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The detachment of rock fragments from fractured bedrock on hillslopes creates sediment with an initial size distribution that sets the upper limits on particle size for all subsequent stages in the evolution of sediment in landscapes. We hypothesize that the initial size distribution should depend on the size distribution of latent sediment (i.e., fracture-bound blocks in unweathered bedrock) and weathering of blocks both before and during detachment (e.g., disintegration along crystal grain boundaries). However, the initial size distribution is difficult to measure because the interface across which sediment is produced is often shielded from view by overlying soil. Here we overcome this limitation by comparing fracture spacings measured from exposed bedrock on cliff faces with particle size distributions in adjacent talus deposits at 15 talus–cliff pairs spanning a wide range of climates and lithologies in California. Median fracture spacing and particle size vary by more than 10-fold and correlate strongly with lithology. Fracture spacing and talus size distributions are also closely correlated in central tendency, spread, and shape, with b-axis diameters showing the closest correspondence with fracture spacing at most sites. This suggests that weathering has not modified latent sediment either before or during detachment from the cliff face. In addition, talus at our sites has not undergone much weathering after deposition and is slightly coarser than the latent sizes because it contains unexploited fractures inherited from bedrock. We introduce a new conceptual framework for understanding the relative importance of latent size and weathering in setting initial sediment size distributions in mountain landscapes. In this framework, hillslopes exist on a spectrum defined by the ratio of two characteristic timescales: the residence time in saprolite and weathered bedrock and the time required to detach a particle of a characteristic size. At one end of the spectrum, where weathering residence times are negligible, the latent size distribution can be used to predict the initial size distribution. At the other end of the spectrum, where weathering residence times are long, the latent size distribution can be erased by weathering in the critical zone.

Erosion at Earth’s surface exposes underlying bedrock to climate-driven chemical and physical weathering, transforming it into a porous, ecosystem-sustaining substrate consisting of weathered bedrock, saprolite, and soil. Weathering in saprolite is typically quantified from bulk geochemistry assuming physical strain is negligible. However, modeling and measurements suggest that strain in saprolite may be common, and therefore anisovolumetric weathering may be widespread. To explore this possibility, we quantified the fraction of porosity produced by physical weathering, FPP, at three sites with differing climates in granitic bedrock of the Sierra Nevada, California, USA. We found that strain produces more porosity than chemical mass loss at each site, indicative of strongly anisovolumetric weathering. To expand the scope of our study, we quantified FPP using available volumetric strain and mass loss data from granitic sites spanning a broader range of climates and erosion rates. FPP in each case is ≥0.12, indicative of widespread anisovolumetric weathering. Multiple regression shows that differences in precipitation and erosion rate explain 94% of the variance in FPP and that >98% of Earth’s land surface has conditions that promote anisovolumetric weathering in granitic saprolite. Our work indicates that anisovolumetric weathering is the norm, rather than the exception, and highlights the importance of climate and erosion as drivers of subsurface physical weathering.

Tributary creeks of the southern Sierra Nevada have pronounced knickpoints that separate the landscape into an alternating sequence of gently sloped treads and steeply sloped risers. These knickpoints and the surrounding “stepped topography” suggest that the landscape is still responding to Pleistocene changes in base level on main-stem rivers. We tested this hypothesis using cosmogenic nuclides and uranium isotopes measured in stream sediment from widely distributed locations. Catchment-scale erosion rates from the cosmogenic nuclides suggest that the treads are relict surfaces that have adjusted to a previous base level. Nevertheless, erosion rates of relict interfluves are similar to canyon incision rates, implying that relief is unchanging in the lower Kings and San Joaquin Rivers. In addition, our results suggest that much of the southern Sierra Nevada is in a state of arrested development: the landscape is not fully adjusted to—and moreover is not responding to— changes in base-level lowering in the canyons. We propose that this can be explained by a paucity of coarse sediment supply, which fails to provide sufficient tools for bedrock channel incision at knickpoints. We hypothesize that the lack of coarse sediment in channels is driven by intense weathering of the local granitic bedrock, which reduces the size of sediment supplied from hillslopes to the channels. Our analysis highlights a feedback in which sediment size reduction due to weathering on hillslopes and transport in channels is both a key response to and control of bedrock channel incision and landscape adjustment to base-level change.

Weathering in mountain landscapes produces sediment with size distributions that evolve as particles are transported down hillslopes, delivered to channels, and carried downstream. The evolving sizes influence rates of river incision into bedrock, which in turn set sediment residence times on hillslopes, with implications for the sizes of sediment produced by weathering. Hence, variations in sediment size are central to feedbacks that link climate, tectonics, and erosion in mountain landscape evolution. However, few studies have quantified how sediment sizes evolve during transport across catchments, focusing instead on rates of erosion and weathering. Yet recent modeling suggests that spatial variations in sediment size can lead to bias in erosion rates from conventional techniques, further highlighting the importance of understanding how sediment size evolves across landscapes.Here we show how a more complete and unbiased picture of sediment production, weathering, and erosion can be obtained by combining field measurements of sediment size together with conventional geochemical proxies in an integrative model that accounts for spatial variations in erosion, weathering, and sediment mixing, while incorporating effects of both abrasion and fragmentation during transport in channels. Our measurements, from a catchment draining the steep eastern Sierra Nevada, California, include particle size distributions of sediment from widely distributed locations. These measurements represent sediment that is produced on hillslopes and delivered to channels, reflecting the combined effects of the initial sediment size distribution (set by bedrock fracture spacing) and subsequent weathering on slopes. Our measurements also include cosmogenic nuclide concentrations and apatite-helium ages in 11 size classes, from sand to boulders, sampled from the creek. The cosmogenic nuclides reveal residence times of sediment in the catchment, while the apatite-helium ages reveal source elevations of sediment eroded into the stream. When combined together, the cosmogenic nuclide and apatite-helium data can be used to quantify altitudinal variations in erosion rates and sediment size distributions.Our measurements from catchment slopes indicate that hillslope sediment size decreases with decreasing elevation, reflecting altitudinal trends in physical, chemical, and biological weathering and producing downvalley fining in hillslope sediment supply. Cosmogenic nuclides in stream sediment decrease by two-fold with increasing particle size, indicating that erosion rates calculated using traditional techniques are sensitive to the size sampled from the creek. Apatite-helium ages suggest that the smallest and largest sizes sediment sizes in the stream originate from lower elevations, where slopes are gentler and soil-mantled. In contrast, coarse gravel and cobbles appear to originate from higher in the catchment, where slopes are steeper and bare bedrock is exposed. The differences in altitudinal trends in sediment size implied by the apatite-helium data and the direct observations from catchment slopes can be reconciled by accounting for particle fragmentation and abrasion during transport from hillslope sources to the sampling point in the creek. Our analysis indicates that each of the unique sources of information in our study are necessary for a complete and unbiased understanding of spatial variations in the production of sediment across the full range of sizes and their evolution during transport across the catchment.

Landscapes evolve through interactions between subsurface processes that move and deform bedrock, and surface processes that redistribute mass through erosion, transport, and deposition of sediment. Sediment is composed of discrete particles that are produced from bedrock and modified during transport by physical and chemical weathering. Sediment particle attributes, including size, angularity, and durability, therefore depend on the climatic, tectonic, and lithologic factors that regulate weathering processes. These attributes, in turn, influence rates and modes of sediment transport, and the tools and cover effects that control rates of river incision into bedrock. Thus the production of sediment helps set the slopes of river channels and the relief structure of landscapes, making it central to the feedbacks between tectonics, climate, and erosion that create topography. Despite their importance, sediment particles are rarely included explicitly in landscape evolution modeling due to gaps in understanding of sediment production on hillslopes, the particle evolution that occurs on hillslopes and in channels, and the implications of sediment attributes for river incision into bedrock. Although these processes have been studied in isolation, they have not been combined together in a comprehensive model of the role of sediment in climate-tectonic-erosion feedbacks. Here we present results from a new, spatially-explicit model that predicts the evolution of individual particle attributes, including size, angularity, and durability. The model also predicts the resulting distributions of particle attributes as sediment from different sources is mixed, and as particles evolve during transport through catchments. The model has two components. The first predicts the initial particle attributes as sediments are produced from bedrock on hillslopes. The initial particle size distribution depends on the spacing of fractures and sizes of mineral grains in crystalline rocks, and on the spacing of bedding planes and the size of cemented particles in clastic sedimentary rocks. Initial size, as well as particle angularity and durability, are also influenced by chemical weathering, which depends on the fraction of soluble minerals, the local climate (parameterized as mean temperature and precipitation), and the residence time of bedrock as it is exhumed through the hillslope weathering engine.The second model component quantifies how particles change as they are transported across hillslopes and through channel networks. Particle sizes are reduced by abrasion as a function of three factors: the potential energy lost in transport; particle angularity; and particle durability, which depends on initial rock tensile strength and subsequent loss of strength due to chemical weathering. Mass lost from abrasion of coarse particles is converted to sand and silt. Particles become less angular as a function of cumulative mass loss. However, high rates of energy loss on steep slopes cause fragmentation, which creates new coarse particles and resets particle angularity. Model relationships are parameterized using published data as well as newly acquired data from laboratory experiments and field studies in the Sierra Nevada, California. We couple the model with the saltation abrasion/bedrock river incision model to simulate evolution of river longitudinal profiles, and explore potential feedbacks between rock uplift, climate, and sediment production.

The generation and transport of mineral dust is strongly related to climate on seasonal, year-to-year, and glacial-interglacial timescales. The modern dust cycle is influenced by soil moisture, which is partly a function of drought duration and severity. The production and transport of dust can therefore be amplified by global and regional droughts, thereby moderating ecosystem vulnerability to disturbance through the influence of dust on nutrient delivery to ecosystems. In this work, we use strontium and neodymium isotopes in combination with trace element concentrations in modern dust samples collected in 2015 to quantify the role of regionally versus globally supplied dust in nutrient delivery to a montane ecosystem. The study sites lie along an elevational transect in the southern Sierra Nevada, USA, with samples spanning the dry seasons of 2014 (Aciego et al., 2017) and 2015 (this study), when the region was experiencing a historic drought. The goal of our research was to quantify the spatial and temporal variability and sensitivity of the dust cycle to short term changes at nutrient-limited sites. We find that, during the dry season of 2015, Asian sources contributed between 10 and 40% of dust to sites located along this elevational transect, and importantly increased in importance during the summer growing season compared to regional dust sources. These changes are likely related to the prolonged drought in Asia in 2015, highlighting both the sensitivity of dust production and transport to drought and the teleconnections of dust transport in terrestrial ecosystems.

Sediments link hillslopes to river channels. The size of sediments entering channels is a key control on river morphodynamics across a range of scales, from channel response to human land use to landscape response to changes in tectonic and climatic forcing. However, very little is known about what controls the size distribution of particles eroded from bedrock on hillslopes, and how particle sizes evolve before sediments are delivered to channels. Here we take the first steps toward building a geomorphic transport law to predict the size distribution of particles produced on hillslopes and supplied to channels. We begin by identifying independent variables that can be used to quantify the influence of five key boundary conditions: lithology, climate, life, erosion rate, and topography, which together determine the suite of geomorphic processes that produce and transport sediments on hillslopes. We then consider the physical and chemical mechanisms that determine the initial size distribution of rock fragments supplied to the hillslope weathering system, and the duration and intensity of weathering experienced by particles on their journey from bedrock to the channel. We propose a simple modeling framework with two components. First, the initial rock fragment sizes are set by the distribution of spacing between fractures in unweathered rock, which is influenced by stresses encountered by rock during exhumation and by rock resistance to fracture propagation. That initial size distribution is then transformed by a weathering function that captures the influence of climate and mineralogy on chemical weathering potential, and the influence of erosion rate and soil depth on residence time and the extent of particle size reduction. Model applications illustrate how spatial variation in weathering regime can lead to bimodal size distributions and downstream fining of channel sediment by down-valley fining of hillslope sediment supply, two examples of hillslope control on river sediment size. Overall, this work highlights the rich opportunities for future research into the controls on the size of sediments produced on hillslopes and delivered to channels.

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.

ABSTRCT Mounting evidence suggests that Earth's cryosphere harbors diverse and active microbial communities. However, our understanding of microbial composition and diversity in seasonal snowpack of montane ecosystems remains limited. We sequenced the 16S rRNA gene to determine microbial structure (composition and diversity) of snow at two depths (0–15 and 15–30 cm) of a subalpine site in the Southern Sierra Critical Zone Observatory, California, U.S.A. Proteobacteria dominated both depths (~72% of sequences), and this phylum was composed mostly of bacteria within the Rhodospirillales order. Cyanobacteria were almost exclusively present in the upper snow layer, while Actinobacteria and Firmicutes were more abundant in the deep snow layer. Many of the most abundant phylotypes were Acetobacteraceae. Phylotype NCR4874, which comprised 22%–32% of the sequences, was most closely related to the N2-fixing bacteria Asaia siamensis, suggesting that N2 fixation may be an important process within the Sierra snowpack...

As bedrock weathers to regolith -defined here as weathered rock, saprolite, and soil - porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Colocated geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from saprolite to weathered bedrock is more gradational, over a depth range of 11-18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth's surface. National Science Foundation (NSF) [EPS 1208909] Department of Energy Office of Basic Energy Sciences [DE-FG02-05ER15675] Susquehanna/Shale Hills and Luquillo Critical Zone Observatory [NSF EAR 1339285, 1331726, NSF EAR 1331841] Duke University Calhoun Critical Zone Observatory [NSF EAR 1331846] [1331939] W.S.H., B.J.C., B.A.F. and C.S.R acknowledge support from the National Science Foundation (NSF) Grant Number EPS 1208909. S.L.B. acknowledges support from funding from the Department of Energy Office of Basic Energy Sciences grant DE-FG02-05ER15675 to SLB and from the Susquehanna/Shale Hills and Luquillo Critical Zone Observatory (NSF EAR 1339285 and 1331726 to S.L.B. and NSF EAR 1331841 to W.H. McDowell at the University of New Hampshire). D.D.R. acknowledges support from Duke University and the Calhoun Critical Zone Observatory (NSF EAR 1331846). C.S.R. acknowledges support from 1331939.

Weathering on mountain slopes converts rock to sediment that erodes into channels and thus provides streams with tools for incision into bedrock. Both the size and flux of sediment from slopes can influence channel incision, making sediment production and erosion central to the interplay of climate and tectonics in landscape evolution. Although erosion rates are commonly measured using cosmogenic nuclides, there has been no complementary way to quantify how sediment size varies across slopes where the sediment is produced. Here we show how this limitation can be overcome using a combination of apatite helium ages and cosmogenic nuclides measured in multiple sizes of stream sediment. We applied the approach to a catchment underlain by granodiorite bedrock on the eastern flanks of the High Sierra, in California. Our results show that higher-elevation slopes, which are steeper, colder, and less vegetated, are producing coarser sediment that erodes faster into the channel network. This suggests that both the size and flux of sediment from slopes to channels are governed by altitudinal variations in climate, vegetation, and topography across the catchment. By quantifying spatial variations in the sizes of sediment produced by weathering, this analysis enables new understanding of sediment supply in feedbacks between climate, tectonics, and mountain landscape evolution.

We present a set of river management tools based on a recently developed method for estimating the amount of salmon spawning habitat in coarse-bedded rivers. The method, which was developed from a mechanistic model of redd building by female salmon, combines empirical relationships between fish length, redd area, and the sizes of particles moved by fish during spawning. Model inputs are the grain-size indices D50 and D84 and an estimate of female fish length, which is used to predict the size of the redd that they will build and the size of the largest particle that they can move on the bed. Outputs include predictions of the fraction of the bed that the fish can use for redd building and the number of redds that they can build within the useable area. We cast the model into easy-to-use look-up tables, charts, an Excel worksheet, a JavaScript web applet, and a MATLAB user interface. We explain how these tools can be used in a new, mechanistic approach to assessing spawning substrates and optimizing gravel augmentation projects in coarse-bedded rivers. Copyright © 2015 John Wiley & Sons, Ltd.

Bedrock weathering runs to the hills Fractures in bedrock drive the breakdown of rock into soil. Soil makes observations of bedrock processes challenging. St. Clair et al. combined a three-dimensional stress model with geophysical measurements to show that bedrock erosion rates mirror changes in topography (see the Perspective by Anderson). Seismic reflection and electromagnetic profiles allowed mapping of the bedrock fracture density. The profiles mirror changes in surface elevation and thus provide a way to study the critical zone between rock and soil. Science , this issue p. 534 ; see also p. 506

Collaborations between biologists and geologists are key to understanding and projecting how landscapes function and change over time. Such collaborations are stimulated by on-going scientific developments, advances in instrumentation and technology, and the growing recognition that environmental problems necessitate interdisciplinary investigation. Here, we show how the biogeosciences are well placed to answer more completely the core questions that motivate the world's invaluable environmental research networks: specifically, the venerable Long Term Ecological Research networks (LTERs), the newer surveillance facilities of the Earth Observatory Networks (EONs including the USA's NEON), and the geosciences' interdisciplinary network of Critical Zone Observatories (CZOs). Because LTER and EON programs have been supported largely by ecological and biological communities and CZOs largely by the geological community, we assert that a concerted biogeoscience approach across these invaluable networks can benefit both their scientific productivity and usefulness to the wider public.

Enhanced understanding of subsurface water storage will improve prediction of future impacts of climate change, including drought, forest mortality, wildland fire, and strained water security. Previous research has examined the importance of plant-accessible water in soil, but in upland landscapes within Mediterranean climates, soil often accounts for only a fraction of subsurface water storage. We draw insights from previous research and a case study of the Southern Sierra Critical Zone Observatory to define attributes of subsurface storage; review observed patterns in their distribu-tion; highlight nested methods for estimating them across scales; and showcase the fundamental processes controlling their formation. We review observations that highlight how forest ecosystems subsist on lasting plant-accessible stores of subsurface water during the summer dry period and during multiyear droughts. The data suggest that trees in these forest ecosystems are rooted deeply in the weathered, highly porous saprolite or saprock, which reaches up to 10–20 m beneath the surface. This review confirms that the system harbors large volumes of subsurface water and shows that they are vital to supporting the ecosystem through the summer dry season and extended droughts. This research enhances understanding of deep subsurface water storage across landscapes and identifies key remaining challenges in predicting and managing response to climate and land use change in mountain ecosystems of the Sierra Nevada and in other Mediterranean climates worldwide. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Extremes Water and Life > Nature of Freshwater Ecosystems.

Long-term environmental research networks are one approach to advancing local, regional, and global environmental science and education. A remarkable number and wide variety of environmental research networks operate around the world today. These are diverse in funding, infrastructure, motivating questions, scientific strengths, and the sciences that birthed and maintain the networks. Some networks have individual sites that were selected because they had produced invaluable long-term data, while other networks have new sites selected to span ecological gradients. However, all long-term environmental networks share two challenges. Networks must keep pace with scientific advances and interact with both the scientific community and society at large. If networks fall short of successfully addressing these challenges, they risk becoming irrelevant. The objective of this paper is to assert that the biogeosciences offer environmental research networks a number of opportunities to expand scientific impact and public engagement. We explore some of these opportunities with four networks: the International Long-Term Ecological Research Network programs (ILTERs), critical zone observatories (CZOs), Earth and ecological observatory networks (EONs), and the FLUXNET program of eddy flux sites. While these networks were founded and expanded by interdisciplinary scientists, the preponderance of expertise and funding has gravitated activities of ILTERs and EONs toward ecology and biology, CZOs toward the Earth sciences and geology, and FLUXNET toward ecophysiology and micrometeorology. Our point is not to homogenize networks, nor to diminish disciplinary science. Rather, we argue that by more fully incorporating the integration of biology and geology in long-term environmental research networks, scientists can better leverage network assets, keep pace with the ever-changing science of the environment, and engage with larger scientific and public audiences. Fil: Richter, Daniel D.. University of Duke; Estados Unidos Fil: Billings, Sharon A.. University of Kansas; Estados Unidos Fil: Groffman, Peter M.. Brooklyn College; Estados Unidos Fil: Kelly, Eugene F.. State University of Colorado - Fort Collins; Estados Unidos Fil: Lohse, Kathleen A.. Idaho State University; Estados Unidos Fil: McDowell, William H.. University Of New Hampshire; Estados Unidos Fil: White, Timothy S.. State University of Pennsylvania; Estados Unidos Fil: Anderson, Suzanne. State University of Colorado at Boulder; Estados Unidos Fil: Baldocchi, Dennis D.. University of California at Berkeley; Estados Unidos Fil: Banwart, Steve. University Of Leeds; Reino Unido Fil: Brantley, Susan. State University of Pennsylvania; Estados Unidos Fil: Braun, Jean J.. University of Yaounide; Camerún. Universite de Toulouse; Francia Fil: Brecheisen, Zachary S.. University of Duke; Estados Unidos Fil: Cook, Charles S.. University of Duke; Estados Unidos Fil: Hartnett, Hilairy E.. Arizona State University; Estados Unidos Fil: Hobbie, Sarah E.. University of Minnesota; Estados Unidos Fil: Gaillardet, Jerome. Institut Universitaire de France. Institut de Physique du Globe de Paris; Francia Fil: Jobbagy Gampel, Esteban Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi". Universidad Nacional de San Luis. Facultad de Ciencias Físico, Matemáticas y Naturales. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi"; Argentina Fil: Jungkunst, Hermann F.. Universitat Koblenz; Alemania Fil: Kazanski, Clare E.. University of Minnesota; Estados Unidos Fil: Krishnaswamy, Jagdish. Ashoka Trust For Research In Ecology And The Environment; India Fil: Markewitz, Daniel. University of Georgia; Estados Unidos Fil: O'Neill, Katherine. Roanoke College; Estados Unidos Fil: Riebe, Clifford S.. University Of Wyoming; Fil: Schroeder, Paul. University of Georgia; Estados Unidos Fil: Siebe, Christina. Universidad Nacional Autónoma de México; México Fil: Silver, Whendee L.. University of California at Berkeley; Estados Unidos Fil: Thompson, Aaron. University of Georgia; Estados Unidos Fil: Verhoef, Anne. University of Reading; Reino Unido Fil: Zhang, Ganlin. Chinese Academy of Sciences; República de China

Dust may serve as a vital nutrient source for many montane ecosystems despite substantial nutrient supply from bedrock., A global compilation of erosion rates and modeled dust fluxes shows that dust inputs can be a large fraction of total soil inputs, particularly when erosion is slow and soil residence time is therefore long. These observations suggest that dust-derived nutrients can be vital to montane ecosystems, even when nutrient supply from bedrock is substantial. We tested this hypothesis using neodymium isotopes as a tracer of mineral phosphorus contributions to vegetation in the Sierra Nevada, California, where rates of erosion and dust deposition are both intermediate within the global compilation. Neodymium isotopes in pine needles, dust, and bedrock show that dust contributes most of the neodymium in vegetation at the site. Together, the global data sets and isotopic tracers confirm the ecological significance of dust in eroding mountain landscapes. This challenges conventional assumptions about dust-derived nutrients, expanding the plausible range of dust-reliant ecosystems to include many temperate montane regions, despite their relatively high rates of erosion and bedrock nutrient supply.

Subsurface porosity production is dominated by volumetric strain in deeply weathered granitic saprolite of the Sierra Nevada., Weathering in the critical zone causes volumetric strain and mass loss, thereby creating subsurface porosity that is vital to overlying ecosystems. We used geochemical and geophysical measurements to quantify the relative importance of volumetric strain and mass loss---the physical and chemical components of porosity---in weathering of granitic saprolite of the southern Sierra Nevada, California, USA. Porosity and strain decrease with depth and imply that saprolite more than doubles in volume during exhumation to the surface by erosion. Chemical depletion is relatively uniform, indicating that changes in porosity are dominated by processes that cause strain with little mass loss. Strain-induced porosity production at our site may arise from root wedging, biotite weathering, frost cracking, and the opening of fractures under ambient topographic stresses. Our analysis challenges the conventional view that volumetric strain can be assumed to be negligible as a porosity-producing mechanism in saprolite.

Detrital thermochronology has revolutionized the study of sediment provenance at orogen scales and spatial patterns in erosion at catchment scales. A strength of the method is that a handful of stream sand can be used to represent processes over landscape scales. However, it relies on both the widely recognized assumption that sand is supplied from every part of the catchment, and the implicit assumption that the fraction of eroded material that is sand-sized does not change across the landscape. These assumptions may be violated when the catchment contains spatial variations in the initial sizes of sediment produced by hillslope weathering or when abrasion during transport causes size reduction of sediment sourced from distal parts of the catchment. In either case, a detrital sample spanning a narrow range of sediment sizes (e.g., sand) may fail to represent the catchment as a whole, leading to bias in thermochronology of erosional and tectonic processes. We used forward modeling to quantify biases that can arise due to plausible abrasion rates and spatial variations in initial sediment size. Our results reveal significant reductions in the chance of detecting age populations originating from the highest, most distal parts of the landscape, leading to potentially erroneous interpretations in provenance studies. In tracer thermochronology, the biases distort detrital age distributions, leading to potentially profound misinterpretation of spatial patterns in erosion rates. Our analysis shows that the sediment size and abrasion biases increase with catchment area and relief but can be significant in small catchments ( 10 km 2 ) with moderate relief (>0.5 km). We show that the biases can be mitigated by analyzing a sufficient number of grains in multiple sediment size classes.

The conversion of rock to soil prepares Earth9s surface for erosion by wind, water, gravity, and life. Together these agents wear down hills and mountains even as the land rises up under the stress of tectonic forces in the crust. Meanwhile, weathering liberates nutrients from minerals and disaggregates rock into regolith, generating hospitable substrates for life. Over the last two decades, geochemists, geomorphologists, and soil scientists have increasingly used cosmogenic nuclides to quantify how fast soils are made, modified, and finally swept away in hilly and mountainous landscapes around the world. These studies are revolutionizing our understanding of soils and their role in feedbacks that shape Earth9s surface, influence overlying ecosystems, and modulate climate over millions of years.

Earth’s land surface teems with life. Although the distribution of ecosystems is largely explained by temperature and precipitation, vegetation can vary markedly with little variation in climate. Here we explore the role of bedrock in governing the distribution of forest cover across the Sierra Nevada Batholith, California. Our sites span a narrow range of elevations and thus a narrow range in climate. However, land cover varies from Giant Sequoia (Sequoiadendron giganteum), the largest trees on Earth, to vegetation-free swaths that are visible from space. Meanwhile, underlying bedrock spans nearly the entire compositional range of granitic bedrock in the western North American cordillera. We explored connections between lithology and vegetation using measurements of bedrock geochemistry and forest productivity. Tree-canopy cover, a proxy for forest productivity, varies by more than an order of magnitude across our sites, changing abruptly at mapped contacts between plutons and correlating with bedrock concentrations of major and minor elements, including the plant-essential nutrient phosphorus. Nutrient-poor areas that lack vegetation and soil are eroding more than two times slower on average than surrounding, more nutrient-rich, soil-mantled bedrock. This suggests that bedrock geochemistry can influence landscape evolution through an intrinsic limitation on primary productivity. Our results are consistent with widespread bottom-up lithologic control on the distribution and diversity of vegetation in mountainous terrain.

Millions of dollars are spent annually on revitalizing salmon spawning in riverbeds where redd building by female salmon is inhibited by sediment that is too big for fish to move. Yet the conditions necessary for productive spawning remain unclear. There is no gauge for quantifying how grain size influences the reproductive potential of coarse-bedded rivers. Hence, managers lack a quantitative basis for optimizing spawning habitat restoration for reproductive value. To overcome this limitation, we studied spawning by Chinook, sockeye, and pink salmon (Oncorhynchus tshawytscha, O. nerka, and O. gorbuscha) in creeks and rivers of California and the Pacific Northwest. Our analysis shows that coarse substrates have been substantially undervalued as spawning habitat in previous work. We present a field-calibrated approach for estimating the number of redds and eggs a substrate can accommodate from measurements of grain size and fish length. Bigger fish can move larger sediment and thus use more riverbed area for spawning. They also tend to have higher fecundity, and so can deposit more eggs per redd. However, because redd area increases with fish length, the number of eggs a substrate can accommodate is maximized for moderate-sized fish. This previously unrecognized tradeoff raises the possibility that differences in grain size help regulate river-to-river differences in salmon size. Thus, population diversity and species resilience may be linked to lithologic, geomorphic, and climatic factors that determine grain size in rivers. Our approach provides a tool for managing grain-size distributions in support of optimal reproductive potential and species resilience.

The conversion of bedrock to regolith marks the inception of critical zone processes, but the factors that regulate it remain poorly understood. Although the thickness and degree of weathering of regolith are widely thought to be important regulators of the development of regolith and its water-storage potential, the functional relationships between regolith properties and the processes that generate it remain poorly documented. This is due in part to the fact that regolith is difficult to characterize by direct observations over the broad scales needed for process-based understanding of the critical zone. Here we use seismic refraction and resistivity imaging techniques to estimate variations in regolith thickness and porosity across a forested slope and swampy meadow in the Southern Sierra Critical Zone Observatory (SSCZO). Inferred seismic velocities and electrical resistivities image a weathering zone ranging in thickness from 10 to 35 m (average = 23 m) along one intensively studied transect. The inferred weathering zone consists of roughly equal thicknesses of saprolite (P-velocity

Dust provides ecosystem-sustaining nutrients to landscapes underlain by intensively weathered soils. Here we show that dust may also be crucial in montane forest ecosystems, dominating nutrient budgets despite continuous replacement of depleted soils with fresh bedrock via erosion. Strontium and neodymium isotopes in modern dust show that Asian sources contribute 18–45% of dust deposition across our Sierra Nevada, California study sites. The remaining dust originates regionally from the nearby Central Valley. Measured dust fluxes are greater than or equal to modern erosional outputs from hillslopes to channels, and account for 10–20% of estimated millennial-average inputs of bedrock P. Our results demonstrate that exogenic dust can drive the evolution of nutrient budgets in montane ecosystems, with implications for predicting forest response to changes in climate and land use., Dust is an important nutrient source to landscapes, but often the source of dust is poorly constrained. Here, the authors quantify the origin of different dust sources in the Sierra Nevada by analysing dust composition and suggest exogenic dust may drive nutrient budgets in montane ecosystems.

Cosmogenic nuclides in rock, soil, and sediment are routinely used to measure denudation rates of catchments and hillslopes. Although it has been shown that these measurements are prone to biases due to chemical erosion in regolith, most studies of cosmogenic nuclides have ignored this potential source of error. Here we quantify the extent to which overlooking effects of chemical erosion introduces bias in interpreting denudation rates from cosmogenic nuclides. We consider two end-member effects: one due to weathering near the surface and the other due to weathering at depth. Near the surface, chemical erosion influences nuclide concentrations in host minerals by enriching (or depleting) them relative to other more (or less) soluble minerals. This increases (or decreases) their residence times relative to the regolith as a whole. At depth, where minerals are shielded from cosmic radiation, chemical erosion causes denudation without influencing cosmogenic nuclide buildup. If this effect is ignored, denudation rates inferred from cosmogenic nuclides will be too low. We derive a general expression, termed the ‘chemical erosion factor’, or CEF, which corrects for biases introduced by both deep and near-surface chemical erosion in regolith. The CEF differs from the ‘quartz enrichment factor’ of previous work in that it can also be applied to relatively soluble minerals, such as olivine. Using data from diverse climatic settings, we calculate CEFs ranging from 1.03 to 1.87 for cosmogenic nuclides in quartz. This implies that ignoring chemical erosion can lead to errors of close to 100% in intensely weathered regolith. CEF is strongly correlated with mean annual precipitation across our sites, reflecting climatic influence on chemical weathering. Our results indicate that quantifying CEFs is crucial in cosmogenic nuclide studies of landscapes where chemical erosion accounts for a significant fraction of the overall denudation. Copyright © 2012 John Wiley & Sons, Ltd.

Mineral weathering plays a prominent role in many biogeochemical and geomorphological processes. It supplies nutrients to soils and streams, accelerates physical erosion by weakening bedrock and producing easily erodible soil, and modulates Earth's long-term climate by drawing down atmospheric carbon dioxide. We calculate mineral-specific chemical weathering rates at two field sites in the Rio Icacos catchment, Puerto Rico, by combining new mineral abundance measurements from quantitative powder X-ray diffraction (XRD) with existing measurements of (i) soil production rates from cosmogenic nuclides, (ii) chemical alteration of the regolith from X-ray fluorescence (XRF), and (iii) dust deposition rates. The central purpose of this paper is to show that combining measurements of cosmogenic nuclides with XRF-based geochemistry and XRD-based mineralogy can, in favorable cases, provide weathering rates of abundant, soluble mineral phases in actively eroding terrain to an accuracy of better than 20% of the mean, even in places with high dust deposition rates. Mineral weathering at our two field sites is dominated by plagioclase, at rates of 3274 ± 575 mol ha − 1 yr − 1 and 3077 ± 541 mol ha − 1 yr − 1 , followed by hornblende, at 187 ± 71 mol ha − 1 yr − 1 and 308 ± 93 mol ha − 1 yr − 1 . Within the uncertainty of our data, all weathering of these primary minerals occurs below the saprolite–soil interface. Our measurements imply that kaolinite production in saprolite is roughly 1.3 times faster than kaolinite weathering in the soil. Our results are the first to show that field measurements of cosmogenic nuclides, XRF, XRD, and dust fluxes can be combined within the geochemical mass balance method to quantify long-term mineral weathering rates, even in locations with high dust deposition rates. This implies that the mass balance method can be a valuable tool for quantifying the effects of climate, vegetation, tectonics, and other factors on weathering rates of individual mineral phases.

The delivery of water, sediment, and solutes by catchments is influenced by the distribution of source elevations and their travel distances to the outlet. For example, elevation affects the magnitude and phase of precipitation, as well as the climatic factors that govern rock weathering, which influence the production rate and initial particle size of sediments. Travel distance, in turn, affects the timing of flood peaks at the outlet and the degree of sediment size reduction by wear, which affects particle size distributions at the outlet. The distributions of elevation and travel distance have been studied extensively but separately, as the hypsometric curve and width function. Yet a catchment can be considered as a collection of points, each with paired values of elevation and travel distance. For every point, the ratio of elevation to travel distance defines the mean slope for transport of mass to the outlet. Recognizing that mean slope is proportional to the average rate of loss of potential energy by water and sediment during transport to the outlet, we use the joint distribution of elevation and travel distance to define two new metrics for catchment geometry: "source-area power", and the corresponding catchment-wide integral "catchment power". We explore patterns in source-area and catchment power across three study catchments spanning a range of relief and drainage area. We then develop an empirical algorithm for generating synthetic source-area power distributions, which can be parameterized with data from natural catchments. This new way of quantifying the three-dimensional geometry of catchments can be used to explore the effects of topography on the distribution on fluxes of water, sediment, isotopes, and other landscape products passing through catchment outlets, and may provide a fresh perspective on problems of both practical and theoretical interest.

Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 °C, average precipitation ranging from 22 to 420 cm·year −1 , and denudation rates ranging from 23 to 755 t·km −2 ·year −1 . Long-term chemical weathering rates range from 0 to 173 t·km −2 year −1 , in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89–95% of the variation in long-term weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, chemical weathering rates increase proportionally with fresh-material supply rates. We refer to this as “supply-limited” weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.

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 long-term rates of chemical weathering and physical erosion is important for under- standing the long-term evolution of soils, landscapes, and Earth's climate. Here we describe how long-term chemical weathering rates can be measured for actively eroding landscapes using cosmogenic nuclides together with a geochemical mass balance of weathered soil and parent rock. We tested this approach in the Rio Icacos watershed, Puerto Rico, where independent studies have estimated weathering rates over both short and long timescales. Results from the cosmogenic/mass balance method are consistent with three independent sets of weathering rate estimates, thus confirming that this approach yields realistic measurements of long-term weathering rates. This approach can separately quantify weathering rates from saprolite and from overlying soil as components of the total. At Rio Icacos, nearly 50% of Si weathering occurs as rock is converted to saprolite; in contrast, nearly 100% of Al weathering occurs in the soil. Physical erosion rates are measured as part of our mass balance approach, making it particularly useful for studying interrelationships between chemical weathering and physical erosion. Our data show that chemical weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and chemical weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. One can normalize for variations in physical erosion rates using the "chemical depletion fraction," which measures the fraction of total denudation that is accounted for by chemical weathering. This measure of chemical weathering intensity increases with increasing average temperature and precipitation in data from climatically diverse granitic sites, including tropical Rio Icacos and six temperate sites in the Sierra Nevada, California. Hence, across a wide range of climate regimes, analysis of chemical depletion fractions appears to effectively account for site-to-site differences in physical erosion rates, which would otherwise obscure climatic effects on chemical weathering rates. Our results show that by quantifying rates of physical erosion and chemical weathering together, our mass balance approach can be used to determine the relative importance of climatic and nonclimatic factors in regulating long-term chemical weathering rates. Copyright © 2003 Elsevier Ltd

The dynamics of granitic landscapes are modulated by bimodal weathering, which produces patchy granular soils and expanses of bare rock ranging from meter-scale boulders to mountain-scale domes. We used terrain analysis and with cosmogenic nuclide measurements of erosion rates to quantitatively explore Wahrhaftig’s decades-old hypothesis for the development of “stepped topography” by differential weathering of bare and soil-mantled granite. According to Wahrhaftig’s hypothesis, bare granite weathers slower than soil-mantled granite; thus random erosional exposure of bare rock leads to an alternating sequence of steep, slowly weathering bedrock “steps” and gently sloped, but rapidly weathering, soil-mantled “treads.” Our investigation focused on the terrain surrounding the Southern Sierra Critical Zone Observatory (CZO), which is underlain by granitic bedrock and lies outside the limits of recent glaciation, in the heart of the stepped topography described by Wahrhaftig. Our digital terrain analysis confirms that steep steps often grade into gentle treads, consistent with Wahrhaftig’s hypothesis. However, we observe a mix-and-match of soil and bare rock on treads and steps, contrary to one of the hypothesis’ major underpinnings – that bare rock should be much more common on steps than on treads. Moreover, the data show that bare rock is not as common as expected at step tops; Wahrhaftig’s hypothesis dictates that step tops should act as slowly eroding base levels for the treads above them. The data indicate that, within each landscape class (i.e., the steps and treads), bare rock erodes more slowly than surrounding soil. This suggests that the coupling between soil production and denudation in granitic landscapes harbors a tipping point wherein erosion rates decrease when soils are stripped to bedrock. Although broadly consistent with the differential weathering invoked by Wahrhaftig, the data also show that steps are eroding faster than treads, undermining Wahrhaftig’s explanation for the origins of the steps. The revised interpretation proposed here is that the landscape evolves by back-wearing of steps in addition to differential erosion due to differences in weathering of bare and soil-mantled granite.

In-situ cosmogenic 26 Al and 10 Be record the residence time of quartz grains near the earth's 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.

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.

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P(CO2) will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

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.

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.