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The crest of the Andes--the trace of the highest mountain topography--weaves back and forth, in places near the coastline, in others farther inland. Its position reflects the asymmetric distribution of orogen mass and coincides with asymmetry of orographic precipitation. This coincidence is thought to reflect a primary influence of orographic precipitation on accumulated orogenic mass whereby the more erosive (wetter) side promotes crest migration toward the less erosive (drier) side. However, whether this remains the case after excluding tectonic controls on the size and asymmetry of the wedge is an open question. We assessed relationships between precipitation, erosion rates, and the macromorphology of the Andes. We find that precipitation rates cannot sufficiently explain orogen asymmetry after statistically controlling for the age or dip of the Nazca slab. Slab age and dip are known to primarily affect mountain building in the Andes by impacting stress and strain propagation into the retro-arc region, thus better explaining the position of the mountain chain within the orogenic wedge. Accordingly, using basin-wide erosion rate, topographic, and precipitation data, we find that precipitation possibly influences erosional efficiency in semi-arid Andean landscapes but falls short in explaining the variability of erosion rates in regions of high orographic precipitation. We conclude that the orographic effect cannot change the Andean macromorphology that is set by bottom-up tectonic processes. [ABSTRACT FROM AUTHOR]

Measuring rates of valley head migration and determining the timing of canyon‐opening are insightful for the evolution of planetary surfaces. Spatial gradients of in situ‐produced cosmogenic nuclide concentrations along horizontal transects provide a framework for assessing the migration of valley networks and similar topographic features. We developed a new derivation for valley head retreat rates from the concentrations of in situ‐produced cosmogenic radionuclides in valley walls. The retreat rate is inversely proportional to the magnitude of the spatial concentration gradient and proportional to local nuclide production rates. By solving for a spatial gradient in concentration along a valley parallel transect, we created an expression for the explicit determination of valley head retreat, which we refer to herein as unzipping. We applied this expression to a seepage‐derived drainage network developing along the Apalachicola River, Florida, USA. Sample concentrations along a valley margin transect varied systematically from 2.9 × 105 to 3.5 × 105 atoms/g resulting in a gradient of 160 atoms/g/m, and from this value a valley head retreat rate of 0.025 m/y was found. The discrepancy between overall network age and current rates of valley head migration suggests intermittent network growth which is consistent with glacial‐interglacial precipitation variations during the Pleistocene. This method can be applied to a wide range of Earth‐surface environments. For the 10Be system, this method should be sensitive to unzipping rates bounded between 10−6 and 100 m/y. Plain Language Summary: The pace at which landforms develop is an important control on many biological, chemical, and physical processes operating at Earth's surface. Rates of landscape change are often quantified by measuring the accumulation of cosmogenic radionuclides in near‐surface Earth materials, as an indicator of the erosion rate and age of landforms. In this study, we advance a method for querying the rate of horizontal topographic changes, such as valley growth or ice margin retreat, by sampling material in a horizontal transect and observing patterns in the nuclide concentrations in a spatial gradient. We present the results of numerical modeling that describe the limits of this approach due to the rate and consistency of ongoing landscape evolution processes. We present the first empirical data on the growth rate of a well‐studied seepage channel network in Florida, which suggests that the time‐averaged channel advance rate is 0.025 m/y. Furthermore, our measurements indicate that the age of the incising plateau surface is between 2 and 2.5 My and that the regional uplift rate is between 27 and 38 m/My. Key Points: Propagation rates of morphologic features determined by nuclide concentration gradients along a horizontal transectUplift rate of the Florida panhandle estimated between 0.027 and 0.038 mm/y from a nuclide depth profileChanges in climate over the Quaternary likely drove variable growth rates of seepage valleys along the Apalachicola River [ABSTRACT FROM AUTHOR]

Meteoric beryllium‐10 (10Bem, t1/2 = 1.4 Myr) is a cosmogenic radionuclide that remains largely underutilized for deriving hillslope‐scale estimates of erosion on uplands under conditions of land use change. We applied two different models for estimating erosion rates from observed 10Bem concentrations (a one‐dimensional model predicting vertical profiles of 10Bem within hillslope soils [loss only, diffusion only, LODO] and a two‐dimensional model predicting the concurrent evolution of hillslope topography and 10Bem distributions via bioturbation, chemical mobility, and surface erosion [Be2D]). Both models were used to derive pre‐European and post‐European settlement erosion rates (Enat and Epost, respectively) across paired cultivated and uncultivated hillslopes in west‐central Minnesota, USA. Epost estimates from 10Bem were compared to Epost estimates derived from 137Cs inventories and the process‐based Water and Tillage Erosion Model (WaTEM). The results from these models suggest that erosion rates from upper positions on the cultivated hillslope have increased from an average of 0.047 mm/year under natural conditions to Epost values of 3.09 mm/year. The Be2D and LODO models, on average, produced Epost estimates that were similar in magnitude to WaTEM and 137Cs conversion models. This numerical convergence does not imply absolute 10Bem model accuracy, particularly when considering the uncertainties inherent in each approach, but it does suggest that the orders of magnitude increase in estimated erosion rates from Enat to Epost is robust. Additionally, the pattern of Epost estimates produced using 10Bem conversion models is supported by the distribution of soil inorganic carbon at the study site. Our results demonstrate that 10Bem can provide reasonable estimates of both predisturbance and postdisturbance erosion rates in landscapes that have undergone extensive human modification. Plain Language Summary: Agricultural practices have substantially changed soil erosion rates in the Midwestern United States. Although much work has been devoted to understanding the changes in soil erosion rates with land cover change, the ability to quantify those changes at discrete locations on the landscape over long periods of time has been limited. We use a set of tracers and models to estimate presettlement and postsettlement erosion rates on a hillslope in west‐central Minnesota, USA, and show that soil erosion has increased by approximately 1 to 2 orders of magnitude over a period of approximately 110 years. This has implications for how we view our current agricultural landscapes and how we think about soil sustainability in the future. Key Points: Meteoric beryllium‐10 is used to generate point‐based estimates of long‐term erosion on paired hillslopesThese estimates suggest that erosion rates have increased by 2 orders of magnitude under agricultural managementModel choice influences erosion rate magnitude, but average rates are similar to estimates from process‐based models and cesium‐137 [ABSTRACT FROM AUTHOR]

At the largest time and space scales, the pace of erosion and chemical weathering is determined by tectonic uplift rates. Deviations from equilibrium arise from the transient response of landscape denudation to climatic and tectonic perturbations. We posit that the constraint of mass balance, however, makes it unlikely that such disequilibrium persists at the global scale over millions of years, as has been proposed for late Cenozoic erosion. We synthesize weathering fluxes, global sedimentation rates, sediment yields and tectonic motions to show a remarkable constancy in the pace of Earth-surface evolution over the last 10 Ma and support the null hypothesis - that global rates of landscape change have remained constant over this time period, despite global climate change and mountain building events. This work undermines the hypothesis that increased weathering due to mountain building or climate change was the primary agent for a decrease in global temperatures. [ABSTRACT FROM AUTHOR]

Over geologic timescales, CO2 is emitted from the Earth’s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial. This balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions. Changes in this balance have been attributed to changes in topographic relief, where varying rates of continental rock weathering and erosion are superimposed on fluctuations in organic carbon burial. Geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation. Widespread observations of a recent (0–5-Myr) fourfold increase in global sedimentation rates require a global mechanism to explain them. Accelerated uplift and global cooling have been given as possible causes, but because of the links between rates of erosion and the correlated rate of weathering, an increase in the drawdown of CO2 that is predicted to follow may be the cause of global climate change instead. However, globally, rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do. Moreover, proxy records of past atmospheric CO2 provide no evidence for this large reduction in recent CO2 concentrations. Here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record. As evidence, we recast the ocean dissolved 10Be/9Be isotope system as a weathering proxy spanning the past ∼12 Myr (ref. 14). This proxy indicates stable weathering fluxes during the late-Cenozoic era. The sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean. We conclude that processes different from an increase in denudation caused Cenozoic global cooling, and that global cooling had no profound effect on spatially and temporally averaged weathering rates. [ABSTRACT FROM AUTHOR]

Abstract: Rainfall scavenges meteoric cosmogenic 10Be from the atmosphere. 10Be falls to the Earth''s surface, where it binds tightly to sediment particles in non-acidic soils over the life-span of those soils. As such, meteoric 10Be has the potential to be an excellent geochemical tracer of erosion and stability of surfaces in a diverse range of natural settings. Meteoric 10Be has great potential as a recorder of first-order erosion rates and soil residence times. Even though this tracer was first developed in the late 1980s and showed great promise as a geomorphic tool, it was sidelined in the past two decades with the rise of the “sister nuclide”, in situ 10Be, which is produced at a known rate inside quartz minerals. Since these early days, substantial progress has been made in several areas that now shed new light on the applicability of the meteoric variety of this cosmogenic nuclide. Here, we revisit the potential of this tracer and we summarize the progress: (1) the atmospheric production and fallout is now described by numeric models, and agrees with present-day measurements and paleo-archives such as from rain and ice cores; (2) short-term fluctuations in solar modulation of cosmic rays or in the delivery of 10Be are averaged out over the time scale soils accumulate; (3) in many cases, the delivery of 10Be is not dependent on the amount of precipitation; (4) we explore where 10Be is retained in soils and sediment; (5) we suggest a law to account for the strong grain-size dependence that controls adsorption and the measured nuclide concentrations; and (6) we present a set of algebraic expressions that allows calculation of both soil or sediment ages and erosion rates from the inventory of meteoric 10Be distributed through a vertical soil column. The mathematical description is greatly simplified if the accumulation of 10Be is at a steady state with its export through erosion. In this case, a surface sample allows for the calculation of an erosion rate. Explored further, this approach allows calculation of catchment-wide erosion rates from river sediment, similar to the approach using 10Be produced in situ. In contrast to the in situ 10Be approach, however, these analyses can be performed on any sample of fine-grained material, even where no quartz minerals are present. Therefore, this technique may serve as a tool to date sediment where no other chronometer is available, to track particle sources and to measure Earth-surface process rates in soil, suspended river sediment, and fine-grained sedimentary deposits. [Copyright &y& Elsevier]