Rosalie Tostevin

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.

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.

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.

Chert is abundant in Archean and Paleoproterozoic rocks and is commonly densely packed with authigenic Fe(II)-silicate nanoparticles such as greenalite, indicating a close relationship between iron and silica deposition. We investigate the relationship between Fe(II)-silicate minerals and dissolved silica during precipitation, settling, and diagenesis using anoxic synthesis, sorption, and heating experiments. Excess silica is associated with the solid during precipitation, resulting in high molar Si/Fe ratios (<1.52) that exceed that of stoichiometric greenalite (0.67). At pH 8–8.5, silica sorbs to the surface, reaching sorption densities of 0.68 mmol Si per mmol Fe(II)-silicate. Furthermore, excess Si is released upon heating as the Fe(II)-silicate gel crystallizes. We suggest that Fe(II)-silicate minerals acted as an effective Si shuttle between the water column and the sediments in Archean and Paleoproterozoic marine environments, providing sites for the growth of early diagenetic chert, consistent with observations from the sedimentary record. Our results explain the exceptional preservation of greenalite in early chert and indicate that these minerals could provide a robust archive of marine geochemical data.

Detailed description of methods, extended results showing the nature of the synthetic precipitate, FTIR results showing changes in the precipitate following sorption, and a discussion of potential controls on sorption behavior.

Detailed description of methods, extended results showing the nature of the synthetic precipitate, FTIR results showing changes in the precipitate following sorption, and a discussion of potential controls on sorption behavior.