Enrichment of Potassium During Simulated Weathering of Chloride Evaporites

This paper reports a simple, prebiotically feasible process for the enrichment of potassium salts from the seepage of water through evaporated mixtures of chlorides thought to dominate Earth’s early ocean. When water passes through a mixture of NaCl and KCl, the solid dissolves and the initial eluate contains NaCl:KCl in a 2.4:1 molar ratio, consistent with the invariant point of the NaCl–KCl–H2O ternary system. The ratio of eluted NaCl:KCl remains constant until the supply of one of the salts in the solid phase is exhausted and the eluate transitions to a saturated solution of the remaining salt. If the initial mixture of salts contains <2.4:1 NaCl:KCl, a pure solution of KCl will eventually elute from the system. Similar behavior extends to the weathering observed in NaCl–KCl–MgCl2–CaCl2–H2O mixtures. For this quinary system, the eluate initially contains NaCl:KCl:MgCl2:CaCl2 in an approximate ratio of 1:1.6:4:4. CaCl2 and MgCl2 are depleted rapidly relative to NaCl and KCl, and once the calcium and magnesium are exhausted, the system returns to the behavior observed of a NaCl–KCl–H2O ternary system. The universal enrichment of potassium ions by cellsdriven by ion transporters and channelssuggests that potassium is critical to life as-we-know-it and was possibly a biochemical relic of the environment where life originated. The weathering experiments described here collectively demonstrate a potential mechanism for the enrichment of potassium salts that could have transpired naturally on the Prebiotic Earth to generate the sort of potassium-rich environment that may have fostered early life on the planet.