The crystallization of sodium carbonate minerals depends on pCO2 and can be used to estimate ancient atmospheric CO2 concentrations. At present atmospheric pCO2 of ~380 ppm, trona (NaHCO3.Na2CO3.2H2O) commonly forms in modern alkaline lakes; natron (Na2CO3.10H2O) occurs in low temperature settings, but nahcolite (NaHCO3) is rare because it precipitates only under elevated pCO2. The dominant sodium carbonate of the Green River Formation, Piceance Creek Basin, Colorado, is nahcolite, in places interlayered with halite (NaCl). Nahcolite laminae, composed of microcrystalline chemical mud, precipitated at the air-water interface of a saline lake from surface brines in contact with the atmosphere, which suggests elevated Eocene atmospheric CO2. The minimum pCO2 at which nahcolite forms is ~1330 ppm; precipitation with halite requires a minimum pCO2 of ~1125 ppm. Crystallization of nahcolite+halite at paleolake temperatures of 20º-35º C fixes early Eocene atmospheric pCO2 at ~1125-2985 ppm, which supports the causal connection between high pCO2 and Eocene global warmth. Trona, not nahcolite, is the dominant sodium carbonate mineral in the coeval Wilkins Peak Member of the Green River Basin, Wyoming. Explanations for the simultaneous crystallization of trona and nahcolite include: Green River Basin trona formed at higher temperatures than Piceance Creek basin nahcolite, and under different geochemical conditions (higher brine aNa+ and pH) or that high pCO2 concentrations were produced by organic processes in the Piceance Creek lake system.

Trona is the dominant sodium carbonate mineral in younger deposits (i.e., Beypazari trona, Turkey, 21.5 Ma), which suggests atmospheric pCO2 decreased below 1125 ppm after the Eocene. Trona is the important sodium carbonate in Pleistocene deposits, Searles Lake, California; trona and natron are the major minerals in modern alkaline saline systems, all of which indicates that sodium carbonate mineralogy over the Cenozoic has been controlled by atmospheric pCO2.