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

Understanding earth\u27s eroding surface with \u3csup\u3e10\u3c/sup\u3eBe

For more than a century, geologists have sought to measure the distribution of erosion rates on Earth\u27s dynamic surface. Since the mid-1980s, measurements of in situ 10Be, a cosmogenic radionuclide, have been used to estimate outcrop and basin-scale erosion rates at 87 sites around the world. Here, we compile, normalize, and compare published 10Be erosion rate data (n = 1599) in order to understand how, on a globalscale, geologic erosion rates integrated over 103 to 106 years varybetween climate zones, tectonic settings, and different rock types. Drainage basins erodemore quickly (mean = 218 m Myr-1; median = 54 m Myr-1) than outcrops (mean = 12 m Myr-1; median = 5.4 m Myr-1), likely reflecting the acceleration of rock weathering rates under soil. Drainage basin and outcrop erosion rates both vary by climate zone, rock type, and tectonic setting. On the global scale, environmental parameters (latitude, elevation, relief, mean annual precipitation and temperature, seismicity, basin slope and area, and percent basin cover by vegetation) explain erosion rate variation better when they are combined in multiple regression analyses than whenconsidered in bivariate relationships. Drainage basin erosion rates are explained well byconsidering these environmental parameters (R2 = 0.60); mean basin slope is the most powerful regressor. Outcrop erosion rates are less well explained (R2 =0.32), and no one parameter dominates. The variance of erosion rates is better explained when subpopulations of the global data are analyzed. While our compilation is global, thegrouped spatial distribution of cosmogenic studies introduces a bias that will only be addressed by research in under-sampled regions

Assessments of human land use, erosion, and sediment deposition in the Southeastern Australian Tablelands

Humans have interacted with their surroundings for over one million years, and researchers have only recently been able to assess the geomorphic impacts indigenous peoples had on their landscapes prior to the onset of European colonialism. The history of human occupation of Australia is noteworthy in that Aboriginal Australians arrived ~50 ka and remained relatively isolated from the rest of the world until the AD 1788 when Europeans established a permanent settlement in Sydney, New South Wales. The southeastern Australian Tablelands landscape, west of Sydney, has seemingly undergone drastic geomorphic change since European arrival. The introduction of European grazing practices reportedly led to the occurrence of deep erosional incisions, gullies, into valley bottoms and hillslopes, releasing sediment, which is subsequently deposited over downstream wetland environments – swampy meadows. This sediment is often called post-settlement alluvium (PSA); however, the age and genesis of PSA in Australia are debated. Questions regarding the geomorphic features and processes in the Tablelands remain unanswered because few studies quantify the timing of gully incision, PSA deposition, or the pre-human rate of landscape change. Erosion rates inferred from concentrations of in situ 10Be measured in fluvial sediment (n = 11) and bedrock outcrops (n = 6) range from 2.9–11.9 mm/kyr and 5.2–13.8 mm/kyr, respectively. The two sample populations are statistically indistinguishable, suggesting no relief has been generated since 600–110 ka. The overall erosion rate in the Tablelands is 7.5 mm/kyr, equal to long-term denudation rates integrated since ~20 Ma. Aboriginal Australians have been present in the Tablelands for at least 30 kyr, ~12–26% of the cosmogenic integration time, yet widespread Aboriginal fire use did not measurably affect landscape erosion until ~5.5 ka, in sync with increased charcoal in the sediment record. Portable optically stimulated luminescence (OSL) reader data from poly-mineral and poly-grain size samples collected from gully wall profiles of PSA and swampy meadow sediment show that swampy meadow environments were buried by PSA and that PSA is alluvium derived from upstream gully erosion. No relationships between bulk OSL and sample grain size or mineralogy exist, and inferences about bulk sediment mineralogy or grain size cannot be determined from portable OSL reader data. Large variability in adjacent PSA sample replicates, however, reveals incomplete sediment bleaching conditions during PSA deposition during floods. Greater bleaching efficiency is inferred from the small variability of bulk OSL data in the uppermost 10s of cm of PSA profiles. Measured concentrations of meteoric 10Be and bulk OSL in two PSA deposits in Birchams Creek show that initial gully incision eroded into weathered sandstone regolith and not swampy meadow environments, as previously believed. Initial gully incision was shallow (<15 cm) and PSA filled ponds in the lower reaches of the catchment. Continued erosion upstream led to a second depositional episode of PSA before headward gully incision from the mouth of Birchams Creek eroded through PSA deposits. Headward erosion of this gully created the continuous gully present at the site today. Initial gully incision was likely the result of livestock trampling in valley bottoms during droughts, creating localised slopes greater than the critical slope threshold required to erode the valley bottom. OSL burial ages of six PSA deposits collected throughout the Tablelands range from 195.1 ± 17.8 to 90.4 ± 8.9 a, corresponding to AD 1800–1932. The OSL burial ages are younger than European arrival in the Tablelands, and the term, PSA, is redefined as post-European settlement alluvium in Australia, recognising the earlier settlement of the region by Aboriginal Australians whose land use did not lead to PSA deposition. PSA burial ages agree with existing quantitative and anecdotal gully incision data. Contrary to previous assertions that gully incision began asynchronously in the Tablelands, three periods of synchronous gully erosion in localised areas within the Tablelands are recognized: 185 a, 158 a, and 94 a (AD 1828, 1855, and 1919, respectively) – in the southern, northern, and central Goulburn Plains, respectively. The AD 1828 and AD 1919 periods of gully incision correspond to the transition from drought-dominated climate regimes to flood-dominated regimes, and the AD 1855 period of gullying corresponds to a flood-dominated regime. Gully incision in the Tablelands is thus a result of European-introduced grazing practices, which primed the landscape for further erosion and degradation during climatic shifts. PSA deposits in the southeastern Australian Tablelands are some of the most recent examples of anthropogenic sedimentation in human history. The earliest preserved examples of PSA-type sediments are ~8,000 years old and found throughout the world. The establishment of an onset date for the Anthropocene is currently debated, and I believe the oldest PSA and PSA-type sediments around the world can define this modern epoch

Rates of erosion and landscape change along the Blue Ridge escarpment, southern Appalachian Mountains, estimated from in situ cosmogenic 10Be

The Blue Ridge escarpment, located within the southern Appalachian Mountains of Virginia and North Carolina, forms a distinct, steep boundary between the lower-elevation Piedmont and higher-elevation Blue Ridge physiographic provinces. To understand better the rate at which this landform and the adjacent landscape are changing, we measured cosmogenic 10Be in quartz separated from sediment samples (n = 50) collected in thirty-two streams and from three exposed bedrock outcrops along four transects normal to the escarpment, allowing us to calculate erosion rates integrated over 104–105 years. These basin-averaged erosion rates (5.4–49 m My-1) are consistent with those measured elsewhere in the southern Appalachians and show a positive relationship between erosion rate and average basin slope. Erosion rates show no relationship with basin size or relative position of the Brevard fault zone, a fundamental structural element of the region. The cosmogenic isotopic data, when considered along with the distribution of average basin slopes in each physiographic province, suggest that the escarpment is eroding on average more rapidly than the Blue Ridge uplands, which are eroding more rapidly than the Piedmont lowlands. This difference in erosion rates by geomorphic setting suggests that the elevation difference between the uplands and lowlands adjacent to the escarpment is being reduced but at extremely slow rates

Landscape preservation under post‐European settlement alluvium in the south‐eastern Australian tablelands, inferred from portable OSL reader data

Human land‐use changes leading to widespread erosion and gully incision have been well studied, but the effects that erosion and sediment mixing, which accompany the deposition of post‐(European) settlement alluvium (PSA), have in valley bottoms and wetlands receive considerably less attention. PSA overlying pre‐disturbance swampy meadow (SM) wetland sediments is commonly exposed along incised stream channel gully walls throughout the south‐eastern Australian Tablelands, providing an ideal setting in which to assess and understand better how PSA deposition affects valley bottoms and the wetland environments that often occupy them. Portable optically stimulated luminescence (pOSL) reader data were measured on bulk sediment samples from SM‐PSA stratigraphies at 16 locations throughout the south‐eastern Australian Tablelands to assess the effects of erosion and sediment mixing at the SM‐PSA boundary. Trends of pOSL data with depth at each profile were used in conjunction with visual profile descriptions to identify the stratigraphic boundary between SM and PSA sediment and to infer the degree of valley bottom erosion and sediment mixing during PSA deposition. At most sites, SM sediments experienced minimal, if any, disturbance during PSA deposition, and we refer to these as non‐eroded sites. Many sites, however, experienced a significant degree of erosion and sediment mixing – eroded sites – often corresponding to visually diffuse sedimentary boundaries between the two stratigraphic units. Our findings demonstrate that SM landscapes in the Tablelands can be preserved with minimal disturbance under PSA at non‐eroded sites and are preserved beneath a mixing zone at all eroded sites. Copyright © 2016 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134129/1/esp3942.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134129/2/esp3942_am.pd

Characterizing landscape-scale erosion using 10Be in detrital fluvial sediment: Slope-based sampling strategy detects the effect of widespread dams

Concentrations of in situ 10Be measured in detrital fluvial sediment are frequently used to estimate long-term erosion rates of drainage basins. In many regions, basin-averaged erosion rates are positively correlated with basin average slope. The slope dependence of erosion allows model-based erosion rate estimation for unsampled basins and basins where human disturbance may have biased cosmogenic nuclide concentrations in sediment. Using samples collected from southeastern North America, we demonstrate an approach that explicitly considers the relationship between average basin slope and erosion rate. Because dams and reservoirs are ubiquitous on larger channels in the field area, we selected 36 undammed headwater subbasins (average area 10.6 km2) from which we collected river sand samples and measured 10Be concentrations. We used these data to train a predictive model that relates average basin slope and 10Be-inferred erosion rate. Applying our model to 28 basins in the same region previously studied with 10Be, we find that the model successfully predicts erosion rates for basins of different sizes if they are undammed or if samples were collected \u3e25 km downstream of dams. For samples collected closer to dams, measured erosion rates exceed modeled erosion rates for two-thirds of the samples. In three of four cases where paired samples were collected upstream of reservoirs and downstream of the impounding dam, 10Be concentrations were lower downstream. This finding has implications for detrital cosmogenic studies, whether or not samples were collected directly downstream of dams, because dams obstruct most major rivers around the world, effectively trapping sediment that originated upstream

A cosmogenic view of erosion, relief generation, and the age of faulting in southern Africa

Southernmost Africa, with extensive upland geomorphic surfaces, deep canyons, and numerous faults, has long interested geoscientists. A paucity of dates and low rates of background seismicity make it challenging to quantify the pace of landscape change and determine the likelihood and timing of fault movement that could raise and lower parts of the landscape and create associated geohazards. To infer regional rates of denudation, we measured 10Be in river sediment samples and found that south-central South Africa is eroding ∼5 m m.y.-1, a slow erosion rate consistent with those measured in other non-tectonically active areas, including much of southern Africa. To estimate the rate at which extensive, fossil, upland, silcrete-mantled pediment surfaces erode, we measured 10Be and 26Al in exposed quartzite samples. Undeformed upland surfaces are little changed since the Pliocene; some have minimum exposure ages exceeding 2.5 m.y. (median, 1.3 m.y.) and maximum erosion rates of \u3c0.2 m m.y.-1 (median, 0.34 m m.y.-1), consistent with no Quaternary movement on faults that displace the underlying quartzite but not the silcrete cover. We directly dated a recent displacement event on the only recognized Quaternary-active fault in South Africa, a fault that displaces both silcrete and the underlying quartzite. The concentrations of 10Be in exposed fault scarp samples are consistent with a 1.5 m displacement occurring ca. 25 ka. Samples from this offset upland surface have lower minimum limiting exposure ages and higher maximum erosion rates than those from undeformed pediment surfaces, consistent with Pleistocene earthquakes and deformation reducing overall landscape stability proximal to the fault zone. Rates of landscape change on the extensive, stable, silcretized, upland pediment surfaces are an order of magnitude lower than basin-average erosion rates. As isostatic response to regional denudation uplifts the entire landscape at several meters per million years, valleys deepen, isolating stable upland surfaces and creating the spectacular relief for which the region is known

Topographic stress and rock fracture: a two-dimensional numerical model for arbitrary topography and preliminary comparison with borehole observations

Theoretical calculations indicate that elastic stresses induced by surface topography may be large enough in some landscapes to fracture rocks, which in turn could influence slope stability, erosion rates, and bedrock hydrologic properties. These calculations typically have involved idealized topographic profiles, with few direct comparisons of predicted topographic stresses and observed fractures at specific field sites. We use a numerical model to calculate the stresses induced by measured topographic profiles and compare the calculated stress field with fractures observed in shallow boreholes. The model uses a boundary element method to calculate the stress distribution beneath an arbitrary topographic profile in the presence of ambient tectonic stress. When applied to a topographic profile across the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania, the model predicts where shear fractures would occur based on a Mohr–Coulomb criterion, with considerable differences in profiles of stresses with depth beneath ridgetops and valley floors. We calculate the minimum cohesion required to prevent shear failure, C[subscript min], as a proxy for the potential for fracturing or reactivation of existing fractures. We compare depth profiles of C[subscript min] with structural analyses of image logs from four boreholes located on the valley floor, and find that fracture abundance declines sharply with depth in the uppermost 15 m of the bedrock, consistent with the modeled profile of C[subscript min]. In contrast, C[subscript min] increases with depth at comparable depths below ridgetops, suggesting that ridgetop fracture abundance patterns may differ if topographic stresses are indeed important. Thus, the present results are consistent with the hypothesis that topography can influence subsurface rock fracture patterns and provide a basis for further observational tests.United States. Army Research Office (Award W911NF-14-1-0037)United States. Dept. of Energy (Award DE-FG01-97ER14760

A framework of single-session problem solving in elite sport: A longitudinal multi-study investigation

In this six-year, multi-study paper we summarize a new and effective framework of single-session problem-solving developed in an elite sport context at a world leading national institute of sport science and medicine (English Institute of Sport: EIS). In Study 1, we used ethnography (3.5 years) to observe how single-session problem-solving methods were being considered, explored, introduced and developed within the EIS. In Study 2, we used case-study methods split into two parts. A multiple case-study design (10 cases) was employed in Part one to evaluate how the approach was refined into an effective framework of practice. An individual case-study is then illustrated to detail the framework in-action. Collectively, findings realized a framework of single-session problem-solving for use both inside and outside of elite sport that focused on ways to reframe clients’ problems into more ‘solvable’ descriptions. Guidance for psychologists wishing to integrate these problem-solving techniques into their practice are offered