Generating Porosity in the Critical Zone: Does Volumetric Strain Dominate Chemical Mass Loss?

Abstract:

The opening of pore space by weathering near Earth’s surface influences a broad array of hydrological and biogeochemical processes, including water storage, surface-groundwater interactions, and nutrient cycling. Understanding how porosity is generated by physical and chemical processes is therefore a fundamental challenge in critical zone science. However, the relative importance and spatial variability of mechanisms that generate porosity remain poorly understood, because the subsurface is hidden from direct observation except by drilling and geophysical measurements. Here we use observations from both drilling and geophysics to quantify the relative importance of volumetric strain and chemical mass loss in the Southern Sierra Critical Zone Observatory. Our results from cores of saprolite show that porosity decreases with depth from 0.65 to 0.35 in the top 11 meters of saprolite. Although seismic refraction surveys reveal velocities that correctly predict the porosity variations, they cannot distinguish between volumetric strain and chemical mass losses. To overcome this limitation, we quantified bulk density and immobile element concentrations in our samples of saprolite. We found that volumetric strain decreases with depth from 1.1 to 0.3. Conversely, the overall mass loss due to chemical weathering shows no trend with depth. Together these results suggest that the changes in porosity are due to physical rather than chemical processes in the upper 11 meters. However neither bioturbation nor frost cracking are likely to extend so deep. Moreover, the paucity of hydrous minerals in the bedrock implies that mineral expansion cannot explain the high volumetric strain in the saprolite. Our results are consistent with porosity generated by the opening of fractures due to the release of confining stress during exhumation of bedrock.