Fractures, Water and Weathering: Geophysical Insights into the Critical Zone by Steve Holbrook

Abstract:

A major challenge in understanding the critical zone (CZ) is measuring the properties of the deep (sub-soil) CZ over its full depth range and at landscape scales. The deep CZ is inaccessible except through (expensive) drill holes, (random) roadcuts and geophysics. Geophysics provides a means of imaging the deep CZ across scales and across gradients in climate, lithology, topography, biology and regional states of stress. Here I present insights into deep CZ structure through the lens of geophysical data (surface and borehole) acquired at six CZO’s by the Wyoming Center for Environmental Hydrology and Geophysics.

Although the critical zone is often defined as extending from the top of vegetation to the lower limits of groundwater, this definition is too expansive, as the “lower limits” of groundwater are inherently difficult to define and can reach depths of kilometers – far deeper than the typical processes of interest in CZ studies. I adopt a different definition: The critical zone comprises surface ecosystems and subsurface material whose bulk chemical and physical properties are altered by virtue of a connection to Earth’s surface. This definition incorporates a surface-process-centric view of the CZ and captures the deep connections that are being discovered between surface ecosystems and subsurface properties. In this presentation I will show geophysical evidence that there is a perceptible zone, 40-50 m thick in places, where these connections are manifest in physical and chemical properties.

Our results show that: (1) Chemical weathering initiates deep in the CZ, in concert with mechanical weathering (opening of fractures). While plagioclase weathering may begin at up to 40 m depth, the principal boundary in porosity, occurs at the saprolite/weathered bedrock boundary. (2) Landscape-scale geophysical surveys show that the ambient stress field (tectonic + topographic stress) may control weathering (both physical and chemical) and porosity in the CZ, especially at sites underlain by crystalline bedrock. (3) Local geological factors, however, can complicate or overwhelm these simple patterns, however: at the Catalina CZO a strong north-south aspect asymmetry is explained by the dip of metamorphic foliation, and at the Reynolds Creek CZO regolith is much thicker in basalt than in granite.