Clay Mineral Reactions and Transport in the Critical Zone

The expression “Rock+Acid+Water -> Clays+Ions+Bicarbonate” may seem simple, but it encompasses many complex processes working in the Earth’s Critical Zone (CZ). The SE United States Calhoun CZ Observatory (O) has been observing the CZ for decades, leading to new understanding of how geological and human factors impact landscape evolution and management. The distribution of clays in the CZO is linked to the response of land to various factors that influence soil formation. These factors include vegetative cover (such as crops, regenerative pines, and old-growth hardwoods), interfluve order (old ridge-crests versus young legacy sediments), underlying geology (felsic versus mafic rocks), climate change (wetter versus dryer seasons), and denudation rates (erosion rates ranging from 1 to 1000 m/Ma).

Through observations, we have found that plant cover and rooting depth affect soil gas (CO₂ and O₂) distributions, which in turn affect clay mineral hydrolysis and redox reactions. We have also found that clay mineral signatures show increasing indices of chemical alteration along increasing interfluve orders. Additionally, felsic rocks weather more deeply into the CZ than mafic rocks under similar biota, climate, relief, and time conditions. Furthermore, we have found that the chemical signatures captured in clays may not reflect average conditions, but rather specific points in soil moisture states. Finally, we have observed that rates of material transport within the CZ vary greatly depending on the chronometer used. The CZ constitutes a small volume of the Earth’s clay budget however it is an important clay factory in exogenic cycles.