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1. The predominant control of hydroclimatic conditions on carbon and weathering fluxes at the hillslope scale

   Wen, Hang; Sullivan, Pamela; Billings, Sharon; Ajami, Hoori; Cueva, Alejandro; Flores, Alejandro; Hirmas, Daniel; Koop, Aaron; Murenbeeld, Kathryn; Zhang, Xi; et al (December 2021, American Geophysical Union annual conference)

   Soil biota generate CO2 that can vertically export to the atmosphere, and dissolved organic and inorganic carbon (DOC and DIC) that can laterally export to streams and accelerate weathering. These processes are regulated by external hydroclimate forcing and internal structures (permeability distribution), the relative influences of which are rarely studied. Understanding these interactions is essential a hydrological extremes intensify in the future. Here we explore the question: How and to what extent do hydrological and permeability distribution conditions regulate soil carbon transformations and chemical weathering? We address the questions using a hillslope reactive transport model constrained by data from the Fitch Forest (Kansas, United States). Numerical experiments were used to mimic hydrological extremes and variable shallow-versus-deep permeability contrasts. Results demonstrate that under dry conditions (0.08 mm/day), long water transit times led to more mineralization of organic carbon (OC) into inorganic carbon (IC) form (>98\%). Of the IC produced, ~ 75\% was emitted upward as CO2 gas and ~ 25\% was exported laterally as DIC into the stream. Wet conditions (8.0 mm/day) resulted in less mineralization (~88\%), more DOC production (~12\%), and more lateral fluxes of IC (~50\% of produced IC). Carbonate precipitated under dry conditions and dissolved under wet conditions as the fast flow rapidly droves the reaction to disequilibrium. The results depict a conceptual hillslope model that prompts four hypotheses for our community to test. H1: Droughts enhance carbon mineralization and vertical upward carbon fluxes, whereas large hydrological events such as storms and flooding enhance subsurface vertical connectivity, reduce transit times, and promote lateral export. H2: The role of weathering as a net carbon sink or source to the atmosphere depends on the interaction between hydrologic flows and lithology: transition from droughts to storms can shift carbonate from a carbon sink (mineral precipitation) to carbon source (dissolution). H3: Permeability contrasts regulate the lateral flow partitioning via shallow flow paths versus deeper groundwater though this alter reaction rates negligibly. H4: Stream chemistry reflect flow paths and can potentially quantify water transit times: solutes enriched in shallow soils have a younger water signature; solutes abundant at depth carry older water signature. 

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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. From on high: Geochemistry of alpine springs, Niwot Ridge, Colorado Front Range, USA

   https://doi.org/10.1002/hyp.13436  

   Fields, Jordan_F; Dethier, David_P (May 2019, Hydrological Processes)

   Abstract Snowmelt‐fed springs and small (0.5 km²) upland catchments in alpine areas of the western United States contribute significantly to the quantity and inorganic chemistry of water delivered to downstream basins but have not been studied extensively. Mineral weathering, transit time, and hydrologic mixing control the solute chemistry of waters that drain the upland zone of Niwot Ridge, Colorado Front Range, and adjacent areas in the granitic core of the Southern Rocky Mountains. Water in 37 springs sampled in this study flows in generally short steep paths (~0.3 km) through shallow regolith with mean transit times (MTT) of weeks to months, producing solutions dominated by Si, Ca²⁺, Na⁺, and HCO₃⁻, locally SO₄²⁻. Rock type is a significant control on spring, surface, and shallow groundwater chemistry, and plagioclase (oligoclase) is the major source of dissolved Na⁺and Si. Concentrations of Ca²⁺exceed stoichiometric predictions of oligoclase weathering by ~3.5×; excess Ca²⁺likely represents weathering of aeolian material, vein calcite, or trace minerals. Concentrations of base cations and Si increase slowly with estimated MTT of 0.2 years for Niwot Ridge spring waters, and several years for shallow groundwater sampled by wells. Chemical weathering of silicate minerals is slow with estimated rates of ~2.0 and 0.2 pmol·m⁻²·s⁻¹for oligoclase and microcline, respectively; the most mineralized spring waters are saturated only with respect to kaolinite and montmorillonite. More than 50% of the dissolved base cations + Si measured in Boulder Creek at Orodell (~25 km downstream) accumulate before water emerges from alpine springs on Niwot Ridge. Warming global temperatures are shifting more high‐elevation precipitation to rain, potentially changing run‐off patterns, transit time, and solute loads. Acquisition of solutes by alpine waters thus has implications far beyond small upland catchments. 

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4. The shallow and deep hypothesis: linking flow paths, biogeochemical reactions, and stream chemistry in the Critical Zone

   Li, Li and (December 2021, American Geophysical Union Annual conference)

   How does the physical and chemical structure of the Critical Zone (CZ), defined as the zone from treetops to the bottom of groundwater, govern its hydro-biogeochemical functioning? Multiple lines of evidence from past and newly emerging research have prompted the shallow and deep partitioning concentration-discharge (C-Q) hypothesis. The hypothesis states that in-stream C-Q relationships are shaped by distinct source waters from flow paths at different depths. Base flows are often dominated by deep groundwater and mostly reflect groundwater chemistry, whereas high flows are often dominated by shallow soil water and thus mostly reflect soil water chemistry. The contrasts between shallow soil water versus deeper groundwater chemistry shape stream solute export patterns. In this context, the vertical connectivity that regulates the shallow and deep flow partitioning is essential in determining chemical contrasts, biogeochemical reaction rates in soils and parent rocks, and ultimately solute export patterns. This talk will highlight insights gleaned from multiple lines of recent studies that include collation of water chemistry data from soils, rocks, and streams in intensively monitored watersheds, meta-analysis of stream chemistry data at the continental scale, and integrated reactive transport modeling at the hillslope and watershed scales. The hypothesis underscores the importance of subsurface vertical structure and connectivity relative to the extensively studied horizontal connectivity. It also alludes to the potential of using streams as mirrors for subsurface water chemistry, and the potential of using C-Q relationships to infer flow paths and biogeochemical reaction rates and the response of earth’s subsurface to climate and human perturbations. Broadly, this simple conceptual framework links CZ subsurface structure to its functioning under diverse climate, geology, and land cover conditions. 

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5. Lithological Control of Stream Chemistry in the Luquillo Mountains, Puerto Rico

   https://doi.org/10.3389/feart.2022.779459  

   Hynek, S. A.; McDowell, W. H.; Bhatt, M. P.; Orlando, J. J.; Brantley, S. L. (May 2022, Frontiers in Earth Science)

   Meteoric waters move along pathways in the subsurface that differ as a function of lithology because of the effects of chemical and physical weathering. To explore how this affects stream chemistry, we investigated watersheds around an igneous intrusion in the Luquillo Mountains (Puerto Rico). We analyzed streams on 1) unmetamorphosed country rock (volcaniclastic sedimentary strata, VC) surrounding an igneous intrusion, 2) the quartz-diorite intrusion (QD), and 3) the metamorphosed aureole rock (hornfels-facies volcaniclastics, HF). These lithologies differ physically and chemically but weather under the same tropical rain forest conditions. The sedimentary VC lithology is pervasively fractured while the massive QD and HF lithologies are relatively unfractured. However, the QD fractures during weathering to produce spheroidally-weathered corestones surrounded by cm-thick rindlets of increasingly weathered rock. Meteoric waters flow pervasively through the network of already-fractured VC rock and the spheroidally weathered rindlets on the QD, but only access a limited fraction of the HF, explaining why streams draining HF are the most dilute in the mountains. This results in various thicknesses of regolith from thick (VC) to moderate (QD) to thin or nonexistent (HF). The pervasive fractures allow groundwater to flow deeply through the VC and then return to the mainstem river (Río Mameyes) at lower elevations. These “rock waters” drive concentrations of rock-derived solutes (silica, base cations, sulfate, phosphate) higher in the lower reaches of the stream. Water also flows through weathering-induced fractures on the QD at high elevations where rindletted corestones are present in stacks, and this water flux dissolves plagioclase and hornblende and oxidizes biotite. This “QD rock water” is not generated at lower elevations in the Río Icacos watershed, where stacks of corestones are absent, and contributions to stream solutes derive from weathering of feldspar- and hornblende-depleted saprolite. The stream chemistry in the QD-dominated watershed (Río Icacos) thus varies from concentrated QD-rock water at channel heads below steep ridgelines toward more diluted “saprolite water” downstream. These observations emphasize the importance of lithology and fracture patterns in dictating water flowpaths, stream chemistry, and regolith development in headwater catchments. 

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