ALBERT

Description: 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 weathering-erosion relationship is capped by certain ‘speed limits’. We estimate a soil production speed limit of between 320 to 450 t km−2 yr−1 and the associated weathering rate speed limit of roughly 150 t km−2 yr−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.

Description: It is broadly understood that external forcings, such as climate and tectonics, influence the pace and pattern of landscape evolution by their control on physical and chemical processes that move soil and dissolve bedrock. Recent work in the Sierra Nevada quantified the potential role of climate in controlling chemical weathering rates, and suggested that strong erosion-weathering feedbacks control the landscape’s response. Here, we explore the tectonic signature on weathering in the San Gabriel Mountains (SGM), using bulk elemental analyses of soils and bedrock and cosmogenic 10Be derived rates of soil production. The SGM are located along a restraining bend in the San Andreas Fault, resulting in tectonic forcing on erosion and relief. We quantified chemical weathering across this landscape using six sites that bracket the low-gradient hillslopes of the stable upland plateau and the transient hillslopes at the margins of the incising landscape. On low gradient hillslopes (〈30°), Si-the fractional loss or gain of Si from parent material-averaged -0.32, and generally increased with increasing distance from the hillcrest and increasing slope. Across a threshold hillslope angle of 30°, soil weathering intensities decreases as erosion rates increase and soils thin. We find distinct patterns of erosion and weathering rates on high and low gradient slopes, and these patterns are consistent with a previously published predictive model for denudation-weathering relationships based on mineral weathering kinetics. Together, these data give new insight into the degree to which tectonics influences soil weathering, through its control on erosion rates, transport processes and soil thickness. Furthermore, this work provides a field-based quantification of the complex relationship between soil erosion and chemical weathering, and supports recent suggestions that this relationship is critical to our understanding of the fundamental controls on chemical weathering.

Description: What controls the chemical weathering of soils in actively eroding landscapes? In this study, we explore the tectonic signature on soil weathering in the San Gabriel Mountains (SGM) of California, where propagating waves of incision triggered by increasing rock uplift have resulted in distinctly different hillslope morphologies and erosion rates across the range. We quantify downslope patterns of soil weathering across this landscape using sites that bracket low-gradient hillslopes of the stable upland plateau and hillslopes near the margins of the incising landscape. We use elemental mass balances in rock and soil to index the weathered extent of soils, and couple these extents with previously measured 10Be-derived soil production rates to calculate rates of soil weathering and erosion. Across all sites, Tau-Si—the fractional loss or gain of Si from parent material—averages -0.32±0.04, and the weathered extent of soils generally increases with increasing distance from the hillcrest. However, weathering intensities decrease as hillslope gradients steepen beyond 30°. Chemical weathering extents on slopes 〈 30° averaged 0.35±0.04, 50% more than steeper slopes (0.23±0.05). Similarly, the relationships between soil weathering and erosion rates show distinct patterns on high and low gradient slopes. Erosion and weathering rates are positively correlated on low gradient hillslopes, and negatively correlated on high gradient hillslopes, likely due to the role of erosion rates in controlling mineral supply and residence time. These patterns are consistent with previously published predictive models for denudation-weathering relationships based on mineral weathering kinetics. Variable weathering extents in soils indicate that soil weathering in the SGM is largely kinetically limited. This work provides a field-based quantification of the complex relationship between soil erosion and chemical weathering, and together our data suggest that tectonic forcing strongly influences soil weathering rates and extents through its control on erosion rates, transport processes and soil thickness.