Controls on the chemical and isotopic composition of carbonate springs during evolution to saturation with respect to calcite

Abongwa, Pride T.; Atekwana, Estella A.

doi:10.1016/j.chemgeo.2015.03.024

Cited by 14 publications

(9 citation statements)

References 22 publications

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“…Instead, it approximates these processes by having a CO 2 source phase CO 2 (g * ) that continuously releases CO 2 (g) and reproduced the observed soil pCO 2 levels at a kinetic rate constant of 10 −9 mol/m 2 /s (Reactions 0-1 in Table 1). This value is at the low end of the reported soil respiration rates (10 −9 -10 −5 mol/m 2 /s) (Bengtson and Bengtsson, 2007;Ahrens et al, 2015;Carey et al, 2016). The dissolution of CO 2 (g) into CO 2 (aq) follows Henry's law C CO 2 (aq) = K 1 pCO 2 .…”

Section: Representation Of Soil Comentioning

confidence: 84%

“…d The kinetic rate parameters and specific surface areas were from Palandri and Kharaka (2004), except Reaction (0). The kinetic rate constant of the source CO 2 (g * ) dissolution (i.e., soil respiration rate constant, Reaction 0) was from Bengtson and Bengtsson (2007), Ahrens et al (2015), and Carey et al (2016); the specific surface area was referred to that of soil organic carbon (Pennell et al, 1995). e These reactions were only used in the base-case model as they occurred in upper soils in Konza.…”

Section: Data Sourcesmentioning

confidence: 99%

“…Values of Q G / Q T estimated through base flow separation vary from 20 % to 90 % in forest/wood-dominated watersheds (Price, 2011), often negatively correlating with the proportion of grasslands (Mazvimavi et al, 2004). Soil respiration rates can vary between 10 −9 and 10 −5 mol/m 2 /s in both grasslands and woodlands (Bengtson and Bengtsson, 2007;Ahrens et al, 2015;Carey et al, 2016). Soil CO 2 levels may vary by 2-3 orders of magnitude depending on vegetation type and climate conditions (Neff and Hooper, 2002;Breecker et al, 2010).…”

Section: Hydrological and Biogeochemical Differences In Grasslands Anmentioning

confidence: 99%

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Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge

et al. 2021

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Abstract. Carbonate weathering is essential in regulating atmospheric CO2 and carbon cycle at the century timescale. Plant roots accelerate weathering by elevating soil CO2 via respiration. It however remains poorly understood how and how much rooting characteristics (e.g., depth and density distribution) modify flow paths and weathering. We address this knowledge gap using field data from and reactive transport numerical experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a site where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots can enhance weathering in two ways. First, deepening roots can control thermodynamic limits of carbonate dissolution by regulating how much CO2 transports vertical downward to the deeper carbonate-rich zone. The base-case data and model from Konza reveal that concentrations of Ca and dissolved inorganic carbon (DIC) are regulated by soil pCO2 driven by the seasonal soil respiration. This relationship can be encapsulated in equations derived in this work describing the dependence of Ca and DIC on temperature and soil CO2. The relationship can explain spring water Ca and DIC concentrations from multiple carbonate-dominated catchments. Second, numerical experiments show that roots control weathering rates by regulating recharge (or vertical water fluxes) into the deeper carbonate zone and export reaction products at dissolution equilibrium. The numerical experiments explored the potential effects of partitioning 40 % of infiltrated water to depth in woodlands compared to 5 % in grasslands. Soil CO2 data suggest relatively similar soil CO2 distribution over depth, which in woodlands and grasslands leads only to 1 % to ∼ 12 % difference in weathering rates if flow partitioning was kept the same between the two land covers. In contrast, deepening roots can enhance weathering by ∼ 17 % to 200 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/a. Weathering rates in these cases however are more than an order of magnitude higher than a case without roots at all, underscoring the essential role of roots in general. Numerical experiments also indicate that weathering fronts in woodlands propagated > 2 times deeper compared to grasslands after 300 years at an infiltration rate of 0.37 m/a. These differences in weathering fronts are ultimately caused by the differences in the contact times of CO2-charged water with carbonate in the deep subsurface. Within the limitation of modeling exercises, these data and numerical experiments prompt the hypothesis that (1) deepening roots in woodlands can enhance carbonate weathering by promoting recharge and CO2–carbonate contact in the deep subsurface and (2) the hydrological impacts of rooting characteristics can be more influential than those of soil CO2 distribution in modulating weathering rates. We call for colocated characterizations of roots, subsurface structure, and soil CO2 levels, as well as their linkage to water and water chemistry. These measurements will be essential to illuminate feedback mechanisms of land cover changes, chemical weathering, global carbon cycle, and climate.

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Section: Representation Of Soil Comentioning

confidence: 84%

Section: Data Sourcesmentioning

confidence: 99%

Section: Hydrological and Biogeochemical Differences In Grasslands Anmentioning

confidence: 99%

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Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge

et al. 2021

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“…Eq. 2 and 3 were tested with compiled soil pCO2 and spring water chemistry data from 8 carbonate-dominated catchments (Dandurand et al, 1982;Lopez-Chicano et al, 2001;Moral et al, 2008;Ozkul et al, 2010;Kanduc et al, 2012;Calmels et al, 2014;Abongwa and Atekwana, 2015;Huang et al, 2015) (also see the SI for details). Plotting spring-water chemistry against pCO2…”

Section: The Thermodynamics Of Carbonate Weathering: Control Of Tempementioning

confidence: 99%

Deepening roots can enhance carbonate weathering

Wen

Sullivan

Macpherson

et al. 2020

Preprint

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Abstract. Carbonate weathering is essential in regulating atmospheric CO2 and carbon cycle at the century time scale. Plant roots have been known to accelerate weathering by elevating soil CO2 via respiration. It however remains poorly understood how and how much rooting characteristics (e.g., depth and density distribution) modify flow paths and weathering. We address this knowledge gap using field data from and reactive transport numerical experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a site where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots potentially enhance weathering in two ways. First, deepening roots can control thermodynamic limits of carbonate dissolution by regulating how much CO2 transports downward to the deeper carbonate-rich zone. The base-case data and model from Konza reveal that concentrations of Ca and Dissolved Inorganic Carbon (DIC) are regulated by soil pCO2 driven by the seasonal fluctuation of soil respiration. This relationship can be encapsulated in equations derived in this work describing the dependence of Ca and DIC on temperature and soil CO2, which has been shown to apply in multiple carbonate-dominated catchments. Second, numerical experiments show that roots control weathering rates by regulating the amount of water fluxes that flush through the carbonate zone and export reaction products at dissolution equilibrium. Numerical experiments explored the potential effects of partitioning 40 % of infiltrated water to depth in woodlands compared to 5 % in grasslands. Soil CO2 data from wood- and grasslands suggest relatively similar soil CO2 distribution over depth, and only led to 1 % to 12 % difference in weathering rates if flow partitioning was kept the same between the two land covers. In contrast, deepening roots can enhance weathering by 17 % to 207 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/yr. Numerical experiments also indicated that weathering fronts in woodlands propagated > 2 times deeper compared to grasslands after 300 years at the infiltration rate of 0.37 m/yr. These differences in weathering fronts are ultimately caused by the contact time of CO2-charged water with carbonate rocks. We recognize that modeling results are subject to limitations in representing processes and parameters, but we propose that the data and numerical experiments allude to the hypothesis that (1) deepening roots can enhance carbonate weathering; (2) the hydrological impacts of rooting characteristics can be more influential than those of soil CO2 distribution in modulating weathering rates. We call for co-located characterizations of roots, subsurface structure, soil CO2 levels, and their linkage to water and water chemistry. These measurements will be essential to improve models and illuminate feedback mechanisms of land cover changes, chemical weathering, global carbon cycle, and climate.

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“…For example, cartography of SIc in different stream sites can be performed for resource spatial evaluation (Herman and Lorah 1987;Pentecost 1992;Hess and White 1993;Keppel et al, 2012). It is also useful when calcite deposition/removal in stream water issued from karstic spring is a concern (Dreybrodt et al 1992;Bono et al 2001;Hattanji et al 2014;Abongwa and Atekwana 2015). The SIc assessments can also describe the severity/progression of the precipitation of calcite enhanced by microbial activities in a supersaturated water (Cantonati et al 2016).…”

Section: Introductionmentioning

confidence: 99%

SIc–Abacus: An in–situ tool for estimating SIc and Pco2 in the context of carbonate karst

Peyraube

Lastennet

Denis

et al. 2019

Journal of Hydrology

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This article aims to provide an on the spot, handy field tool that can be used in estimating the Saturation Index with respect to calcite in karst aquifers in a limestone context. It relies on an abacus that gives the SIc by using measured pH and bicarbonate concentration as coordinates. The analytical expressions of the carbonate equilibrium equations are derived in order to calculate SIc from values that can be measured on field. Using these established relationships, we propose an Abacus built from the SIc=f(Pco 2 eq) reference frame, hence called SIc-Abacus. Accuracy of SIc calculation is also discussed. SIc is calculated with values measured considering uncertainty coming from pH (±0.05 pH units), temperature (±0.1 °C), bicarbonate and calcium concentrations (±2.5 mg/L and ±2 mg/L). This gives, for each sample, a range of possible SIc values of ±0.060 pH units (for a bicarbonate concentration close to 300 mg/L). The effect on SIc range of possible values given by bicarbonate and calcium uncertainties is about 10 times lower than the effect brought by pH uncertainty. Also, bicarbonate and calcium concentration can be estimated from electrical conductivity leading to a possible range of SIc value of ±0.087 pH units. In both cases, the range of possible values of SIc varies according to the bicarbonate concentration. Using Cussac and Lascaux sites, an evaluation was done. Results show the goodness of fit (R 2 =0.96) between the calculated SIc from measured pH, bicarbonate and calcium concentration and the estimated SIc values generated from the SIc-Abacus. Apart from calculating SIc, another method which is an alternative use of the generated SIc equation focused on obtaining pH using parameters such as electrical conductivity and dissolved CO 2 measurements is presented. The study shows that SIc-Abacus can be a new in-situ tool for karst aquifer surveys. It can facilitate in making a swift decision that tackles carbonate karst issues (e.g. when selecting sites or in deciding if extensive monitoring should be performed or not based from a specific objective).

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Controls on the chemical and isotopic composition of carbonate springs during evolution to saturation with respect to calcite

Cited by 14 publications

References 22 publications

Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge

Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge

Deepening roots can enhance carbonate weathering

SIc–Abacus: An in–situ tool for estimating SIc and Pco2 in the context of carbonate karst

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Controls on the chemical and isotopic composition of carbonate springs during evolution to saturation with respect to calcite

Cited by 14 publications

References 22 publications

Deepening roots can enhance carbonate weathering by amplifying CO&lt;sub&gt;2&lt;/sub&gt;-rich recharge

Deepening roots can enhance carbonate weathering by amplifying CO&lt;sub&gt;2&lt;/sub&gt;-rich recharge

Deepening roots can enhance carbonate weathering

SIc–Abacus: An in–situ tool for estimating SIc and Pco2 in the context of carbonate karst

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Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge

Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge