Response to Concept 3, One of Many Possible Responses

These ubiquitous minerals are often referred to as “evaporites” because the flow system transporting the solutes forming minerals is driven by the combination of an upward hydraulic gradient and the removal of water by evaporation. Evaporites, however, may not be an appropriate term as calcite, dolomite, gypsum, and anhydrite commonly form in the fully saturated capillary zone a few tens of centimeters below the surface thus, are not directly impacted by evaporation. That is, the capillary zone is fully saturated so it is not clear how evaporation at the surface would impact their development without some kind of density driven convection on a local scale transporting solutes from the surface downward a few centimeters and the physics of such a process is very murky! Given Occam’s Razor, it is believed that in many systems the presence of these common minerals is a result of the annual thermal pulse from solar heating propagating downward. These four minerals are unusual in exhibiting retrograde solubility, that is, they become less soluble with increasing temperature in contrast to nearly all other 4000 naturally occurring minerals that are more soluble with increasing temperature. In summer the propagation of the annual solar pulse causes precipitation of theses minerals. In winter, the propagation of cooler temperatures causes solution of these same retrograde minerals thus, net accumulation over time is a delicate balance of greater groundwater flux at times of mineral precipitation (when the summer solar pulse arrives at that depth) and less at times of mineral dissolution (when the winter low temperatures prevail). Temperature is also a factor in precipitation of normal soluble minerals on the surface because they precipitate as the daily or annual temperature cools. Thus, both temperature variation and the solute flux control mineral accumulation in this environment.

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