GSA Connects 2021 in Portland, Oregon

Paper No. 214-3
Presentation Time: 8:35 AM


TOSCA, Nicholas1, BRADY, Matthew P.1 and TOSTEVIN, Rosalie2, (1)Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, United Kingdom, (2)Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa

The universal and deep-seated importance of phosphorus in biology has long suggested that it was incorporated early in the history of life. More recently, advances in prebiotic systems chemistry have shown that at high concentration, PO4 performs an array of chemical functions; it orchestrates the selective formation of amino acids, lipid precursors, and nucleotides from one multi-component reaction network, assuming a central role in the chemical origins of life. Nevertheless, soluble PO4 is widely thought to have been scarce on the prebiotic Earth, confining scenarios for the chemical origins of life to specific environments where PO4 may have reached high concentration.

To quantify the behaviour of PO4 in prebiotic environments, we determined vivianite solubility in synthetic seawater solutions at 25oC as a function of salinity and pH. New and existing experimental data were then used to develop and optimise a Pitzer ion interaction-based thermodynamic model including aqueous Fe-phosphate complexation. The model closely reproduces measured stoichiometric dissociation constants of phosphoric acid in seawater media as a function of salinity, and predicts a strong salinity control on vivianite saturation (arising from aqueous metal-phosphate complexation), consistent with an apparent locus of vivianite deposition in brackish and lacustrine systems. Application of the model to published modern marine pore water datasets reveals that equilibrium vivianite solubility is precisely maintained across depth intervals where the mineral has been unambiguously identified.
These data predict prebiotic marine PO4 concentrations 3-4 orders of magnitude higher than currently estimated, and highlight seawater as a principal PO4 source to a range of terrestrial environments. Reaction path models show that small changes in the alkalinity of the initial fluid in turn produce PO4-rich fluids with a range of pH values, salinities, and/or composition upon evaporation. This expands the combinatorial possibilities for prebiotic synthesis and suggests that seawater-dominated fluids could have sustained primitive cellular systems dependent on variable physico-chemical conditions for RNA separation and replication, peptide elongation, and fatty acid vesicle formation. Together, the chemistry of ancient Fe-rich environments ensured that PO4 was an integral part of prebiotic and early biotic landscapes, endowing life on Earth with its most important and versatile building block.