In upland watersheds, depletion of essential nutrients due to physical erosion and chemical weathering can be compensated by exogenous inputs such as aeolian dust deposition. The presence and chemical composition of exogenous dust arriving in natural environments is commonly analyzed in soil profiles using a suite of geochemical and isotopic tracers. However, it remains an outstanding challenge to describe the impacts of dust on the reaction rates that produce these profiles and how this cascades into ecosystem function and water chemistry. As increasingly intense and episodic periods of drought and aridity are promoted by a warming climate, the role of dust production and deposition in Critical Zone structure and function requires improved modeling techniques to facilitate rigorous quantification and prediction. Here we present a newly developed process-based reactive transport framework by modifying the open source CrunchTope software in order to quantitatively interpret the impacts of dust deposition and solubilization in stream water chemistry, regolith weathering rates, and ecosystem nutrient availability. We describe two simulations: (1) a generic model demonstrating a simplified system in which bedrock uplift and soil erosion occur in tandem with solid phase dust deposition at the land surface; (2) a case study based on a small (0.54 km2) upland Mediterranean watershed located on Mont Lozère in the National Park of Les Cévennes, France. In the absence of an exogenous dust input, long-term field observations of calcium in stream water, rain, bedrock, soil, and plant samples cannot be produced from reactive transport simulations of the weathering profile. By adding a carbonate-rich depositional input consistent with the composition of Saharan dust, both stream water chemistry and elemental mass-transfer coefficients in the soil profile better align with field observations, suggesting that dust has become a significant input to this field site in the last ~10 ka. Over this period, the deposition of exogenous carbonates has introduced far more calcium into the system than what could be supplied by the Ca-poor granitic bedrock. This highly soluble carbonate also limits the reactive potential of infiltrating precipitation, ultimately inhibiting chemical weathering rates and hence the component of elemental export fluxes derived from local bedrock. This is the first demonstration of solid-phase dust deposition incorporated into a multi-component reactive transport framework. Our update to the CrunchTope source code allows us to show how dust incorporation affects geochemical cycling across upland watersheds beyond the prohibitive limitations of simplified steady-state assumptions, a feature that will allow further research of a variety of Critical Zone systems subject to the effects of environmental change scenarios. 
How flowing water and organisms can shape Earth9s surface, the Critical Zone, depends on how fast this layer is turned over by erosion. To quantify the dependence of rock weathering and the cycling of elements through ecosystems on erosion we have used existing and new metrics that quantify the partitioning and cycling of elements between rock, saprolite, soil, plants, and river dissolved and solid loads. We demonstrate their utility at three sites along a global transect of mountain landscapes that differ in erosion rates – an "erodosequence". These sites are the Swiss Central Alps, a rapidly-eroding, post-glacial mountain belt; the Southern Sierra Nevada, USA, eroding at moderate rates; and the slowly-eroding tropical Highlands of Sri Lanka. The backbone of this analysis is an extensive data set of rock, saprolite, soil, water, and plant geochemical and isotopic data. This set of material properties is converted into process rates by using regolith production and weathering rates from cosmogenic nuclides and river loads, and estimates of biomass growth. Combined, these metrics allow us to derive elemental fluxes through regolith and vegetation. The main findings are: 1) the rates of weathering are set locally in regolith, and not by the rate at which entire landscapes erode; 2) the degree of weathering is mainly controlled by regolith residence time. This results in supply-limited weathering in Sri Lanka where weathering runs to completion in the regolith, and kinetically-limited weathering in the Alps and Sierra Nevada where soluble primary minerals persist; 3) these weathering characteristics are reflected in the sites9 ecosystem processes, namely in that nutritive elements are intensely recycled in the supply-limited setting, and directly taken up from soil and rock in the kinetically settings; 4) the weathering rates are not controlled by biomass growth; 5) at all sites we find a deficit in river solute export when compared to solute production in regolith, the extent of which differs between elements. Plant uptake followed by litter export might explain this deficit for biologically utilized elements of high solubility, and rare, high-discharge flushing events for colloidal-bound elements of low solubility. Our data and new metrics have begun to serve for calibrating metal isotope systems in the weathering zone, the isotope ratios of which depend on the flux partitioning between the compartments of the Critical Zone. We demonstrate this application in several isotope geochemical companion papers.
<p><span>Lateritic soils are deep weathering profiles, developed in tectonically quiescent areas under tropical conditions and over long timescales. Laterites are key components in the regulation of element cycle in the Earth&#8217;s history but, the timing between climatic changes and lateritic weathering episodes remains unconstrained. The combination of chronometric and weathering proxies is one way to build a comprehensive story of laterite formation.</span></p><p><span>In this study, two lateritic vertical profiles were targeted on the outer part of the Guyana Shield in the Amazon Basin. This region is tectonically stable and subjected to a rainy tropical climate since the Cretaceous. The first soil profile, located in the Brownsberg Mountains, Suriname, is developed on Proterozoic Greenstone [1]. The second lateritic cover, already studied and dated using EPR technique [2], is developed over the Cretaceous sedimentary Alter do Chao formation, Brazil. Both lateritic profiles are characterized by 1/ a total depletion of soluble elements and weathering of primary minerals at the base of the profile and 2/ a desilication followed by the formation of Fe and Al duricrusts on top. Here, traditional geochemical budgets are seconded by measurements of Si isotopes in both soils (bulk and/or clay fractions) and laterite draining streams. Silicon isotopes (&#948;</span><sup><span>30</span></sup><span>Si) are known to be an excellent weathering proxy, fractionated during clay mineral formation [3]. </span></p><p><span>In Suriname bulk soils, heavier &#948;</span><sup><span>30</span></sup><span>Si is associated with lateritization due to the &#8220;buffering&#8221; quartz exerts on bulk &#948;</span><sup><span>30</span></sup><span>Si. However, if clay fractions are isolated, the observed strong enrichment in light Si (&#916;</span><span>&#948;</span><sup><span>30</span></sup><span>Si<sub>clay fraction-bedrock</sub> up to -0.9&#8240;) is in line with the weathering of primary minerals and the formation of kaolinite. The dating of this intense weathering episode is c.a. 2-9 Ma based on preliminary EPR dating of kaolinites. </span></p><p><span>Regarding the Brazilian laterite, the material forming the Alter do Chao formation already suffered weathering episodes before deposition. The combination of EPR dating [2] and &#948;</span><sup><span>30</span></sup><span>Si measurements on the clay fraction reveals two distinct formation phases. First, chemical weathering is limited to the 37-22 Ma period. Second, the progressive depletion of</span><span> &#948;</span><sup><span>30</span></sup><span>Si from the bottom to the top of the lateritic profile highlights a replacement of a first kaolinite generation by a second population through dissolution-reprecipitation around 6 Ma, as previously inferred by EPR dating [2]. </span></p><p><span>These results, in combination with elemental mass budgets, give us better constraints to estimate the intensity and the timing of element mass transfers during laterite formation.</span><span>&#160;</span></p><p><span>[1] Monsels & van Bergen (2017) <em>Journal of Geochemical Exploration </em>180, 71-90. [2] Balan et al. (2005) <em>GCA</em> 69 (9), 2193-2204. [3] Opfergelt & Delmelle (2012) <em>Comptes Rendus Geoscience</em> 334 (11), 723-738.</span></p>
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