Geophysical constraints on deep weathering and water storage potential in the Southern Sierra Critical Zone Observatory

Abstract

The conversion of bedrock to regolith marks the inception of critical zone processes, but the factors that regulate it remain poorly understood. Although the thickness and degree of weathering of regolith are widely thought to be important regulators of the development of regolith and its water-storage potential, the functional relationships between regolith properties and the processes that generate it remain poorly documented. This is due in part to the fact that regolith is difficult to characterize by direct observations over the broad scales needed for process-based understanding of the critical zone. Here we use seismic refraction and resistivity imaging techniques to estimate variations in regolith thickness and porosity across a forested slope and swampy meadow in the Southern Sierra Critical Zone Observatory (SSCZO). Inferred seismic velocities and electrical resistivities image a weathering zone ranging in thickness from 10 to 35 m (average = 23 m) along one intensively studied transect. The inferred weathering zone consists of roughly equal thicknesses of saprolite (P-velocity < 2 km s À1 ) and moderately weathered bedrock (P-velocity = 2-4 km s À1 ). A minimum-porosity model assuming dry pore space shows porosities as high as 50% near the surface, decreasing to near zero at the base of weathered rock. Physical properties of saprolite samples from hand augering and push cores are consistent with our rock physics model when variations in pore saturation are taken into account. Our results indicate that saprolite is a crucial reservoir of water, potentially storing an average of 3 m 3 m À2 of water along a forested slope in the headwaters of the SSCZO. When coupled with published erosion rates from cosmogenic nuclides, our geophysical estimates of weathering zone thickness imply regolith residence times on the order of 10 5 years. Thus, soils at the surface today may integrate weathering over glacial-interglacial fluctuations in climate.

Bedrock composition regulates mountain ecosystems and landscape evolution

“…There is some indication from the vicinity of the CZO that variations in regolith can be high within a single geochemically homogeneous pluton. For example, seismic refraction surveys indicate that depth-integrated regolith water-holding capacity can vary by more than an order of magnitude in the space of 100 m on a single slope (44). This intrapluton variability appears to be widespread based on road cuts in the area.…”

Recent Patterns in Climate, Vegetation, and Forest Water Use in California Montane Watersheds

“…Recent research has suggested high subsurface storage potential in the mid-elevation Sierra Nevada [57], and large summer dry season soil water fluxes because of vegetation use in California mountain regions [20]. Higher elevation catchments such as Bull have a shallow soil layer and lower carryover storage potential with higher risk to subsurface storage losses from climate warming and a smaller winter snowpack [19].…”

“…Garcia and Tague [54] also show that subsurface plant available water capacity and lateral redistribution of water can act as a major control of ET response to climate in the Sierra Nevada. Despite the importance of subsurface properties and water storage on vegetation and water balance response to climate variability, few studies have direct measurements (e.g., Holbrook et al [57]). Future research focused on hydrologic response in this region should focus on quantifying and constraining the subsurface storage potential and contribution to interannual water balance variability.…”

Using an Integrated Hydrology Model to Elucidate Plant Water Use in a Headwaters Research Catchment

“…The regolith subsurface layer is more broken up than the underlying bedrock and feeds the supply of soil. Tree roots take advantage of the highly chemically weathered and more easily fractured regolith (Graham, Tice, and Guertal 1994;Holbrook et al 2014). Accurate characterization elucidates subsurface water storage.…”

“…Understanding mountain vegetation water use is an essential component for predicting the vegetative response to stress. This study is motivated by evidence of drought-induced tree mortality in some Sierran catchments, a high degree of uncertainty in mountain regolith groundwater storage, and the impacts of subsurface characterization on mountain ecohydrology (Holbrook et al 2014;Jepsen et al 2016). …”