Climate-driven processes of hillslope weathering

Abstract

Climate controls erosion and weathering on soil-mantled landscapes through diverse processes that have remained diffi cult to disentangle due to their complex interactions. We quantify denudation, soil and saprolite weathering, and soil transport near the base and crest of the western slope of the Sierra Nevada to examine how large differences in climate affect these processes. Depth profi les of fallout radionuclides and fi eld observations show relative differences in erosion and weathering processes at these two climatically diverse sites, and our data suggest fundamentally different patterns of soil production and transport mechanisms: biotically driven soil transport at low elevation, and surface erosion driven by overland fl ow at high elevation. Soil production rates from cosmogenic 10 Be decrease from 31.3 to 13.6 m/Ma with increasing soil depth at low elevation, but show uncertain depth dependence at the high elevation site. Our data also show a positive correlation between physical erosion and saprolite weathering at both sites. Highly weathered saprolites are overlain by weakly weathered and rapidly eroding soils, while chemically less depleted saprolites are overlain by slowly eroding, more weathered soils. Our data are among the fi rst to quantify the critical role of saprolite weathering in the evolution of actively eroding upland landscapes, and our results provide quantitative constraints on how different climates can shape hillslopes by driving processes of erosion and weathering.

Bedrock composition regulates mountain ecosystems and landscape evolution

“…Meanwhile, low bedrock P has been implicated in the nutrient-limited fynbos ecosystem of South Africa (8) and some vegetation-free bedrock of the Colorado Plateau (7); each of these nutrient-limited landscapes are developed on quartz arenites with P concentrations similar to the barren granite in Fig. 1 C and D. However, in granitic terrain, the evidently crucial role of lithology in regulating pedogenesis, erosion, weathering, and ecosystem development has gone undetected until now, perhaps because previous studies have largely focused on quantifying effects of gradients in climate, tectonics, and time (e.g., 15,[56][57][58][59]. Here, we held these state factors (60) constant across a lithosequence of varying granitic bedrock.…”

Colluvial deposits as a possible weathering reservoir in uplifting mountains

“…Other models 25 argue that when the regolith vanishes at large erosion rates, weathering becomes significant in the fractured bedrock (Maher, 2011;Calmels et al, 2011;West, 2012), or that high reliefs consume more CO 2 than low reliefs during wetter periods (Maher and Chamberlain, 2014). Datasets from soil pits and riverine fluxes show a monotonic relationship between both the denudation rate and weathering rate in some cases (Millot et al, 2002;Dixon et al, 2009b;West et al, 2005), but also 30 evidence a possible maximum erosion rate above which the weathering rate decreases (Dixon and von Blanckenburg, 2012). Recent data from the Southern Alps in New Zealand have challenged the existence of this erosion rate limit by demonstrating that weathering was able to continue increasing at the highest erosion rates when rainfall is abundant (Larsen et al, 2014).…”

“…As a result, the debate on the locus of weathering in mountains is still open and different weathering reservoirs from the hillslopes to plains may dominate at different stages of the mountain evolution. Until now, four main weathering reservoirs have been identified: soils (Dixon et al, 2009b), fractured bedrock (Calmels et al,40 2011), basins (Bouchez et al, 2012), which also trap a considerable amount of organic carbon (Galy et al, 2015), and oceans (Oelkers et al, 2011). In this paper, we address the particular question of the relative contributions of in situ produced regolith and colluvial deposits in the weathering outflux of an uplifting mountain under a cooling climate.…”

“…CC BY 4.0 License. enough to significantly weather, producing a hump in the weathering-erosion relationship (Dixon et al, 2009b;Ferrier and Kirchner, 2008;Hilley et al, 2010;Gabet and Mudd, 2009). Other models 25 argue that when the regolith vanishes at large erosion rates, weathering becomes significant in the fractured bedrock (Maher, 2011;Calmels et al, 2011;West, 2012), or that high reliefs consume more CO 2 than low reliefs during wetter periods (Maher and Chamberlain, 2014).…”

Estimating millennial-scale rates of dust incorporation into eroding hillslope regolith using cosmogenic nuclides and immobile weathering tracers

“…As many of these studies have noted, incorporation of dust into regolith has consequences for geomorphic and geochemical studies that use the rock-to-regolith enrichment of chemically immobile elements as a tool for inferring rates and processes of regolith formation, chemical weathering, and physical erosion [e.g., Brimhall and Dietrich, 1987;Brimhall et al, 1992;White et al, 1998;Riebe et al, 2001Riebe et al, , 2003Riebe et al, , 2004aRiebe et al, , 2004bGreen et al, 2006;Yoo et al, 2007Yoo et al, , 2009Burke et al, 2007Burke et al, , 2009Dixon et al, 2009aDixon et al, , 2009b. Because the elements typically considered to be immobile are present in small quantities in most regoliths, their concentrations in regolith can be altered by small additions of atmospheric dust, especially if the dust is rich in these elements.…”