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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Machine learning deciphers CO<sub>2</sub> sequestration and subsurface flowpaths from stream chemistry
Abstract. Endmember mixing analysis (EMMA) is often used by hydrogeochemiststo interpret the sources of stream solutes, but variations in streamconcentrations and discharges remain difficult to explain. We discoveredthat machine learning can be used to highlight patterns in stream chemistrythat reveal information about sources of solutes and subsurface groundwaterflowpaths. The investigation has implications, in turn, for the balance ofCO2 in the atmosphere. For example, CO2-driven weathering ofsilicate minerals removes carbon from the atmosphere over ∼106-year timescales. Weathering of another common mineral, pyrite, releases sulfuricacid that in turn causes dissolution of carbonates. In that process,however, CO2 is released instead of sequestered from the atmosphere. Thus, understanding long-term global CO2 sequestration by weatheringrequires quantification of CO2- versus H2SO4-drivenreactions. Most researchers estimate such weathering fluxes from streamchemistry, but interpreting the reactant minerals and acids dissolved in streams has been fraught with difficulty. We apply a machine-learningtechnique to EMMA in three watersheds to determine the extent of mineraldissolution by each acid, without pre-defining the endmembers. The resultsshow that the watersheds continuously or intermittently sequester CO2, but the extent of CO2 drawdown is diminished in areas heavily affectedby acid rain. Prior to applying the new algorithm, CO2 drawdown wasoverestimated. The new technique, which elucidates the importance ofdifferent subsurface flowpaths and long-timescale changes in the watersheds,should have utility as a new EMMA for investigating water resourcesworldwide.
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- NSF-PAR ID:
- 10313541
- Date Published:
- Journal Name:
- Hydrology and Earth System Sciences
- Volume:
- 25
- Issue:
- 6
- ISSN:
- 1607-7938
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/hess-25-3397-2021
