Process‐based modeling of silicate mineral weathering responses to increasing atmospheric CO2 and climate change (original) (raw)
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Response of Soil Mineral Weathering to Elevated Carbon Dioxide
2002
Understanding the rates of weathering of soil minerals and the factors that may either enhance or inhibit these rates is a crucial part of understanding many processes from the watershed to the global scale. One potentially important factor in mineral weathering that is not yet well understood is the effect of elevated CO2 concentrations on weathering rates. Here, the direct and indirect effects of elevated soil CO2 are examined in field and laboratory-based studies, and the incorporation of the relationship between CO2 and mineral weathering in soil chemistry models is critically evaluated. At Mammoth Mountain, California, volcanic ash soil is exposed to naturally occurring high levels of CO2 from a magmatic source. Comparative analyses of chemical and mineralogical characteristics of exposed and control soils suggest that decade-long exposure to elevated CO2 concentrations has altered soil dissolution rates. Indirect effects of elevated soil CO2 at this site, including vegetation ...
Effects of carbon dioxide on mineral weathering rates at earth surface conditions
The weathering of silicate minerals is an important long-term control on the global carbon budget. While the rate of mineral weathering is influenced by the atmospheric variations in atmospheric carbon dioxide, the only measurements of those effects have occurred during dissolution experiments at temperatures much higher than earth surface conditions. Thus, any climate models that include such a relationship may not be able to fully couple variations in atmospheric carbon dioxide with the lithospheric sinks. Our study presents a relationship for the dependence of plagioclase dissolution rates on PCO2 based on field data from a site in the southeastern Sierra Nevada drainages. A series of canyons that have similar drainages show wide variability in water chemistry that is attributed to variations in PCO2 from geothermal sources. This setting allowed us to isolate the effect of PCO2 on weathering rates in conditions relevant to climate models. The results show that mineral dissolution rates are proportional to PCO2 0.45 when the observed variations are attributed solely to variations in PCO2. This relationship is likely to be more applicable to climate models than prior laboratory derived data.
Impact of atmospheric CO 2 levels on continental silicate weathering
Geochemistry, Geophysics, Geosystems, 2010
1] Anthropogenic sources are widely accepted as the dominant cause for the increase in atmospheric CO 2 concentrations since the beginning of the industrial revolution. Here we use the B-WITCH model to quantify the impact of increased CO 2 concentrations on CO 2 consumption by weathering of continental surfaces. B-WITCH couples a dynamic biogeochemistry model (LPJ) and a process-based numerical model of continental weathering (WITCH). It allows simultaneous calculations of the different components of continental weathering fluxes, terrestrial vegetation dynamics, and carbon and water fluxes. The CO 2 consumption rates are estimated at four different atmospheric CO 2 concentrations, from 280 up to 1120 ppmv, for 22 sites characterized by silicate lithologies (basalt, granite, or sandstones). The sensitivity to atmospheric CO 2 variations is explored, while temperature and rainfall are held constant. First, we show that under 355 ppmv of atmospheric CO 2 , B-WITCH is able to reproduce the global pattern of weathering rates as a function of annual runoff, mean annual temperature, or latitude for silicate lithologies. When atmospheric CO 2 increases, evapotranspiration generally decreases due to progressive stomatal closure, and the soil CO 2 pressure increases due to enhanced biospheric productivity. As a result, vertical drainage and soil acidity increase, promoting CO 2 consumption by mineral weathering. We calculate an increase of about 3% of the CO 2 consumption through silicate weathering (mol ha −1 yr −1 ) for 100 ppmv rise in CO 2 . Importantly, the sensitivity of the weathering system to the CO 2 rise is not uniform and heavily depends on the climatic, lithologic, pedologic, and biospheric settings.
Global carbon sequestration through continental chemical weathering in a climatic change context
Scientific Reports, 2021
This study simulates carbon dioxide (CO 2) sequestration in 300 major world river basins (about 70% of global surface area) through carbonates dissolution and silicate hydrolysis. For each river basin, the daily timescale impacts under the RCP 2.6 and RCP 8.5 climate scenarios were assessed relative to a historical baseline (1969-1999) using a cascade of models accounting for the hydrological evolution under climate change scenarios. Here we show that the global temporal evolution of the CO 2 uptake presents a general increase in the annual amount of CO 2 consumed from 0.247 ± 0.045 Pg C year −1 to 0.261 and 0.273 ± 0.054 Pg C year −1 , respectively for RCP 2.6 and RCP 8.5. Despite showing a general increase in the global daily carbon sequestration, both climate scenarios show a decrease between June and August. Such projected changes have been mapped and evaluated against changes in hydrology, identifying hot spots and moments for the annual and seasonal periods. Chemical weathering of rocks has a significant impact on long-term global climate regulation 1. It transforms soil CO 2 into inorganic dissolved carbon (such as HCO 3 − and CO 3
Coupled modeling of biospheric and chemical weathering processes at the continental scale
Global Biogeochemical Cycles, 2010
1] In this contribution, a reactive-transport model describing weathering in soil profiles and at the watershed scale is coupled to a dynamic global vegetation model to calculate the dissolved load of continental waters on a 0.5°latitude × 0.5°longitude grid. The so-called Biosphere-Weathering at the Catchment Scale (B-WITCH) model is applied to the Orinoco watershed (South America). We show that B-WITCH is able to reproduce the main cation composition of the surface waters over the watershed. Sensitivity tests demonstrate that clay mineral reactivities are key factors controlling the calculated discharge of dissolved species. More specifically, our simulations show that the dissolution and precipitation rates of clay minerals in the weathering profiles are strongly intertwined, and that this coupling must be accurately described when modeling the weathering fluxes at the continental scale. A second set of sensitivity tests show that, for the tropical environment, land plants control the total base cation discharge through their impact on the soil hydrology, rather than through enhanced soil CO 2 pressures. Indeed, the complete removal of the continental vegetation leads to an increase in the dissolved fluxes to the ocean by 80% because of the collapse in the evapotranspiration, resulting in a more efficient drainage of the weathering profiles. On the other hand, neglecting the root respiration and setting the soil CO 2 pressure to the atmospheric level forces the total base cation discharge to decrease by only 20%.
Contribution of carbonate weathering to the CO2 efflux from temperate forest soils
Biogeochemistry, 2015
Temperate forests provide favorable conditions for carbonate bedrock weathering as the soil CO 2 partial pressure is high and soil water is regularly available. As a result of weathering, abiotic CO 2 can be released and contribute to the soil CO 2 efflux. We used the distinct isotopic signature of the abiotic CO 2 to estimate its contribution to the total soil CO 2 efflux. Soil cores were sampled from forests on dolomite and limestone and were incubated under the exclusion of atmospheric CO 2 . Efflux and isotopic signatures of CO 2 were repeatedly measured of cores containing the whole mineral soil and bedrock material (heterotrophic respiration ? CO 2 from weathering) and of cores containing only the mineral top-soil layer (A-horizon; heterotrophic respiration). An aliquot of the cores were let dry out during incubation to assess effects of soil moisture. Although the d 13 C values of the CO 2 efflux from the dolomite soil cores were within a narrow range (A-horizon -26.2 ± 0.1 %; whole soil profile wet -25.8 ± 0.1 %; whole soil profile dry -25.5 ± 0.1 %) the CO 2 efflux from the separated A-horizons was significantly depleted in 13 C when compared to the whole soil profiles (p = 0.015). The abiotic contribution to the total CO 2 efflux from the dolomite soil cores was 2.0 ± 0.5 % under wet and 3.4 ± 0.5 % under dry conditions. No abiotic CO 2 efflux was traceable from the limestone soil cores. An overall low contribution of CO 2 from weathering was affirmed by the amount and 13 C signature of the leached dissolved inorganic carbon (DIC) and the radiocarbon signature of the soil CO 2 efflux in the field. Together, our data point towards no more than 1-2 % contribution of abiotic CO 2 to the growing season soil CO 2 efflux in the field.
Journal of Ecology, 2011
1. The relative constancy of the lower limit on Earth's atmospheric CO 2 concentration ([CO 2 ] a) during major tectonic episodes over the final 24 million years (Ma) of the Cenozoic is surprising because they are expected to draw-down [CO 2 ] a by enhancing chemical weathering and carbonate deposition on the seafloor. That [CO 2 ] a did not drop to extremely low values suggests the existence of feedback mechanisms that slow silicate weathering as [CO 2 ] a declines. One proposed mechanism is a negative feedback mediated through CO 2 starvation of land plants in active orogenic regions compromising the efficiency of the primary carboxylating enzyme in C 3 plants (Rubisco) and diminishing productivity and terrestrial weathering. 2. The CO 2 starvation hypothesis is developed further by identifying four key related mechanisms: decreasing net primary production leading to (i) decreasing below-ground C allocation, reducing the surface area of contact between minerals and roots and mycorrhizal fungi and (ii) reduced demand for soil nutrients decreasing the active exudation of protons and organic acids by fine roots and mycorrhizas; (iii) lower carbon cost-for-nutrient benefits of arbuscular mycorrhizas (AM) favouring AM over ectomycorrhizal root-fungal symbioses, which are less effective at mineral weathering, and (iv) conversion of forest to C 3 and C 4 grassland arresting Ca leaching from soils. 3. We evaluated the global importance of mechanisms 1 and 2 in silicate weathering under a changing late Miocene [CO 2 ] a and climate using a process-based model describing the effects of plants and mycorrhizal fungi on the biological proton cycle and soil chemistry. The model captures what we believe are the key processes controlling the pH of the mycorrhizosphere and includes numerical routines for calculating weathering rates on basalt and granite using simple yet rigorous equilibrium chemistry and rate laws. 4. Our simulations indicate a reduction in the capacity of the terrestrial biosphere to weather continental silicate rocks by a factor of four in response to successively decreasing [CO 2 ] a values (400, 280, 180 and 100 p.p.m.) and associated late Miocene (11.6-5.3 Ma) cooling. Marked reductions in terrestrial weathering could effectively limit biologically mediated long-term carbon sequestration in marine sediments. 5. These results support the idea of terrestrial vegetation acting as a negative feedback mechanism that counteracts substantial declines in [CO 2 ] a linked to increased production of fresh weatherable minerals in warm, low-latitude, active orogenic regions.