Robert C. Finkel

The soil production function and landscape equilibrium

Hilly and mountainous landscapes are partially to completely covered with soil under a wide range of erosion and uplift rates, bedrock type and climate. For soil to persist it must be replenished at a rate equal to or greater than that of erosion. Although it has been assumed for over 100 years that bedrock disintegration into erodable soil declines with increasing soil mantle thickness, no field data have shown this relationship. Here we apply two independent field methods for determining soil production rates to hillslopes in northern California. First, we show that hillslope curvature (a surrogate for soil production) varies inversely with soil depth. Second, we calculate an exponential decline of soil production rates with increasing soil depth from measurements of the in situ produced cosmogenic 10Be and 26Al concentrations in bedrock sampled under soils of different depths. Results from both methods agree well and yield the first empirical soil production function. We also illustrate how our methods can determine whether a landscape is in morphological equilibrium or not.

Spatially Averaged Long-Term Erosion Rates Measured from in Situ-Produced Cosmogenic Nuclides in Alluvial Sediment

Spatially averaged erosion rates of small catchments can be accurately inferred from the concentrations of cosmogenic nuclides in stream sediment. Here we test this technique at two catchments by comparing erosion rates inferred from cosmogenic nuclides with rates of alluvial fan deposition over the past 16,000 years. These two independent estimates agree within one standard error, demonstrating that cosmogenic nuclide signatures of stream sediment can be used to measure spatially averaged long-term erosion rates. Using this technique, we show that long-term erosion rates are an exponential function of average hillslope gradient at these sites.

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.

Cosmogenic nuclides, topography, and the spatial variation of soil depth

Abstract If the rate of bedrock conversion to a mobile layer of soil depends on the local thickness of soil, then hillslopes on uniform bedrock in a landscape approaching dynamic equilibrium should be mantled by a uniform thickness of soil. Conversely, if the depth of soil varies across an actively eroding landscape, then rates of soil production will also vary and, consequently the landscape will not be in morphologic equilibrium. The slow evolution of hillslopes relative to the tempo of climatic variations and tectonic adjustments would suggest that local morphologic disequilibrium may be expected in many landscapes. Here, we explore this issue of equilibrium landscapes through a previously developed model that predicts the spatial variation in thickness of soil as a consequence of the local balance between soil production and erosion. First, we confirm the assumption in the model that soil production varies inversely with the thickness of soil using two independent methods. One method uses the theoretical prediction that at local steady state (soil production equals removal), the depth of soil should vary inversely with hillslope curvature. The second method relies on direct measurements of in situ produced concentrations of cosmogenic 10 Be and 26 Al in bedrock at the base of the soil column. For our study site in Northern California, the two methods agree and yield the expression that the rate of soil production declines exponentially with the thickness of soil from 0.077 mm/year with no soil mantle to 0.0077 mm/year under 1 m of soil. We then use this function of soil production in a coupled production and diffusive model of sediment transport to explore the controls on the spatial variation of the depth of soil on four separate spur ridges (noses) where we measured the data for the function of soil production. Model predictions are sensitive to boundary conditions, grid scale, and run time. Nonetheless, we found good agreement between predicted and observed depths of soil as long as we used the observed function of soil production. The four noses each have spatially varying curvature and, consequently, have varying depths of soil, implying morphologic disequilibrium. We suggest that our study site has been subjected to a wave of incision and varying intensities of erosion because of tectonic and climatic oscillations that have a frequency shorter than the morphologic response time of the landscape.

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.

Soil production on a retreating escarpment in southeastern Australia

The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega Valley, southeastern Australia. Concentrations of 10 Be and 26 Al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth. These data define a soil production function with a maximum soil production rate of 53 m/m.y. under no soil mantle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining landscape evolution rates subsequent to escarpment retreat. The form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes. Spatial variation of soil production shows a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 Be and 26 Al on the surfaces of emergent tors.

High-frequency Holocene glacier fluctuations in New Zealand differ from the northern signature.

Vive La Différence How closely do climate changes in the Northern and Southern Hemispheres resemble each other? Much discussion has concentrated on the Holocene, the warm period of the past 11,500 years in which we now live, which represents a baseline to which contemporary climate change can be compared. Schaefer et al. (p. 622; see the Perspective by Balco) present a chronology of glacial movement over the last 7000 years in New Zealand, which they compare to similar records from the Northern Hemisphere. Clear differences are observed between the histories of glaciers in the opposing hemispheres, which may be owing to regional controls. Thus, neither of two popular arguments—that the hemispheres change in-phase or that they change in an anti-phased manner—appear to be correct. The patterns of glacial advances and retreats in New Zealand during the Holocene contrast markedly with those of the Northern Hemisphere. Understanding the timings of interhemispheric climate changes during the Holocene, along with their causes, remains a major problem of climate science. Here, we present a high-resolution 10Be chronology of glacier fluctuations in New Zealand’s Southern Alps over the past 7000 years, including at least five events during the last millennium. The extents of glacier advances decreased from the middle to the late Holocene, in contrast with the Northern Hemisphere pattern. Several glacier advances occurred in New Zealand during classic northern warm periods. These findings point to the importance of regional driving and/or amplifying mechanisms. We suggest that atmospheric circulation changes in the southwest Pacific were one important factor in forcing high-frequency Holocene glacier fluctuations in New Zealand.

Holocene left-slip rate determined by cosmogenic surface dating on the Xidatan segment of the Kunlun fault (Qinghai, China)

Cosmogenic dating, using in situ {sup 26}Al and {sup 10}Be in quartz pebbles from alluvial terrace surfaces, constrains the late Holocene slip rate on the Xidatan segment of the Kunlun fault in northeastern Tibet. Two terrace risers offset by 24 {+-} 3 and 33 {+-} 4 m, having respective ages of 1799 {+-} 388 and 2914 {+-} 471 yr, imply a slip rate of 12.1 {+-} 2.6 mm/yr. The full range of ages obtained ({le}22.8 k.y., most of them between 6.7 and 1.4 k.y.) confirm that terrace deposition and incision, hence landform evolution, are modulated by post-glacial climate change. Coupled with minimum offsets of 9--12 m, this slip rate implies that great earthquakes (M {approximately}8) with a recurrence time of 800--1000 yr, rupture the Kunlun fault near 94 E.

EROSION RATES OF ALPINE BEDROCK SUMMIT SURFACES DEDUCED FROM IN SITU 10BE AND 26AL

We have measured the concentration of in situ produced cosmogenic 10Be and 26Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My−1. Individual measurements vary between 2 and 19 m My−1. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates (∼ 50 m My−1). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing. We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.

Quantification of soil production and downslope creep rates from cosmogenic 10Be accumulations on a hillslope profile

Average soil transport rates over a period of ∼3500 yr on a convex soil-mantled hillslope have been quantified using a mass-balance model that incorporates the soil concentration of the cosmogenic isotope 10 Be. The 10 Be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient. The predicted diffusion coefficient is 360 ±55 cm 3 ⋅ yr -1 ⋅ cm -1 contour length and the average rate of soil production is 0.026 ±0.007 cm/yr. Within the uncertainty of this technique, the data do not reject G. K. Gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate. However, the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil-production rate law.