Paper No. 240-1
Presentation Time: 1:35 PM
LOOKING AT THE VIRGINIA PIEDMONT WITH MILAN
As mineral assemblages re-equilibrate at the surface, minerals interact with water, oxygen and carbon dioxide (as well as organic acids). The ratio of the capacity to consume oxygen to carbon dioxide differs between diabase (0.04) and granite (0.02). However the ratios of actual consumption of oxygen (FeO oxidation) to carbon dioxide (silicate weathering), R, are observed to be identical, 0.02, in samples of regolith cored by Pavich and coworkers in the 1980s. For Fe-rich diabase, the oxygen in the soil atmosphere must become depleted at shallower depths than carbon dioxide. This is documented in the regolith as deep dissolution of ferrous iron-containing pyroxene due to acid-promoted dissolution which allowed mobilization of ferrous Fe out of the rock. This deep dissolution was observed as the first weathering reaction at depth. In contrast, for granite (which is Fe-poor compared to its base cation content), Fe is retained in the deep weathering rock as ferric oxides, attesting to the presence of oxygen at depth. Volume constraints during deep oxidation in the granite promote fracturing. In turn, fracturing promotes advection and development of thick regolith. This is one reason that regolith on granitic rock is thicker than on diabase in the Virginia Piedmont. Pavich also pointed out that quartz in the felsic rock acts to hold open porosity and maintain the thick regolith. In addition smectitic clays form on the diabase, impeding deep infiltration of water into that impermeable rock. Therefore, both initial characteristics (presence of quartz in granite and high ferrous iron content in diabase), bottom-up characteristics (deep microfracturing during deep oxidation on the granite), and top-down characteristics (formation of smectite on the diabase) conspire so that regolith is thicker on granite but thinner on diabase in the Virginia Piedmont despite similar erosion rates. We are pursuing more research on these crystalline rocks in the Piedmont by drilling to sample water and by deep measurements of soil gas to understand how these characteristics can be quantitatively understood. Such research builds on the contributions of Pavich throughout his career.