The hypothesis that abrupt spatial gradients in erosion can cause high strain rates in active orogens has been supported by numerical models that couple erosional processes with lithospheric deformation via gravitational feedbacks. Most such models invoke a 'stream-power' rule, in which either increased discharge or steeper channel slopes cause higher erosion rates. Spatial variations in precipitation and slopes are therefore predicted to correlate with gradients in both erosion rates and crustal strain. Here we combine observations from a meteorological network across the Greater Himalaya, Nepal, along with estimates of erosion rates at geologic timescales (greater than 100,000 yr) from low-temperature thermochronometry. Across a zone of about 20 km length spanning the Himalayan crest and encompassing a more than fivefold difference in monsoon precipitation, significant spatial variations in geologic erosion rates are not detectable. Decreased rainfall is not balanced by steeper channels. Instead, additional factors that influence river incision rates, such as channel width and sediment concentrations, must compensate for decreasing precipitation. Overall, spatially constant erosion is a response to uniform, upward tectonic transport of Greater Himalayan rock above a crustal ramp.

▪ Abstract  Plants and animals exploit the soil for food and shelter and, in the process, affect it in many different ways. For example, uprooted trees may break up bedrock, transport soil downslope, increase the heterogeneity of soil respiration rates, and inhibit soil horizonation. In this contribution, we review previously published papers that provide insights into the process of bioturbation. We focus particularly on studies that allow us to place bioturbation within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production. Using geometrical relationships and data from others' work, we derive simple sediment flux equations for tree throw and root growth and decay.

The successful quantification of long-term erosion rates underpins our understanding of landscape. formation, the topographic evolution of mountain ranges, and the mass balance within active orogens. The measurement of in situ-produced cosmogenic radionuclides (CRNs) in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide such longterm erosion rates. In active orogens, however, deep-seated bedrock landsliding is an important erosional process, the effect of which on CRN-derived erosion rates is largely unquantified. We present a numerical simulation of cosmogenic nuclide production and distribution in landslide-dominated catchments to address the effect of bedrock landsliding on cosmogenic erosion rates in actively eroding landscapes. Results of the simulation indicate that the temporal stability of erosion rates determined from CRN concentrations in sediment decreases with increased ratios of landsliding to sediment detachment rates within a given catchment arcs and that larger catchment areas must be sampled with increased frequency of landsliding in order to accurately evaluate long-term erosion rates. In addition, results of this simulation suggest that sediment sampling for CRNs is the appropriate method for determining long-term erosion rates in regions dominated by mass-wasting processes, while bedrock surface sampling for CRNs is generally an ineffective means of determining long-term erosion rates. Response times of CRN concentrations to changes in erosion rate indicate that climatically driven cycles of erosion may be detected relatively quickly after such changes occur, but that complete equilibration of CRN concentrations to new erosional conditions may take tens of thousands of years. Simulation results of CRN erosion rates are compared with a new, rich dataset of CRN concentrations from the Nepalese Himalaya, supporting conclusions drawn from the simulation.

[1] Dry ravel is a general term that describes the rolling, bouncing, and sliding of individual particles down a slope and is a dominant hillslope sediment transport process in steep arid and semiarid landscapes. During fires, particles can be mobilized by the collapse of sediment wedges that have accumulated behind vegetation. On a daily basis, particles may be mobilized by bioturbation and by small landslides. Experiments on a dry ravel flume indicate that a basic expression of the momentum equation predicts the distance traveled by particles propelled down a rough surface. This equation is further elaborated to produce a nonlinear slope-dependent transport equation for dry ravel that represents the rate at which sediment crosses a contour width of slope. Sediment traps installed on two hillslope transects near Santa Barbara, California, measured the flux from dry ravel initiated by bioturbation, and the data support the form of the equation. Additionally, a physical model, based on the infinite slope stability analysis, is proposed for the initiation of dry ravel by landsliding. The analytical result from this model is supported by experiments and field data reported by others.INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: Geomorphology (1625); 5120 Physical Properties of Rocks: Plasticity, diffusion, and creep; 5415 Planetology: Solid Surface Planets: Erosion and weathering Citation: Gabet, E. J., Sediment transport by dry ravel,

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Uplift of the Himalayas has been proposed to have locally accelerated chemical weathering, thus leading to enhanced CO 2 sequestration and global cooling. This hypothesis assumes that rapid erosion exposes fresh, highly reactive minerals at Earth's surface. Empirical studies quantifying the relationship between erosion and weathering have produced apparently confl icting results, where the nature of the relationship is dependent on the weathering regime of ACKNOWLEDGMENTS We are very grateful to P. Almond, S. Brantley, and an anonymous reviewer for their helpful and incisive reviews. We also thank T. Niemi for shepherding the manuscript through the review process.

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It has generally been assumed that diffusive sediment transport on soil-mantled hillslopes is linearly dependent on hillslope gradient. Fieldwork was done near Santa Barbara, California, to develop a sediment transport equation for bioturbation by the pocket gopher (Thomomys bottae) and to determine whether it supports linear diffusion. The route taken by the sediment is divided into two parts, a subsurface path followed by a surface path. The first is the transport of soil through the burrow to the burrow opening. The second is the discharge of sediment from the burrow opening onto the hillslope surface. The total volumetric sediment flux, as a function of hillslope gradient, is found to be:2 68(dz/dx) 34(dz/dx) 0Á4 . This result does not support the use of linear diffusion for hillslopes where gopher bioturbation is the dominant mode of sediment transport. A one-dimensional hillslope evolution program was used to evolve hillslope profiles according to non-linear and linear diffusion and to compare them to a typical hillslope. The non-linear case more closely resembles the actual profile with a convex cap at the divide leading into a straight midslope section.