Chemical reactions, porosity, and microfracturing in shale during weathering: The effect of erosion rate

Gu, Xiaoxiong; Rempe, D. M.; Dietrich, W. E.; West, A. Joshua; Lin, Teng Chiu; Jin, Lixin; Brantley, Susan L.

doi:10.1016/j.gca.2019.09.044

Cited by 84 publications

(152 citation statements)

References 121 publications

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“…4A). Finally, Gu et al (2020) showed that pyrite oxidation drives the calcite dissolution at Shale Hills, and this is corroborated by NMF results showing that samples near calcite equilibrium have the highest pyrite-derived sulfate concentrations (Fig. 4B).…”

Section: Synthetic Data Modelsupporting

confidence: 62%

“…3), we interpreted the two endmembers as deep and shallow weathering along the 215 two flowpaths, respectively (Jin et al, 2014;Sullivan et al, 2016 only dissolve at depth ( Fig. 3; Gu et al, 2020). The high [Cl -]/[SO4 2-] endmember was attributed to shallow water dominated by Cl-containing rain (interflow).…”

Section: Synthetic Data Modelmentioning

confidence: 94%

“…We focus first on Shale Hills, a small (0.08 km 2 ), acid-rain impacted forested watershed underlain by Rose Hill Shale located in central Pennsylvania, USA (Brantley et al, 2018). The Rose Hill Formation shale contains ~0.14 wt% S as pyrite (FeS2) (Gu et al, 2020).…”

Section: Study Sitesmentioning

confidence: 99%

“…valley. In these deeper zones, calcite (CaCO3), ankerite (Ca(Fe0.34Mg0.62Mn0.04)(CO3)2), and pyrite (FeS2) dissolve in regional groundwaters that flow to the stream (Brantley et al, 2013a;Gu et al, 2020). These groundwaters thus contribute DIC, Ca 2+ , Mg 2+ , and SO4 2into the stream.…”

Section: Synthetic Data Modelmentioning

confidence: 99%

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Machine Learning Deciphers CO<sub>2</sub> Sequestration and Subsurface Flowpaths from Stream Chemistry

Shaughnessy¹,

Gu²,

Wen³

et al. 2020

Preprint

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Abstract. Endmember mixing analysis (EMMA) is often used by hydrogeochemists to interpret the sources of stream solutes, but variations in stream concentrations and discharges remain difficult to explain. We discovered that machine learning can be used to reveal patterns in stream chemistry that pertain to information about weathering sources of solutes and also about subsurface groundwater flowpaths. The investigation has implications, in turn, for the balance of CO2 in the atmosphere. For example, CO2-driven weathering of silicate minerals removes carbon from the atmosphere over ~106-yr timescales. Weathering of another common mineral, pyrite, releases sulfuric acid that in turn causes dissolution of carbonates. In that process, however, CO2 is released instead of sequestered from the atmosphere. Thus, to understand long-term global CO2 sequestration by weathering requires quantification of CO2-versus H2SO4-driven reactions. Most researchers estimate such weathering fluxes from stream chemistry but interpreting the reactant minerals and acids dissolved in streams has been fraught with difficulty. We use a new machine learning technique in three watersheds to determine the minerals dissolved by each acid. The results show that the watersheds continuously or intermittently sequester CO2 but the extent of CO2 drawdown is diminished in areas heavily affected by acid rain. Sulfide oxidation contributes ~23 % to 62 % of sulfate fluxes. Without the new algorithm to deconvolve the mineral weathering, CO2 drawdown was always overestimated. The new technique, which also elucidated the importance of different subsurface flowpaths and long-timescale changes in the watersheds, should have great utility as a new EMMA for investigating water resources worldwide.

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Section: Synthetic Data Modelsupporting

confidence: 62%

Section: Synthetic Data Modelmentioning

confidence: 94%

Section: Study Sitesmentioning

confidence: 99%

Section: Synthetic Data Modelmentioning

confidence: 99%

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Machine Learning Deciphers CO<sub>2</sub> Sequestration and Subsurface Flowpaths from Stream Chemistry

Shaughnessy¹,

Gu²,

Wen³

et al. 2020

Preprint

Self Cite

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show abstract

“…Recent studies using major oxygen and sulfur isotope compositions (δ 18 O, δ 34 S; Materials and Methods) suggest that pyrite oxidation in some modern river systems occurs in suboxic groundwater aquifers and quantitatively incorporates meteoric water-derived oxygen (18)(19)(20). Consistent with this interpretation, detailed studies of shale bedrock drill cores indicate that pyrite oxidation occurs in low-O2 pores and microfractures within a deep, sharp reaction front independent of erosion rate (21). Such results raise the question as to why direct O2 incorporation into sulfate is evident in the geologic past but is apparently absent from modern pyrite weathering environments.…”

Section: Significancementioning

confidence: 63%

Triple oxygen isotope insight into terrestrial pyrite oxidation

Hemingway

Olson

Turchyn

et al. 2020

Proc. Natl. Acad. Sci. U.S.A.

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The mass-independent minor oxygen isotope compositions (Δ′17O) of atmospheric O2andCO2are primarily regulated by their relative partial pressures,pO2/pCO2. Pyrite oxidation during chemical weathering on land consumesO2and generates sulfate that is carried to the ocean by rivers. The Δ′17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes directO2incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation—including oxygen sources—in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Δ′17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ′17O values imply no direct troposphericO2incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative,17O-enriched sources such as reactive oxygen species. Sulfate Δ′17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ′17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ′17O as a proxy for pastpO2/pCO2should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.

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Relating the depth of the water table to the depth of weathering

Lebedeva

Brantley

2020

Earth Surf Processes Landf

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Weathering of bedrock creates and occludes permeability, affecting subsurface water flow. Often, weathering intensifies above the water table. On the contrary, weathering can also commence below the water table. To explore relationships between weathering and the water table, a simplified weathering model for an eroding hillslope was formulated that takes into account both saturated and unsaturated subsurface water flow (but does not fully account for changes in dissolved gas chemistry). The phreatic line was calculated using solutions to mathematical treatments for both zones. In the model, the infiltration rate at the hill surface sets both the original and the eventual steady‐state position of the water table with respect to the weathering reaction front. Depending on parameters, the weathering front can locate either above or below the water table at steady state. Erosion also affects the water table position by changing porosity and permeability even when other hydrological conditions (e.g. hydraulic conductivity of parent material, infiltration rate at the surface) do not change. The total porosity in a hill (water storage capacity) was found to increase with infiltration rate (all else held constant). This effect was diminished by increasing the erosion rate. We also show examples of how the infiltration rate affects the position of the water table and how infiltration rate affects weathering advance. Published 2020. This article is a U.S. Government work and is in the public domain in the USA

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Chemical reactions, porosity, and microfracturing in shale during weathering: The effect of erosion rate

Cited by 84 publications

References 121 publications

Machine Learning Deciphers CO<sub>2</sub> Sequestration and Subsurface Flowpaths from Stream Chemistry

Machine Learning Deciphers CO<sub>2</sub> Sequestration and Subsurface Flowpaths from Stream Chemistry

Triple oxygen isotope insight into terrestrial pyrite oxidation

Relating the depth of the water table to the depth of weathering

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Chemical reactions, porosity, and microfracturing in shale during weathering: The effect of erosion rate

Cited by 84 publications

References 121 publications

Machine Learning Deciphers CO&lt;sub&gt;2&lt;/sub&gt; Sequestration and Subsurface Flowpaths from Stream Chemistry

Machine Learning Deciphers CO&lt;sub&gt;2&lt;/sub&gt; Sequestration and Subsurface Flowpaths from Stream Chemistry

Triple oxygen isotope insight into terrestrial pyrite oxidation

Relating the depth of the water table to the depth of weathering

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Product

Resources

About

Machine Learning Deciphers CO<sub>2</sub> Sequestration and Subsurface Flowpaths from Stream Chemistry

Machine Learning Deciphers CO<sub>2</sub> Sequestration and Subsurface Flowpaths from Stream Chemistry