Insights into chemical weathering patterns from geophysical data

Abstract:

Rock weathering, clay mineral formation, and subsequent soil forming processes change not only rock chemistry but also rock properties. These changes affect the way water is transmitted and stored in the shallow subsurface, which in turn alters weathering reactions. Understanding the relationships between rock weathering and water flux in the critical zone will advance our understanding of both the geochemical and hydrological processes that operate in the critical zone. Geophysical tools provide a way to see into the subsurface and have been developed for understanding hydrology of subsurface systems. Geophysical data also has the potential to inform our understanding of coupled geochemical and hydrologic processes across landscapes in ways that point measurements of geochemical data never will.

Samples from 4, 10-13 meter deep boreholes in the Boulder Creek Critical Zone Observatory were evaluated for evidence of chemical weathering with bulk chemistry, clay fraction, and exchangeable cation concentrations. Unfortunately, heterogeneity of the metamorphic bedrock obscures the 1D weathering profile signal we expect commonly used mass balance methods to define. We find that on north facing slopes exchangeable cation concentrations are similar regardless of depth, compared to a decreasing trend in exchangeable cations with depth on south facing slopes. Clay fractions do not show these trends clearly, possibly due to mechanical grinding of rocks during drilling. If we use exchangeable cation concentrations to estimate weathering patterns on north vs. south facing slopes it appears that north facing slopes are weathered to depths >10 meters, but that weathering extends to depths of ~ 7 meters on south facing slopes. Seismic anisotropy data show that depths of broken up rock (or saprolite) on both slope aspects are the same, but that saprolite on northern facing slopes is more fractured and granulated than south facing slopes. Comparing geochemical observations with seismic observations tells a story of water infiltration through a connected network to greater depths on north facing slopes pushing chemical weathering deeper into the critical zone, with water infiltration through a less connected fracture network on the south facing slopes limiting chemical weathering to shallower depths.