Enhanced Phosphorous Solubility in Sulfate Brines and the Potential for Prebiotic Synthesis on Ancient Mars.

Session: Advancing Mineral Science and Exploring Planetary Surfaces: In Honor of MSA Dana Medalist, Elizabeth B. Rampe, Part I

Presenting Author:

Authors:

Abstract:

Phosphorous (P) is a vital ingredient for life. Experimental synthesis of the building blocks of life apparently requires P concentrations upwards of 100s of millimolar, but low solubility of apatite in most natural water’s limits P concentrations to micromolar levels. This conundrum, known as the ‘phosphate-problem’, is overcome in waters where conditions enhance phosphate solubility. For instance, Toner and Catling (2020, PNAS) observed that alkaline soda lakes, which have low calcium concentrations due to elevated alkalinity and associated low solubility of calcium carbonate, have the highest phosphate concentrations of any known natural waters. However, mineralogical observations of the Martian surface imply that soda lakes may have been rare on ancient Mars, diminishing the chances of this soda lake mechanism for enhanced P solubility. Yet, widespread evidence of co-occurring Ca-sulfate (e.g., gypsum) and Mg-sulfate (e.g., starkeyite) minerals indicate a mechanism analogous to that operating in soda lakes could similarly enhance P concentrations in Mars-relevant Mg-sulfate brines. In Mg-sulfate brines, the reduced concentrations of Ca resulting from gypsum precipitation during evaporation could enhance apatite solubility, like calcium carbonate precipitation during evaporation enhances apatite solubility in soda lake brines.

We explore the plausibility of this sulfate-based mechanism for enhanced P solubility through investigations of the natural, Mg-sulfate-rich Basque Lakes of British Columbia, Canada. Mineralogical analysis of Basque Lake sediments indicates that the main carbonate sink is magnesite, whose precipitation diminishes alkalinity and permits subsequent precipitation of gypsum and Mg-sulfate minerals, such as epsomite, during evaporation. Porewater samples reveal P concentrations of up to 700 µmol/L, and lake waters up to 100 µmol/L. These both sit far below the modelled P concentrations of up to 20 mmol/kg when assessing hydroxyapatite solubility in Mg-Sulfate brines, potentially due to biological uptake. Preliminary experiments and modelling show the interplays between gypsum and epsomite ultimately control apatite solubility and consequently maximum achievable P concentrations in prebiotic Mg-sulfate brines. Thick, Mg-sulfate-bearing successions identified on Mars therefore indicate the possibility of evaporative intervals where P solubility could have risen and increased the likelihood of prebiotic building block synthesis on ancient Mars. Our investigations thus imply that the deposition of Mg-sulfate-bearing strata on ancient Mars, while indicative of broader transition towards diminished habitability, may also provide evidence of a period conducive to prebiotic synthesis reactions.