Lithologic, Climatic and Depth Controls on Critical Zone Transformations

Abstract

Biotite content was positively related to regolith thickness in granitic terrain. Depth regulates regolith transformations by dampening subsurface temperature. The degree of soil development does not reflect deep regolith characteristics. We evaluated the effects of temperature and subtle differences in lithology (biotite content) on the degree of pedogenesis in regolith (soils + saprock) in granitic terrain of the southern Sierra Nevada. Deep regolith was sampled from summit and backslope landscape positions in two catchments representing the transition from rain‐dominated (1100 m elevation) to snow‐dominated (2000 m elevation) precipitation. Regolith thickness varied regardless of degree of soil development (e.g., Alfisols vs. Inceptisols), ranging from 1.4 to 7.6 m at the 1100 m site and 1.5 to 10.7 m at the 2000‐m site. Biotite content in fine sand fractions of saprock was positively correlated with regolith thickness at both sites. In the rain‐dominated catchment (1100 m elevation), particle grain size was negatively correlated with regolith thickness. Evidence of pedogenic transformations was evaluated based on clay content and dithionite extractable iron (Fed). The degree of regolith transformation was controlled by heat energy, which was related to the temperature mediated by depth. Annual sum of heat energy modeled from surface to hard bedrock showed a significant positive correlation with clay content and Fed concentration. The slopes of regression lines between annual heat energy load and clay and Fed were steeper at the 1100 m site, where the dominant form of precipitation was rain. This trend is a result of the greater degree of pedogenesis and higher temperatures in soils compared with saprock at 2000 m.

Deep in the Sierra Nevada critical zone: saprock represents a large terrestrial organic carbon stock

“…Soils include Entisols, Inceptisols and Alfisols, representing a range of soil properties. Soils reflecting the highest degree of development (as indicated by clay content and pedogenic iron) are found at mid elevations (Tian et al 2019). All sites are underlain by granodiorite bedrock, except the lowest elevation site that is tonalite, a slightly more felsic rock (Dahlgren et al 1997).…”

“…Climate indirectly controls OC in regolith along the western slope of the southern Sierra Nevada through its impact on vegetation and weathering (figures 1 and 5). Climate exerts strong controls on saprock weathering primarily through thickening and subtle transformations of primary minerals (Dahlgren et al 1997, Tian et al 2019). Trends in regolith stocks are also influenced by GPP, which was highest at mid-elevations, contributing to high C inputs, and perhaps, to regolith thickening through biophysical weathering processes (figure 1) (Hinsinger et al 2001, Goulden et al 2012, Kelly and Goulden 2016, O'Geen et al 2018.…”

Determining How Critical Zone Structure Constrains Hydrogeochemical Behavior of Watersheds: Learning From an Elevation Gradient in California's Sierra Nevada

“…This study is based on the long-term monitoring of the KREW and on the database from the Southern Sierra Critical Zone Observatory (SSCZO, Liu et al, 2013;Holbrook et al, 2014;Safeeq and Hunsaker, 2016;Hunsaker and Johnson, 2017;Bales et al, 2018). Recent efforts have been undertaken to characterize the CZ structure and regolith properties along the SSCZO elevation gradient (O'Geen et al, 2018;Tian et al, 2019).…”

“…The mean value of K sat was 6 × 10 −6 m.s −1 for the entire regolith column. Soil and regolith samples were also collected in the KREW for geochemical and mineralogical analysis from soil-pit, handauger and/or Geoprobe campaigns (Johnson et al, 2011;O'Geen et al, 2018;Tian et al, 2019). Sampling depth was between 0 and 2 m, with a few samples from Geoprobe campaigns reaching 10 m in Providence.…”

“…Sampling depth was between 0 and 2 m, with a few samples from Geoprobe campaigns reaching 10 m in Providence. Mineralogical composition of regolith samples was determined by the quantitative interpretation of X-ray analysis reported by Tian et al (2019), following Ackerer et al (2016). Elemental concentrations in bulk regolith samples were also determined by ICP-AES following Tian et al (2019).…”