Publications
131 results found
Stirling MW, Oskin ME, Arrowsmith JR, et al., 2021, Evaluation of seismic hazard models with fragile geologic features, Seismological Research Letters, Vol: 92, Pages: 314-324, ISSN: 0895-0695
We provide an overview of a 2019 workshop on the use of fragile geologic features (FGFs) to evaluate seismic hazard models. FGFs have been scarcely utilized in the evaluation of seismic hazard models, despite nearly 30 yr having passed since the first recognition of their potential value. Recently, several studies have begun to focus on the implementation of FGFs in seismic hazard modeling. The workshop was held to capture a “snapshot” of the state‐of‐the‐art in FGF work and to define key research areas that would increase confidence in FGF‐based evaluation of seismic hazard models. It was held at the annual meeting of the Southern California Earthquake Center on 8 September 2019, and the conveners were Mark Stirling (University of Otago, New Zealand) and Michael Oskin (University of California, Davis). The workshop attracted 44 participants from a wide range of disciplines. The main topics of discussion were FGF fragility age estimation (age at which an FGF achieved its current fragile geometry), fragility estimation, FGF‐based evaluation of seismic hazard models, and ethical considerations relating to documentation and preservation of FGFs. There are now many scientists working on, or motivated to work on, FGFs, and more types of FGFs are being worked on than just the precariously balanced rock (PBR) variety. One of the ideas presented at the workshop is that fragility ages for FGFs should be treated stochastically rather than assuming that all share a common age. In a similar vein, new studies propose more comprehensive methods of fragility assessment beyond peak ground acceleration and peak ground velocity‐based approaches. Two recent studies that apply PBRs to evaluate probabilistic seismic hazard models use significantly different methods of evaluation. Key research needs identified from the workshop will guide future, focused efforts that will ultimately facilitate the uptake of FGFs in seismic hazard analysis.
Rood AH, Rood DH, Stirling MW, et al., 2020, Earthquake hazard uncertainties improved using precariously balanced rocks, AGU Advances, Vol: 1, Pages: 1-24, ISSN: 2576-604X
Probabilistic seismic hazard analysis (PSHA) is the state‐of‐the‐art method to estimate ground motions exceeded by large, infrequent, and potentially damaging earthquakes; however, a fundamental problem is the lack of an accepted method for both quantitatively validating and refining the hazard estimates using empirical geological data. In this study, to reduce uncertainties in such hazard estimates, we present a new method that uses empirical data from precariously balanced rocks (PBRs) in coastal Central California. We calculate the probability of toppling of each PBR at defined ground‐motion levels and determine the age at which the PBRs obtained their current fragile geometries using a novel implementation of cosmogenic 10Be exposure dating. By eliminating the PSHA estimates inconsistent with at least a 5% probability of PBR survival, the mean ground‐motion estimate corresponding to the hazard level of 10−4 yr−1 (10,000 yr mean return period) is significantly reduced by 27%, and the range of estimated 5th–95th fractile ground motions is reduced by 49%. Such significant reductions in uncertainties make it possible to more reliably assess the safety and security of critical infrastructure in earthquake‐prone regions worldwide.
Simms AR, Rood DH, Rockwell TK, 2020, Correcting MIS5e and 5a sea-level estimates for tectonic uplift, an example from southern California, Quaternary Science Reviews, Vol: 248, Pages: 1-9, ISSN: 0277-3791
Along tectonically active margins, the difference in elevations between global sea levels during highstands and uplifted marine terraces is a function of both tectonics and glacial-isostatic adjustment (GIA). However, disentangling the relative influence of these two processes remains a challenge for those trying to gain insights into either process. In this study, we outline a strategy for isolating the tectonic contribution to marine isotope stage (MIS) 5e and 5a marine terrace elevations for the southern California coast by determining the cosmogenic radionuclide burial age and elevation of the early Pleistocene (1.48 ± 0.17 Ma) Clairemont Terrace in San Diego. Using this older terrace as a datum for calculating tectonic uplift rate provides a much longer time period to average out uncertainties in past local or relative sea levels (RSL) that arise from ambiguities in GIA parameters and global meltwater volumes. The assumption of constant uplift rates is warranted for this portion of the California coast given its relatively simple tectonic setting on the rift flank of the Salton Trough. From this approach, we determine an average uplift rate of 0.066 ± 0.020 mm/yr or 0.055 ± 0.013 mm/yr, depending on the RSL model used for the time of the Clairemont Terrace formation, for much of the San Diego coastline. Correcting for this tectonic uplift rate leaves an estimate of 15.1 + 2.6/-3.1 m (16.4 + 1.9/-2.6 m) and 4.8 ± 1.9 m (5.6 ± 1.5 m) for RSL during MIS5e and MIS5a, respectively. These new estimates of MIS5e and MIS5a sea levels along the southern California coast provide important constraints on GIA parameters and former ocean and ice volumes.
Johnson JS, Roberts SJ, Rood DH, et al., 2020, Deglaciation of pope glacier implies widespread early holocene ice sheet thinning in the Amundsen sea sector of Antarctica, Earth and Planetary Science Letters, Vol: 548, Pages: 1-13, ISSN: 0012-821X
The Amundsen Sea sector of the Antarctic ice sheet presently dominates the contribution from Antarctica to sea level rise. Several large ice streams that currently drain the sector have experienced rapid flow acceleration, grounding line retreat and thinning during the past few decades. However, little is known of their longer-term – millennial-scale – retreat history, despite the reliance of several ice sheet and glacial-isostatic adjustment models on such data for improving sea level prediction from this critical region. This study investigates the timing and extent of surface lowering of one of those ice streams, Pope Glacier, since the Last Glacial Maximum (LGM), using glacial-geological evidence for former ice cover. We present a new deglacial chronology for the glacier, derived from surface exposure dating of glacially-deposited cobbles and ice-scoured bedrock from Mount Murphy and its surrounding peaks. Cosmogenic 10Be exposure ages from 44 erratic cobbles and 5 bedrock samples, and in situ 14C exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5.5-16 ka. Although 10Be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples, none of the ages pre-date the LGM. From these results we infer that the surface of Pope Glacier lowered by 560 m during the early- to mid-Holocene (9-6 ka), at an average rate of 0.13 ± 0.09/0.04 m yr−1. The lowering coincided with a period of enhanced upwelling of warm Circumpolar Deep Water onto the continental shelf in the region. A reduction in buttressing − facilitated by such upwelling − by an ice shelf that is thought to have spanned the embayment until 10.6 cal kyr BP could have triggered simultaneous early Holocene thinning of Pope Glacier and glaciers elsewhere in the Amundsen Sea Embayment.
Pacheco M, Plattner AM, Stock GM, et al., 2020, Surface exposure dating and geophysical tomography of the royal arches meadow rock avalanche, Yosemite Valley, California, Frontiers in Earth Science, Vol: 8, Pages: 1-12, ISSN: 2296-6463
Since the retreat of glaciers after the Last Glacial Maximum, rock avalanches haveoccurred intermittently in Yosemite Valley, California. We investigated the distal portionof the oldest of these, the Royal Arches Meadow rock avalanche, which has beenpartially buried by sediment aggradation. Cosmogenic 10Be exposure ages of boulderswithin the deposit indicate that the rock avalanche occurred at 16.1 ± 0.3 ka, immediatelyafter deglaciation and thus prior to most aggradation. The interface between the rockavalanche deposit and the underlying glaciofluvial sediments therefore provides anelevation marker of the valley floor at the time of deposition. To identify the elevationof this interface, we collected eight Ground Penetrating Radar (GPR) and five ElectricalResistivity Tomography (ERT) profiles across the rock avalanche. Both methods aresensitive to contrasts between the granitic avalanche deposit and the underlyingsediments. By constraining ERT inversions with GPR interfaces that are continuousacross the profiles, we identified a single interface, interpreted as the basal contact of therock avalanche, that separates resistive material from conductive material underneath.The elevation of this approximately horizontal interface is between 1,206 and 1,209m, roughly 10 m below the modern ground surface, indicating ≈ 10 m of sedimentaggradation since deglaciation. Based on topographic expression and depth to thiscontact, we determined a minimum volume estimate of between 8.1 × 105 m3 and9.7 × 105 m3, nearly three times larger than what would be estimated from surfaceexpression alone. Our findings allow reconstruction of the sedimentation history ofYosemite Valley, inform hazard and risk assessment, and confirm that geophysicalmethods are valuable tools for three-dimensional investigations of rock avalanches,particularly those buried by younger sediments.
Swirad ZM, Rosser NJ, Brain MJ, et al., 2020, Publisher Correction: Cosmogenic exposure dating reveals limited long-term variability in erosion of a rocky coastline., Nat Commun, Vol: 11
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
Swirad ZM, Rosser NJ, Brain MJ, et al., 2020, Cosmogenic exposure dating reveals limited long-term variability in erosion of a rocky coastline., Nature Communications, Vol: 11, Pages: 3804-3804, ISSN: 2041-1723
Predicted sea-level rise and increased storminess are anticipated to lead to increases in coastal erosion. However, assessing if and how rocky coasts will respond to changes in marine conditions is difficult due to current limitations of monitoring and modelling. Here, we measured cosmogenic 10Be concentrations across a sandstone shore platform in North Yorkshire, UK, to model the changes in coastal erosion within the last 7 kyr and for the first time quantify the relative long-term eros0ive contribution of landward cliff retreat, and down-wearing and stripping of rock from the shore platform. The results suggest that the cliff has been retreating at a steady rate of 4.5 ± 0.63 cm yr-1, whilst maintaining a similar profile form. Our results imply a lack of a direct relationship between relative sea level over centennial to millennial timescales and the erosion response of the coast, highlighting a need to more fully characterise the spatial variability in, and controls on, rocky coast erosion under changing conditions.
Hughes A, Bell RE, Mildon ZK, et al., 2020, Three‐dimensional structure, ground rupture hazards, and static stress models for complex non‐planar thrust faults in the Ventura basin, southern California, Journal of Geophysical Research: Solid Earth, Vol: 125, ISSN: 2169-9313
To investigate the subsurface geometry of a recently discovered, seismically‐active fault in the Ventura basin, southern California, USA, we present a series of cross sections and a new three‐dimensional fault model across the Southern San Cayetano fault (SSCF) based on integration of surface data with petroleum industry well‐log data. Additionally, the fault model for the SSCF, along with models of other regional faults extracted from the Southern California Earthquake Center three‐dimensional Community Fault Model, are incorporated in static Coulomb stress modeling to investigate static Coulomb stress transfer between thrust faults with complex geometry and to further our understanding of stress transfer in the Ventura basin. The results of the subsurface well investigation provide evidence for a low‐angle SSCF that dips ~15° north and connects with the western section of the San Cayetano fault around 1.5–3.5 km depth. We interpret the results of static Coulomb stress models to partly explain contrasting geomorphic expression between different sections of the San Cayetano fault and a potential mismatch in timings between large‐magnitude uplift events suggested by paleoseismic studies on the Pitas Point, Ventura, and San Cayetano faults. In addition to new insights into the structure and potential rupture hazard of a recently discovered active reverse fault in a highly populated area of southern California, this study provides a simple method to model static Coulomb stress transfer on complex geometry faults in fold and thrust belts.
Regalla C, Bierman P, Rood DH, 2019, Meteoric Be-10 reveals a young, active accretionary prism and structurally complex decollement in the vicinity of the 2011 Tohoku Earthquake rupture, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 20, Pages: 4956-4971, ISSN: 1525-2027
We present new meteoric 10Be concentration data from marine sediment cores recovered during Integrated Ocean Drilling Program Expedition 343 (JFAST) that help constrain the age, origin, and internal structure of the frontal prism at the Japan trench in the vicinity of the 2011 Tohoku‐oki M9 earthquake rupture. 10Be sediment ages from the lower portion of the frontal prism range from ~0–10 Ma, with >60% of analyzed samples above the décollement yielding young ages <2 Ma. Repetition and inversion of high‐over‐low 10Be‐concentration sediments indicate the presence of stratigraphic inversions that correspond to faulting and imbrication of late Miocene and Quaternary sediments. The density of faults inferred indicates that the frontal prism has a fault spacing on the order of 10 s of meters, and the identification of faults in the underthrust section suggests that the plate boundary décollement may be a zone with multiple slip surfaces. Comparison of 10Be concentrations in the frontal prism with those of the incoming and forearc slope sediments indicates that the majority of the prism is sourced from accretion of Pacific Plate sediments, rather than from reworked frontal prism or slope sediments. These data suggest that over at least the past ~1–2 Ma, the décollement preferentially has localized at or near the base of the incoming sediment section, with relatively efficient sediment accretion occurring even in the presence of subducted horst‐and‐graben topography.
McCarthy JA, Schoenbohm LM, Bierman PR, et al., 2019, Late quaternary tectonics, incision, and landscape evolution of the calchaqui river catchment, Eastern Cordillera, NW Argentina, Journal of Geophysical Research: Earth Surface, Vol: 124, Pages: 2265-2287, ISSN: 2169-9011
Unraveling the relative impacts of climate, tectonics, and lithology on landscape evolution is complicated by the temporal and spatial scale over which observations are made. We use soil and desert pavement classification, longitudinal river profiles, 10Be‐derived catchment mean modern and paleo‐erosion rates, and vertical incision rates to test whether, if we restrict our analyses to a spatial scale over which climate is relatively invariant, tectonic and lithologic factors will dominate the late Quaternary landscape evolution of the Calchaquí River Catchment, NW Argentina. We find that the spatial distribution of erosion rates, normalized channel steepness indices, and concavity indices reflect active tectonics and lithologic resistance. Knickpoints are spatially coincident with tectonic and/or lithologic discontinuities, indicating local base‐level control by faulting. Catchment mean erosion rates, ranging from 22.5 ± 2.6 to 121.9 ± 13.7 mm/kyr, and paleo‐erosion rates, ranging from 56 +43/‐19 to 105 +60/‐33 mm/kyr, are similar, possibly suggesting that Quaternary climate changes have not had a strong enough influence on erosion rates to be detected using cosmogenic 10Be. However, punctuated abandonment of pediment and strath terraces at 43.6 +15.0/‐11.6, 91.2 +54.2/‐22.2, and 151 +92.7/‐34.1 ka and disparities between vertical incision rates and catchment mean erosion rates could suggest periods of landscape transience, possibly reflecting climate cyclicity. Our results emphasize the role of tectonic uplift and lithologic contrasts in shaping long‐term erosion rates and channel morphology at the relatively local scale of the Calchaqui River Catchment, in contrast to regional‐scale studies which find precipitation to exert the dominant control.
Portenga EW, Bierman PR, Trodick CD, et al., 2019, Erosion rates and sediment flux within the Potomac River basin quantified over millennial timescales using beryllium isotopes, Geological Society of America Bulletin, Vol: 131, Pages: 1295-1311, ISSN: 0016-7606
Beryllium isotopes measured in detrital river sediment are often used to estimate rates of landscape change at a basin scale, but results from different beryllium isotope systems have rarely been compared. Here, we report measurements of in situ and meteoric 10Be (10Bei and 10Bem, respectively) along with measurements of reactive and mineral phases of 9Be (9Bereac and 9Bemin, respectively) to infer long-term rates of landscape change in the Potomac River basin, North America. Using these data, we directly compare results from the two different 10Be isotope systems and contextualize modern sediment flux from the Potomac River basin to Chesapeake Bay.Sixty-two measurements of 10Bei in river sand show that the Potomac River basin is eroding on average at 29.6 ± 14.1 Mg km–2 yr–1 (11 ± 5.2 m m.y.–1 assuming a rock density of 2700 kg m–3)—a rate consistent with other estimates in the mid-Atlantic region. 10Bei erosion rates correlate with basin latitude, suggesting that periglacial weathering increased with proximity to the former Laurentide Ice Sheet margin. Considering the 10Bei-derived erosion rate as a sediment flux over millennia, rates of sediment delivery from the Potomac River to Chesapeake Bay are up to ∼5× lower than contemporary sediment yields implying modern land-use practices have accelerated erosion and sediment transport over background rates. However, 10Bei erosion rate data suggest that regulatory benchmark levels used to manage sediment export from the Potomac River basin to Chesapeake Bay are set appropriately to reduce sedimentation and restore the Bay’s ecological health.The mean of 56 10Bem/9Bereac-derived denudation rates (40.0 ± 21.7 Mg km–2 yr–1) is higher than, but statistically indistinguishable from, the mean 10Bei erosion rate (29.6 ± 14.1 Mg km–2 yr–1; p = 0.003). However, when considered basin by basin, 10Bem/9Bereac-determined denudation rate
Stucky de Quay G, Roberts GG, Rood DH, et al., 2019, Holocene uplift and rapid fluvial erosion of Iceland: a record of post-glacial landscape evolution, Earth and Planetary Science Letters, Vol: 505, Pages: 118-130, ISSN: 0012-821X
In actively deforming regions fluvial systems are strongly regulated by uplift. River geometries record histories of vertical motions that can be used to examine the driving forces generating topographic relief. Iceland's rapidly evolving landscapes provide an opportunity to disentangle histories of uplift generated by postglacial rebound, volcanism, dynamic support, and plate spreading. Broad knickzones observed along Iceland's large rivers, and its powerful waterfalls and deep canyons, hint that regional processes have generated significant relief. We combine high-resolution drone photogrammetry and cosmogenic 3He dating of fluvial terraces to measure the erosional history of one of Iceland's largest knickzones, Jökulsárglúfur, in the northeast part of the island. Progressive younging of terraces indicates knickpoint propagation rates of up to ∼70 cm a−1 during the last 8 ka. Knickpoint velocities appear to be controlled partly by toppling of basalt columns. These rates were used to calibrate a model that inverts Iceland's drainage networks for uplift rate histories. Calculated uplift and isostatic calculations indicate that rifting, sub-plate support, and isostatic adjustment resulted in tens to hundreds of meters of regional Holocene uplift. Our results suggest regional uplift and fluvial erosion can rapidly generate hundreds of meters of relief in post-glacial landscapes.
Hughes A, Rood DH, Whittaker AC, et al., 2018, Geomorphic evidence for the geometry and slip rate of a young, low-angle thrust fault: Implications for hazard assessment and fault interaction in complex tectonic environments, Earth and Planetary Science Letters, Vol: 504, Pages: 198-210, ISSN: 0012-821X
We present surface evidence and displacement rates for a young, active, low-angle (∼20°) reverse thrust fault in close proximity to major population centers in southern California (USA), the Southern San Cayetano fault (SSCF). Active faulting along the northern flank of the Santa Clara River Valley displaces young landforms, such as late Quaternary river terraces and alluvial fans. Geomorphic strain markers are examined using field mapping, high-resolution lidar topographic data, 10Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over 103–104 yr timescales with maximum slip rates for the central SSCF of 1.9[Formula presented] mm yr−1 between ∼19–7 ka and minimum slip rates of 1.3[Formula presented] mm yr−1 since ∼7 ka. Uplift rates for the central SSCF have not varied significantly over the last ∼58 ka, with a maximum value of 1.7[Formula presented] mm yr−1 for the interval ∼58–19 ka, and a minimum value of 1.2±0.3 mm yr−1 since ∼7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some time after ∼58 ka. The geometry for the SSCF presented here, with a ∼20° north dip in the subsurface, is the first interpretation of the SSCF based on geological field data. Our new interpretation is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45–60° and intersects the surface in the middle of the Santa Clara River Valley. We suggest that the SSCF may rupture in tandem with the main San Cayetano fault. Additionally, the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multi-fault earthquakes in southern California. However, given structural complexities, including significant changes
Rainsley E, Menviel L, Fogwill CJ, et al., 2018, Greenland ice mass loss during the Younger Dryas driven by Atlantic Meridional Overturning Circulation feedbacks, SCIENTIFIC REPORTS, Vol: 8, ISSN: 2045-2322
Understanding feedbacks between the Greenland Ice Sheet (GrIS) and the Atlantic Meridional Overturning Circulation (AMOC) is crucial for reducing uncertainties over future sea level and ocean circulation change. Reconstructing past GrIS dynamics can extend the observational record and elucidate mechanisms that operate on multi-decadal timescales. We report a highly-constrained last glacial vertical profile of cosmogenic isotope exposure ages from Sermilik Fjord, a marine-terminating ice stream in the southeast sector of the GrIS. Our reconstruction reveals substantial ice-mass loss throughout the Younger Dryas (12.9-11.7 ka), a period of marked atmospheric and sea-surface cooling. Earth-system modelling reveals that southern GrIS marginal melt was likely driven by strengthening of the Irminger Current at depth due to a weakening of the AMOC during the Younger Dryas. This change in North Atlantic circulation appears to have drawn warm subsurface waters to southeast Greenland despite markedly cooler sea surface temperatures, enhancing thermal erosion at the grounding lines of palaeo ice-streams, supporting interpretation of regional marine-sediment cores. Given current rates of GrIS meltwater input into the North Atlantic and the vulnerability of major ice streams to water temperature changes at the grounding line, this mechanism has important implications for future AMOC changes and northern hemisphere heat transport.
Bierman PR, Rood DH, Shakun JD, et al., 2018, Directly dating postglacial Greenlandic land-surface emergence at high resolution using in situ 10Be, Quaternary Research, Vol: 90, Pages: 110-126, ISSN: 0033-5894
Postglacial emergence curves are used to infer mantle rheology, delimit ice extent, and test models of the solid Earth response to changing ice and water loads. Such curves are rarely produced by direct dating of land emergence; rather, most rely on the presence of radiocarbon-datable organic material and inferences made between the age of sedimentary deposits and landforms indicative of former sea level. Here, we demonstrate a new approach, 10Be dating, to determine rates of postglacial land emergence in two different settings. In southern Greenland (Narsarsuaq/Igaliku), we date directly the exposure, as relative sea level fell, of gravel beaches and rocky outcrops allowing determination of rapid, post–Younger Dryas emergence. In western Greenland (Kangerlussuaq), we constrain Holocene isostatic response by dating the sequential stripping of terrace sediment driven by land-surface uplift, relative sea-level fall, and resulting fluvial incision. The technique we employ provides high temporal and elevation resolution important for quantifying rapid emergence immediately after deglaciation and less rapid uplift during the middle Holocene. 10Be-constrained emergence curves can improve knowledge of relative sea-level change by dating land emergence along rocky coasts, at elevations and locations where radiocarbon-datable sediments are not present, and without the lag time needed for organic material to accumulate.
Schmidt AH, Gonzalez VS, Bierman PR, et al., 2018, Agricultural land use doubled sediment loads in western China's rivers, Anthropocene, Vol: 21, Pages: 95-106, ISSN: 2213-3054
Land use changes, such as deforestation and agricultural expansion, increase soil erosion on the scale of hillslopes and small drainage basins. However, the effects of these changes on the sediment load in rivers is poorly quantified, with a few studies scattered globally, and only 10 data points in the world's most populous nation, China. At 20 different sites in western China, we compare contemporary fluvial sediment yield data collected daily over 4 to 26 years between 1945 and 1987 (median=19years) to long-term measures of sediment generation based on new isotopic measurements of in situ 10 Be (beryllium-10) in river sediments. We find that median sediment yield at these sites exceeds background sediment generation rates by a factor of two (from 0.13 to 5.79 times, median 1.85 times) and that contemporary sediment yield is statistically significantly different from long-term sediment generation rates (p < 0.05). Agricultural land use is directly and significantly proportional to the ratio of contemporary sediment yield to long term sediment generation rates (Spearman correlation coefficient rho=0.52, p < 0.05). We support these findings by calculating erosion indices, which compare the delivery of meteoric 10 Be to each watershed with the export of meteoric 10 Be bound to riverine sediment. Erosion indices are also directly and significantly proportional to agricultural land use (rho=0.58, p < 0.05). Together, these data sets suggest that upstream agricultural land use has significantly increased sediment supply to rivers in western China, likely increasing turbidity and decreasing ecosystem services such as fisheries.
Neilson TB, Schmidt AH, Bierman PR, et al., 2017, Efficacy of in situ and meteoric ¹⁰Be mixing in fluvial sediment collected from small catchments in China, Chemical Geology, Vol: 471, Pages: 119-130, ISSN: 0009-2541
Using measurements of in situ and meteoric 10Be in fluvial sand to measure erosion rates, quantify soil loss, and trace sediment sources and sinks relies on the assumption that such sediment is well-mixed and representative of the upstream area. We test this assumption at 13 river junctions in three tributary watersheds (200–2500 km2) to the Mekong River, Yunnan, China, where human alteration of the landscape is significant and widespread.We find that two of the three watersheds mix well for in situ 10Be and none mix well for meteoric 10Be when considering the concentration of 10Be at the outlet compared to the area-weighted mean of headwater samples. We also assessed mixing at 13 river junctions by comparing the erosion rate-weighted isotopic concentration of sediment taken from tributaries upstream of a junction to the concentration in a sample taken downstream of the junction. With this metric, mixing is generally poor for both in situ and meteoric 10Be but is better for in situ 10Be than for meteoric 10Be (p < 0.05). This is likely because in situ 10Be is measured in quartz, which is resilient to physical and chemical breakdown in river systems whereas meteoric 10Be is measured in grain coatings which can abrade and dissolve.Basins eroding faster (> 100 mm/kyr) tend to mix better than slowly eroding basins. We find no evidence that agricultural land use in sampled basins affects sediment mixing downstream. Mixing improves with increased basin area (particularly > 200 km2), increased sampling distance downstream from an upstream junction (> 500 m), and increased difference in size between tributaries (one tributary > 3 times larger than the other). The most important factor affecting mixing efficacy for both in situ and meteoric 10Be is the fraction of the basin area contributing to the downstream sample that does not contribute to the upstream samples. Junctions with > 2% of the basin area unsampled by upstream samples tend not to mix as
Sosa Gonzalez V, Schmidt AH, Bierman PR, et al., 2017, Spatial and temporal replicability of meteoric and in situ ¹⁰Be concentrations in fluvial sediment, Earth Surface Processes and Landforms, Vol: 42, Pages: 2570-2584, ISSN: 0197-9337
Cosmogenic isotopes, short-lived radionuclides, elemental concentrations and thermochronometric indicators are measured in river sand to quantify erosion rates and trace sediment sources, and/or infer erosional processes. Interpretations of detrital sediment analyses are often based on the rarely tested assumption of time-invariant tracer concentration. A better understanding of when and where this assumption breaks down and what sampling strategies minimize temporal and small-scale spatial variance will improve science done using detrital river sediment. Here, we present new and previously published spatial and temporal replicates measured for in situ and meteoric 10 Be ( 10 Be i and 10 Be m , respectively). Our new data include 113 replicate pairs, taken from agricultural and/or tectonically active watersheds in China months to millennia apart and spatial replicates taken up to 2 km apart on the same day. The mean percentage difference is 10% (-122% to 150%) for both systems considered together; the mode is close to 0% for both systems; and 36% of pairs of samples replicate within our analytical accuracy at 2σ. We find that 10 Be i replicates better than 10 Be m (p < 0.01). 10 Be i replicability is worse in steeper basins, suggesting that stochastic processes (i.e. landslides) affect reproducibility. 10 Be m replicability is worse in larger basins, suggesting non-conservative behavior of 10 Be m as sediment moves downstream. Our results are consistent with the few previously published replicate studies. Considering all replicate data in a wide range of landscapes, in areas with deep erosional processes, replicability is poor; in other areas, replicability is good. This suggests that, in steep, tectonically active, and/or agricultural landscapes, individual detrital sediment measurements do not represent upstream rates as well as larger populations of samples. To ensure that measurements are representative of the upstream watershed, our data
Koffman TNB, Schaefer JM, Putnam AE, et al., 2017, A beryllium-10 chronology of late-glacial moraines in the upper Rakaia valley, Southern Alps, New Zealand supports Southern-Hemisphere warming during the Younger Dryas, Quaternary Science Reviews, Vol: 170, Pages: 14-25, ISSN: 0277-3791
Interhemispheric differences in the timing of pauses or reversals in the temperature rise at the end of the last ice age can help to clarify the mechanisms that influence glacial terminations. Our beryllium-10 (10Be) surface-exposure chronology for the moraines of the upper Rakaia valley of New Zealand's Southern Alps, combined with glaciological modeling, show that late-glacial temperature change in the atmosphere over the Southern Alps exhibited an Antarctic-like pattern. During the Antarctic Cold Reversal, the upper Rakaia glacier built two well-defined, closely-spaced moraines on Reischek knob at 13,900 ± 120 [1σ; ± 310 yrs when including a 2.1% production-rate (PR) uncertainty] and 13,140 ± 250 (±370) yrs ago, in positions consistent with mean annual temperature approximately 2 °C cooler than modern values. The formation of distinct, widely-spaced moraines at 12,140 ± 200 (±320) and 11,620 ± 160 (±290) yrs ago on Meins Knob, 2 km up-valley from the Reischek knob moraines, indicates that the glacier thinned by ∼250 m during Heinrich Stadial 0 (HS 0, coeval with the Younger Dryas 12,900 to 11,600 yrs ago). The glacier-inferred temperature rise in the upper Rakaia valley during HS 0 was about 1 °C. Because a similar pattern is documented by well-dated glacial geomorphologic records from the Andes of South America, the implication is that this late-glacial atmospheric climate signal extended from 79°S north to at least 36°S, and thus was a major feature of Southern Hemisphere paleoclimate during the last glacial termination.
Reusser LJ, Bierman PR, Rizzo DM, et al., 2017, Characterizing landscape-scale erosion using Be-10 in detrital fluvial sediment: Slope-based sampling strategy detects the effect of widespread dams, WATER RESOURCES RESEARCH, Vol: 53, Pages: 4476-4486, ISSN: 0043-1397
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 >25 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.
Portenga EW, Bishop P, Rood DH, et al., 2017, Combining bulk sediment OSL and meteoric Be-10 fingerprinting techniques to identify gully initiation sites and erosion depths, Journal of Geophysical Research. Earth Surface, Vol: 122, Pages: 513-527, ISSN: 2169-9003
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.
Corbett LB, Bierman PR, Rood DH, et al., 2017, Cosmogenic 26Al/10Be surface production ratio in Greenland, GEOPHYSICAL RESEARCH LETTERS, Vol: 44, Pages: 1350-1359, ISSN: 0094-8276
The assumed value for the cosmogenic 26Al/10Be surface production rate ratio in quartz is an important parameter for studies investigating the burial or subaerial erosion of long-lived surfaces and sediments. Recent models and data suggest that the production ratio is spatially variable and may be greater than originally thought. Here we present measured 26Al/10Be ratios for 24 continuously exposed bedrock and boulder surfaces spanning ~61–77°N in Greenland. Empirical measurements, such as ours, include nuclides produced predominately by neutron-induced spallation with percent-level contributions by muon interactions. The slope of a York regression line fit to our data is 7.3 ± 0.3 (1σ), suggesting that the 26Al/10Be surface production ratio exceeds the commonly used value of 6.75, at least in the Arctic. A higher 26Al/10Be production ratio has implications for multinuclide cosmogenic isotope studies because it results in greater modeled burial durations and erosion rates.
Portenga EW, Bierman PR, Trodick CD, et al., 2017, Background rates of erosion and sediment generation in the potomac river basin, USA, derived using in situ 10 Be, meteoric 10 Be, and 9 Be, Bulletin of the Geological Society of America, Vol: 129, Pages: 1-15, ISSN: 0016-7606
Beryllium isotopes are often used to estimate rates of landscape change, but results from different beryllium isotope systems have rarely been compared. Here, we combine measurements of in situ and meteoric 10 Be ( 10 Be i and 10 Be m , respectively) with the reactive and mineral phases of 9 Be ( 9 Be reac and 9 Be min , respectively) to elucidate shortand long-term rates of erosion and sediment transport in the Potomac River basin on the North American passive margin. Sixty-two measurements of 10 Be i in alluvium show that the Potomac watershed is eroding on average at 11 m m.y. -1 (~30 Mg km -2 yr -1 ), which is consistent with regional erosion rate estimates. The 10 Be i erosion rates correlate with basin latitude, suggesting that periglacial weathering increased proximal to the Laurentide ice sheet. The average of 55 10 B em / 9 Be reac -derived sediment generation rates (26.2 ± 18.3 Mg km -2 yr -1 ) is indistinguishable from the average of 62 10 Be i rates; however, 10 Be m / 9 Be reac - and 10 Be i -based sediment generation rates are uncorrelated for individual basins. The lack of correlation on a basin-by-basin basis suggests biogeochemical assumptions inherent to the 10 Be m / 9 Be reac technique are not valid everywhere. Contemporary sediment yields (n = 10) are up to 10 times greater than 10 Be i -or 10 Be m -derived sediment generation rates. However, we find that benchmark levels set to manage sediment export into Chesapeake Bay are within the uncertainty of long-term sediment generation rates. Erosion indices derived from 10 Be m measurements range from 0.07 to 1.24, signifying that sediment retention occurs throughout the basin, except in the Appalachian Plateau. Paleo-erosion indices, calculated from the 150 k.y. Hybla Valley sediment core, suggest sediment excavation and storage under colder and warmer climate conditions, respectively.
Hurst MD, Rood DH, Ellis MA, 2017, Controls on the distribution of cosmogenic 10Be across shore platforms, Earth Surface Dynamics, Vol: 5, Pages: 67-84, ISSN: 2196-6311
Quantifying rates of erosion on cliffed coasts across a range of timescales is vital for understanding the drivers and processes of coastal change and for assessing risks posed by future cliff retreat. Historical records cover at best the last 150 years; cosmogenic isotopes, such as 10Be could allow us to look further into the past to assess coastal change on millennial timescales. Cosmogenic isotopes accumulate in situ near the Earth surface and have been used extensively to quantify erosion rates, burial dates and surface exposure ages in terrestrial landscapes over the last 3 decades. More recently, applications in rocky coast settings have quantified the timing of mass wasting events, determined long-term averaged rates of cliff retreat and revealed the exposure history of shore platforms. In this contribution, we develop and explore a numerical model for the accumulation of 10Be on eroding shore platforms. In a series of numerical experiments, we investigated the influence of topographic and water shielding, dynamic platform erosion processes, the presence and variation in beach cover, and heterogeneous distribution of erosion on the distribution of 10Be across shore platforms. Results demonstrate that, taking into account relative sea level change and tides, the concentration of 10Be is sensitive to rates of cliff retreat. Factors such as topographic shielding and beach cover act to reduce 10Be concentrations on the platform and may result in overestimation of cliff retreat rates if not accounted for. The shape of the distribution of 10Be across a shore platform can potentially reveal whether cliff retreat rates are declining or accelerating through time. Measurement of 10Be in shore platforms has great potential to allow us to quantify long-term rates of cliff retreat and platform erosion.
Bierman PR, Shakun JD, Corbett LB, et al., 2016, A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years, Nature, Vol: 540, Pages: 256-260, ISSN: 0028-0836
Climate models show that ice-sheet melt will dominate sea-level rise over the coming centuries, but our understanding of ice-sheet variations before the last interglacial 125,000 years ago remains fragmentary. This is because terrestrial deposits of ancient glacial and interglacial periods1, 2, 3 are overrun and eroded by more recent glacial advances, and are therefore usually rare, isolated and poorly dated4. In contrast, material shed almost continuously from continents is preserved as marine sediment that can be analysed to infer the time-varying state of major ice sheets. Here we show that the East Greenland Ice Sheet existed over the past 7.5 million years, as indicated by beryllium and aluminium isotopes (10Be and 26Al) in quartz sand removed by deep, ongoing glacial erosion on land and deposited offshore in the marine sedimentary record5, 6. During the early Pleistocene epoch, ice cover in East Greenland was dynamic; in contrast, East Greenland was mostly ice-covered during the mid-to-late Pleistocene. The isotope record we present is consistent with distinct signatures of changes in ice sheet behaviour coincident with major climate transitions. Although our data are continuous, they are from low-deposition-rate sites and sourced only from East Greenland. Consequently, the signal of extensive deglaciation during short, intense interglacials could be missed or blurred, and we cannot distinguish between a remnant ice sheet in the East Greenland highlands and a diminished continent-wide ice sheet. A clearer constraint on the behaviour of the ice sheet during past and, ultimately, future interglacial warmth could be produced by 10Be and 26Al records from a coring site with a higher deposition rate. Nonetheless, our analysis challenges the possibility of complete and extended deglaciation over the past several million years.
Middleton TA, Walker RT, Rood DH, et al., 2016, The tectonics of the western Ordos Plateau, Ningxia, China: Slip rates on the Luoshan and East Helanshan Faults, Tectonics, Vol: 35, Pages: 2754-2777, ISSN: 0278-7407
Analysis of the locus, style, and rate of faulting is fundamental to understanding thekinematics of continental deformation. The Ordos Plateau lies to the northeast of Tibet, within theIndia-Eurasia collision zone. Previous studies have suggested that it behaves rigidly and rotatesanticlockwise within a large-scale zone of ENE-WSW left-lateral shearing. For this rotation to beaccommodated, the eastern and western margins of the Ordos Plateau should be undergoing right-lateralshearing and yet the dominant faulting style appears to be extensional. We focus specifically on thekinematics of the faults bounding the western margin of the Ordos Plateau and make new slip rateestimates for two of the major faults in the region: the right-lateral strike-slip Luoshan Fault and thenormal-slip East Helanshan Fault. We use a combination of infrared stimulated luminescence dating ofoffset landforms with high-resolution imagery and topography from the Pleiades satellites to determinean average right-lateral slip rate of 4.3 ± 0.4 mm/a (1 uncertainty) on the Luoshan Fault. Similarly,we use10Be exposure dating to determine a vertical throw rate on the East Helanshan Fault of<0.6 ± 0.1 mm/a, corresponding to an extension rate of <0.7 ± 0.1 mm/a (1 uncertainty). Both ofthese results agree well with slip rates determined from the latest campaign GPS data. We thereforeconclude that right-lateral shearing is the dominant motion occurring in the western Ordos region,supporting a kinematic model of large-scale anticlockwise rotation of the whole Ordos Plateau.
Hurst MD, Rood DH, Ellis MA, et al., 2016, Recent acceleration in coastal cliff retreat rates on the south coast of Great Britain, Proceedings of the National Academy of Sciences of the United States of America, Vol: 113, Pages: 13336-13341, ISSN: 1091-6490
Rising sea levels and increased storminess are expected to accelerate the erosion of soft-cliff coastlines, threatening coastal infrastructure and livelihoods. To develop predictive models of future coastal change we need fundamentally to know how rapidly coasts have been eroding in the past, and to understand the driving mechanisms of coastal change. Direct observations of cliff retreat rarely extend beyond 150 y, during which humans have significantly modified the coastal system. Cliff retreat rates are unknown in prior centuries and millennia. In this study, we derived retreat rates of chalk cliffs on the south coast of Great Britain over millennial time scales by coupling high-precision cosmogenic radionuclide geochronology and rigorous numerical modeling. Measured 10Be concentrations on rocky coastal platforms were compared with simulations of coastal evolution using a Monte Carlo approach to determine the most likely history of cliff retreat. The 10Be concentrations are consistent with retreat rates of chalk cliffs that were relatively slow (2–6 cm⋅y−1) until a few hundred years ago. Historical observations reveal that retreat rates have subsequently accelerated by an order of magnitude (22–32 cm⋅y−1). We suggest that acceleration is the result of thinning of cliff-front beaches, exacerbated by regional storminess and anthropogenic modification of the coast.
Schmidt AH, Neilson TB, Bierman PR, et al., 2016, Influence of topography and human activity on apparent in situ <sup>10</sup>Be-derived erosion rates in Yunnan, SW China, Earth Surface Dynamics, Vol: 4, Pages: 819-830, ISSN: 2196-632X
In order to understand better if and where erosion rates calculated using in situ 10Be are affected by contemporary changes in land use and attendant deep regolith erosion, we calculated erosion rates using measurements of in situ 10Be in quartz from 52 samples of river sediment collected from three tributaries of the Mekong River (median basin area = 46.5 km2). Erosion rates range from 12 to 209 mm kyr−1 with an area-weighted mean of 117 ± 49 mm kyr−1 (1 standard deviation) and median of 74 mm kyr−1. We observed a decrease in the relative influence of human activity from our steepest and least altered watershed in the north to the most heavily altered landscapes in the south. In the areas of the landscape least disturbed by humans, erosion rates correlate best with measures of topographic steepness. In the most heavily altered landscapes, measures of modern land use correlate with 10Be-estimated erosion rates but topographic steepness parameters cease to correlate with erosion rates. We conclude that, in some small watersheds with high rates and intensity of agricultural land use that we sampled, tillage and resultant erosion has excavated deeply enough into the regolith to deliver subsurface sediment to streams and thus raise apparent in situ 10Be-derived erosion rates by as much as 2.5 times over background rates had the watersheds not been disturbed.
Singleton AA, Schmidt AH, Bierman PR, et al., 2016, Effects of grain size, mineralogy, and acid-extractable grain coatings on the distribution of the fallout radionuclides <sup>7</sup>Be, <sup>10</sup>Be, <sup>137</sup>Cs, and <sup>210</sup>Pb in river sediment, Geochimica et Cosmochimica Acta, Vol: 197, Pages: 71-86, ISSN: 0016-7037
Grain-size dependencies in fallout radionuclide activity have been attributed to either increase in specific surface area in finer grain sizes or differing mineralogical abundances in different grain sizes. Here, we consider a third possibility, that the concentration and composition of grain coatings, where fallout radionuclides reside, controls their activity in fluvial sediment. We evaluated these three possible explanations in two experiments: (1) we examined the effect of sediment grain size, mineralogy, and composition of the acid-extractable materials on the distribution of 7Be, 10Be, 137Cs, and unsupported 210Pb in detrital sediment samples collected from rivers in China and the United States, and (2) we periodically monitored 7Be, 137Cs, and 210Pb retention in samples of known composition exposed to natural fallout in Ohio, USA for 294 days. Acid-extractable materials (made up predominately of Fe, Mn, Al, and Ca from secondary minerals and grain coatings produced during pedogenesis) are positively related to the abundance of fallout radionuclides in our sediment samples. Grain-size dependency of fallout radionuclide concentrations was significant in detrital sediment samples, but not in samples exposed to fallout under controlled conditions. Mineralogy had a large effect on 7Be and 210Pb retention in samples exposed to fallout, suggesting that sieving sediments to a single grain size or using specific surface area-based correction terms may not completely control for preferential distribution of these nuclides. We conclude that time-dependent geochemical, pedogenic, and sedimentary processes together result in the observed differences in nuclide distribution between different grain sizes and substrate compositions. These findings likely explain variability of measured nuclide activities in river networks that exceeds the variability introduced by analytical techniques as well as spatial and temporal differences in erosion rates and processes. In short, we s
Sosa Gonzalez V, Bierman PR, Fernandes NF, et al., 2016, Long-term background denudation rates of southern and southeastern Brazilian watersheds estimated with cosmogenic 10Be, Geomorphology, Vol: 268, Pages: 54-63, ISSN: 1872-695X
In comparison to humid temperate regions of the Northern Hemisphere, less is known about the long-term (millennial scale) background rates of erosion in Southern Hemisphere tropical watersheds. In order to better understand the rate at which watersheds in southern and southeastern Brazil erode, and the relationship of that erosion to climate and landscape characteristics, we made new measurements of in situ produced 10Be in river sediments and we compiled all extant measurements from this part of the country.New data from 14 watersheds in the states of Santa Catarina (n = 7) and Rio de Janeiro (n = 7) show that erosion rates vary there from 13 to 90 m/My (mean = 32 m/My; median = 23 m/My) and that the difference between erosion rates of basins we sampled in the two states is not significant. Sampled basin area ranges between 3 and 14,987 km2, mean basin elevation between 235 and 1606 m, and mean basin slope between 11 and 29°. Basins sampled in Rio de Janeiro, including three that drain the Serra do Mar escarpment, have an average basin slope of 19°, whereas the average slope for the Santa Catarina basins is 14°. Mean basin slope (R2 = 0.73) and annual precipitation (R2 = 0.57) are most strongly correlated with erosion in the basins we studied. At three sites where we sampled river sand and cobbles, the 10Be concentration in river sand was greater than in the cobbles, suggesting that these grain sizes are sourced from different parts of the landscape.Compiling all cosmogenic 10Be-derived erosion rates previously published for southern and southeastern Brazil watersheds to date (n = 76) with our 14 sampled basins, we find that regional erosion rates (though low) are higher than those of watersheds also located on other passive margins including Namibia and the southeastern North America. Brazilian basins erode at a pace similar to escarpments in southeastern North America. Erosion rates in southern and southeastern Brazil are directly and positively relat
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