Soils as pacemakers and limiters of global silicate weathering

Abstract

The weathering and erosion processes that produce and destroy regolith are widely recognized to be positively correlated across diverse landscapes. However, conceptual and numerical models predict some limits to this relationship that remain largely untested. Using new global data compilations of soil production and weathering rates from cosmogenic nuclides and silicate weathering fluxes from global rivers, we show that the weatheringerosion relationship is capped by certain 'speed limits'. We estimate a soil production speed limit of between 320 to 450 t km -2 y -1 and the associated weathering rate speed limit of roughly 150 t km -2 y -1. These limits appear to be valid for a range of lithologies, and also extend to mountain belts, where soil cover is not continuous and erosion rates outpace soil production. We argue that the presence of soil and regolith is a requirement for high weathering fluxes from a landscape, and that rapidly eroding, active mountain belts are not the most efficient sites for weathering. . Ces limites semblent être valides pour une large gamme de roches-mères, et s'appliquent aussi aux zones montagneuses, où les sols ne recouvrent pas toute la surface et où les taux d'érosion dépassent ceux de la production du sol. Nous avançons que la présence d'un sol est un prérequis pour qu'un paysage exporte de fort flux d'altération, et que les chaînes de montagne actives présentant de forts taux d'érosion ne sont pas les sites les plus efficaces pour l'altération.

Oxidation of sulfides and rapid weathering in recent landslides

“…We propose a conceptual model of a rapidly eroding landscape where the background weathering is set by lithology and climate, overprinted by the impact of landslides. The importance of reactive mineral phases, weathering rapidly in small and specific parts of the landscape affected by deep-seated erosion, is a contrast to existing concepts, which do not encompass the stochastic nature of weathering (Dixon and von Blanckenburg, 2012;Maher and Chamberlain, 2014;Riebe et al, 2003). A full description of the impact of landsliding on weathering should include the effect on the availability of reactive phases in addition to the previously modelled spatial stochasticity (Gabet, 2007).…”

“…It has been shown in the western Southern Alps (WSA) of New Zealand (Emberson et al, 2016) that this can give rise to strong gradients in dissolved solid concentrations in surface waters on hillslopes, with highest concentrations in seepage from landslides, and that the rate of landsliding is an important control on the chemistry of the rivers draining the mountain belt. Localised landslide weathering, prone to the spatial and temporal variability of mass wasting, contrasts with common models in which weathering is controlled by steady and distributed erosion of a laterally continuous soil and regolith mantle (Dixon and von Blanckenburg, 2012;Hilley et al, 2010;Ferrier and Kirchner, 2008;West, 2012;Lebedeva et al, 2010;D. D. Li et al, 2014).…”

An attempt to estimate tolerable soil erosion rates by matching soil formation with denudation in Alpine grasslands

“…P soil =F soil +D soil (dashed line). d Erosion rates (E) related to P soil as calculated in c, assuming that E is in the range of 0.5-0.9×D (Dixon and von Blanckenburg 2012). Dashed line: E=0.9×D…”

“…produced soil mass or denudation divided by the surface age); b Erosion rates (E; Eq. 5 and 6) related to soil depth (calculated by dividing P soil(I) by soil density (1.2 t/m 3 ; an average value considering top-and subsoil material) assuming that E is in the range of 0.5-0.9×D (Dixon and von Blanckenburg 2012). Dashed line: E=0.9×D.…”

“…Most soils feature a maximum possible CDF of 0.5 to 0.1 (that finally depends on climate and parent material: Dixon and von Blanckenburg 2012;Larsen et al 2014); consequently soil erosion accounts for at least half of the denudation.…”