of the oceans, microbial and diagenetic processes, sea level fluctuations, and seasonal or tidal forcing. However, their potential link to past climate oscillations remains unexplored. Here we use cyclostratigraphic analysis combined with high-precision uranium-lead dating to investigate the potential influence of Milankovitch forcing on their deposition. Field exposures of the 2.48-billion-year-old Kuruman Banded Iron Formation reveal a well-defined hierarchical cycle pattern in weathering profile that is laterally continuous over at least 250 kilometres. The isotopic ages constrain the sedimentation rate at 10 m/Myr and link the observed cycles to known eccentricity oscillations with periods of 405 thousand and about 1.4 to 1.6 million years. We conclude that long-period, Milankovitch-forced climate cycles exerted a primary control on large-scale compositional variations in banded iron formations.