Combining bulk sediment OSL and meteoric 10 Be fingerprinting techniques to identify gully initiation sites and erosion depths

Deep erosional gullies dissect landscapes around the world. Existing erosion models focus on predicting where gullies might begin to erode, but identifying where existing gullies were initiated and under what conditions is difficult, especially when historical records are unavailable. Here we outline a new approach for fingerprinting alluvium and tracing it back to its source by combining bulk sediment optically stimulated luminescence (bulk OSL) and meteoric 10Be (10Bem) measurements made on gully-derived alluvium samples. In doing so, we identify where gully erosion was initiated and infer the conditions under which such erosion occurred. As both 10Bem and bulk OSL data have distinctive depth profiles in different uneroded and depositional settings, we are able to identify the likely incision depths in potential alluvium source areas. We demonstrate our technique at Birchams Creek in the southeastern Australian Tablelands—a well-studied and recent example of gully incision that exemplifies a regional landscape transition from unchanneled swampy meadow wetlands to gully incision and subsequent wetland burial by post-European settlement alluvium. We find that such historic alluvium was derived from a shallow erosion of valley fill upstream of former swampy meadows and was deposited down the center of the valley. Incision likely followed catchment deforestation and the introduction of livestock, which overgrazed and congregated in valley bottoms in the early 20th century during a period of drought. As a result, severe gully erosion was likely initiated in localized, compacted, and oversteepened reaches of the valley bottom

Forecasting the response of Earth\u27s surface to future climatic and land use changes: A review of methods and research needs

In the future, Earth will be warmer, precipitation events will be more extreme, global mean sea level will rise, and many arid and semiarid regions will be drier. Human modifications of landscapes will also occur at an accelerated rate as developed areas increase in size and population density. We now have gridded global forecasts, being continually improved, of the climatic and land use changes (C&LUC) that are likely to occur in the coming decades. However, besides a few exceptions, consensus forecasts do not exist for how these C&LUC will likely impact Earth-surface processes and hazards. In some cases, we have the tools to forecast the geomorphic responses to likely future C&LUC. Fully exploiting these models and utilizing these tools will require close collaboration among Earth-surface scientists and Earth-system modelers. This paper assesses the state-of-the-art tools and data that are being used or could be used to forecast changes in the state of Earth\u27s surface as a result of likely future C&LUC. We also propose strategies for filling key knowledge gaps, emphasizing where additional basic research and/or collaboration across disciplines are necessary. The main body of the paper addresses cross-cutting issues, including the importance of nonlinear/threshold-dominated interactions among topography, vegetation, and sediment transport, as well as the importance of alternate stable states and extreme, rare events for understanding and forecasting Earth-surface response to C&LUC. Five supplements delve into different scales or process zones (global-scale assessments and fluvial, aeolian, glacial/periglacial, and coastal process zones) in detail. © 2015 The Authors. Earth\u27s Future published by Wiley on behalf of the American Geophysical Union

Cosmogenic nuclides indicate that boulder fields are dynamic, ancient, multigenerational features

Boulder fields are found throughout the world; yet, the history of these features, as well as the processes that form them, remain poorly understood. In high and mid-latitudes, boulder fields are thought to form and be active during glacial periods; however, few quantitative data support this assertion. Here, we use in situ cosmogenic 10Be and 26Al to quantify the near-surface history of 52 samples in and around the largest boulder field in North America, Hickory Run, in central Pennsylvania, USA. Boulder surface 10Be concentrations (n = 43) increase downslope, indicate minimum near-surface histories of 70-600 k.y., and are not correlated with lithology or boulder size. Measurements of samples from the top and bottom of one boulder and three underlying clasts as well as 26Al/10Be ratios (n = 25) suggest that at least some boulders have complex exposure histories caused by flipping and/or cover by other rocks, soil, or ice. Cosmogenic nuclide data demonstrate that Hickory Run, and likely other boulder fields, are dynamic features that persist through multiple glacial-interglacial cycles because of boulder resistance to weathering and erosion. Long and complex boulder histories suggest that climatic interpretations based on the presence of these rocky landforms are likely over simplifications

Low rates of bedrock outcrop erosion in the central Appalachian mountains inferred from in situ 10Be

Bedrock outcrops are common on central Appalachian Mountain ridgelines. Because these ridgelines define watersheds, the rate at which they erode influences the pace of landscape evolution. To estimate ridgeline erosion rates, we sampled 72 quartz-bearing outcrops from the Potomac and Susquehanna River Basins and measured in situ–produced 10Be. Ridgeline erosion rates average 9 ± 1 m m.y.−1 (median = 6 m m.y.−1), similar to 10Be-derived rates previously reported for the region. The range of erosion rates we calculated reflects the wide distribution of samples we collected and the likely inclusion of outcrops affected by episodic loss of thick slabs and periglacial activity. Outcrops on main ridgelines erode slower than those on mountainside spur ridges because ridgelines are less likely to be covered by soil, which reduces the production rate of 10Be and increases the erosion rate of rock. Ridgeline outcrops erode slower than drainage basins in the Susquehanna and Potomac River watersheds, suggesting a landscape in disequilibrium. Erosion rates are more similar for outcrops meters to tens of meters apart than those at greater distances, yet semivariogram analysis suggests that outcrop erosion rates in the same physiographic province are similar even though they are hundreds of kilometers apart. This similarity may reflect underlying lithological and/or structural properties common to each physiographic province. Average 10Be-derived outcrop erosion rates are similar to denudation rates determined by other means (sediment flux, fission-track thermochronology, [U-Th]/He dating), indicating that the pace of landscape evolution in the central Appalachian Mountains is slow, and has been since post-Triassic rifting events

Coupling meteoric 10Be with pedogenic losses of 9Be to improve soil residence time estimates on an ancient North American interfluve

We couple meteoric 10Be measurements with mass balance analysis of 9Be to estimate the soil residence time (SRT) of a biogeomorphically stable Ultisol in the Southern Piedmont physiographic region of the southeastern United States. We estimate SRT after correcting the meteoric 10Be inventory to account for observed 9Be losses, which indicate that more than half of the 9Be weathered from primary minerals has been leached from the upper 18.3 m of the Ultisol. Our estimates of minimum SRT range between 1.3–1.4 Ma and between 2.6–3.1 Ma under high and low (2.0 and 1.3 × 106 atoms cm–2 yr–1, respectively), estimates of 10Be delivery. Denudation rates of the physiographic region corroborate our estimates. We redefine pedogenic time constraints in the Southern Piedmont, and demonstrate that the assumption of complete meteoric 10Be retention in acidic soil systems cannot always be made; the latter has far-reaching consequences for soil, sediment, river, and ocean research using meteoric 10Be

Holocene paleostorms identified by particle size signatures in lake sediments from northern eastern United States

The frequency and timing of Holocene paleofloods in the hilly terrain of New Hampshire and Maine are identified usin

Constraining landscape history and glacial erosivity using paired cosmogenic nuclides in Upernavik, northwest Greenland

High-latitude landscape evolution processes have the potential to preserve old, relict surfaces through burial by cold-based, nonerosive glacial ice. To investigate landscape history and age in the high Arctic, we analyzed in situ cosmogenic 10Be and 26Al in 33 rocks from Upernavik, northwest Greenland. We sampled adjacent bedrock-boulder pairs along a 100 km transect at elevations up to 1000 m above sea level. Bedrock samples gave signifi cantly older apparent exposure ages than corresponding boulder samples, and minimum limiting ages increased with elevation. Two-isotope calculations (26Al/10Be) on 20 of the 33 samples yielded minimum limiting exposure durations up to 112 k.y., minimum limiting burial durations up to 900 k.y., and minimum limiting total histories up to 990 k.y. The prevalence of 10Be and 26Al inherited from previous periods of exposure, especially in bedrock samples at high elevation, indicates that these areas record long and complex surface exposure histories, including signifi cant periods of burial with little subglacial erosion. The long total histories suggest that these highelevation surfaces were largely preserved beneath cold-based, nonerosive ice or snowfi elds for at least the latter half of the Quaternary. Because of high concentrations of inherited nuclides, only the six youngest boulder samples appear to record the timing of ice retreat. These six samples suggest deglaciation of the Upernavik coast at 11.3 ± 0.5 ka (average ± 1 standard deviation). There is no difference in deglaciation age along the 100 km sample transect, indicating that the ice-marginal position retreated rapidly at rates of ~120 m yr−1

Basins and bedrock: spatial variation in 10Be erosion rates and increasing relief in the southern Rocky Mountains, USA

We used measurements of cosmogenic 10Be in alluvium to estimate erosion rates on a 103–104 yr time scale for small (0.01–47 km2), unglaciated basins in northern Colorado, southern Wyoming, and adjacent western Nebraska (western United States). Basins formed in Proterozoic cores of Laramide ranges are eroding more slowly (23 ± 7 mm k.y.–1, n = 19) than adjacent basins draining weakly lithifi ed Cenozoic sedimentary rocks (75 ± 36 mm k.y. –1, n = 20). Erosion rates show a relationship to rock resistance and, for granitic rocks, to basin slope, but not to mean annual precipitation. We estimated longer-term (>105 yr time scale) erosion rates for the granitic core of the Front Range by measuring the concentration of 10Be and 26Al produced mainly by muon interactions at depths 1.7–10 m below the surface. Concentrations imply erosion rates of 9–31 mm k.y. –1, similar to shorter-term erosion rates inferred from alluvial sediment. The spatial distribution of erosion rates and stratigraphic evidence imply that relief in the southern Rocky Mountains increased in the late Cenozoic; modern relief probably dates from post-middle Miocene time

Age of the Fjord Stade moraines in the Disko Bugt region, western Greenland, and the 9.3 and 8.2 ka cooling events

Retreat of the western Greenland Ice Sheet during the early Holocene was interrupted by deposition of the Fjord Stade moraine system. The Fjord Stade moraine system spans several hundred kilometers of western Greenland’s ice-free fringe and represents an important period in the western Greenland Ice Sheet’s deglaciation history, but the origin and timing of moraine deposition remain uncertain. Here, we combine new and previously published 10Be and 14C ages from Disko Bugt, western Greenland to constrain the timing of Fjord Stade moraine deposition at two locations w60 km apart. At Jakobshavn Isfjord, the northern of two study sites, we show that Jakobshavn Isbræ advanced to deposit moraines ca 9.2 and 8.2e8.0 ka. In southeastern Disko Bugt, the ice sheet deposited moraines ca 9.4e9.0 and 8.5e8.1 ka. Our ice-margin chronology indicates that the Greenland Ice Sheet in two distant regions responded in unison to early Holocene abrupt cooling 9.3 and 8.2 ka, as recorded in central Greenland ice cores. Although the timing of Fjord Stade moraine deposition was synchronous in Jakobshavn Isfjord and southeastern Disko Bugt, within uncertainties, we suggest that Jakobshavn Isbræ advanced while the southeastern Disko Bugt ice margin experienced stillstands during the 9.3 and 8.2 ka events based on regional geomorphology and the distribution of 10Be ages at each location. The contrasting style of ice-margin response was likely regulated by site-specific ice-flow characteristics. Jakobshavn Isbræ’s high ice flux results in an amplified ice-margin response to a climate perturbation, both warming and cooling, whereas the comparatively low-flux sector of the ice sheet in southeastern Disko Bugt experiences a more subdued response to climate perturbations. Our chronology indicates that the western Greenland Ice Sheet advanced and retreated in concert with early Holocene temperature variations, and the 9.3 and 8.2 ka events, although brief, were of sufficient duration to elicit a significant response of the western Greenland Ice Sheet

Paired bedrock and boulder 10Be concentrations resulting from early Holocene ice retreat near Jakobshavn Isfjord, western Greenland

We measured in situ cosmogenic 10Be in 16 bedrock and 14 boulder samples collected along a 40-km transect outside of and normal to the modern ice margin near Sikuijuitsoq Fjord in central-west Greenland (69°N). We use these data to understand better the efficiency of glacial erosion and to infer the timing, pattern, and rate of ice loss after the last glaciation. In general, the ages of paired bedrock and boulder samples are in close agreement (r2 = 0.72). Eleven of the fourteen paired bedrock and boulder samples are indistinguishable at 1σ; this concordance indicates that subglacial erosion rates are sufficient to remove most or all 10Be accumulated during previous periods of exposure, and that few, if any, nuclides are inherited from pre-Holocene interglaciations. The new data agree well with previously-published landscape chronologies from this area, and suggest that two chronologically-distinct land surfaces exist: one outside the Fjord Stade moraine complex (∼10.3 ± 0.4 ka; n = 7) and another inside (∼8.0 ± 0.7 ka; n = 21). Six 10Be ages from directly outside the historic (Little Ice Age) moraine show that the ice margin first reached its present-day position ∼7.6 ± 0.4 ka. Early Holocene ice margin retreat rates after the deposition of the Fjord Stade moraine complex were ∼100–110 m yr−1. Sikuijuitsoq Fjord is a tributary to the much larger Jakobshavn Isfjord and the deglaciation chronologies of these two fjords are similar. This synchronicity suggests that the ice stream in Jakobshavn Isfjord set the timing and pace of early Holocene deglaciation of the surrounding ice margin