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1. Regolith Weathering, Plastic Deformation and Hydrologic Evolution in a Volcanic Landscape

   Derry, Louis A; Perez-Fodich, Alida (December 2024, American Geophysical Union)

   Plain-Language Summary: Volcanic landscapes begin with high permeability, but with time develop a weathered surface that reduces permeability and diverts increasing amounts of water to stream runoff. On the island of Hawai’i the young volcanoes have no permanent streams; stream incision becomes important the older surfaces (more than about 20,000 years). By treating the weathered surface as a porous-plastic medium we Wnd that weathering can induce compaction of the soil that reduces permeability. The reduction in inWltration and initiation of stream incision fundamentally changes the hydrologic and geomorphic evolution of the landscape. Weathering affects both the chemistry and material properties of the surface and strongly inNuences landscape development, in ways that can be predicted with reactive transport and mechanical modeling. Geochemical tracers can be used to identify and quantify weathering processes and constrain these models. 

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2. A low‐dimensional model of bedrock weathering and lateral flow coevolution in hillslopes: 2. Controls on weathering and permeability profiles, drainage hydraulics, and solute export pathways

   https://doi.org/10.1002/hyp.13385  

   Harman, Ciaran J.; Cosans, Cassandra L. (February 2019, Hydrological Processes)

   Abstract The advance of a chemical weathering front into the bedrock of a hillslope is often limited by the rate weathering products that can be carried away, maintaining chemical disequilibrium. If the weathering front is within the saturated zone, groundwater flow downslope may affect the rate of transport and weathering—however, weathering also modifies the rock permeability and the subsurface potential gradient that drives lateral groundwater flow. This feedback may help explain why there tends to be neither “runaway weathering” to great depth nor exposed bedrock covering much of the earth and may provide a mechanism for weathering front advance to keep pace with incision of adjacent streams into bedrock. This is the second of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory. Here, we show how a simplified kinetic model of 1‐D rock weathering can be extended to consider lateral flow in a 2‐D hillslope. Exact and approximate analytical solutions for the location and thickness of weathering within the hillslope are obtained for a number of cases. A location for the weathering front can be found such that lateral flow is able to export weathering products at the rate required to keep pace with stream incision at steady state. Three pathways of solute export are identified: “diffusing up,” where solutes diffuse up and away from the weathering front into the laterally flowing aquifer; “draining down,” where solutes are advected primarily downward into the unweathered bedrock; and “draining along,” where solutes travel laterally within the weathering zone. For each pathway, a different subsurface topography and overall relief of unweathered bedrock within the hillslope is needed to remove solutes at steady state. The relief each pathway requires depends on the rate of stream incision raised to a different power, such that at a given incision rate, one pathway requires minimal relief and, therefore, likely determines the steady‐state hillslope profile. 

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3. Quantifying Depth‐Dependent Seismic Anisotropy in the Critical Zone Enhanced by Weathering of a Piedmont Schist

   https://doi.org/10.1029/2021JF006289  

   Eppinger, B. J.; Hayes, J. L.; Carr, B. J.; Moon, S.; Cosans, C. L.; Holbrook, W. S.; Harman, C. J.; Plante, Z. T. (October 2021, Journal of Geophysical Research: Earth Surface)

   Abstract Weathering processes weaken and break apart rock, freeing nutrients and enhancing permeability through the subsurface. To better understand these processes, it is useful to constrain physical properties of materials derived from weathering within the critical zone. Foliated rocks exhibit permeability, strength and seismic anisotropy–the former two bear hydrological and geomorphological consequences while the latter is geophysically quantifiable. Each of these types of anisotropy are related to rock fabric (fractures and foliation); thus, characterizing weathering‐dependent changes in rock fabric with depth may have a range of implications (e.g., landslide susceptibility, groundwater modeling, and landscape evolution). To better understand how weathering effects rock fabric, we quantify seismic anisotropy in saprolite and weathered bedrock within two catchments underlain by the Precambrian Loch Raven schist, located in Oregon Ridge Park, MD. Using circular geophone arrays and perpendicular seismic refraction profiles, anisotropy versus depth functions are created for material 0–25 m below ground surface (bgs). We find that anisotropy is relatively low (0%–15%) in the deepest material sampled (12–25 m bgs) but becomes more pronounced (29%–33%) at depths corresponding with saprolite and highly weathered bedrock (5–12 m bgs). At shallow soil depths (0–5 m bgs), material is seismically isotropic, indicating that mixing processes have destroyed parent fabric. Therefore, in situ weathering and anisotropy appear to be correlated, suggesting that in‐place weathering amplifies the intrinsic anisotropy of bedrock. 

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4. A bottom-up control on fresh-bedrock topography under landscapes

   https://doi.org/10.1073/pnas.1404763111  

   Rempe, Daniella M.; Dietrich, William E. (May 2014, Proceedings of the National Academy of Sciences)

   The depth to unweathered bedrock beneath landscapes influences subsurface runoff paths, erosional processes, moisture availability to biota, and water flux to the atmosphere. Here we propose a quantitative model to predict the vertical extent of weathered rock underlying soil-mantled hillslopes. We hypothesize that once fresh bedrock, saturated with nearly stagnant fluid, is advected into the near surface through uplift and erosion, channel incision produces a lateral head gradient within the fresh bedrock inducing drainage toward the channel. Drainage of the fresh bedrock causes weathering through drying and permits the introduction of atmospheric and biotically controlled acids and oxidants such that the boundary between weathered and unweathered bedrock is set by the uppermost elevation of undrained fresh bedrock, Z b . The slow drainage of fresh bedrock exerts a “bottom up” control on the advance of the weathering front. The thickness of the weathered zone is calculated as the difference between the predicted topographic surface profile (driven by erosion) and the predicted groundwater profile (driven by drainage of fresh bedrock). For the steady-state, soil-mantled case, a coupled analytical solution arises in which both profiles are driven by channel incision. The model predicts a thickening of the weathered zone upslope and, consequently, a progressive upslope increase in the residence time of bedrock in the weathered zone. Two nondimensional numbers corresponding to the mean hillslope gradient and mean groundwater-table gradient emerge and their ratio defines the proportion of the hillslope relief that is unweathered. Field data from three field sites are consistent with model predictions. 

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5. Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya

   https://doi.org/10.1029/2021JF006240  

   Medwedeff, William G.; Clark, Marin K.; Zekkos, Dimitrios; West, A. Joshua; Chamlagain, Deepak (February 2022, Journal of Geophysical Research: Earth Surface)

   Abstract Shallow bedrock strength controls both landslide hazard and the rate and form of erosion, yet regional patterns in near‐surface mechanical properties are rarely known quantitatively due to the challenge in collectingin situmeasurements. Here we present seismic and geomechanical characterizations of the shallow subsurface across the central Himalayan Range in Nepal. By pairing widely distributed 1D shear wave velocity surveys and engineering outcrop descriptions per the Geological Strength Index classification system, we evaluate landscape‐scale patterns in near‐surface mechanical characteristics and their relation to environmental factors known to affect rock strength. We find that shallow bedrock strength is more dependent on the degree of chemical and physical weathering, rather than the mineral and textural differences between the metamorphic lithologies found in the central Himalaya. Furthermore, weathering varies systematically with topography. Bedrock ridge top sites are highly weathered and have S‐wave seismic velocities and shear strength characteristics that are more typical of soils, whereas sites near valley bottoms tend to be less weathered and characterized by high S‐wave velocities and shear strength estimates typical of rock. Weathering on hillslopes is significantly more variable, resulting in S‐wave velocities that range between the ridge and channel endmembers. We speculate that variability in the hillslope environment may be partly explained by the episodic nature of mass wasting, which clears away weathered material where landslide scars are recent. These results underscore the mechanical heterogeneity in the shallow subsurface and highlight the need to account for variable bedrock weathering when estimating strength parameters for regional landslide hazard analysis. 

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