Anisovolumetric Weathering in Granitic Saprolite Controlled by Climate and Erosion Rate Clifford S

https://doi.org/10.1130/G48191.1

Manuscript received 17 July 2020 Revised manuscript received 31 October 2020 Manuscript accepted 8 November 2020

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 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA 2Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania 17013, USA 3Department 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

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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 vegetation. Conditions are warmer and drier still, and vegetation is sparsest at Fine Gold, the lowest- elevation site. The three sites have similar erosion 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 influence 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 during 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- centrations of immobile elements (including Zr sites. Our results indicate that weathering in ated with the chemical mass loss is equal to the and Ti) in protolith sampled from outcrops at the granitic saprolite is commonly anisovolumet- fractional mass loss, represented by −τ. There- surface and in representative saprolite aliquots

ric and that climate and erosion rate strongly fore, when ϕp = 0, the fraction of porosity from from cores. A geochemical mass balance shows

regulate the relative importance of subsurface chemical weathering (FPC) is equal to −τ/ϕw. that ε and τ can be quantified from the immobile physical and chemical weathering. element enrichment that occurs in saprolite as −ττ−+()ε 1 soluble elements are dissolved and removed in MATHEMATICAL FRAMEWORK FPC = = (2) solution (Brimhall and Dietrich, 1987). φw ετ− . Porosity in weathered rock (φw) equals the

initial protolith porosity (φp) plus the integrated The fraction of porosity from physical RESULTS AND DISCUSSION

effects of physical and chemical weathering, weathering (FPP) equals 1 − FPC. Porosity ranges from 0.15 to 0.78 across the which open voids via volumetric strain (ε) and samples and increases toward the surface at both chemical mass losses that can be quantified Soaproot and P301, the two sites with saprolite 1Supplemental Material. Supplemental methods using τ, the bulk mass transfer coefficient (Ague, and Tables S1–S4. Please visit https://doi.org/10.1130/ thick enough to show meaningful downhole 1991). The relationship among these variables, GEOL.S.13377221 to access the supplemental material, trends in weathering (Fig. 2A). These trends derived from mass balance principles, is shown and contact [email protected] with any questions. reflect the cumulative effects of weathering

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/551/5272857/551.pdf by guest on 28 September 2021 AB CD Figure 2. (A–D) Poros- ity, strain, mass loss (–τ), and fraction of porosity from both physical strain

(FPP) and chemical mass loss (FPC) versus depth at Sierra Nevada (California, USA) sites. Uncertainties were propagated as one EF GH standard error in protolith concentrations. (E–H) Mean ± standard error of mean (s.e.m.) across all depths for each site.

during exhumation, which are also evident in example, Soaproot and P301 are high enough at low elevations within the range (Dixon et al.,

upwards increases in mass loss at Soaproot and in elevation to have supported cyclical freezing, 2009), which may also help to explain why FPP strain at both P301 and Soaproot (Figs. 2B and thawing, and frost cracking over the glacial part at Fine Gold is greatest on average (Fig. 2H). 5 6 2C). At Soaproot, FPP is roughly uniform with of the 10 –10 yr time scales required to produce Meanwhile, average porosity, strain, and mass depth (Fig. 2D), because increases in porosity the thick saprolite of these sites (Callahan et al., loss are all greatest at Soaproot (Figs. 2E–2G), from strain (Fig. 2B) are matched by increases 2019). However, the finding that Fine Gold has where forest cover is also greatest due to trade-

from mass loss (Fig. 2C). At P301, where mass the highest FPP, even though its elevation is well offs in moisture and cold limitations that maxi- loss is roughly uniform with depth (Fig. 2C), the below the limits of seasonal freezing (both now mize ecosystem productivity at midelevations in increase in strain toward the surface (Fig. 2B) and in the Pleistocene), indicates that porosity the region (Goulden et al., 2012). We hypoth- is too small to cause a statistically significant production can be driven by other strain-induc- esize that this climatic optimum for vegetation

increase in FPP (Fig. 2D), despite contributing ing mechanisms such as root wedging (Brantley also promotes a vegetation-weathering feedback to increased porosity (Fig. 2A). This illustrates et al., 2017), biotite expansion during weather- that maximizes porosity, volumetric strain, and

the greater sensitivity of ϕw to −τ relative to ε ing (Goodfellow et al., 2016), and exhumation mass loss at midelevations. This optimum in in the mass balance formulation (Equation 1) through the ambient stress field (St Clair et al., weathering is broadly consistent with altitudi-

and moreover highlights the value of FPP over 2015). nal trends in soil production, clay content, and either ε or −τ alone in quantifying the relative The Sierra Nevada sites show several other exchangeable iron in the region (Dahlgren et al., importance of physical and chemical weathering potentially climate-related differences in sub- 1997; Dixon et al., 2009).

in saprolite porosity production. surface weathering. For example, Fine Gold has The finding that average FPP is highest where

At each site, FPP is greater than 0.5 on aver- the lowest annual average precipitation (AAP), AAP is lowest raises the possibility of climatic age (Fig. 2H). Hence, more than half of poros- highest mean annual temperature (MAT), and control on the relative importance of physical ity production in saprolite at these sites is due lowest forest cover, and it also has the lowest and chemical weathering. To explore this pos- to volumetric strain, indicating that weathering near-surface porosity (Fig. 2E), the lowest mass sibility, we compiled available data on strain and can be strongly anisovolumetric across a range loss (Fig. 2G), and the shallowest refusal depths. mass loss in granitic saprolite—i.e., from below of climates. This may reflect a variety of cli- This may reflect precipitation and vegetation the top 1–2 m of soil. This yielded estimates of

mate-driven, strain-inducing mechanisms. For limitations on subsurface chemical weathering average FPP from four additional sites, including

TABLE 1. VOLUMETRIC STRAIN AND MASS LOSS IN SAPROLITE PRODUCED IN CRYSTALLINE BEDROCK

§§ §§ ††† Site Lithology N Latitude Longitude AAP MAT Erosion rate Bulk mass transfer Volumetric FPP*** Porosity, φ (°N) (°W) (mm/yr) (°C) (mm/k.y.) coefficient, τ## strain, ε## (m3/m3) Fine Gold* Tonalite 5 37.0594 119.6423 473 1 7. 1 30 –0.10 ± 0.04 0.59 ± 0.16 0.77 ± 0.09 0.43 ± 0.06 Soaproot* Tonalite 22 37.0311 119.2564 883 13.8 42 –0.28 ± 0.02 0.84 ± 0.12 0.54 ± 0.03 0.61 ± 0.03 P301† Granodiorite 26 37.0675 119.1950 1034 9.2 40 –0.22 ± 0.01 0.53 ± 0.05 0.55 ± 0.03 0.49 ± 0.02 Davis Run§ Foliated granite 5 38.7083 77.2917 1046 13.0 4 –0.22 ± 0.06 0.21 ± 0.07 0.39 ± 0.10 0.35 ± 0.06 Crabtree Quarry# Granite 16 35.8459 78.7164 1147 15.3 3.2 –0.29 ± 0.05 0.30 ± 0.08 0.36 ± 0.07 0.46 ± 0.05 Panola Mountain** Granodiorite 18 33.6329 84.1671 1266 16.4 7 –0.21 ± 0.02 0.07 ± 0.03 0.21 ± 0.07 0.27 ± 0.03 Rio Icacos†† Tonalite 17 18.2813 65.7882 4200 22.0 43 –0.47 ± 0.02 0.14 ± 0.07 0.12 ± 0.04 0.54 ± 0.03 *Lithology from Bateman (1992); erosion rates from Callahan et al. (2019); τ, ε, and ϕ from this study (see Tables S2 and S3 in the Supplemental Material [see text footnote 1]). †Lithology from Bateman (1992); erosion rate from Callahan et al., (2019); τ, ε, and ϕ from Hayes et al. (2019). §Lithology, τ, and ε from Pavich et al. (1989); erosion rate from White et al. (1998). #Lithology, τ, and ε from Calvert et al. (1980); erosion rate from Reusser et al. (2015). **Lithology, erosion rate, τ, and ε from White et al. (2001). ††Lithology from Hayes et al. (2020); precipitation, temperature, τ, and ε from White et al. (1998); erosion rate from Brown et al. (1995). §§Average annual precipitation and mean annual temperature are 30 yr normals from PRISM Model (PRISM Climate Group, 2020), except as noted above. ##For individual values from profiles, see Table S4 in the Supplemental Material (see text footnote 1). Uncertainties are reported as one standard error of the mean ofN measurements. ***Fraction of porosity from physical weathering calculated from Equation 3, with uncertainties propagated from uncertainties in τ and ε. ††† Calculated from Equation 1 assuming ϕp = 0, with uncertainties propagated from uncertainties in τ and ε.

Geological Society of America | GEOLOGY | Volume 49 | Number 5 | www.gsapubs.org 553

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/551/5272857/551.pdf by guest on 28 September 2021 A B Figure 3. Variations in fraction of porosity from

physical weathering (FPP) in granitic bedrock. (A) Volumetric strain and mass loss in saprolite at seven granitic sites in North America, showing the relative importance of physical and chemical porosity production. Dotted contours of total porosity (right axis) show the sum of contributions from volumetric strain and mass loss (Equation 1). Solid contours and color

shades show FPP (top axis) calculated from Equation 3. Symbol colors repre- sent erosion rate (color bar). (B) Joint distribu- tion of annual average precipitation (AAP) and mean annual temperature (MAT) from 2.5 arc-minute C E climate model of Earth’s D land surface (Fick and Hijmans, 2017) scaled by area (color shade, note log scale) and overlain by Whittaker’s biomes (Whittaker, 1970; Rick- lefs, 2008), with study site symbols colored by

FPP. Top and right axes show physical and chemical weathering regimes.

(C–D) Decrease in FPP with AAP and increase in

FPP residuals with erosion rate are consistent with a multiple regression model (p = 0.0035) in which AAP and erosion rate explain 94% of the variance in

FPP, yielding the predictive relationship in E. (E)

Modeled predictive relationship. Symbols in E show measured FPP at each site, so the mismatch in color reflects model misfit. Empty regions span conditions predicted to induce 100% physical (top left) and isovolumetric (bottom right) saprolite weathering. AAP is >3 m/yr across <2% of Earth’s land area (B), implying that isovolumetric weathering in granitic terrain is rare.

Rio Icacos, in Puerto Rico, and three sites in the climatic and erosional control on the relative provides a framework for quantifying the rela- southeastern United States (Table 1). These sites importance of physical and chemical weathering tive contributions of physical and chemical encompass a wider range of climates and erosion in saprolite (see the Supplemental Material for weathering to saprolite porosity production.

rates and thus span a wider range of weather- details). The predicted FPP from the model shows Despite differences in mineralogy across the

ing conditions that might influence the relative that FPP = 1, corresponding to physical weather- seven granitic sites where both strain and mass importance of physical and chemical weathering. ing without mass loss, when erosion rates are loss data are available, 94% of the variance in

FPP is greater than zero on average at all sites high or when AAP is low (Fig. 3E). Conversely, FPP can be explained by differences in average in the compilation, ranging from 0.12 to 0.77 purely chemical, isovolumetric weathering, with precipitation and erosion rate. This may reflect

(Fig. 3A). This suggests that anisovolumetric FPP = 0, is predicted where AAP > 3000 mm/ a moisture limit on chemical depletion at rela- weathering is widespread, arising from a diver- yr, which occurs across <2% of Earth’s sur- tively arid sites and a time limit on chemical sity of physical and chemical mechanisms acting face (cf. Figs. 3B and 3E). Hence, the model weathering at rapidly eroding sites (Ferrier

across the climates and biomes spanned by the predicts that anisovolumetric weathering is the et al., 2016). Although FPP is lower at wetter,

sites (Fig. 3B). FPP correlates strongly with AAP norm rather than the exception at granitic sites more slowly eroding sites, volumetric strain is across the sites (Fig. 3C), and the residuals of a around the world. predicted to be a nonnegligible part of granitic

semi-log regression between FPP and AAP corre- saprolite porosity production in climates that late strongly with erosion rates (Fig. 3D). When CONCLUSIONS prevail across >98% of Earth’s land surface. combined together in a multiple regression The fraction of porosity from physical To test these predictions, new measurements of

model (Fig. 3E), AAP and erosion rate explain weathering, FPP, can be calculated from esti- both strain and mass loss are needed to quantify

94% of the variance in FPP, indicative of strong mates of volumetric strain and mass loss and the relative importance of physical processes

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/551/5272857/551.pdf by guest on 28 September 2021 such as freezing, thawing, frost cracking, root vertical rock-saprolite-soil sequence in the North Maher, K., and Chamberlain, C.P., 2014, Hydrologic wedging, mineral expansion, and exhumation Carolina Piedmont: I. Profile morphology, chemi- regulation of chemical weathering and the geo- through the ambient stress field. These processes cal composition, and mineralogy: Soil Science logic carbon cycle: Science, v. 343, p. 1502– Society of America Journal, v. 44, p. 1096–1103, 1504, https://doi.org/10.1126/science.1250770. are widespread in mountain landscapes irrespec- https://doi.org/10.2136/sssaj1980.0361599500 National Agriculture Imagery Program, 2017, 1-m- tive of lithology, indicating that anisovolumet- 4400050044x. 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