Clifford S. Riebe

Mountain erosion over 10 yr, 10 k.y., and 10 m.y. time scales

We used cosmogenic 10 Be to measure erosion rates over 10 k.y. time scales at 32 Idaho mountain catchments, ranging from small experimental watersheds (0.2 km 2 )t o large river basins (35 000 km 2 ). These long-term sediment yields are, on average, 17 times higher than stream sediment fluxes measured over 10‐84 yr, but are consistent with 10 m.y. erosion rates measured by apatite fission tracks. Our results imply that conventional sediment-yield measurements—even those made over decades—can greatly underestimate long-term average rates of sediment delivery and thus overestimate the life spans of engineered reservoirs. Our observations also suggest that sediment delivery from mountainous terrain is extremely episodic, sporadically subjecting mountain stream ecosystems to extensive disturbance.

Strong tectonic and weak climatic control of long-term chemical weathering rates

The relationships among climate, physical erosion, and chemical weathering have remained uncertain, because long-term chemical weathering rates have been difficult to measure. Here we show that long-term chemical weathering rates can be measured by combining physical erosion rates, inferred from cosmogenic nuclides, with dissolution losses, inferred from the rock-to-soil enrichment of insoluble elements. We used this method to measure chemical weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates. Chemical weathering rates correlate strongly with physical erosion rates but only weakly with climate, implying that, by regulating erosion rates, tectonic uplift may significantly accelerate chemical weathering rates in granitic landscapes.

Minimal climatic control on erosion rates in the Sierra Nevada, California

Climate is widely thought to regulate erosion rates, but the relationships among precipitation, temperature, and erosion rate have remained speculative, because long-term erosion rates have been difficult to measure. We used cosmogenic nuclides to measure long-term erosion rates at climatically diverse sites in the Sierra Nevada, California, spanning 20‐180 cm/yr in annual precipitation and 4‐15 8C in mean annual temperature. Average erosion rates vary by only 2.5 fold across these sites and are not correlated with climate, indicating that climate only weakly regulates nonglacial erosion rates in mountainous granitic terrain.

Erosional equilibrium and disequilibrium in the Sierra Nevada, inferred from cosmogenic 26Al and 10Be in alluvial sediment

We used cosmogenic 26 Al and 10 Be in stream sediment to measure landscape-scale erosion rates for topographically diverse catchments at seven Sierra Nevada sites. At three sites, erosion rates and hillslope gradients are strongly correlated, increasing with proximity to fault scarps and river canyons, which appear to have accelerated local base-level lowering rates, and thus increased catchment erosion rates by up to 15-fold. At four other sites, far from fault scarps and river canyons, erosion rates are much more uniform and less sensitive to average hillslope gradient. Our measurements show that contrasts in landscape erosion rates cannot be inferred from hillslope gradients alone, because landscapes can evolve toward a state of erosional equilibrium, in which steep and gentle slopes erode at similar rates.

Modulation of erosion on steep granitic slopes by boulder armoring, as revealed by cosmogenic 26Al and 10Be

Cosmogenic 26 Al and 10 Be in quartz from boulders, bedrock and sandy sediment from 21 small watersheds in the Diamond Mountains batholith, CA, USA, and two small watersheds from the nearby Fort Sage Mountains confirm that exposed granitic bedrock and boulders erode more slowly than the catchments in which they are found. Exposed bedrock and boulders are more abundant on steep slopes and may play an important role in regulating mountain erosion rates. Rapid transport of fine sediment on steep slopes exhumes resistant corestones which accumulate on the surface. The resulting boulder lag apparently shields the underlying soil and bedrock from erosion, even when the bedrock is deeply weathered and friable. Where steep slopes have an abundant boulder lag, they erode as slowly as gentler slopes nearby. In contrast, steep slopes lacking a boulder lag erode much more quickly than gentle slopes. Boulder armoring can modulate hillslope erosion such that erosion rates of summits, steep mountain flanks, and gentle footslopes are indistinguishable, thus permitting local relief and steep mountain slopes to persist for long periods of time. fl 2001 Elsevier Science B.V. All rights reserved.

Sharp decrease in long-term chemical weathering rates along an altitudinal transect

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and physical erosion across a steep climatic gradient in the Santa Rosa Mountains, Nevada. Our study sites are distributed along a 2 km ridgeline transect that spans 2090 to 2750 m in altitude, and encompasses marked contrasts in both vegetative cover and snow depth, but is underlain by a single, roughly uniform, granodiorite bedrock. Cosmogenic nuclides in colluvial soils reveal that denudation rates vary by less than a factor of 1.4 (104–144 t/km2/yr) along this transect. Bulk elemental analyses indicate that, relative to the parent rock, soils are less intensively weathered with increasing altitude, and show little evidence of weathering-related mass losses near the top of the ridge. Chemical weathering rates decrease rapidly with increasing altitude, both in absolute terms (from 24 to 0 t/km2/yr) and as a fraction of total denudation rates (from 20 to 0%). Thus these results indicate an increasing dominance of physical erosion with altitude. The observed decrease in chemical weathering rates is greater than one would predict from the decrease in mean annual temperature using simple weathering kinetics, suggesting that weathering rates along our transect may also be affected by the progressive decline in vegetative cover and increase in snow depth with increasing altitude. These results, considered together with weathering rate measurements for a wide range of climates in the Sierra Nevada, USA, suggest that chemical weathering rates may be particularly sensitive to differences in climate at higher-altitude sites. Consistent with this hypothesis, chemical weathering rates fall virtually to zero at the highest sites on our transect, suggesting that sparsely vegetated, high-altitude crystalline terrain may often be characterized by extremely slow silicate weathering rates.

Quantifying quartz enrichment and its consequences for cosmogenic measurements of erosion rates from alluvial sediment and regolith

Abstract In-situ cosmogenic 26 Al and 10 Be record the residence time of quartz grains near the earths surface, and thus can be used to measure whole-catchment erosion rates averaged over millennial time scales. Quartz is enriched in hillslope regolith by the dissolution of more soluble minerals; thus, its residence time will be longer than the regolith average. It has been noted that this introduces a bias into erosion rate estimates derived from cosmogenic nuclide concentrations in regolith or alluvium [Geomorphology 27 (1999) 131], but the magnitude of this bias has not previously been measured. The enrichment of quartz in regolith, and the resulting bias in cosmogenic erosion rate estimates, can be quantified using concentrations of immobile elements (such as zirconium) in bedrock and regolith. Here we show that the erosion rate bias introduced by regolith dissolution is less than 12%, across 22 granitic catchments that span a wide range of temperate climates. Except in extreme weathering environments, biases due to regolith dissolution will be a small component of the overall uncertainty in cosmogenic erosion rate measurements.