https://doi.org/10.1130/G48191.1 Manuscript received 17 July 2020 Revised manuscript received 31 October 2020 Manuscript accepted 8 November 2020 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 12 January 2021 Anisovolumetric weathering in granitic saprolite controlled by climate and erosion rate Clifford S. Riebe1*, Russell P. Callahan1, Sarah B.-M. Granke1, Bradley J. Carr1, Jorden L. Hayes2, Marlie S. Schell1 and Leonard S. Sklar3 1 Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA 2 Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania 17013, USA 3 Department of Geography, Planning and Environment, Concordia University, Montréal, Quebec H3G 1M8, Canada ABSTRACT compared to bulk geochemistry, bulk density Erosion at Earth’s surface exposes underlying bedrock to climate-driven chemical and is more difficult to quantify because it requires physical weathering, transforming it into a porous, ecosystem-sustaining substrate consist- intact samples collected without disruption that ing of weathered bedrock, saprolite, and soil. Weathering in saprolite is typically quantified renders volume measurements incorrect. This from bulk geochemistry assuming physical strain is negligible. However, modeling and mea- is particularly problematic in saprolite, which surements suggest that strain in saprolite may be common, and therefore anisovolumetric is difficult to sample for bulk density except by weathering may be widespread. To explore this possibility, we quantified the fraction of po- expensive push-core extraction. Without bulk rosity produced by physical weathering, FPP, at three sites with differing climates in granitic density measurements, subsurface weather- bedrock of the Sierra Nevada, California, USA. We found that strain produces more porosity ing studies have assumed volumetric strain is than chemical mass loss at each site, indicative of strongly anisovolumetric weathering. To negligible, which also simplifies reactive trans- expand the scope of our study, we quantifiedF PP using available volumetric strain and mass port models of chemical weathering (Lebedeva loss data from granitic sites spanning a broader range of climates and erosion rates. FPP in et al., 2007). However, recent modeling of strain each case is ≥0.12, indicative of widespread anisovolumetric weathering. Multiple regression caused by freezing, thawing, and frost crack- shows that differences in precipitation and erosion rate explain 94% of the variance in FPP ing challenges the assumption of isovolumetric and that >98% of Earth’s land surface has conditions that promote anisovolumetric weath- weathering, particularly in climates with sea- ering in granitic saprolite. Our work indicates that anisovolumetric weathering is the norm, sonal freezing (Anderson et al., 2013; Rempel rather than the exception, and highlights the importance of climate and erosion as drivers et al., 2016). In addition, a recent push-core of subsurface physical weathering. study showed that granitic saprolite doubled in volume during exhumation at a forested site in INTRODUCTION subsurface weathering (Brantley et al., 2017). the Sierra Nevada, California (USA), possibly In eroding landscapes, chemical and physical Over millions of years, solutes from subsurface reflecting seasonal freezing or the effects of weathering transform unaltered protolith into weathering affect atmospheric CO2, which in other strain-inducing mechanisms such as root weathered bedrock, saprolite, and soil as it is turn influences Earth’s weathering-climate ther- wedging, biotite expansion, and exhumation exhumed by erosion and gradually exposed to mostat (Maher and Chamberlain, 2014). Quan- through the ambient stress field (Hayes et al., climate. This subsurface weathering weakens tifying subsurface weathering, and its sensitiv- 2019). Thus, modeling, observations, and the the rock (Goodfellow et al., 2016), making it ity to factors such as climate, vegetation, and abundance of potential strain-inducing mecha- more susceptible to erosion (Dixon et al., 2009), erosion rates, is therefore important for under- nisms suggest that anisovolumetric weather- which in turn influences weathering by setting standing geomorphological, hydrological, and ing may be more widespread than previously the duration of chemical and physical attack biogeochemical feedbacks across broad spatio- appreciated. (Yoo and Mudd, 2008). The resulting dissolution temporal scales. To explore this possibility, we quanti- and volumetric expansion open subsurface pore Subsurface weathering is commonly studied fied subsurface porosity, strain, and mass space (Callahan et al., 2020), enabling through- using the bulk geochemistry of weathered rock loss using push cores from two sites near the flow of ecosystem-sustaining water (Klos et al., and saprolite to quantify chemical mass losses Sierra Nevada site of Hayes et al. (2019), but 2018), which promotes additional subsurface accrued during exhumation of protolith by ero- at lower elevations, thus widening the range weathering. Weathering also liberates nutrients, sion (Riebe et al., 2017). This mass loss accu- of climates spanned by the measurements thus influencing biogeochemical cycles (Uhlig rately quantifies the chemical part of subsurface (Fig. 1). We interpret the results in a non- et al., 2017) and fueling ecosystem productiv- weathering but ignores the physical part caused dimensional framework that quantifies the ity (Hahm et al., 2014), which drives biological by volumetric strain. Strain can also be quanti- relative contributions of volumetric strain fied if in situ measurements of bulk density are and mass loss to saprolite porosity produc- *E-mail: [email protected] available (Brimhall and Dietrich, 1987), but, tion. Our analysis shows that volumetric strain CITATION: Riebe, C.S., et al., 2021, Anisovolumetric weathering in granitic saprolite controlled by climate and erosion rate: Geology, v. 49, p. 551–555, https:// doi.org/10.1130/G48191.1 Geological Society of America | GEOLOGY | Volume 49 | Number 5 | www.gsapubs.org 551 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/551/5272857/551.pdf by guest on 28 September 2021 ετ()+1 FPP = (3) ετ− . FPP and FPC express the relative importance of physical and chemical weathering in porosity production nondimensionally. When physical weathering dominates, FPP is high, and FPC is therefore low (and vice versa). Thus, quantifica- tion of FPP and FPC provides a basis for testing assumptions about isovolumetric weathering in saprolite. STUDY SITES AND METHODS To encompass weathering environments that are likely to include a range of strain-inducing mechanisms, our analysis combined two new sites together with the P301 site of Hayes et al. (2019) to span a range in elevation, and thus climate, in the Sierra Nevada Batholith (Fig. 1). P301, the highest-elevation site, is the coolest and wettest. Soaproot, the middle-elevation site, is warmer and drier but has denser vegeta- tion. Conditions are warmer and drier still, and vegetation is sparsest at Fine Gold, the lowest- elevation site. The three sites have similar ero- sion rates (Callahan et al., 2019), have tonalitic or granodioritic bedrock (Bateman, 1992), and were not glaciated in the Pleistocene (Gillespie and Zehfuss, 2004), permitting us to focus on climate and vegetation as factors that may influ- ence the relative importance of subsurface phys- Figure 1. Study locations in the southern Sierra Nevada Batholith (pink, inset), California, USA. ical and chemical weathering. Annual average precipitation (AAP) and mean annual temperature (MAT) are 30 yr normals from We quantified strain, mass loss, and poros- the Parameter Regressions on Independent Slopes (PRISM) model (PRISM Climate Group, ity in saprolite collected to refusal depth using 2020). 1-m-resolution aerial imagery from 2012 shows differences in vegetation (National Agri- culture Imagery Program, 2017). Nev.—Nevada; Ariz.—Arizona; Calif.—California. a Geoprobe 7822DT direct push system dur- ing July 2019, which yielded two 2-m-long cores at Fine Gold and one 4-m-long and two has contributed more to porosity than mass in Equation 1 (Hayes et al., 2019; see Item S1 in 7-m-long cores at Soaproot (see Item S1 in the loss at all three Sierra Nevada sites, with the the Supplemental Material1 for details). Supplemental Material for detailed sampling highest fractional contribution at the lowest- methods). Cores were collected in 0.5 m incre- elevation site, where average precipitation τ +1 ments and were weighed after drying for 24 h φwp=−1 1− φ , (1) is lowest and temperatures are highest. We ε +1() at 105 °C to obtain dry material mass, which we also analyzed available volumetric strain and used with core volume to calculate bulk density. mass loss data from four other granitic sites A chemical mass loss in the subsurface pro- Calculated bulk density was used to estimate ϕw and found that 94% of the variance in frac- duces an effective loss of protolith volume equal assuming a 2.65 g cm–3 average mineral density. tional porosity from volumetric strain can be to the mass loss divided by the density of the To estimate ε and τ, we used X-ray fluorescence explained by precipitation and erosion rate, protolith. Hence, the production of porosity (i.e., following standard procedures to quantify con- despite variations in mineralogy across the the added fractional volume of voids) associ-