Exploring the Effect of Aspect to Inform Future Earthcasts of Climate‐Driven Changes in Weathering of Shale

Abstract

Projections of future conditions within the critical zone—earthcasts—can be used to understand the potential effects of changes in climate on processes affecting landscapes. We are developing an approach to earthcast how weathering will change in the future using scenarios of climate change. As a first step here, we use the earthcasting approach to model aspect‐related effects on soil water chemistry and weathering on hillsides in a well‐studied east‐west trending watershed (Shale Hills, Pennsylvania, USA). We completed model simulations of solute chemistry in soil water with and without the effect of aspect for comparison to catchment observations. With aspect included, aqueous weathering fluxes were higher on the sunny side of the catchment. But the effect of aspect on temperature (0.8 °C warmer soil on sunny side) and recharge (100 mm/year larger on shaded side) alone did not explain the magnitude of the observed higher weathering fluxes on the sunny side. Modeled aspect‐related differences in weathering fluxes only approach field observations when we incorporated the measured differences in clay content observed in augered soils on the two hillslopes. We also had to include a biolifting module to accurately describe cation concentrations in soil water versus depth. Biolifting lowered some mineral dissolution rates while accelerating kaolinite precipitation. These short‐duration simulations also highlighted that the inherited differences in particle size on the two sides of the catchment might in themselves be explained by weathering under different microclimates caused by aspect—over longer durations than simulated with our models.

Soil CO2 and O2 Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

“…Silicate weathering has been documented in the soil of the SSHCZO and accounts for some of the consumed CO 2 in the soils of Garner Run and Shale Hills (Angert et al, 2015; Jin et al, 2014). However, the rate of these weathering reactions is slow, ranging from 0.001 to 0.005 mol Si m −2 yr −1 (Sullivan et al, 2019), and at most could only consume about 0.3 mg C m −2 d −1 (Jin et al, 2014). Although model results indicate that the silicate dissolution rate is fastest in the early growing season when we observe ARQ <1, this relatively small consumption potential could not explain our observations.…”

“…This is due to the greater reactivity and higher surface area of metals in the soils of the SSHCZO than the clay minerals and quartz that largely dominate the soils in both rock types. Based on published estimates, silicate weathering at its fastest rate in Shale Hills has the potential to consume about 0.3 mg C m −2 d −1 (Sullivan et al, 2019). On the other hand, oxidation of 0.1 mmol Fe (II) kg −1 soil over a month, when assuming the reaction stoichiometry in Table 1, a soil depth of 100 cm, and bulk density of 1.2 g cm −3 , would consume about 32 mg O 2 m −2 d −1 .…”

Strong slope‐aspect control of regolith thickness by bedrock foliation

“…In mountain landscapes, regolith thickness is a fundamental control on water storage space and flow paths (Holbrook et al, 2014;Grant and Dietrich, 2017;Rempe and Dietrich, 2018), which together influence fluid-rock reactions (Navarre-Sitchler and Brantley, 2015) and connect ecosystems to rock-derived nutrients (Graham et al, 2010) and vital subsurface water stores (Klos et al, 2018). Studying the processes responsible for regolith architecture is therefore essential for understanding how the critical zone functions, and how it may respond to future changes (Goddéris and Brantley, 2013;Sullivan et al, 2019).…”

3D Seismic Anatomy of a Watershed Reveals Climate‐Topography Coupling That Drives Water Flowpaths and Bedrock Weathering

“…Throughout the periglacial periods during the last 1 My, the depth of frost‐cracking under the shaded N‐facing hillside remained greater than under the S‐facing side, allowing deeper hydrogeochemically driven weathering of the subsurface under the more shaded side of the catchment. This climate‐driven hydrogeochemical asymmetry can thus explain the asymmetry in erosional efficiency, hillslope, clay content, and base cation content (Sullivan et al., 2019; West et al., 2019).…”