The Pedologic Nature of Weathered Rock

Graham, Robert C.; Tice, Kathy R.; Guertal, William R.

doi:10.2136/sssaspecpub34.c2

Cited by 17 publications

(17 citation statements)

References 50 publications

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“…This follows a long string of studies by Bob Graham, one of the study's coauthors, who has helped spotlight the vital importance of deep CZ processes in the development of overlying soils and the ecosystems they support (e.g. Jones and Graham, 1993;Graham et al, 1994;Zanner and Graham, 2005;Graham et al, 2010).…”

Section: Studies Of Erosion Weathering and Soil Developmentmentioning

confidence: 99%

Controls on deep critical zone architecture: a historical review and four testable hypotheses

Riebe

Hahm

Brantley

2016

Earth Surf Processes Landf

265

263

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The base of Earth's critical zone (CZ) is commonly shielded from study by many meters of overlying rock and regolith. Though deep CZ processes may seem far removed from the surface, they are vital in shaping it, preparing rock for infusion into the biosphere and breaking Earth materials down for transport across landscapes. This special issue highlights outstanding challenges and recent advances of deep CZ research in a series of articles that we introduce here in the context of relevant literature dating back to the 1500s. Building on several contributions to the special issue, we highlight four exciting new hypotheses about factors that drive deep CZ weathering and thus influence the evolution of life-sustaining CZ architecture. These hypotheses have emerged from recently developed process-based models of subsurface phenomena including: fracturing related to subsurface stress fields; weathering related to drainage of bedrock under hydraulic head gradients; rock damage from frost cracking due to subsurface temperature gradients; and mineral reactions with reactive fluids in subsurface chemical potential gradients. The models predict distinct patterns of subsurface weathering and CZ thickness that can be compared with observations from drilling, sampling and geophysical imaging. We synthesize the four hypotheses into an overarching conceptual model of fracturing and weathering that occurs as Earth materials are exhumed to the surface across subsurface gradients in stress, hydraulic head, temperature, and chemical potential. We conclude with a call for a coordinated measurement campaign designed to comprehensively test the four hypotheses across a range of climatic, tectonic and geologic conditions.

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Section: Studies Of Erosion Weathering and Soil Developmentmentioning

confidence: 99%

Controls on deep critical zone architecture: a historical review and four testable hypotheses

Riebe

Hahm

Brantley

2016

Earth Surf Processes Landf

265

263

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“…Recognizing that weathering rates are partially a function of rock structure, mineral grain size and composition (Graham et al, 1994; Frazier and Graham, 2000; Mareschal et al, 2015), we hypothesized that lithology was an important factor for the prediction of regolith evolution, a term we consider to include thickening and pedogenic transformations and translocations. Elemental composition using the bottom depth of all cores showed no correlation with thickness, and thus, within the range of the geochemical composition of granitic rock studied here, total elemental composition appears less important (Table S3).…”

Section: Discussionmentioning

confidence: 99%

Lithologic, Climatic and Depth Controls on Critical Zone Transformations

Tian

Hartsough

O’Geen

2019

Soil Science Soc of Amer J

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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.

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“…Weathering of bedrock to produce porous regolith, the precursor to biologically active soil and soluble mineral nutrients, creates the life-supporting matrix upon which Earth's Critical Zone-the thin surface layer where rock meets life-develops (Ollier 1985;Graham et al 1994;Taylor and Eggleston 2001). Water and nutrients locked up in low porosity bedrock are biologically inaccessible until weathering helps transform the inert rock into a rich habitat for biological activity.…”

Section: Introductionmentioning

confidence: 99%

10. How Porosity Increases During Incipient Weathering of Crystalline Silicate Rocks

Navarre‐Sitchler¹,

Brantley²,

Rother³

2015

Pore Scale Geochemical Processes

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these porosities as nanoporosity and microporosity, respectively. Although this terminology is loose, it is operationally useful because different techniques are used in the measurement of the differently sized pores, as discussed below. The International Union of Pure and Applied Chemistry (IUPAC) defi nes micropores as pores with width smaller than 2 nm, mesopores have pore widths of 2-50 nm, and macropores have widths larger than 50 nm (Sing et al. 1985; Rouquerol et al. 1994). METHODS FOR POROSITY AND PORE-SIZE DISTRIBUTION QUANTIFICATION Detailed analysis and characterization of natural pore systems requires a multitude of techniques that are capable of interrogating different aspects of the pore network across many orders of magnitude length scale. Some of the most widely used techniques include gas sorption, fl uid intrusion (including mercury porosimetry), various microscopy and image analysis approaches, and X-ray and neutron scattering. Sorption and intrusion techniques Gas sorption techniques, especially nitrogen sorption measurements at 77 K, are routinely used for the measurement of internal surface area and pore size in the region of 2-200 nm. Nitrogen gas sorption analysis (often referred to as BET analysis) of an intact sample yields the internal surface area and pore size distribution of accessible, connected pore spaces, while analysis of fi nely powdered samples yields the total porosity and surface area, plus the surface

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The Pedologic Nature of Weathered Rock

Cited by 17 publications

References 50 publications

Controls on deep critical zone architecture: a historical review and four testable hypotheses

Controls on deep critical zone architecture: a historical review and four testable hypotheses

Lithologic, Climatic and Depth Controls on Critical Zone Transformations

10. How Porosity Increases During Incipient Weathering of Crystalline Silicate Rocks

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