A window into transition metal availability in early palaeoproterozoic seawater.

Metals are required by all life to build metalloproteins, but the metal preferences of the dominant microbes have evolved over geological time. Consistent with this, experiments and models predict that metal availability in anoxic seawater during the Archean and Proterozoic eons (4.0-0.541 billion years ago) would have been radically different to today. Corroborating this in the geological record is challenging because bulk rock geochemistry reflects complex histories. Here we take a novel approach, determining the transition metal content of micron-scale laths of greenalite, a primary Fe(II)-silicate mineral, from the Kuruman Formation of the Transvaal Supergroup, South Africa. Our data provide a high-resolution snapshot of seawater chemistry ~ 2.46 Ga, and reveal striking compositional differences compared to today: Zn and V were relatively scarce, Ni was similar, Co was enriched, and Mn was highly-enriched. Our data are largely consistent with chemical predictions and overlap with constraints from a range of different geological archives. Ancient seawater was therefore dominated by Fe and Mn, consistent with evidence that early life preferentially utilised these transition metals. Extremely high Mn concentrations could have interfered with cellular homeostasis, as well as disrupting DNA synthesis, potentially driving faster rates of evolution.