Earth's temperature is thought to be regulated by a negative feedback between atmospheric CO2 levels and chemical weathering of silicate rocks that operates over million-year time scales. To explain variations in the strength of the weathering feedback, we present a model for silicate weathering that regulates climatic and tectonic forcing through hydrologic processes and imposes a thermodynamic limit on weathering fluxes, based on the physical and chemical properties of river basins. Climate regulation by silicate weathering is thus strongest when global topography is elevated, similar to the situation today, and lowest when global topography is more subdued, allowing planetary temperatures to vary depending on the global distribution of topography and mountain belts, even in the absence of appreciable changes in CO2 degassing rates.

https://science.sciencemag.org/content/sci/343/6178/1502.full.pdf

Hydrologic Regulation of Chemical Weathering and the Geologic Carbon Cycle

Soil pH regulates the capacity of soils to store and supply nutrients, and thus contributes substantially to controlling productivity in terrestrial ecosystems. However, soil pH is not an independent regulator of soil fertility-rather, it is ultimately controlled by environmental forcing. In particular, small changes in water balance cause a steep transition from alkaline to acid soils across natural climate gradients. Although the processes governing this threshold in soil pH are well understood, the threshold has not been quantified at the global scale, where the influence of climate may be confounded by the effects of topography and mineralogy. Here we evaluate the global relationship between water balance and soil pH by extracting a spatially random sample (n = 20,000) from an extensive compilation of 60,291 soil pH measurements. We show that there is an abrupt transition from alkaline to acid soil pH that occurs at the point where mean annual precipitation begins to exceed mean annual potential evapotranspiration. We evaluate deviations from this global pattern, showing that they may result from seasonality, climate history, erosion and mineralogy. These results demonstrate that climate creates a nonlinear pattern in soil solution chemistry at the global scale; they also reveal conditions under which soils maintain pH out of equilibrium with modern climate.

/pdf/water-balance-creates-a-threshold-in-soil-ph-at-the-global-4n0jyfl1dl.pdf

Water balance creates a threshold in soil pH at the global scale

http://home.iitk.ac.in/~rsinha/Publication/2012_GCA_Lupker.pdf

Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin)

The base of Earth's critical zone (CZ) is commonly shielded from study by many meters of overlying rock and regolith. Though deep CZ processes may seem far removed from the surface, they are vital in shaping it, preparing rock for infusion into the biosphere and breaking Earth materials down for transport across landscapes. This special issue highlights outstanding challenges and recent advances of deep CZ research in a series of articles that we introduce here in the context of relevant literature dating back to the 1500s. Building on several contributions to the special issue, we highlight four exciting new hypotheses about factors that drive deep CZ weathering and thus influence the evolution of life-sustaining CZ architecture. These hypotheses have emerged from recently developed process-based models of subsurface phenomena including: fracturing related to subsurface stress fields; weathering related to drainage of bedrock under hydraulic head gradients; rock damage from frost cracking due to subsurface temperature gradients; and mineral reactions with reactive fluids in subsurface chemical potential gradients. The models predict distinct patterns of subsurface weathering and CZ thickness that can be compared with observations from drilling, sampling and geophysical imaging. We synthesize the four hypotheses into an overarching conceptual model of fracturing and weathering that occurs as Earth materials are exhumed to the surface across subsurface gradients in stress, hydraulic head, temperature, and chemical potential. We conclude with a call for a coordinated measurement campaign designed to comprehensively test the four hypotheses across a range of climatic, tectonic and geologic conditions. This article is protected by copyright. All rights reserved.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/esp.4052

Controls on deep critical zone architecture: a historical review and four testable hypotheses

Quantitative understanding of variability in weathering fluxes on the modern Earth is limited because little is known about where the most important weathering reactions take place. This is partly because the locus of weathering is difficult to measure empirically. Inverse analysis of a parametric model presented here provides first-order constraints on variability in the thickness of the zone of active weathering. Results suggest that the effective thickness of the weathering zone varies relatively little across several orders of magnitude of denudation rate. At low to moderate denudation rates, reactions in soils may dominate weathering fluxes at the catchment scale, but the contribution from soil weathering decreases at higher denudation rates. Consequently, increased erosion leads to higher weathering fluxes, sustained by progressively greater contributions from weathering in bedrock. The effect of climate (temperature and runoff) on weathering fluxes is apparently weaker at low denudation rates than at high denudation rates, such that erosion, and potentially associated bedrock weathering, may be important for maintaining climate-stabilizing feedbacks in Earth’s carbon cycle.

Thickness of the chemical weathering zone and implications for erosional and climatic drivers of weathering and for carbon-cycle feedbacks

http://pangea.stanford.edu/~hilley/REPRINTS/Hilleyetal_EPSL2010.pdf

C.P. Chamberlain

Papers

Competition between erosion and reaction kinetics in controlling silicate-weathering rates