Linking soil chemistry, treeline shifts and climate change: scenario modeling using an experimental approach (original) (raw)
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Geoderma, 2015
Climate change and a related increase in temperature, particularly in alpine areas, force both flora and fauna to adapt to the new conditions. These changes should in turn affect soil formation processes. The aim of this study was to identify possible consequences for soils in a dry-alpine region with respect to weathering of primary minerals and leaching of elements under expected vegetation and climate changes. To achieve this, a field empirical approach investigating an altitudinal sequence was used in combination with laboratory weathering experiments simulating several scenarios. The study sites are located in Sinks Canyon and Stough Basin of the Wind River Range, Wyoming, USA. The following sites (from moist to dry with increasing temperature along the sequence) were investigated: 10 soil profiles (Typic Haplocryoll) in a tundra ecotone, 10 soil profiles (Ustic Haplocryoll) in a pine-fir forest and 20 soil profiles (Ustic Argicryoll) in sagebrush. All soils developed on granitoid moraines. Soil mineralogy was analysed using cathodoluminescence and X-ray diffraction. This revealed that biotite and plagioclase were both weathered to smectite while plagioclase also weathered to kaolinite. Cooler, wetter, altitude-dependent conditions promoted weathering of primary minerals. Furthermore, the soils of the tundra and forest zone exhibited a higher acidity and more organic carbon. In a series of wet laboratory batch experiments, materials from topsoils (A horizons) and subsoils (B horizons) in each ecotone were examined alone or in combination with other samples. In a first step, aqueous extracts of the topsoil samples were generated in batch reactors and analysed for the main ions. In a second and a third step the topsoil extracts were reacted with the subsoil samples of the same ecotone, and with the subsoil samples of the ecotones at higher altitude. The total duration of these batch experiments was 1800 h, and the solutes were measured using ICP-OES and ion chromatography. Dissolved Ca, Mg and K were mainly controlled by the chemical weathering of oligoclase, K-feldspar and biotite. With increasing altitude the total concentrations of Ca, Mg and K in the aqueous extracts decreased, the relative ionic contribution from K decreased, while the ionic contribution from Ca increased. Climate change (warming, changed precipitation) potentially will reduce weathering intensity, soil acidity and the content of organic carbon. An altitudinal shift in vegetation due to climate change seems to affect the ionic composition of the soil solution. In the case of a shift from forest to sagebrush and tundra to forest or sagebrush, the relative contribution from K would increase at the expense of Ca. We hypothesise that K will play an important role in future biogeochemical cycles under the assumptions of climate warming and subsequent vegetation shifts to higher altitudes.
Geochemical mass balance applied to the study of weathering and evolution of soils
2018
Soil is viewed as an open system with addi¬tions, losses, translocations and transformations of materi¬als. The purpose of this research is to assess the geochemi¬cal mass balance and weathering intensity of Vertisols (Typic Haplusterts) and Entisols (Lithic Ustorthents) developed in a Quaternary-age basaltic toposequence under semi-humid conditions in the central Black Sea region of Turkey. We used mass-balance analysis with a view to measuring ele¬mental gains and losses along with alterations concerning soil forming processes. To this end, geochemical properties, elemental mass-balance changes and certain physicochem¬ical features were identified to benchmark the weathering levels of the profiles. Lithic Ustorthents are distinguished by having a rough texture along with a low organic substance ingredient, whereas Typic Haplusterts have a high clay tex¬ture with low bulk density and slickenside features. X-ray diffraction showed that smectites were the prevailing miner¬als inside the Typic Haplusterts, while a significant amount of kaolinite and illite was observed in the Lithic Ustorthents. Mass-balance computations indicated that massive mineral weathering resulted in substantial Si losses through leach¬ing as well as an exchange of cations, such as Na+,K+ , Ca2+ and Mg2+, particularly from the upper horizons. The study also took into account other features such as the pe¬dogenic evolution of soils using weathering indices such as CIA (chemical index of alteration), CIW (chemical in¬dex of weathering), bases/R2O3, WIP (weathering index of Parker), P (product index), PIA (plagioclase index of alter¬ation). According to the results, CIA, CIW, PIA, P, WIP and bases/R2O3 (Fe2O3 + Al2O3) index values of all soils varied from 42.33 to 73.83, 44.46 to 80.43, 37.53 to 65.63, 75.39 to 84.31 and 0.45 to 1.27 respectively, to solum depth. This result indicated that soils classified as Entisol and Vertisol have similar pedochemical properties in terms of weathering indices. In spite of similar weathering rates, the soils were classified under different groups as a result of erosion. This showed that environmental conditions for soil development in the studied area had a far more impact on weathering and elemental loss than the parent material on the site.
2009
This study is the geochemical examination of mineral weathering and its path from hinterland, through sediment deposition and pedogenesis, to its dissolution and eventual uptake into plants or precipitation as carbonate minerals. The three papers examine the rate and character of carbonate and silicate mineral weathering over a wide range of climatic and tectonic regimes, time periods, and lithologies, and focus on very different questions. Examination of the 87 Sr/ 86 Sr ratios of architectural ponderosa pine in Chaco Canyon, New Mexico confirms a societally complex style of timber procurement from the 10 th to the 12 th centuries. In El Malpais National Monument, New Mexico, we measured the 87 Sr/ 86 Sr ratios in local bedrock and soils and compared them to the leaf/wood cellulose of four conifers (Pinus ponderosa, Pinus edulis, Juniperus monosperma, Juniperus scopulorum), a deciduous tree (Populus tremuloides), three shrubs (Chrysothamus nauseosus, Fallugia paradoxa, Rhus trilobata), and an annual grass (Bouteloua gracilis) and a lichen (Xanthoparmelia lineola). We found that plant 87 Sr/ 86 Sr ratios covaried with variations in plant physiognomy, life history, and rooting depth. In addition, the proportion of atmospheric dust and bedrock mineral contributions to soil water 87 Sr/ 86 Sr ratios varied predictably with landscape age and bedrock lithology. On the Himalayan floodplain, soils and paleosol silicate weathering intensities were measured along a climatic transect and through time. Overall, carbonate weathering dominates floodplain weathering. But, periods of more intense silicate weathering between 9-2 Ma, identified in soil profile and in the 87 Sr/ 86 Sr ratios of pedogenic carbonates, appear to be driven by changes in tectonic, rather than climatic, regime. All 12 three papers are good examples of how 87 Sr/ 86 Sr isotopic tracer studies can shed light on pedogenic formation rates and internal processes. The complexity of each system warns against generalizations based on just one locale, one species or lithology, or a few isotopic ratios.
Modeling Soil Solution, Mineral Formation and Weathering
SSSA special publication series, 2015
Simulation models are valuable tools to increase understanding of complex soil chemical processes. Initially modeling was restricted by limited computational power; thus, complex natural systems were often simulated with site-specific, regression-based models. These models (classified as functional models by Hoosbeek & Bryant, 1992) do not deal with the actual processes but were designed to simulate the response of the studied system to specific variables. Although useful for the conditions represented by the collected data set, these models are usually not suited for other environments. Often the process controlling variables or responses to these variables are different in different environments. Nonetheless, due to the complexity of natural systems, quantitative mechanistic models often do not exist. The development of quantitative models has evolved from initial attempts to determine species distributions and ion activity, to models that attempt to represent chemical processes including soil mineral formation and prediction of soil solution composition and rates of mineral weathering. In this chapter we present an overview of the types of quantitative models suitable for describing soil chemical processes including mineral weathering. Generally following a chronological development, the models can be classified into a series of increasingly complex treatments of the natural system. EQUILIBRIUM Solution Speciation Models The conceptual framework of solution speciation model was presented by Garrels and Christ (1965). One of the first chemical equilibrium models was IONPAIR developed by Thrailkill (1970). This relatively simple model required input of pH and alkalinity and computed the saturation status of a water with respect to calcite. The more extensive model WATEQ, developed by Truesdell and Jones (1974), included a complete speciation scheme for the major, naturally occurring chemical species and calculations of saturation status with respect to many important silicate minerals as well as some oxides and carbonates. This program has been updated periodically (WATEQ4F; Ball et al., 1987) and is still
Geochemistry, the key to understanding environmental chemistry
Science of The Total Environment, 1996
Weathering is the most mysterious but also the most mishandled process in soil chemistry modelling. The myth arose that it could never be determined, nor ever modelled. Thus it became the obvious parameter to use for tuning the models, since the real value was unknown. This is not true. New models such as SAFE and PROFILE have made possible modeling of weathering rates in the field from geochemical and geophysical properties. The weathering model in PROFILE is a reductionistic type of model, where the main mechanism is based on a set of parallel molecular mechanisms at the mineral surface. Rate coefficients in the models have been taken from laboratory studies. In field tests, the PROFILE model has performed well and is capable of predicting the weathering rate well within the range determined by other methods. PROFILE is operationally used for regional weathering rate mapping and is currently applied in 37 countries. Through feedback mechanisms in the model, the weathering rate can be used to affect biotic processes.
Geochimica et Cosmochimica Acta, 2010
Anthropogenic and natural climate change affect processes in the atmosphere, biosphere, hydrosphere, and pedosphere. The impact of climate on soil evolution has not been well-explored, largely due to slow rates and the complexity of coupled processes that must be observed and simulated. The rates of mineral weathering in loess deposited 23 kyr ago and experiencing soil formation for 13 kyr are explored here using the WITCH model for weathering and the GENESIS model for climate simulation. The WITCH model, which uses rigorous kinetic parameters and laws with provision for the effect on rates of deviation from equilibrium, can successfully simulate the depletion profiles in the soil for dolomite and albite if soil CO 2 is assumed to rise over the last 10 kyr up to about 30-40Â the present atmospheric pressure, and if the solubility product of the Ca-smectite is assumed equal to that of an Fe(III)-rich Ca-montmorillonite. Such simulations document that dissolution behavior for silicates and carbonates are strongly coupled through pH, and for Ca-smectite and feldspars through dissolved silica. Such coupling is not incorporated in simple geometric and analytical models describing mineral dissolution, and therefore probably contributes to the long-standing observation of discrepancies among laboratory and field mineral dissolution rates.
Reservoir theory for studying the geochemical evolution of soils
Journal of Geophysical Research, 2010
1] Linking mineral weathering rates measured in the laboratory to those measured at the landscape scale is problematic. In laboratory studies, collections of minerals are exposed to the same weathering environment over a fixed amount of time. In natural soils, minerals enter, are mixed within, and leave the soil via erosion and dissolution/leaching over the course of soil formation. The key to correctly comparing mineral weathering studies from laboratory experiments and field soils is to consistently define time. To do so, we have used reservoir theory. Residence time of a mineral, as defined by reservoir theory, describes the time length between the moment that a mineral enters (via soil production) and leaves (via erosion and dissolution/leaching) the soil. Age of a mineral in a soil describes how long the mineral has been present in the soil. Turnover time describes the time needed to deplete a species of minerals in the soil by sediment efflux from the soil. These measures of time are found to be sensitive to not only sediment flux, which controls the mineral fluxes in and out of a soil, but also internal soil mixing that controls the probability that a mineral survives erosion. When these measures of time are combined with published data suggesting that a mineral's dissolution reaction rate decreases during the course of weathering, we find that internal soil mixing, by partially controlling the age distribution of minerals within a soil, might significantly alter the soil's mass loss rate via chemical weathering.
Biogeosciences Discussions, 2019
This study explored the influence of uncertainty in quantitative mineralogy on PROFILE base cation (Ca, Mg, K, Na) weathering rates obtained using normative mineralogy compared to those obtained using measured mineralogy, which was taken as a reference. Weathering rates were determined for two sites, one in Northern (Flakaliden) and one in Southern (Asa) Sweden. At each site, 3-4 soil profiles were analyzed at 10 cm depth intervals. Normative quantitative mineralogy was calculated from geochemical data and qualitative mineral data with the "Analysis to Mineralogy" program ('A2M') using two sets of qualitative mineralogical data inputs to A2M: A site-specific mineralogy determined from X-ray powder diffraction (XRPD) analyses, and regional mineralogy, representing the assumed mineral identity and compositions for larger geographical areas in Sweden. For the site-specific mineral input the precise elemental compositions of minerals were determined by microprobe analysis, whereas for the regional mineralogy the compositions were as assumed in previous studies. A2M does not provide a unique mineralogical solution and one thousand random mineralogical solutions were calculated by A2M for each soil unit in order to include the full space of quantitative mineralogies in model outcome, all equally possible. A corresponding number of PROFILE runs were made to estimate weathering rates. The contribution of individual minerals to the release of base cations was also quantified by using a version of PROFILE which outputs this detail. A discrepancy between weathering rates calculated from XRPD data (WXRPD) and weathering rates based on A2M (WA2M) was only considered significant if the former was outside the full range of the latter. Arithmetic means of WA2M were generally in relatively close agreement with WXRPD. The hypothesis that using site-specific instead of regional mineralogy will improve the confidence in mineral data input to PROFILE was supported for Flakaliden. However, at Asa, site-specific mineralogies reduced the discrepancy for Na between WA2M and WXRPD but produced larger and significant discrepancies for K, Ca and Mg. For Ca and Mg the differences between weathering rates based on different mineralogies could be explained by differences in the content of some specific Ca-and Mg-bearing minerals, in particular amphibole, apatite, pyroxene and illite. It was concluded that improving the precision in the content of those minerals would reduce weathering uncertainties. High uncertainties in mineralogy, due for example to different A2M assumptions, had surprisingly low effect on the weathering from Na-and K-bearing minerals. This can be explained by the fact that the weathering rate constants for the minerals involved, e.g. K-feldspar and micas, are similar in PROFILE. Improving the description of the dissolution rate kinetics of the plagioclase mineral group as well as major K-bearing minerals (K-feldspars and micas) should be of particular importance to future weathering estimates.
Soil chemistry in lithologically diverse datasets: The quartz dilution effect
Applied Geochemistry, 2009
National-and continental-scale soil geochemical datasets are likely to move our understanding of broad soil geochemistry patterns forward significantly. Patterns of chemistry and mineralogy delineated from these datasets are strongly influenced by the composition of the soil parent material, which itself is largely a function of lithology and particle size sorting. Such controls present a challenge by obscuring subtler patterns arising from subsequent pedogenic processes. Here the effect of quartz concentration is examined in moist-climate soils from a pilot dataset of the North American Soil Geochemical Landscapes Project. Due to variable and high quartz contents (6.2-81.7 wt.%), and its residual and inert nature in soil, quartz is demonstrated to influence broad patterns in soil chemistry. A dilution effect is observed whereby concentrations of various elements are significantly and strongly negatively correlated with quartz. Quartz content drives artificial positive correlations between concentrations of some elements and obscures negative correlations between others. Unadjusted soil data show the highly mobile base cations Ca, Mg, and Na to be often strongly positively correlated with intermediately mobile Al or Fe, and generally uncorrelated with the relatively immobile high-field-strength elements (HFS) Ti and Nb. Both patterns are contrary to broad expectations for soils being weathered and leached. After transforming bulk soil chemistry to a quartz-free basis, the base cations are generally uncorrelated with Al and Fe, and negative correlations generally emerge with the HFS elements. Quartz-free element data may be a useful tool for elucidating patterns of weathering or parent-material chemistry in large soil datasets.
Biogeosciences Discussions
The PROFILE model, now incorporated in the ForSAFE model can accurately reproduce the chemical and mineralogical evolution of the soil unsaturated zone. However, in deeper soil layers and in groundwater systems, it appears to overestimate weathering rates. This overestimation has been corrected by improving the kinetic expression describing mineral dissolution by adding or upgrading 'breaking functions'. The base cation and aluminium brakes have been strengthened, and an additional silicate brake has been developed, improving the ability to describe mineral-water reactions in deeper soils. These brakes are developed from a molecular-level model of the dissolution mechanisms. Equations, parameters and constants describing mineral dissolution kinetics have now been obtained for 102 different minerals from 12 major structural groups, comprising all types of minerals encountered in most soils. The PROFILE and ForSAFE weathering sub-model was extended to cover two-dimensional catchments, both in the vertical and the horizontal direction, including the hydrology. Comparisons between this improved model and field observations is available in Erlandsson Lampa et al. (2019, This special issue). The results showed that the incorporation of a braking effect of silica concentrations was necessary and helps obtain more accurate descriptions of soil evolution rates at greater depths and within the saturated zone. 1. Introduction Chemical weathering of silicate minerals, and notably the dissolution rates of these minerals are one of the most important factors shaping soil chemistry over longer time periods. The quality of the kinetic database in most cases determines the quality of the simulations. In the 1980's, the need arose to mitigate acid deposition, to set critical loads for acid deposition, and to set limits for sustainable forest growth and nitrogen critical loads. The critical loads depend directly on the ability of the soil to neutralize the incoming acid, thus the critical load depends on the weathering rate. It became apparent that the usual approach to soil geochemical modelling of using the weathering rate as the adjustable parameter to make the simulations fit the data, would be inadequate for estimating the critical loads. As a consequence, a quest for creating a weathering rate models that would accurately reproduce field observations and based on fundamental principles was started (Warfvinge and Sverdrup 1985, Sverdrup and Warfvinge 1987).