Soil Cycles of Elements simulator for Predicting TERrestrial regulation of greenhouse gases: SCEPTER v0.9

Abstract

Abstract. The regulation of anthropogenic carbon dioxide (CO2) is an urgent issue – continuously increasing atmospheric CO2 from burning fossil fuels is leading to significant warming and acidification of the surface ocean. Timely and effective measures to curb CO2 increases are thus needed in order to mitigate the potential degradation of natural ecosystems, food security, and livelihood caused by anthropogenic release of CO2. Enhanced rock weathering (ERW) on croplands and hinterlands may be one of the most economically and ecologically effective ways to sequester CO2 from the atmosphere, given that these soil environments generally favor mineral dissolution and because amending soils with crushed rock can result in a number of co-benefits to plant growth and crop yield. However, robust quantitative evaluation of CO2 capture by ERW in terrestrial soil systems that can lead to coherent policy implementation will require an ensemble of traceable mechanistic models that are optimized for simulating ERW in managed systems. Here, we present a new 1D reactive transport model – SCEPTER. The model is designed to (1) mechanistically simulate natural weathering, including dissolution/precipitation of minerals along with uplift/erosion of solid phases, advection plus diffusion of aqueous phases and diffusion of gas phases, (2) allow targeted addition of solid phases at the soil–atmosphere interface, including multiple forms of organic matter (OM) and crushed mineral/rock feedstocks, (3) implement a range of soil mixing regimes as catalyzed by soil surface fauna (e.g., bioturbation) or humans (e.g., various forms of tilling), and (4) enable calculation of solid mineral surface area based on controlled initial particle size distributions coupled to a shrinking core framework. Here we describe the model structure and intrinsic thermodynamic/kinetic data, provide a series of idealized simulations to demonstrate the basic behavior of the code, and evaluate the computational and mechanistic performance of the model against observational data. We also provide selected example applications to highlight model features particularly useful for future prediction of CO2 sequestration by ERW in soil systems.

Impact of Climate on the Global Capacity for Enhanced Rock Weathering on Croplands

“…The above-described procedures successfully simulate ERW on 929 (927) out of the 963 sites (>96% success rate) under the RCP8.5 (RCP4.5) scenario; SCEPTER did not converge for the other 34 ( 36) sites (representing <4% of locations), mostly due to (a) climate boundary conditions that make convergence untenable during the 100 Kyr spin-up experiment (e.g., zero/NaN values for runoff [mm/s], soil moisture [unitless fraction], or temperature [K]) and/or (b) high assumed organic matter flux (e.g., >3,000 g C/m 2 year). Spin-up experiments (which do not have basalt application) and basalt application simulations described above are conducted in the same way as the "pulsed basalt application" example in Kanzaki et al (2022), with full tracking of particle size distributions of individual minerals; however, tilling is represented by an inversion mixing in this paper (see Equation S1 in Supporting Information S1), rather than as homogeneous mixing. We also track the evolution of solid, aqueous, and gaseous species.…”

“…Tracked solid species include: albite, anorthite, K-feldspar, forsterite, fayalite, diopside, phlogopite, tremolite, hedenbergite, quartz, goethite, kaolinite, Ca-beidellite, Mg-beidellite, gypsum, calcite, aragonite, dolomite, and one class of soil organic matter; tracked aqueous species include: Ca, Mg, Na, K, Si, Al, Fe(II), Fe(III), and SO 4 ; tracked gaseous species include: O 2 and CO 2 ; Fe(II) oxidation to Fe(III) by O 2 is also explicitly simulated as an extra reaction in addition to dissolution/precipitation of the above-listed solid species. Fundamental thermodynamic (e.g., particle density, molar volume/weight, chemical formula, and solubility of solid species; thermodynamic constants for formation of dependent aqueous species) and kinetic (e.g., rate constants of dissolution/precipitation for solid species and extra reactions; molecular diffusion coefficients for aqueous/gaseous species) data are given in Kanzaki et al (2022).…”

“…Organic matter is applied continuously at the surface at a rate described above and mixed to a 25 cm depth via natural bioturbation (Fickian mixing: see Kanzaki et al (2022) for detailed formulation) along with other solid species (listed above), whether basalt is additionally applied or not. When applied, basalt is assumed to have the same mineralogical composition as those adopted by Beerling et al (2020) and Kanzaki et al (2022) (Table S1 in Supporting Information S1), with 0.37 g CO 2 as the maximum capture capacity per 1 g basalt. For our "fine-grained" basalt feedstock, initial particle size distributions are defined as a function of particle radius, equivalent to the sum of 4 log-normal distributions centered at 1, 2, 5, and 20 μm, all with 0.2 log unit standard deviations (f basl (r) ∝ 𝐴𝐴 ∑ 4 𝑖𝑖=1 exp(−(log r − log r i ) 2 /0.4), where f basI (r) is the initial number of particles as a function of particle radius r in meters and r i = 1 × 10 −6 , 2 × 10 −6 , 5 × 10 −6 , and 20 × 10 −6 for i = 1, …, 4).…”

“…Although a coarser discretization of the soil column may suffice, finer discretization is still desirable given that basalt may accumulate on topsoil when mixing is inefficient. Organic matter is applied continuously at the surface at a rate described above and mixed to a 25 cm depth via natural bioturbation (Fickian mixing: see Kanzaki et al (2022) for detailed formulation) along with other solid species (listed above), whether basalt is additionally applied or not. When applied, basalt is assumed to have the same mineralogical composition as those adopted by Beerling et al (2020) and Kanzaki et al (2022) (Table S1 in Supporting Information S1), with 0.37 g CO 2 as the maximum capture capacity per 1 g basalt.…”

River chemistry constraints on the carbon capture potential of surficial enhanced rock weathering

“…Considering extra CO 2 emission during mining, crushing/grinding, transporting, and spreading of rock powder on land should decrease the overall net CDR efficiency, largely depending on the choice of source rocks and comminution techniques (Renforth 2012; Moosdorf et al 2014; Strefler et al 2018). In any case, further exploration of the kinetics of feedstock dissolution and secondary mineral formation in soil in a reaction‐transport framework (Taylor et al 2016; Beerling et al 2020; Kantzas et al 2022; Kanzaki et al 2022), understanding the impacts of processes in the soil‐to‐river continuum, and better constraints on CO 2 emissions during the large‐scale implementation of ERW will all be critical for the continued development of a holistic picture of ERW as a CDR strategy.…”