Kyle K. Nichols

Quantifying sediment transport on desert piedmonts using 10Be and 26Al

In situ produced 10 Be and 26 Al, measured in 40 sediment samples collected from the Iron and Granite Mountain piedmonts, eastern Mojave Desert, provide a unique view of piedmont modification processes and process rates over the 10 3 to 10 5 year time scale. Cosmogenic nuclide-based models suggest that the Iron and Granite Mountains generate 0.11–0.13 and 0.082–0.097 m 3 of sediment per year per meter of rangefront, respectively. The sediment moves down the piedmont in an active transport layer (ATL), which is 20 to 30 cm thick (based on visual observations, measurements of depth to a buried B-horizon, cosmogenic nuclide data, and maximum ephemeral channel depths). Sediment in this layer is well-mixed vertically and horizontally on the 10 2 year time scale, indicating that the small ephemeral channels, which dominate the piedmont surface migrate quickly. Interpretive models of increasing nuclide activities at depth in two pits suggest steady sediment deposition on the piedmont (at rates between 17–21 and 38–45 m Ma � 1 ) until the late Pleistocene epoch, when a discontinuity to markedly lower nuclide activities in the isotopically well-mixed active transport layer suggests that deposition stopped, a significant change in piedmont behavior. Nuclide activities in 10 amalgamated surface samples, each collected along a different 4-km-long transect, increase steadily away from the mountain front. Thus, we infer that sediment is uniformly dosed by cosmic rays as it is transported down the Iron and Granite Mountain piedmonts. Interpretive models suggest that long-term average sediment speeds down the Iron and Granite Mountain piedmonts are a few decimeters to a meter per year. D 2002 Published by Elsevier Science B.V.

Cosmogenically enabled sediment budgeting

We used 10 Be and 26 Al to constrain the millennial-scale sedi- ment and nuclide budget for a common, long-studied, but poorly understood landform in arid regions, the desert piedmont. We sampled the Chemehuevi Mountain piedmont, a complex multi- surfaced landform in the Mojave Desert, western United States. The nuclide data indicate that sediment is produced more rapidly (1.1 3 10 5 kg·yr 21 ·km 22 ) in steep mountain source basins than on the low-gradient pediment (4.0 3 10 4 kg·yr 21 ·km 22 ) or the intra- piedmont mountain range (2.5 3 10 4 kg·yr 21 ·km 22 ). However, the bulk of the sediment in transport is derived from erosion of the large abandoned alluvial surface (3.9 3 10 4 kg·yr 21 ·km 22 ). The combination of mass and nuclide budgeting suggests that sediment transport speeds decrease downslope from tens of meters per year in confined channels on the proximal pediment to decimeters per year in unconfined distributaries on distal wash surfaces. The sed- iment and nuclide budgeting approach we use is particularly valu- able in arid regions where geomorphically significant events are infrequent and dating control is poor, thus confounding traditional sediment-budgeting techniques.

Using Cosmogenic Nuclide Measurements In Sediments To Understand Background Rates Of Erosion And Sediment Transport

Understanding the tempo of sediment generation and transport is fundamental to understanding Earth as a system. For land managers, knowing rates of landscape change is important as they consider human impact on landscapes in a long-term context. Numerous means have been employed to estimate basin-scale erosion rates (Saunders and Young, 1983); many of these methods, such as calculations based on river sediment and solute transport rates, are influenced by human impacts or are useful only over short (10 to 100 y) time scales (Trimble, 1977). Other techniques involve reconstruction of initial topography or definition of sediment volumes and source areas; however, these techniques are feasible only in particular environments and geologic settings, many of which are uncommon (Bishop, 1985). Sediment transport rates can also be estimated using tracers (e.g., Lekach and Schick, 1995) and sediment traps. The traditional means by which basin-scale erosion and sediment transport rates are estimated remain uncertain and thus are not widely applied.

Dates and rates of arid region geomorphic processes

Analysis of in situ–produced cosmogenic nuclides, including Be, Al, and cl, has changed how geologists understand desert surface processes. Here, we provide a series of examples from arid mountain-piedmont systems that illustrate both the power and limitations of this geochronometer. Analyses of samples collected from bare bedrock surfaces at the Alabama Hills, california, demonstrate slow but variable (1.4–20 m m.y.) rates of erosion, whereas cosmogenic dating of the Blackhawk landslide debris (~6.5–31 k.y.) and the castle Dome piedmont allows linkages between landscape-scale processes and climate change. However, data show that nuclides inherited from prior periods of exposure, as well as the effect of post-depositional surface change, limit the accuracy and precision of exposure dating in some settings. On the broad castle Dome piedmont, detailed isotopic stratigraphies, coupled with analysis of desert soils, indicate depositional histories over the past ~70 k.y. in the absence of radiocarbon-datable organic material. transect-based amalgamated sampling techniques allow for estimation of sediment velocity down mountain-fringing piedmonts. in drainage basins, the concentration of Be in fluvial sediment demonstrates the efficacy of fluvial mixing even in areas where surface flow is intermittent. considered together, these applications of the cosmogenic technique allow the delineation of sediment budgets in areas where no other technique has been useful. Such data are important for the arid Southwest, where population is increasing rapidly, as is the interaction of society and surface processes. INTRODUCTION Desert landscapes contain a rich record of geomorphic and geologic change (cooke et al., 1993). Over the past century, geomorphologists and pedologists have used a variety of approaches, such as interpreting and dating sediments from dry-lake playas (enzel, 1992; lowenstein, 2002; Anderson and Wells, 2003), alluvial fans (Harvey and Wells, 2003; McDonald et al., 2003), and landforms offset by fault systems (Weldon et al., 2004; Matmon et al., 2005), to determine the effects of climate change on sediment generation and transport systems and to quantify process rates. Such studies allow us to understand the broad timing of sediment deposition and erosion, their drivers, and the overall rates and processes of soil development. However, at finer temporal and spatial resolution, there is significant variability in the data, which often makes it difficult to interpret because numeric age-control is frequently lacking. Furthermore, the timing of older events is often inferred solely from the behavior of the system during more recent c-datable climatic and tectonic episodes. Quantifying rates and dates beyond the 40–50 k.y. limit of radiocarbon dating not only allows geologists to test long-standing hypotheses regarding desert process behavior during climate change (e.g., Bull, 1991), but also allows for systematic evaluation of the effects of lithology, nonglacial climate change, tectonics, and other potential drivers of landscape change. Determining rates of surface change in the desert is no simple task. Most desert surfaces change imperceptibly over human time scales (Webb, 1996) because much geomorphic work in arid climates is accomplished during large but infrequent storm events (Schick, 1977; cooke et al., 1993). Quantifying the effects of such storms, both spatially and temporally, requires expensive and time-consuming monitoring programs (Schick, 1977; Persico et al., 2005). Over millennia, the timing of such events is difficult to establish because dry desert climates are not conducive to the generation or preservation of plant material for radiocarbon analysis, the standard means by which late Quaternary deposits are dated and rates of surface change are often calculated (Bull, 1991). it is clear that major advances in the understanding of desert landforms and the rate at which they shed sediment require widely applicable, quantitative, and reliable chronometers. in this paper, we present new data to illustrate both the promise and limitations of cosmogenic nuclides when combined with field data as a tool for understanding arid-region geomorphic systems. Many of the approaches we present are also applicable to other climatic and tectonic settings (Bierman and nichols, 2004). CASE STUDIES Fundamental to the application of cosmogenic nuclides as a monitor of desert surface processes is understanding that (1) most production of cosmogenic nuclides occurs near earth’s surface and (2) production decreases to minimal rates at depths of several meters. the measured concentration of isotopes, such as Be, reflects the near-surface residence time, or cosmic-ray dosing, of a mineral grain. this leads to an inverse relationship between nuclide concentration and erosion rate and a direct relationship between surface age and nuclide conGSA Today: v. 16, no. 8, doi: 10.1130/GSAt01608.1 *e-mails: knichols@skidmore.edu; Paul.Bierman@uvm.edu current address: PriMe laboratory, Purdue University, West lafayette, indiana 47907, USA Dates and rates of arid region geomorphic processes

Quantifying Urban Land Use and Runoff Changes Through Service-Learning Hydrology Projects

We have used land use change, driven by development of the University of Vermont campus and recent student occupancy of surrounding neighborhoods in Burlington, Vermont, as an opportunity for service learning and for teaching fundamental hydrologic and geologic skills in two undergraduate Geology courses. Two students, from a Geomorphology class, used historical maps and aerial photographs of the University campus to document the dramatic increase in impermeable surfaces on campus from 4% of the land area in 1869 to 42% in 1999. In Geohydrology, student teams used aerial photographs, field mapping, and door-to-door surveys to document green space losses of 40 to 50% over the past 20 years in neighborhoods inhabited predominantly by students, despite zoning controls enacted in 1973. Students used simple hydrologic calculations to demonstrate that this unregulated change in land use increased both the volume and peak flow of stormwater runoff. Senior research projects have also made field and demographic studies of individual neighborhoods and examined the percent of land use change. In each of these studies, students worked closely with City and University staff and presented results at local forums, professional national meetings, and on the World Wide Web. These service-learning projects have received positive feedback from the students, city officials, and community members.

Long-Term Sediment Generation Rates for the Upper Río Chagres Basin

In situ-produced cosmogenic 10Be was measured in 17 sediment samples to estimate the rate and distribution of sediment generation in the upper Rio Chagres basin over the last 10 to 20 kyr. Results indicate that the upper Rio Chagres basin is generating sediment uniformly. Nuclide activities suggest basin-wide sediment generation rates of 143 and 354 tons/km/yr (avg. = 234 ± 74 tons/km/yr; n = 7) for small tributary basins and 248 to 281 tons/km/yr (avg. = 267 ± 97 tons/km/yr; n = 3) for large tributary basins. The weighted average of all tributaries is 269 ± 63 tons/km/yr; n = 10). A sample collected upstream of Lago Alhajuela suggests that the entire basin is exporting sediment at a rate of 275 ± 62 tons/km/yr. These cosmogenic nuclide measurements all suggest that the upper Rio Chagres basin (when considered on scales 350 km2) is generating sediment at ∼270 tons/km/yr. This long-term (1 −20 kyr) sediment generation rate that is equivalent to the estimate derived from suspended sediment yield measured below the upper Rio Chagres- Rio Chico confluence from 1981–96 (289 tons tons/km/yr). Such similarity implies that decadal and millennial sediment yields are similar. Thus, short-term sediment yields and long-term sediment generations are in balance, implying steady landscape behavior over time. The background sediment yield suggests that it would take ∼3,600 years to completely fill Lago Alhajuela, the reservoir for the Panama Canal. Taking into account the present day 2- to 3- fold increase in sediment yields for adjacent human-impacted Rio Boqueron and Rio Pequeni basins, the filling time is reduced to ∼2,000 years. However, it would only take between 250 to 600 years to reduce the reservoir capacity (69% of maximum) enough to drain the entire reservoir for precipitation conditions similar to the 1982 El Nino event. Such models highlight the importance of proper watershed management in order to reduce the sedimentation of Lago Alhajuela.

Teaching Winter Geohydrology Using Frozen Lakes and Snowy Mountains

We have developed two ice- and snow-dependent geohydrology projects for the long, chilly Vermont winter. Both projects are field-oriented and allow the students to do original research. The first project involves investigating a pond and its surrounding drainage basin. In the second project, students explore the snowpack hydrology of a local skiing area. At the pond, the students learn to survey, make bathymetric maps, measure water temperature and conductivity, calculate lake volume and water residence times, and collect and analyze a sediment core. In the skiing area, students calculate the volume of water in the snowpack, establish the relationship between water equivalent of snow and elevation, record snow stratigraphy, and calculate the effects of a hypothetical late-winter rainfall event on the snowpack. Both projects emphasize a hands-on, interactive-learning style based on data collection, field observation, and the application of geohydrologic principles rather than the memorization of information.

The Anthropocene’s dating problem: Insights from the geosciences and the humanities

The Anthropocene, generally defined, is the time when human activities have a significant impact on the Earth System. However, the natural sciences, the humanities, and the social sciences have different understandings of how and when human activities affected the Earth System. Humanities and social science scholars tend to approach the Anthropocene from a wide range of moral-political concerns including differential responsibility for the change in the Earth System and social implications going forward. Geologists, on the other hand, see their work as uninfluenced by such considerations, instead concerning themselves with empirical data that might point to a ‘golden spike’ in the geologic record – the spike indicating a change in the Earth System. Thus, the natural sciences and the humanities/social sciences are incongruent in two important ways: (1) different motivations for establishing a new geologic era, and (2) different parameters for identifying it. The Anthropocene discussions have already hinted at a paradigm shift in how to define geologic time periods. Several articles suggest a mid-20th century commencement of the Anthropocene based on stratigraphic relationships identified in concert with knowledge of human history. While some geologists in the Anthropocene Working Group have stated that the official category should be useful well beyond geology, they continue to be guided by the stratigraphic conventions of defining the epoch. However, the methods and motivations that govern stratigraphers are different from those that govern humanists and social scientists. An Anthropocene defined by stratigraphic convention would supersede many of the humanities/social science perspectives that perhaps matter more to mitigating and adapting to the effects of humans on Earth’s System. By this reasoning, the impetus for defining the Anthropocene ought to be interdisciplinary, as traditional geologic criteria for defining the temporal scale might not meet the aspirations of a broad range of Anthropocene thinkers.