Abstract
Recent modeling and comparison with field results showed that soil formation by chemical weathering, from bedrock or unconsolidated material, is limited largely by solute transport. Chemical weathering rates are proportional to solute velocities. Nonreactive solute transport described by non-Gaussian transport theory appears compatible with soil formation rates. This change in understanding opens new possibilities for predicting soil production and depth across orders of magnitude of time scales. Percolation theory for modeling the evolution of soil depth and production was applied to new and published data for alpine and Mediterranean soils. The first goal was to check whether the empirical data conform to the theory. Secondly we analyzed discrepancies between theory and observation to find out if the theory is incomplete, if modifications of existing experimental procedures are needed and what parameters might be estimated improperly. Not all input parameters required for current theoretical formulations (particle size, erosion and infiltration rates) are collected routinely in the field; thus, theory must address how to find these quantities from existing climate and soil data repositories, which implicitly introduces some uncertainties. Existing results for soil texture, typically reported at relevant field sites, had to be transformed to results for a median particle size, d50, a specific theoretical input parameter. The modeling tracked reasonably well the evolution of the alpine and Mediterranean soils. For the Alpine sites we found, however, that we consistently overestimated soil depths by approximately 45%. Particularly during early soil formation, chemical weathering is more severely limited by reaction kinetics than by solute transport. The kinetic limitation of mineral weathering can affect the system until 1kyr to a maximum of 10kyr of soil evolution. Thereafter, solute transport seems dominant. The trend and scatter of soil depth evolution is well captured, particularly for Mediterranean soils. We assume that some neglected processes, such as bioturbation, tree throw, and land use change contributed to local reorganization of the soil and thus to some differences to the model. Nonetheless, the model is able to generate soil depth and confirms decreasing production rates with age. A steady state for soils is not reached before about 100 kyr.
Highlights
We applied a theoretical model of soil formation rates as limited by chemical weathering to prediction of the evolution of soil depths at about 250 sites in two climate regimes, Mediterranean and alpine
The chemical weathering was assumed to be limited by advective solute transport, a result which made relevant the rate of the mean vertical flow through the soil column over time
We found that the model matched soil depths in the Mediterranean climate zone, but overpredicted depths in alpine regions by ∼45%
Summary
SettingThe importance of quantifying chemical weathering and soil formation rates has as its basis their relevance across a wide range of fields of study, from agricultural engineering (Montgomery, 2007a,b; Lal, 2010) through climate change (Berner, 1992; Raymo, 1994; Algeo and Scheckler, 1998; Molnar and Cronin, 2015) and geochemistry (White and Brantley, 2003; Anderson and Anderson, 2010) to geomorphology (Heimsath et al, 1997, 1999; Burke et al, 2006; Dixon et al, 2009; Egli et al, 2012, 2014; Amundson et al, 2015). Soil forms in unconsolidated sediments developed through processes such as alluvial deposition (White et al, 1996) tree-throw (Borman et al, 1995), landslides (Trustrum and de Rose, 1988; Smale et al, 1997), mining, (Frouz et al, 2006), and exposure of glacial sediments by glacial retreat (Mavris et al, 2010; Egli et al, 2014). In the latter, the role of any gradual physical weathering processes is correspondingly reduced, since water penetration is guaranteed
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