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Pfeiffer, AM, Finnegan NJ, Willenbring JK.  2017.  Sediment supply controls equilibrium channel geometry in gravel rivers. Proceedings of the National Academy of Sciences of the United States of America. 114:3346-3351.   10.1073/pnas.1612907114   AbstractWebsite

In many gravel-bedded rivers, floods that fill the channel banks create just enough shear stress to move the median-sized gravel particles on the bed surface (D-50). Because this observation is common and is supported by theory, the coincidence of bankfull flow and the incipient motion of D50 has become a commonly used assumption. However, not all natural gravel channels actually conform to this simple relationship; some channels maintain bankfull stresses far in excess of the critical stress required to initiate sediment transport. We use a database of > 300 gravel-bedded rivers and > 600 10Be-derived erosion rates from across North America to explore the hypothesis that sediment supply drives the magnitude of bankfull shear stress relative to the critical stress required to mobilize the median bed surface grain size (tau*(bf) = tau*(c))We find that tau*(bf) = tau*(c) is significantly higher in West Coast river reaches (2.35, n = 96) than in river reaches elsewhere on the continent (1.03, n = 245). This pattern parallels patterns in erosion rates (and hence sediment supplies). Supporting our hypothesis, we find a significant correlation between upstream erosion rate and local tau*(bf) = tau*(c) at sites where this comparison is possible. Our analysis reveals a decrease in bed surface armoring with increasing tau*(bf) = tau*(c), suggesting channels accommodate changes in sediment supply through adjustments in bed surface grain size, as also shown through numerical modeling. Our findings demonstrate that sediment supply is encoded in the bankfull hydraulic geometry of gravel bedded channels through its control on bed surface grain size.

Gonneau, C, Mohanty SK, Dietterich LH, Hwang W-T, Willenbring JK, Casper BB.  2017.  Differential elemental uptake in three pseudo-metallophyte C4 grasses in situ in the eastern USA. Plant and Soil. :1-15.   10.1007/s11104-017-3198-9   Abstract

Elemental uptake in serpentine floras in eastern North America is largely unknown. The objective of this study was to determine major and trace element concentrations in soil and leaves of three native pseudo-metallophyte C4 grasses in situ at five sites with three very different soil types, including three serpentine sites, in eastern USA.

Garcin, Y, Schildgen TF, Torres Acosta V, Melnick D, Guillemoteau J, Willenbring J, Strecker MR.  2017.  Short-lived increase in erosion during the African Humid Period: Evidence from the northern Kenya Rift. Earth and Planetary Science Letters. 459:58-69.   10.1016/j.epsl.2016.11.017   Abstract

The African Humid Period (AHP) between ∼15 and 5.5 cal. kyr BP caused major environmental change in East Africa, including filling of the Suguta Valley in the northern Kenya Rift with an extensive (∼2150 km2), deep (∼300 m) lake. Interfingering fluvio-lacustrine deposits of the Baragoi paleo-delta provide insights into the lake-level history and how erosion rates changed during this time, as revealed by delta-volume estimates and the concentration of cosmogenic 10Be in fluvial sand. Erosion rates derived from delta-volume estimates range from 0.019 to 0.03 mm yr−1. 10Be-derived paleo-erosion rates at ∼11.8 cal. kyr BP ranged from 0.035 to 0.086 mm yr−1, and were 2.7 to 6.6 times faster than at present. In contrast, at ∼8.7 cal. kyr BP, erosion rates were only 1.8 times faster than at present. Because 10Be-derived erosion rates integrate over several millennia, we modeled the erosion-rate history that best explains the 10Be data using established non-linear equations that describe in situ cosmogenic isotope production and decay. Two models with different temporal constraints (15–6.7 and 12–6.7 kyr) suggest erosion rates that were ∼25 to ∼300 times higher than the initial erosion rate (pre-delta formation). That pulse of high erosion rates was short (∼4 kyr or less) and must have been followed by a rapid decrease in rates while climate remained humid to reach the modern 10Be-based erosion rate of ∼0.013 mm yr−1. Our simulations also flag the two highest 10Be-derived erosion rates at ∼11.8 kyr BP related to non-uniform catchment erosion. These changes in erosion rates and processes during the AHP may reflect a strong increase in precipitation, runoff, and erosivity at the arid-to-humid transition either at ∼15 or ∼12 cal. kyr BP, before the landscape stabilized again, possibly due to increased soil production and denser vegetation.

Willenbring, JK, Jerolmack DJ.  2016.  The null hypothesis: globally steady rates of erosion, weathering fluxes and shelf sediment accumulation during Late Cenozoic mountain uplift and glaciation. Terra Nova. 28:11-18.   10.1111/ter.12185   AbstractWebsite

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 10Ma 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.

Boschi, V, Willenbring JK.  2016.  Beryllium desorption from minerals and organic ligands over time. Chemical Geology. 439:52-58.   10.1016/j.chemgeo.2016.06.009   AbstractWebsite

Beryllium isotopes sorbed to sediments have provided useful tools in the field of geochronology and geomorphology over the last few decades. The use of beryllium isotopes relies on the premise that beryllium sorbed to sediments is unaltered over large timescales. Changes in the environmental chemistry, either in-situ or en route from soil to fluvial system, to the ocean, can cause beryllium desorption and may preclude some beryllium isotopic applications. Four mechanisms were tested to determine the relative desorption potential of beryllium including a reduction in pH, an increase in ionic strength (NaCl) and complexation by soluble organic (malonic acid) and inorganic species (NaF). To assess the relative effect of each mechanism on beryllium desorption from both organic and mineral fractions, we prepared separate solutions of beryllium bound to minerals and organic compounds and measured beryllium concentrations in solution before and after each chemical perturbation. We conclude a reduction in pH resulted in the greatest amount of desorption among the four treatments, removing 97% and 75% of sorbed beryllium from illite and montmorillonite, respectively, and none from the organic ligands tested. The addition of malonic acid and increasing the ionic strength also resulted in desorption from montmorillonite. Although increasing the ionic strength did remove 32% and 8.4% of beryllium from montmorillonite and sulfonate, respectively, the presence of sodium significantly enhanced sorption to illite. The addition of NaF did not result in any beryllium desorption. Our results demonstrate that various chemical processes can promote the exchange of beryllium between solid and dissolved phases, the extent to which depends on the composition of the system. We also related differences in beryllium desorption behavior to complexation mechanisms driving retention among organic and mineral species. We estimate inner sphere complexation is the predominant sorption mechanism among the organic ligands tested due to the minimal amounts of desorption and the large stability constants previously reported in the literature. Additionally, we found that different complexation processes are involved in beryllium sorption to illite versus montmorillonite. Because beryllium desorbed from montmorillonite due to changes in pH, ionic strength and organic acid complexation, we hypothesize that a portion of beryllium-montmorillonite associations involve outer sphere processes, driven by weaker electrostatic attractions. However, beryllium exhibited a unique relationship with illite in that sorption not only involves inner sphere processes but also physical inclusion within collapsed interlayer spaces.

Wolf, J, Brocard G, Willenbring J, Porder S, Uriarte M.  2016.  Abrupt change in forest height along a tropical elevation gradient detected using airborne lidar. Remote Sensing. 8   10.3390/rs8100864   AbstractWebsite

Most research on vegetation in mountain ranges focuses on elevation gradients as climate gradients, but elevation gradients are also the result of geological processes that build and deconstruct mountains. Recent findings from the Luquillo Mountains, Puerto Rico, have raised questions about whether erosion rates that vary due to past tectonic events and are spatially patterned in relation to elevation may drive vegetation patterns along elevation gradients. Here we use airborne light detection and ranging (LiDAR) technology to observe forest height over the Luquillo Mountain Range. We show that models with different functional forms for the two prominent bedrock types best describe the forest height-elevation patterns. On one bedrock type there are abrupt decreases in forest height with elevation approximated by a sigmoidal function, with the inflection point near the elevation of where other studies have shown there to be a sharp change in erosion rates triggered by a tectonic uplift event that began approximately 4.2 My ago. Our findings are consistent with broad geologically mediated vegetation patterns along the elevation gradient, consistent with a role for mountain building and deconstructing processes.

Salamatipour, A, Mohanty SK, Pietrofesa RA, Vann DR, Christofidou-Solomidou M, Willenbring JK.  2016.  Asbestos fiber preparation methods affect fiber toxicity. Environmental Science & Technology Letters. 3:270-274.: American Chemical Society   10.1021/acs.estlett.6b00174   AbstractWebsite

To measure the toxic potential of asbestos fibers—a known cause of asbestosis, lung cancer, and malignant mesothelioma—asbestos minerals are generally first ground down to small fibers, but it is unknown whether the grinding condition itself changes the fiber toxicity. To evaluate this, we ground chrysotile ore with or without water for 5–30 min and quantified asbestos-induced reactive oxygen species generation in elicited murine peritoneal macrophages as an indicator of fiber toxicity. The toxicity of dry-ground fibers was higher than the toxicity of wet-ground fibers. Grinding with or without water did not materially alter the mineralogical properties. However, dry-ground fibers contained at least 7 times more iron than wet-ground fibers. These results indicate that grinding methods significantly affect the surface concentration of iron, resulting in changes in fiber-induced reactive oxygen species generation or toxicity. Therefore, fiber preparation conditions should be accounted for when comparing the toxicity of asbestos fibers between reported studies.

Brocard, GY, Willenbring JK, Miller TE, Scatena FN.  2016.  Relict landscape resistance to dissection by upstream migrating knickpoints. Journal of Geophysical Research: Earth Surface. 121:1182-1203.   10.1002/2015JF003678   AbstractWebsite

Expanses of subdued topographies are common at high elevation in mountain ranges. They are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds. Preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography. However, relict fragments could also possess intrinsic characteristics that make them resilient to dissection. We document here the propagation of a wave of dissection across an uplifted relict landscape in Puerto Rico. Using 10Be-26Al burial dating on cave sediments, we show that uplift started 4 Ma and that river knickpoints have since migrated very slowly across the landscape. Modern detrital 10Be erosion rates are consistent with these long-term rates of knickpoint retreat. Analysis of knickpoint distribution, combined with visual observations along the streambeds, indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat. The studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks. Earlier studies have shown that vegetation over the relict topography efficiently limits erosion, allowing for the formation of a thick saprolite underneath. Such slow erosion reduces streambed load fluxes delivered to the knickpoints, as well as bed load grain size. Both processes limit abrasion. Compounding the effect of slow abrasion, wide joint spacing in the bedrock makes plucking infrequent. Thus, the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream, reducing the celerity at which the relict, subdued landscape is dissected. We conclude that similar top-down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies.

Boschi, V, Willenbring JK.  2016.  The effect of pH, organic ligand chemistry and mineralogy on the sorption of beryllium over time. Environmental Chemistry. 13:711-722.   10.1071/EN15107   AbstractWebsite

Understanding the chemical controls on beryllium sorption is fundamental when assessing its mobility as a pollutant and interpreting its concentration as a geochemical tracer of erosion, weathering and landscape surface stability. In order to evaluate the interactions of beryllium with soil- and aquatic-related materials, we selected model organic compounds and minerals to perform sorption experiments. The retention of beryllium by each of these compounds and minerals was evaluated over a pH range of 3–6 and at various equilibration times to determine which conditions allowed the greatest retention of beryllium. We conclude that most beryllium sorption occurred within 24 h for both organic and mineral materials. However, equilibration required longer periods of time and was dependent on the solution pH and sorbent material. The pH exhibited a strong control on beryllium sorption with distribution coefficient (Kd) values increasing non-linearly with increasing pH. A system with a pH of 6 is likely to retain 79–2270 % more beryllium than the same system at a pH of 4. Phosphonate retained the greatest amount of beryllium, with Kd values 2–30× greater than all other materials tested at a pH of 6. Therefore, soils containing larger amounts of phosphorus-bearing minerals could result in greater retention of beryllium relative to phosphorus-limited soils. Overall, soil composition, with an emphasis on phosphorus oxide content and pH, is an important property to consider when evaluating the capacity of a system to retain beryllium.

McClintock, MA, Brocard G, Willenbring J, Tamayo C, Porder S, Pett-Ridge JC.  2015.  Spatial variability of African dust in soils in a montane tropical landscape in Puerto Rico. Chemical Geology. 412:69-81.   10.1016/j.chemgeo.2015.06.032   AbstractWebsite

Dust deposition provides rock-derived nutrients such as phosphorus (P) to terrestrial ecosystems. Over pedogenic timescales, as bedrock sources of P are depleted, dust sources of P may support productivity in certain ecosystems, but controls on the spatial variability of dust in montane forested systems are largely unknown. Here, we use neodymium (Nd) isotope ratios in 31 ridgetop surface soils to evaluate the spatial variability of dust contributions to soil across similar to 100 km(2) in the Luquillo Mountains, Puerto Rico. Dust from the Sahara-Sahel region of Africa carries a distinct isotopic signature of - 12 epsilon(Nd). Local bedrock, in contrast, has a eNd value of similar to+ 7. End-member mixing calculations based on eNd reveal a wide range in dust influence on surface soils, with between 0% and 92% of the top 20 cm of ridgetop soil Nd derived from African dust. Using eNd paired with both dust and soil Nd content, the current soil dust content was calculated, ranging from 0 to 8%. There were no correlations between current dust content of soil and Be-10-based denudation rate, elevation, rainfall, longitude, or forest type. Current soil dust content in the Luquillo Mountains is significantly higher in soils developed on volcaniclastic sandstone, breccia and mudstone than in soils developed on quartz diorite bedrock, which we attribute to greater retention capacity in the volcaniclastic soils. Current soil dust content also increases with increasing ridge-width, implying that small-scale topographic effects and other factors such as wind speed and turbulence influence local dust deposition rates. Higher current dust content of soil is also positively correlated with biologically cycled fractions of soil P on quartz diorite bedrock (r(2) = 0.24 and p = 0.002 for sum of extractable NaHCO3-P + NaOH-P), suggesting that atmospheric dust inputs contribute to the fertility of Luquillo Mountain ecosystems on the relatively P-poor quartz diorite bedrock. (C) 2015 Elsevier B.V. All rights reserved.

Brocard, GY, Willenbring JK, Scatena FN, Johnson AH.  2015.  Effects of a tectonically-triggered wave of incision on riverine exports and soil mineralogy in the Luquillo Mountains of Puerto Rico. Applied Geochemistry. 63:586-598.   10.1016/j.apgeochem.2015.04.001   AbstractWebsite

We document the long-term response of a tropical mountain stream to tectonics and show how this response exerts a first-order spatial control on the delivery of weatherable minerals to forest soils in its catchment. These minerals, in turn, affect nutrient availability and biogeochemical cycles. The Luquillo Mountains is a rainforest-covered isolated massif on Puerto Rico Island. Instead of displaying typical concave-up long profiles of most mountain streams, the rivers draining the southern flank of these mountains display a systematic downstream increase in gradient. Using concentrations of in situ-produced cosmogenic Be-10 in quartz from river sediments, we find that the downstream steepening is due to the propagation of an erosion wave along the rivers from the coastal plain upstream, toward the headwaters. Decreased Be-10 concentration along the steepened reaches (knickpoint faces or knickzones) results from faster denudation of the valley sides around the steepened reaches. The upper portions of the steepened reaches (knickpoint lips) cluster in elevation around the altitude of a flat-lying, dissected surface traceable around the Luquillo Mountains. Inspection of similar platforms over other parts of Puerto Rico and in the surrounding seas suggests that the dissected surface is an uplifted shore platform formed in the Early Pliocene (similar to 4 My ago). Upstream of the knickpoints, rivers possess alluvial reaches still graded to this uplifted platform. They represent relict profiles of the rivers that once drained an island that was surrounded by the now-uplifted platform. We conclude that the knickpoints initiated when the platform started to rise above the Caribbean Sea. The knickpoints then started propagating upstream, defining the front of a slow-moving wave of erosion. Be-10-derived catchment-scale denudation rates measured above and below the knickpoints indicate a 30-210% increase in denudation associated to the passage of the knickpoints. Mineralogical analysis of the soils show that soils upstream of the knickpoints overlie deeply depleted saprolite and only contain recalcitrant minerals, whereas soils downstream of the knickpoints experience an input of fresh, weatherable minerals. This influx considerably increases nutrient availability (Porder et al., 2015) in a forest where atmospheric inputs are otherwise the main source of nutrients (McDowell et al., 1990; Zarin and Johnson, 1995; Pett-Ridge, 2009). (C) 2015 Elsevier Ltd. All rights reserved.

Valletta, RD, Willenbring JK, Lewis AR, Ashworth AC, Caffee M.  2015.  Extreme decay of meteoric beryllium-10 as a proxy for persistent aridity. Scientific Reports. 5:7.   10.1038/srep17813   AbstractWebsite

The modern Antarctic Dry Valleys are locked in a hyper-arid, polar climate that enables the East Antarctic Ice Sheet (EAIS) to remain stable, frozen to underlying bedrock. The duration of these dry, cold conditions is a critical prerequisite when modeling the long-term mass balance of the EAIS during past warm climates and is best examined using terrestrial paleoclimatic proxies. Unfortunately, deposits containing such proxies are extremely rare and often difficult to date. Here, we apply a unique dating approach to tundra deposits using concentrations of meteoric beryllium-10 (Be-10) adhered to paleolake sediments from the Friis Hills, central Dry Valleys. We show that lake sediments were emplaced between 14-17.5 My and have remained untouched by meteoric waters since that time. Our results support the notion that the onset of Dry Valleys aridification occurred -14 My, precluding the possibility of EAIS collapse during Pliocene warming events. Lake fossils indicate that >14 My ago the Dry Valleys hosted a moist tundra that flourished in elevated atmospheric CO2 (>400 ppm). Thus, Dry Valleys tundra deposits record regional climatic transitions that affect EAIS mass balance, and, in a global paleoclimatic context, these deposits demonstrate how warming induced by 400 ppm CO2 manifests at high latitudes.

Carrasco, RM, Pedraza J, Dominguez-Villar D, Willenbring JK, Villa J.  2015.  Sequence and chronology of the Cuerpo de Hombre paleoglacier (Iberian Central System) during the last glacial cycle. Quaternary Science Reviews. 129:163-177.   10.1016/j.quascirev.2015.09.021   AbstractWebsite

The Cuerpo de Hombre paleoglacier occupies the upper sector of the Cuerpo de Hombre river basin, located on the northwest slope of the Sierra de Bejar Mountains (Iberian Central System). At the stage of the maximum ice extent during the last glacial cycle, this paleoglacier was one of the longest tongues emerging from the Sierra de Bejar plateau glacier. The study of the morphostratigraphic succession and the geometric and genetic relations between the geomorphological indicators of this paleoglacier has revealed its evolutionary sequence during the last glacial cycle. The comparison between this sequence and the one previously established by a regional evolutionary pattern shows that although they both coincide in general terms, some stages/substages of this pattern must be corrected or more clearly defined. The absolute chronology of the different stages was obtained using terrestrial cosmogenic nuclides (Be-10). The maximum ice extent of Cuerpo de Hombre paleoglacier has been dated to similar to 25.0 ka (MIS2 and concurrent with the LGM). This chronology coincides with date obtained for other paleoglaciers in the Iberian Central System, but is slightly more modern than the regional chronology estimated as most likely for the maximum ice extent in these areas. Subsequent to reaching the maximum extent, the glacier had a retreat (minimum age similar to 20.6 ka), followed by another stage of expansion or readvance, after which it stabilised until the start of the deglaciation stage (similar to 17.8 ka). In all previous work, the deglaciation stages in the Iberian Central System have been described as one continuous recession process. However, in the Cuerpo de Hombre paleoglacier, all the data point to stabilisations of considerable magnitude, and particularly to another stage of readvance of the glacier. Based on its chronology (minimum age similar to 11.1 ka) and its evolutionary significance, this new readvance has been correlated with the Older Dryas stadia]. Finally, the evolutionary context marked by the indicators and its chronology confirm that the whole of the Late Glacial sequence identified in Cuerpo de Hombre can be correlated with the traditionally established in northern Europe (climatic periods or stadials and interstadials): Oldest Dryas-Balling-Older Dryas-Allerod-Younger Dryas. The minimum ages obtained for the cold periods (stadials) in this sequence in the Cuerpo de Hombre paleoglacier are (respectively): similar to 17.5 ka, similar to 13.9 ka and similar to 11.1 ka. This complete sequence of the Late Glacial Period, including the stadials Older and Younger Dryas, had not been previously identified in any of the paleoglaciers in the Iberian Central System. (C) 2015 Elsevier Ltd. All rights reserved.

Scholl, MA, Shanley JB, Murphy SF, Willenbring JK, Occhi M, Gonzalez G.  2015.  Stable-isotope and solute-chemistry approaches to flow characterization in a forested tropical watershed, Luquillo Mountains, Puerto Rico. Applied Geochemistry. 63:484-497.   10.1016/j.apgeochem.2015.03.008   AbstractWebsite

The prospect of changing climate has led to uncertainty about the resilience of forested mountain watersheds in the tropics. In watersheds where frequent, high rainfall provides ample runoff, we often lack understanding of how the system will respond under conditions of decreased rainfall or drought. Factors that govern water supply, such as recharge rates and groundwater storage capacity, may be poorly quantified. This paper describes 8-year data sets of water stable isotope composition (delta H-2 and delta O-18) of precipitation (4 sites) and a stream (1 site), and four contemporaneous stream sample sets of solute chemistry and isotopes, used to investigate watershed response to precipitation inputs in the 1780-ha Rio Mameyes basin in the Luquillo Mountains of northeastern Puerto Rico. Extreme delta H-2 and delta O-18 values from low-pressure storm systems and the deuterium excess (d-excess) were useful tracers of watershed response in this tropical system. A hydrograph separation experiment performed in June 2011 yielded different but complementary information from stable isotope and solute chemistry data. The hydrograph separation results indicated that 36% of the storm rain that reached the soil surface left the watershed in a very short time as runoff. Weathering-derived solutes indicated near-stream groundwater was displaced into the stream at the beginning of the event, followed by significant dilution. The more biologically active solutes exhibited a net flushing behavior. The d-excess analysis suggested that streamflow typically has a recent rainfall component (similar to 25%) with transit time less than the sampling resolution of 7 days, and a more well-mixed groundwater component (similar to 75%). The contemporaneous stream sample sets showed an overall increase in dissolved solute concentrations with decreasing elevation that may be related to groundwater inputs, different geology, and slope position. A considerable amount of water from rain events runs off as quickflow and bypasses subsurface watershed flow-paths, and better understanding of shallow hillslope and deeper groundwater processes in the watershed will require sub-weekly data and detailed transit time modeling. A combined isotopic and solute chemistry approach can guide further studies to a more comprehensive model of the hydrology, and inform decisions for managing water supply with future changes in climate and land use. Published by Elsevier Ltd.

Wu, L, Ortiz C, Xu Y, Willenbring J, Jerolmack D.  2015.  In situ liquid cell observations of asbestos fiber diffusion in water. Environmental Science & Technology. 49:13340-13349.: American Chemical Society   10.1021/acs.est.5b03839   AbstractWebsite

We present real-time observations of the diffusion of individual asbestos fibers in water. We first scaled up a technique for fluorescent tagging and imaging of chrysotile asbestos fibers and prepared samples with a distribution of fiber lengths ranging from 1 to 20 μm. Experiments were then conducted by placing a 20, 100, or 150 ppm solution of these fibers in a liquid cell mounted on a spinning-disk confocal microscope. Using automated elliptical-particle detection methods, we determined the translation and rotation and two-dimensional (2D) trajectories of thousands of diffusing chrysotile fibers. We find that fiber diffusion is size-dependent and in reasonable agreement with theoretical predictions for the Brownian motion of rods. This agreement is remarkable given that experiments involved non-idealized particles at environmentally relevant concentrations in a confined cell, in which particle–particle and particle–wall interactions might be expected to cause deviations from theory. Experiments also confirmed that highly elongated chrysotile fibers exhibit anisotropic diffusion at short time scales, a predicted effect that may have consequences for aggregate formation and transport of asbestos in confined spaces. The examined fibers vary greatly in their lengths and were prepared from natural chrysotile. Our findings thus indicate that the diffusion rates of a wide range of natural colloidal particles can be predicted from theory, so long as the particle aspect ratio is properly taken into account. This is an important first step for understanding aggregate formation and transport of non-spherical contaminant particles, in the environment and in vivo.

von Blanckenburg, F, Willenbring JK.  2014.  Cosmogenic nuclides: Dates and rates of Earth-surface change. Elements. 10:341-346.   10.2113/gselements.10.5.341   AbstractWebsite

Cosmogenic nuclides are very rare isotopes that are produced when particles generated in supernovas in our galaxy hit the atmosphere and then the Earth's surface. When the rocks and soils in this thin, ever-changing surface layer are bombarded by such cosmic radiation, the nuclide clock begins to tick, thus providing dates and rates of Earth-surface processes. The measurement of cosmogenic nuclides tells us when earthquakes created topography at faults, when changing climate led to the growth of glaciers, how fast rivers grind mountains down, and how fast rocks weather to soil and withdraw atmospheric CO2. The use of cosmogenic nuclides is currently revolutionizing our understanding of Earth-surface processes and has significant implications for many Earth science disciplines.

Stout, JC, Belmont P, Schottler SP, Willenbring JK.  2014.  Identifying sediment sources and sinks in the Root River, southeastern Minnesota. Annals of the Association of American Geographers. 104:20-39.   10.1080/00045608.2013.843434   AbstractWebsite

Excessive loading of fine sediment is a prominent cause of river impairment, not only due to direct effects on biota and habitat but because sediment is often laden with excess nutrients, metals, and toxic substances. Determining the sources and transport pathways of sediment has proven challenging. The Root River watershed in southeastern Minnesota was listed under section 303d of the U.S. Clean Water Act as having forty-three impaired reaches, raising these questions: Where is the fine sediment coming from? What proportions of the sediment are from uplands versus near-channel erosion? How much of the excess sediment loading is caused by modern land use and water management versus the legacy of past land use? Managing fine sediment at the watershed scale requires that we identify potential sources and sinks throughout the watershed, measure source contributions, and understand transport pathways of fine sediment. Here we utilize sediment fingerprinting techniques involving long- and short-lived radionuclide tracers, specifically beryllium-10 (Be-10), excess lead-210 (Pb-210(ex)), and cesium-137 (Cs-137), in combination with other supporting data sets to address the preceding questions. We document a shift in hydrologic regime and that sediment fluxes are sensitive to both magnitude and sequence of flood events. Geomorphic analysis indicates that many river reaches have accessible near-channel sources that contribute the dominant proportion of the washload flux in subwatersheds. Lastly, geochemical tracer analyses of floodplains and hillslope soils indicate that historic erosion has been variable across the landscape and the majority of suspended sediment in the river today is sourced from floodplains and terraces.

Belmont, P, Willenbring JK, Schottler SP, Marquard J, Kumarasamy K, Hemmis JM.  2014.  Toward generalizable sediment fingerprinting with tracers that are conservative and nonconservative over sediment routing timescales. Journal of Soils and Sediments. 14:1479-1492.   10.1007/s11368-014-0913-5   AbstractWebsite

The science of sediment fingerprinting has been evolving rapidly over the past decade and is well poised to improve our understanding, not only of sediment sources, but also the routing of sediment through watersheds. Here, we discuss channel-floodplain processes that may convolute or modify the sediment fingerprinting signature of alluvial bank/floodplain sources and explore the use of nonconservative tracers for differentiating sediment derived from surface soil erosion from that of near-channel fluvial erosion. We use a mathematical model to demonstrate the theoretical effects of channel-floodplain exchange on conservative and nonconservative tracers. Then, we present flow, sediment gauging data, and geochemical measurements of long- (meteoric beryllium-10, Be-10) and short-lived (excess lead-210 and cesium-137, Pb-210(ex) and Cs-137, respectively) radionuclide tracers from two study locations: one above, and the other below, a rapidly incising knick zone within the Maple River watershed, southern Minnesota. We demonstrate that measurements of Be-10, Pb-210(ex), and Cs-137 associated with suspended sediment can be used to distinguish between the three primary sediment sources (agricultural uplands, bluffs, and banks) and estimate channel-floodplain exchange. We observe how the sediment sources systematically vary by location and change over the course of a single storm hydrograph. While sediment dynamics for any given event are not necessarily indicative of longer-term trends, the results are consistent with our geomorphic understanding of the system and longer-term observations of sediment dynamics. We advocate for future sediment fingerprinting studies to develop a geomorphic rationale to explain the distribution of the fingerprinting properties for any given study area, with the intent of developing a more generalizable, process-based fingerprinting approach. We show that measurements of conservative and nonconservative tracers (e.g., long- and short-lived radionuclides) can provide spatially integrated, yet temporally discrete, insights to constrain sediment sources and channel-floodplain exchange at the river network-scale. Fingerprinting that utilizes nonconservative tracers requires that the nonconservative behavior is predictable and verifiable.

von Blanckenburg, F, Willenbring JK.  2014.  Cosmogenic Nuclides: Earth’s Surface Clock [Special issue]. Elements. 10: Mineralogical Society of America Abstract


Granger, DE, Lifton NA, Willenbring JK.  2013.  A cosmic trip: 25 years of cosmogenic nuclides in geology. Geological Society of America Bulletin. 125:1379-1402.   10.1130/b30774.1   AbstractWebsite

Terrestrial cosmogenic nuclides, produced by secondary cosmic-ray interactions in the atmosphere and in situ within minerals in the shallow lithosphere, are widely used to date surface exposure of rocks and sediments, to estimate erosion and weathering rates, and to date sediment deposition or burial. Their use has transformed geomorphology and Quaternary geology, for the first time allowing landforms to be dated and denudation rates to be measured over soil-forming time scales. The application of cosmogenic nuclides to geology began soon after the invention of accelerator mass spectrometry (AMS) in 1977 and increased dramatically with the measurement of in situ-produced nuclides in mineral grains near Earth's surface in the 1980s. The past 25 yr have witnessed the development of cosmogenic nuclides from their initial detection to their prevalence today as a standard geochronological and geochemical tool. This review covers the major developments of the past 25 yr by comparing the state of the field in 1988 with that of today, and by identifying key advances in that period that moved the field forward. We emphasize the most commonly used in situ-produced nuclides measured by AMS for geological applications, but we also discuss other nuclides where their applications overlap. Our review covers AMS instrumentation, cosmogenic nuclide production rates, the methods of surface exposure dating, measurement of erosion and weathering, and burial dating, and meteoric Be-10.

Carrasco, RM, Pedraza J, Dominguez-Villar D, Willenbring JK, Villa J.  2013.  Supraglacial Debris Supply in the Cuerpo de Hombre paleoglacier (Spanish Central System): Reconstruction and Interpretation of a Rock Avalanche Event. Geografiska Annaler Series a-Physical Geography. 95:211-226.   10.1111/geoa.12010   AbstractWebsite

During the deglaciation stages of the last glacial period a rock avalanche took place on the glacier that occupied the upper sector of the Cuerpo de Hombre Valley (Sierra de Bejar). The material displaced during the avalanche fell onto the ice, was transported by the glacier and later deposited as supraglacial ablation till. The cause of the avalanche was the decompression of the valley slopes after they were freed from the glacier ice (stress relaxation). Reconstruction of the ice masses has been carried out to quantify the stress relaxation that produced the collapse. The rock avalanche took place on a lithologically homogeneous slope with a dense fracture network. The avalanche left a 0.4ha scar on the slope with a volume of displaced material of 623 +/- 15x10(3)m(3). The deposit is an accumulation of large, angular, heterometric boulders (1-100m(3) in volume) with a coarse pebble-size matrix. The avalanche can be explained as a relaxation process. This implies rock decompression once the glacier retreat left the wall ice free (debuttressing). Calculations show that the avalanche took place where the decompression stresses were highest (130-170kPa). In the Spanish Central System paleoglaciers the largest accumulation of morainic deposits occurred after the glacial maximum and the earliest stages of the ice retreat. The process described here is used as an example to formulate a hypothesis that the largest accumulations of tills were formed in relation to enhanced slope dynamics once some glacier retreat had occurred.

Willenbring, JK, Codilean AT, McElroy B.  2013.  Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales. Geology. 41:343-346.   10.1130/g33918.1   AbstractWebsite

We use a new compilation of global denudation estimates from cosmogenic nuclides to calculate the apportionment and the sum of all sediment produced on Earth by extrapolation of a statistically significant correlation between denudation rates and basin slopes to watersheds without denudation rate data. This robust relationship can explain approximately half of the variance in denudation from quartz-bearing topography drained by rivers using only mean slopes as the predictive tool and matches a similar fit for large river basins. At slopes >200 m/km, topography controls denudation rates. Controls on denudation in landscapes where average slopes are 10 mm/k.y. We use global topographic data to show that the vast majority of the Earth's surface consists of these gently sloping surfaces with modest, but positive, gross denudation rates, and that these areas contribute the most sediment to the oceans. Because of the links between silicate weathering rates and denudation rates, the predominance of low sloping areas on the Earth's surface compared to areas of steep mountainous topography implies that mountain uplift contributes little to drawdown of CO2 at cosmogenic nuclide time scales of 10(3)-10(6) yr. The poorly understood environmental controls that set the pace of denudation for the largest portion of Earth's surface hold the key to understanding the feedbacks between erosion and climate.

Dominguez-Villar, D, Carrasco RM, Pedraza J, Cheng H, Edwards RL, Willenbring JK.  2013.  Early maximum extent of paleoglaciers from Mediterranean mountains during the last glaciation. Scientific Reports. 3:6.   10.1038/srep02034   AbstractWebsite

Mountain glaciers respond directly to changes in precipitation and temperature, thus their margin extent is a high-sensitivity climate proxy. Here, we present a robust Be-10 chronology for the glacier maximum areal extent of central Spain paleoglaciers dated at 26.1 +/- 1.3 ka BP. These glaciers reached their maximum extent several thousand years earlier than those from central Europe due to the increased precipitation within a cold period between 25 to 29 ka BP, as confirmed by a local speleothem record. These paleoclimate conditions impacted the maximum extent of mountain glaciers along the western and central Mediterranean region. The cause and timing of the enhanced precipitation implies a southward shift of the North Atlantic Polar Front followed by storm tracks in response to changes in insolation via orbital parameters modulation. Thus, these mountain paleoglaciers from the Mediterranean region record an ocean-continent climate interaction triggered by external forcing.

Willenbring, JK, Gasparini NM, Crosby BT, Brocard G.  2013.  What does a mean mean? The temporal evolution of detrital cosmogenic denudation rates in a transient landscape Geology. 41:1215-1218.   10.1130/g34746.1   AbstractWebsite

In equilibrium landscapes, Be-10 concentrations within detrital quartz grains are expected to quantitatively reflect basin-wide denudation rates. In transient landscapes, though detrital quartz is derived from both the incising, adjusting lowland and the unadjusted, relict upland, the integrated Be-10 concentrations still provide a denudation rate averaged across the two domains. Because field samples can provide only a snapshot of the current upstream-averaged erosion rate, we employ a numerical landscape evolution model to explore how Be-10-derived denudation rates vary over time and space during transient adjustment. Model results suggest that the longitudinal pattern of mean denudation rates is generated by the river's progressive dilution of low-volume, high-concentration detritus from relict uplands by the integration of high-volume, low-concentration detritus from adjusting lowlands. The proportion of these materials in any detrital sample depends on what fraction of the upstream area remains unadjusted. Because the boundary of the adjusting part of the landscape changes over time, the longitudinal trend in cosmogenic nuclide-derived erosion rates changes over time. These insights are then used to guide our interpretation of geomorphic and longitudinal cosmogenic nuclide data from the South Fork Eel River (SFER) in the California Coast Range (United States). The northward-propagating crustal thickening and rock uplift associated with the passage of the Mendocino triple junction generates a mobile wave of uplift that progressively sweeps longitudinally down the SFER. The consequences of this forcing can be both replicated in the model environment and observed in the field. The SFER contains transient landforms including knickpoints and river terraces along mainstem and tributary channels that define a clear boundary between an incised, adjusting lowland and an unadjusted, relict upland. We report nine nested, basin-wide denudation rates in the mainstem of the SFER using terrestrial cosmogenic Be-10 in river-borne sediment. We find that denudation rates increase in the downstream direction from similar to 0.2 mm/yr in the upper catchment to similar to 0.5 mm/yr at the outlet. Using comparisons to the modeled landscape, we show that this pattern of denudation rates, paired with the distribution of relict topography throughout the watershed, reflect the immaturity of the landscape's transient adjustment. Later in this modeled transient, the predicted erosion rates decrease downstream before they become uniform. This interpretation of our data has potentially far-reaching implications for quantifying the uplift history and response time of transient landscapes using cosmogenic nuclides.