Greenalite provides a snapshot of metal availability in an Archean shelf environment.

Thermodynamic models predict that marine metal availability has changed over geological time, particularly in the Archean Eon (4.0 – 2.5 billion years ago), when seawater was anoxic and Fe²⁺-rich. Since metals are essential micronutrients required to build metalloproteins, changes in metal availability in seawater would have influenced evolving microbial ecosystems. Recent work on Archean rocks has highlighted the role of greenalite as an abundant, primary precipitate in Archean seawater, and its potential as a faithful geochemical archive. Greenalite can be exceptionally well preserved in early diagenetic chert, providing protection from diagenesis and metamorphic alteration. Furthermore, experimental work has demonstrated that several key metals enter the greenalite precursor phase during precipitation, and the associated partition coefficients are consistent under a range of conditions. Furthermore, most metals are retained in the structure during heating and crystallisation, suggesting that greenalite could represent a robust archive of the metal content of early oceans. Here, we present mineral-specific laser ablation ICP-MS data for natural greenalite from the ~2.5 Ga Transvaal Supergroup, South Africa. Petrographic relationships and rare earth element patterns suggest this greenalite precipitated from seawater in a shelf environment. We place metal abundance into a quantitative framework to predict metal availability in Archean seawater. Our calculations suggest that V and Zn were depleted, Ni was similar, Co was enriched, and Mn was super-enriched in this setting compared to modern marine environments. These results are consistent with predictions based on marine chemistry and proteomics, as well as some bulk geochemical records.