Publications
131 results found
Matmon A, Fink D, Davis M, et al., 2014, Unraveling rift margin evolution and escarpment development ages along the Dead Sea fault using cosmogenic burial ages, Quaternary Research (United States), Vol: 82, Pages: 281-295, ISSN: 0033-5894
The Dead Sea fault (DSF) is one of the most active plate boundaries in the world. Understanding the Quaternary history and sediments of the DSF requires investigation into the Neogene development of this plate boundary. DSF lateral motion preceded significant extension and rift morphology by ~10Ma. Sediments of the Sedom Formation, dated here between 5.0±0.5Ma and 6.2-2.1+infMa, yielded extremely low 10Be concentrations and 26Al is absent. These reflect the antiquity of the sediments, deposited in the Sedom Lagoon, which evolved in a subdued landscape and was connected to the Mediterranean Sea. The base of the overlying Amora Formation, deposited in the terminal Amora Lake which developed under increasing relief that promoted escarpment incision, was dated at 3.3-0.8+0.9Ma. Burial ages of fluvial sediments within caves (3.4±0.2Ma and 3.6±0.4Ma) represent the timing of initial incision. Initial DSF topography coincides with the earliest Red Sea MORB's and the East Anatolian fault initiation. These suggest a change in the relative Arabian-African plate motion. This change introduced the rifting component to the DSF followed by a significant subsidence, margin uplift, and a reorganization of relief and drainage pattern in the region resulting in the topographic framework observed today. © 2014 University of Washington.
McPhillips D, Bierman PR, Crocker T, et al., 2013, Landscape response to Pleistocene-Holocene precipitation change in the Western Cordillera, Peru: 10Be concentrations in modern sediments and terrace fills, Journal of Geophysical Research: Earth Surface, Vol: 118, Pages: 2488-2499, ISSN: 2169-9011
The landscape response to climate change is frequently investigated with models because natural experiments on geologic timescales are rare. In Quebrada Veladera, in the western Andes Mountains, the formation of alluvial terraces during periods of high precipitation presents opportunities for such an experiment. We compare drainage-average erosion rates during Pleistocene terrace deposition with Holocene rates, using cosmogenic 10Be samples for seven pairs of quartz sand taken from the trunk and tributaries of Quebrada Veladera and adjacent terraces. Each pair consists of sediment collected from the modern channel and excavated from an adjacent fill terrace. The terrace fill was deposited at ~16 ka and preserved an isotopic record of paleoerosion rates in the Late Pleistocene. Modern sands yield 10Be concentrations between 1.68 × 105 and 2.28 × 105 atoms/g, corresponding to Holocene erosion rates between 43 ± 3 and 58 ± 4 mm/kyr. The 10Be concentrations in terrace sands range from 9.46 × 104 to 3.73 × 105 atoms/g, corresponding to paleoerosion rates from 27 ± 2 to 103 ± 8 mm/kyr. Smaller, upstream tributaries show a substantial decline in erosion rate following the transition from a wet to dry climate, but larger drainage areas show no change. We interpret this trend to indicate that the wetter climate drove landscape dissection, which ceased with the return to dry conditions. As channel heads propagated upslope, erosion accelerated in low-order drainages before higher-order ones. This contrast disappeared when the drainage network ceased to expand; at that point, erosion rates became spatially uniform, consistent with the uniformity of modern hillslope gradients. Key Points Landscape response to climate change evaluated with 10Be erosion rates Wetter climate associated with rapid erosion in smaller, upstream drainages Drier, Holocene climate associated with spatially uniform erosion rates ©2013. American Geop
Putnam AE, Schaefer JM, Denton GH, et al., 2013, Warming and glacier recession in the Rakaia valley, Southern Alps of New Zealand, during Heinrich Stadial 1, Earth and Planetary Science Letters, Vol: 382, Pages: 98-110, ISSN: 0012-821X
The termination of the last ice age featured a major reconfiguration of Earth's climate and cryosphere, yet the underlying causes of these massive changes continue to be debated. Documenting the spatial and temporal variations of atmospheric temperature during deglaciation can help discriminate among potential drivers. Here, we present a 10Be surface-exposure chronology and glaciological reconstruction of ice recession following the Last Glacial Maximum (LGM) in the Rakaia valley, Southern Alps of New Zealand. Innermost LGM moraines at Big Ben have an age of 17,840 ± 240 yrs, whereas ice-marginal moraines or ice-molded bedrock surfaces at distances up-valley from Big Ben of 12.5 km (Lake Coleridge), ~25 km (Castle Hill), ~28 km (Double Hill), ~43 km (Prospect Hill), and ~58 km (Reischek knob) have ages of 17,020 ± 70 yrs, 17,100 ± 110 yrs, 16,960 ± 370 yrs, 16,250 ± 340 yrs, and 15,660 ± 160 yrs, respectively. These results indicate extensive recession of the Rakaia glacier, which we attribute primarily to the effects of climatic warming. In conjunction with geomorphological maps and a glaciological reconstruction for the Rakaia valley, we use our chronology to infer timing and magnitude of past atmospheric temperature changes. Compared to an overall temperature rise of ~4.65°C between the end of the LGM and the start of the Holocene, the glacier recession between ~17,840 and ~15,660 yrs ago is attributable to a net temperature increase of ~4.0°C (from -6.25 to -2.25°C), accounting for ~86% of the overall warming. Approximately 3.75°C (~70%) of the warming occurred between ~17,840 and ~16,250 yrs ago, with a further 0.75°C (~16%) increase between ~16,250 and ~15,660 yrs ago. A sustained southward shift of the Subtropical Front (STF) south of Australia between ~17,800 and ~16,000 yrs ago coincides with the warming over the Rakaia valley, and suggests a close link between Southern Ocean frontal boundary pos
Amidon WH, Bookhagen B, Avouac JP, et al., 2013, Late Pleistocene glacial advances in the western Tibet interior, Earth and Planetary Science Letters, Vol: 381, Pages: 210-221, ISSN: 0012-821X
It has long been observed that the timing of glacial advances is asynchronous across the Himalaya-Karakoram-Tibet Plateau (HKTP) but the climatic implications, if any, remain unclear. Resolving this question requires additional glacial chronologies from unique spatial and climatic regimes as well as an analysis of how glaciers within different regimes are likely to have responded to past climate changes. This study presents a 10Be-21Ne chronology from the Mawang Kangri range of western Tibet (~34°N, 80°E); an arid high-elevation site. We identify advances at ~123, 83, and 56 kyr, which agree reasonably well with sites in the immediate vicinity, but are asynchronous relative to sites across the entire HKTP, and relative to sites in the western HKTP. To evaluate HKTP-wide asynchroneity, we compile dated glacial chronologies and classify them by the approximate timing of their maximum recent advance. This result shows a strong spatial clustering of young (MIS 1-2) relative to older (MIS 3-5) maximum advances. Further comparison with modern precipitation, temperature, and topographic data show that the pattern of HKTP-wide asynchroneity is broadly independent of topography and can potentially be explained by local responses to changes in temperature at either very warm-wet or cold-dry sites. Sites that receive intermediate amounts of precipitation are more ambiguous, although spatial clustering of MIS 1-2 vs. MIS 3-5 advances is suggestive of past variations in precipitation at these sites. In western Tibet, no spatial or climatic correlation is observed with the timing of maximum glacial advances. We suggest this could arise from mis-interpretation of disparate boulder ages generated by a prolonged MIS-3/4 glacial advance in the western HKTP. © 2013 Elsevier B.V.
West N, Kirby E, Bierman P, et al., 2013, Regolith production and transport at the Susquehanna Shale Hills Critical Zone Observatory, Part 2: Insights from meteoric 10Be, Journal of Geophysical Research: Earth Surface, Vol: 118, Pages: 1877-1896, ISSN: 2169-9011
Regolith-mantled hillslopes are ubiquitous features of most temperate landscapes, and their morphology reflects the climatically, biologically, and tectonically mediated interplay between regolith production and downslope transport. Despite intensive research, few studies have quantified both of these mass fluxes in the same field site. Here we present an analysis of 87 meteoric <sup>10</sup>Be measurements from regolith and bedrock within the Susquehanna Shale Hills Critical Zone Observatory (SSHO), in central Pennsylvania. Meteoric <sup>10</sup>Be concentrations in bulk regolith samples (n=73) decrease with regolith depth. Comparison of hillslope meteoric <sup>10</sup>Be inventories with analyses of rock chip samples (n=14) from a 24 m bedrock core confirms that >80% of the total inventory is retained in the regolith. The systematic downslope increase of meteoric <sup>10</sup>Be inventories observed at SSHO is consistent with <sup>10</sup>Be accumulation in slowly creeping regolith (∼ 0.2 cm yr<sup>-1</sup>). Regolith flux inferred from meteoric <sup>10</sup>Be varies linearly with topographic gradient (determined from high-resolution light detection and ranging-based topography) along the upper portions of hillslopes at SSHO. However, regolith flux appears to depend on the product of gradient and regolith depth where regolith is thick, near the base of hillslopes. Meteoric <sup>10</sup>Be inventories at the north and south ridgetops indicate minimum regolith residence times of 10.5 ± 3.7 and 9.1 ± 2.9 ky, respectively, similar to residence times inferred from U-series isotopes in Ma et al. (2013). The combination of our results with U-series-derived regolith production rates implies that regolith production and erosion rates are similar to within a factor of two on SSHO hillcrests. ©2013. American Geophysical Union. All Rights Reserved.
Corbett LB, Bierman PR, Graly JA, et al., 2013, Constraining landscape history and glacial erosivity using paired cosmogenic nuclides in upernavik, northwest greenland, Bulletin of the Geological Society of America, Vol: 125, Pages: 1539-1553, ISSN: 0016-7606
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 significantly 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 significant 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 snowfields 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. © 2013 Geological Society of America.
Gudmundsdottir MH, Blisniuk K, Ebert Y, et al., 2013, Restraining bend tectonics in the santa cruz mountains, california, imaged using <sup>10</sup> be concentrations in river sands, Geology, Vol: 41, Pages: 843-846, ISSN: 0091-7613
Reverse faults frequently generate large and destructive earthquakes, yet their seismic hazard remains difficult to assess with traditional paleoseismic tools because their surface expressions are often complex and subtle. This contribution assesses the utility of millennial- scale denudation rates derived from in-situ cosmogenic 10Be for revealing the spatial patterns and magnitudes of rock uplift produced by slip along reverse faults. We present seventeen basin-averaged denudation rates from rivers draining the Santa Cruz Mountains along the San Andreas fault (California, USA) which closely reproduce known uplift rate patterns associated with a restraining bend along the fault. An additional component of vertical deformation appears to be superposed on the uplift due to the restraining bend; this may result from regional transpression, further irregularities in the fault trace, or interactions with neighboring faults. Our results indicate that 10Be-derived denudation rates can reveal patterns of rock uplift adjacent to reverse faults over length-scales relevant for characterizing their seismic hazard potential. © 2013 Geological Society of America.
Amos CB, Brownlee SJ, Rood DH, et al., 2013, Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California, Bulletin of the Geological Society of America, Vol: 125, Pages: 1187-1202, ISSN: 0016-7606
New geochronologic and geomorphic constraints on the Little Lake fault in the Eastern California shear zone reveal steady, modest rates of dextral slip during and since the midto-late Pleistocene. We focus on a suite of offset fluvial landforms in the Pleistocene Owens River channel that formed in response to peri odic interaction with nearby basalt flows, thereby recording displacement over multiple time intervals. Overlap between 40Ar/39Ar ages for the youngest intracanyon basalt flow and 10Be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the Little Lake channel at ca. 64 ka. Older basalt flows flanking the upper and lower canyon margins indicate localization of the Owens River in its current position between 212 ± 14 and 197 ± 11 ka. Coupled with terrestrial light detection and ranging (lidar) and digital topographic measurements of dextral offset, the revised Little Lake chronology indicates average dextral slip rates of at least ~0.6-0.7 mm/yr and <1.3 mm/yr over intervals ranging from ~104 to 105 yr. Despite previous geodetic observations of relatively rapid interseismic strain along the Little Lake fault, we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles. Instead, our results indicate that accelerated fault loading may be transient over much shorter periods (~101 yr) and perhaps indicative of time-dependent seismic hazard associated with Eastern California shear zone faults. © 2013 Geological Society of America.
Refsnider KA, Miller GH, Fréchette B, et al., 2013, A chronological framework for the Clyde Foreland Formation, Eastern Canadian Arctic, derived from amino acid racemization and cosmogenic radionuclides, Quaternary Geochronology, Vol: 16, Pages: 21-34, ISSN: 1871-1014
The most extensive terrestrial outcrops of glacial and glaciomarine deposits in the Eastern Canadian Arctic are exposed in sea cliffs along the Clyde Foreland and Qivitu Peninsula of Baffin Island. Collectively known as the Clyde Foreland Formation (CFF), these stacked deposits record at least seven glacial advances. Despite having been the focus of numerous investigations spanning nearly 50 years, no numerical chronological framework for the age of the deposits has been established. Previous studies relied on biostratigraphy and amino acid racemization (AAR) geochronology and postulated that the oldest units were Late Pliocene to Mid-Pleistocene in age. In this paper, we use a cosmogenic radionuclide isochron approach to determine a minimum age for the burial of a paleosol preserved within the CFF. Abundant palynomorphs in the paleosol are dominated by cool-climate taxa. Combining the paleosol burial age with a compilation of published and new CFF AAR data for marine bivalves Hiatella arctica and Mya truncata, we statistically define seven CFF aminozones and develop a piecewise isoleucine AAR calibration model for Baffin Island. From this, we estimate the minimum age of each aminozone, although the propagation of errors through all calculations produces large uncertainties for each age estimate. The youngest three CFF units, known as the Kogalu, Kuvinilk, and Cape Christian members, were most likely deposited during glaciations in the Mid- to Late-Pleistocene. The paleosol formed prior to 1.15 ± 0.20 Ma, and the underlying aminozones represent sedimentation during Early Pleistocene or latest Pliocene glaciations and record early advances of Laurentide ice across Baffin Island. © 2012 Elsevier B.V.
Boroda R, Matmon A, Amit R, et al., 2013, Long-term talus flatirons formation in the hyperarid northeastern Negev, Israel, Quaternary Research (United States), Vol: 79, Pages: 256-267, ISSN: 0033-5894
Colluvial sediments of talus relicts ("talus flatirons") around mesas preserve a record that sheds light on slope-forming processes at temporal scales >103yr. The sedimentology and soil stratigraphy of two groups of talus flatirons in the northeastern hyperarid Negev desert reveal four deposition events in the younger talus and at least two in the older one. Numerical modeling of high-resolution 10Be depth profiles suggests that these taluses were deposited during the middle Pleistocene; the younger talus group first depositional event occurred at 551 -142+80ka and its abandonment occurred at 270 -38+17ka. The abandonment of the older talus group and stabilization of its surface occurred at 497 -114+176ka. These ages indicate that the development of the studied talus sequence is not specifically associated with Pleistocene glacial-interglacial cycles. The 10Be modeled concentrations indicate significant differences in the average inheritance of talus flatirons of different groups. These differences can be attributed to variability in the transport distance and duration of gravel exposure during transport but could also reflect some temporal variability in cliff retreat. Our results also demonstrate that talus slopes in hyperarid areas, despite their steepness, can store sediment for long periods (~500ka) and thus constitute a valuable archive. © 2012 University of Washington.
Hidy AJ, Gosse JC, Froese DG, et al., 2013, A latest Pliocene age for the earliest and most extensive Cordilleran Ice Sheet in northwestern Canada, Quaternary Science Reviews, Vol: 61, Pages: 77-84, ISSN: 0277-3791
The Klondike gravel is a widespread glaciofluvial gravel marking the earliest and most extensive Cordilleran Ice Sheet (CIS) in NW North America. New terrestrial cosmogenic nuclide (TCN) burial ages indicate this gravel was emplaced 2.64+0.20/-0.18 Ma (1σ). Coupled with previously interpreted paleomagnetic stratigraphy, this numerical age constrains the timing of the earliest CIS to the late Gauss Chron and provides a minimum age for the Upper White Channel gravel, a significant placer gold source in the Yukon. This implies the first CIS glacial maximum pre-dates the maximum extent of the Laurentide Ice Sheet, indicating that during the initial stages of northern hemisphere glaciation, the most extensive glaciers were present in the relatively cold and high elevation northern Cordillera. Our results verify the CIS as a likely source of persistent coeval ice-rafted debris in the northern Pacific, and suggest that the first CIS formed as a response to the establishment of the northern Pacific halocline and emergence of the 41 ka obliquity cycle during the Plio-Pliocene transition. © 2012 Elsevier Ltd.
Young NE, Briner JP, Rood DH, et al., 2013, Age of the Fjord Stade moraines in the Disko Bugt region, western Greenland, and the 9.3 and 8.2 ka cooling events, Quaternary Science Reviews, Vol: 60, Pages: 76-90, ISSN: 0277-3791
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 ∼60 km apart. At Jakobshavn Isfjord, the northern of two study sites, we show that Jakobshavn Isbræ advanced to deposit moraines ca 9.2 and 8.2-8.0 ka. In southeastern Disko Bugt, the ice sheet deposited moraines ca 9.4-9.0 and 8.5-8.1 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 du
Portenga EW, Bierman PR, Rizzo DM, et al., 2013, Low rates of bedrock outcrop erosion in the Central Appalachian mountains inferred from in situ <sup>10</sup>Be, Bulletin of the Geological Society of America, Vol: 125, Pages: 201-215, ISSN: 0016-7606
Bedrock outcrops are common on central Appalachian Mountain ridgelines. Because these ridgelines define watersheds, the rate at which they erode infl uences 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 theregion. The range of erosion rates we calculated refl ects 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 reducesthe production rate of 10Be and increases the erosion rate of rock. Ridgeline outcrops erode slower than drainage basinsin 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 refl ect 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 fl ux, 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. © 2013 Geological Society of America.
Tumey SJ, Brown TA, Finkel RC, et al., 2013, The feasibility of isobaric suppression of <sup>26</sup>Mg via post-accelerator foil stripping for the measurement of <sup>26</sup>Al, Pages: 406-409, ISSN: 0168-583X
Most accelerator mass spectrometry measurements of 26Al utilize the Al- ion despite lower source currents compared with AlO - since the stable isobar 26Mg does not form elemental negative ions. A gas-filled magnet allows sufficient suppression of 26Mg thus enabling the use of the more intense 26AlO - ion. However, most AMS systems do not include a gas-filled magnet. We therefore explored the feasibility of suppressing 26Mg by using a post-accelerator stripping foil. With this approach, combined with the use of alternative cathode matrices, we were able to suppress 26Mg by a factor of 20. This suppression was insufficient to enable the use of 26AlO-, however further refinement of our system may permit its use in the future. © 2012 Elsevier B.V. All rights reserved.
Rood DH, Brown TA, Finkel RC, et al., 2013, Poisson and non-Poisson uncertainty estimations of <sup>10</sup>Be/ <sup>9</sup>Be measurements at LLNL-CAMS, Pages: 426-429, ISSN: 0168-583X
We quantify the routine performance and uncertainties of 10Be measurements made on the CAMS FN accelerator mass spectrometer in combination with the CAMS high-intensity cesium sputter source. Our analysis compiles data from 554 primary and secondary standard targets measured on 47 different wheels in nine different run campaigns over a 1-year interval (September 2009-September 2010). The series includes 87, 86, and 85 measurements of each of three different secondary standards and 296 measurements of our primary standard, KNSTD3110 (01-5-4). The average initial 9Be3+ beam current is 22 ± 3 μA (1 standard deviation). Secondary standard targets, which are measured as unknowns in each of the wheels, have average statistical uncertainties based on counting statistics of 1.8%, 1.3%, and 0.8% (1σ) (September 2009-March 2010) and 1.3%, 1.0%, and 0.6% (April 2010-September 2010) for standard materials with 10Be/9Be = 5.35 × 10-13, 9.72 × 10-13, and 8.56 × 10 -12, respectively. The mean measured ratio for each of the secondary standards (normalized to the primary standard) falls within the 1.1% uncertainties of the reported values for each standard material. The weighted standard deviation around the mean of this large number of runs is 2.5%, 2.0%, and 1.2% (September 2009-March 2010) and 1.5%, 1.1%, and 1.2% (April 2010-September 2010) for each secondary standard. These data indicate an additional source of uncertainty, 0.9-1.8% (April 2010-September 2010) and 0.2-1.0% (April 2010-September 2010), above that calculated from counting statistics alone. These 10Be AMS results demonstrate the precision and accuracy of the LLNL-CAMS system. © 2012 Elsevier B.V. All rights reserved.
Merchel S, Bremser W, Akhmadaliev S, et al., 2012, Quality assurance in accelerator mass spectrometry: Results from an international round-robin exercise for <sup>10</sup>Be, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, Vol: 289, Pages: 68-73, ISSN: 0168-583X
The first international round-robin exercise for the measurement of the long-lived radionuclide 10Be has been conducted. Ten participating accelerator mass spectrometry (AMS) facilities have each measured three samples at the 10 -12 to 10 -1410Be/ 9Be level. All results have been made traceable to the NIST SRM 4325 standard to avoid additional discrepancies that arise when different facilities use different calibration materials. Hence, the data concentrates on pure measurement distinctions. Multivariate statistical investigations have been performed to reveal a bias between facilities, i.e. two distinguished groups could be identified. Maximum discrepancies between two single facilities are in the range of 6-31% depending on the absolute 10Be/ 9Be value. These findings should be considered when comparing 10Be data produced at one AMS facility with that produced at another facility, which is e.g. often the case for in situ 10Be dating studies. Round-robin exercises are a very helpful tool as part of an overall quality assurance scheme to improve the accuracy, and not only the precision, of AMS data. © 2012 Elsevier B.V. All rights reserved.
Young NE, Briner JP, Rood DH, et al., 2012, Glacier extent during the Younger Dryas and 8.2-ka event on Baffin Island, Arctic Canada, Science, Vol: 337, Pages: 1330-1333, ISSN: 0036-8075
Greenland ice cores reveal that mean annual temperatures during the Younger Dryas (YD) cold interval - about 12.9 to 11.7 thousand years ago (ka) - and the ∼150-year-long cold reversal that occurred 8.2 thousand years ago were ∼15° and 3° to 4°C colder than today, respectively. Reconstructing ice-sheet response to these climate perturbations can help evaluate ice-sheet sensitivity to climate change. Here, we report the widespread advance of Laurentide Ice Sheet outlet glaciers and independent mountain glaciers on Baffin Island, Arctic Canada, in response to the 8.2-ka event and show that mountain glaciers during the 8.2-ka event were larger than their YD predecessors. In contrast to the wintertime bias of YD cooling, we suggest that cooling during the 8.2-ka event was more evenly distributed across the seasons.
Bacon AR, Richter DDB, Bierman PR, et al., 2012, Coupling meteoric <sup>10</sup>be with pedogenic losses of <sup>9</sup>be to improve soil residence time estimates on an ancient North American interfluve, Geology, Vol: 40, Pages: 847-850, ISSN: 0091-7613
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. © 2012 Geological Society of America.
Mercader J, Gosse JC, Bennett T, et al., 2012, Cosmogenic nuclide age constraints on Middle Stone Age lithics from Niassa, Mozambique, Quaternary Science Reviews, Vol: 47, Pages: 116-130, ISSN: 0277-3791
The late phases of the Middle Stone Age (MSA) in the East African Rift System (EARS) are known for their evolutionary shifts and association with bottlenecks, transcontinental expansion, and climatic fluctuations. The chronology of MSA sites contemporaneous with these eco-demographic upheavals is uncertain because of the scarcity of datable sites and the poor understanding of their depositional and erosional histories. We apply terrestrial cosmogenic nuclide dating in a stratigraphic section with a complex exposure history to the study of the Luchamange Beds, a widespread sedimentological unit underlying MSA sites from the shores of Lake Niassa (Mozambican EARS). We use an innovative approach, which may be applicable elsewhere, to calculate their age using a Monte Carlo-based Bayesian model that links depth profiles of 26Al and 10Be, and uses other geomorphic and cosmogenic nuclide age constraints on episodic erosion and burial. The age of the basal Luchamange Beds is 42 + 77/-15. ka, and the MSA occupation on top is 29 + 3/-11 ka. These dates suggest temporal overlap between MSA and the earliest Later Stone Age and diversity in cultural manifestations at the end of the MSA. © 2012 Elsevier Ltd.
Balco G, Purvance MD, Rood DH, 2012, Corrigendum to " Exposure dating of precariously balanced rocks" [Quaternary Geochronology 6 (2011) 295-303], Quaternary Geochronology, Vol: 9, ISSN: 1871-1014
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- Citations: 2
Gourbet L, Shuster DL, Balco G, et al., 2012, Neon diffusion kinetics in olivine, pyroxene and feldspar: Retentivity of cosmogenic and nucleogenic neon, Geochimica et Cosmochimica Acta, Vol: 86, Pages: 21-36, ISSN: 0016-7037
We performed stepwise degassing experiments by heating single crystals of neutron- or proton-irradiated olivine, pyroxene and feldspar to study diffusion kinetics of neon. This is important in evaluating the utility of these minerals for cosmogenic 21Ne measurements and, potentially, for Ne thermochronometry. Degassing patterns are only partially explained by simple Arrhenius relationships; most samples do not exhibit a precisely-determined activation energy in an individual diffusion domain. Regardless, we find clear differences in diffusion kinetics among these minerals. Based on sub-selected data, our estimates for neon diffusion kinetics (activation energy E a and pre-exponential factor D o, assuming the analyzed fragments approximate the diffusion domain) in each mineral are as follows: for the feldspars, E a ranges from ~65 to 115kJ/mol and D o from 3.9×10 -3 to 7.1×10 2cm 2s -1; for the pyroxenes, E a ranges from ~292 to 480kJ/mol and D o from 1.6×10 2 to 2.9×10 11cm 2s -1; for the olivines, E a ranges from ~360 to 370kJ/mol and D o from 1.5×10 6 to 5.0×10 6cm 2s -1. Differences in these parameters are broadly consistent with the expected effect of structural differences between feldspar, and olivine and pyroxene. These results indicate that cosmogenic 21Ne will be quantitatively retained within olivine and pyroxene at Earth surface temperatures over geological timescales. The diffusion kinetics for feldspars, on the other hand, predicts that 21Ne retention at Earth surface temperatures will vary significantly with domain size, crystal microtexture, surface temperature, and exposure duration. Quantitative retention is expected only in favorable conditions. This conclusion is reinforced by additional measurements of cosmogenic 21Ne in coexisting quartz and feldspar from naturally irradiated surface samples; sanidine from a variety of rhyolitic ignimbrites exhibits quantitative retention, whereas alkali-feldspar from several
Davis M, Matmon A, Rood DH, et al., 2012, Constant cosmogenic nuclide concentrations in sand supplied from the Nile River over the past 2.5 m.y., Geology, Vol: 40, Pages: 359-362, ISSN: 0091-7613
Quartz sand in the eastern Mediterranean coastal plain is supplied through an extended transport system, which includes the Nile River, east Mediterranean longshore currents, and inland eolian transport. While the concentrations of cosmogenic nuclides (26Al and 10Be), and their ratio, in modern sand deposited along the coast of the eastern Mediterranean reflect the combined effect of exposure and burial during transport, the concentrations of these nuclides in buried sands are the result of decay of this initial dosing. Samples of modern exposed sand (n = 3) collected from the coastal plain of Israel yield an average 26Al/10Be ratio of 4.8 ± 0.2, significantly lower than the expected ratio of 6.8 for exposed quartz grains at the surface. A similar ratio of 4.5 ± 0.3 was measured in a late Pleistocene sand sample, indicating similar exposure-burial histories during transport in spite of the difference in climatic conditions. The results imply a steady, preburial cosmogenic nuclide ratio related to the Nile River's ability, through storage and recycling, to buffer the effects of climatic and tectonic perturbations on cosmogenic nuclide concentrations in the transported quartz. All ancient and buried sand samples (n = 11) fall on a decay path that originates from the concentrations and ratio of 26Al and 10Be in modern sand, suggesting steady preburial concentrations of cosmogenic nuclides in quartz sand over the past 2.5 m.y. © 2012 Geological Society of America.
Johnson JS, Everest JD, Leat PT, et al., 2012, The deglacial history of NW Alexander Island, Antarctica, from surface exposure dating, Quaternary Research, Vol: 77, Pages: 273-280, ISSN: 0033-5894
Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic 10Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the 10Be ages are in the range 10.2-21.7ka, showing a general trend of progressive ice-sheet thinning since at least 22ka until 10ka. The data also provide a minimum estimate (490m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic 3He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46ka, probably reflecting exhumation by periglacial processes. © 2011 University of Washington.
Young NE, Briner JP, Axford Y, et al., 2011, Response of a marine-terminating Greenland outlet glacier to abrupt cooling 8200 and 9300 years ago, Geophysical Research Letters, Vol: 38, ISSN: 0094-8276
Long-term records of Greenland outlet-glacier change extending beyond the satellite era can inform future predictions of Greenland Ice Sheet behavior. Of particular relevance is elucidating the Greenland Ice Sheet's response to decadal-and centennial-scale climate change. Here, we reconstruct the early Holocene history of Jakobshavn Isbr, Greenland's largest outlet glacier, using <sup>10</sup>Be surface exposure ages and <sup>14</sup>C-dated lake sediments. Our chronology of ice-margin change demonstrates that Jakobshavn Isbr advanced to deposit moraines in response to abrupt cooling recorded in central Greenland ice cores ca. 8,200 and 9,300 years ago. While the rapid, dynamically aided retreat of many Greenland outlet glaciers in response to warming is well documented, these results indicate that marine-terminating outlet glaciers are also able to respond quickly to cooling. We suggest that short lag times of high ice flux margins enable a greater magnitude response of marine-terminating outlets to abrupt climate change compared to their land-terminating counterparts. Copyright 2011 by the American Geophysical Union.
Rood DH, Burbank DW, Herman SW, et al., 2011, Rates and timing of vertical-axis block rotations across the central Sierra Nevada-Walker Lane transition in the Bodie Hills, California/Nevada, Tectonics, Vol: 30, ISSN: 0278-7407
We use paleomagnetic data from Tertiary volcanic rocks to address the rates and timing of vertical-axis block rotations across the central Sierra Nevada-Walker Lane transition in the Bodie Hills, California/Nevada. Samples from the Upper Miocene (∼9 Ma) Eureka Valley Tuff suggest clockwise vertical-axis block rotations between NE-striking left-lateral faults in the Bridgeport and Mono Basins. Results in the Bodie Hills suggest clockwise rotations (R ± δR, 95% confidence limits) of 74 ± 8° since Early to Middle Miocene (∼12-20 Ma), 42 ± 11° since Late Miocene (∼8-9 Ma), and 14 ± 10° since Pliocene (∼3 Ma) time with no detectable northward translation. The data are compatible with a relatively steady rotation rate of 5 ± 2° Ma<sup>-1</sup> (2σ) since the Middle Miocene over the three examined timescales. The average rotation rates have probably not varied by more than a factor of two over time spans equal to half of the total time interval. Our paleomagnetic data suggest that block rotations in the region of the Mina Deflection began prior to Late Miocene time (∼9 Ma), and perhaps since the Middle Miocene if rotation rates were relatively constant. Block rotation in the Bodie Hills is similar in age and long-term average rate to rotations in the Transverse Ranges of southern California associated with early transtensional dextral shear deformation. We speculate that the age of rotations in the Bodie Hills indicates dextral shear and strain accommodation within the central Walker Lane Belt resulting from coupling of the Pacific and North America plates. Copyright 2011 by the American Geophysical Union.
Moon S, Page Chamberlain C, Blisniuk K, et al., 2011, Climatic control of denudation in the deglaciated landscape of the Washington Cascades, Nature Geoscience, Vol: 4, Pages: 469-473, ISSN: 1752-0894
Since the Last Glacial Maximum, the extent of glaciers in many mountainous regions has declined, and erosion driven by glacial processes has been supplanted by fluvial incision and mass wasting processes. This shift in the drivers of erosion is thought to have altered the rate and pattern of denudation of these landscapes. The Washington Cascades Mountains in the northwestern USA still bear the topographic imprint of Pleistocene glaciations, and are affected by large variations in precipitation, making them an ideal setting to assess the relative controls of denudation. Here we show that denudation rates over the past millennia, as determined by 10 Be exposure ages, range from 0.08 to 0.57 yr-1, about four times higher than the rates inferred for million-year timescales. We find that the millennial timescale denudation rates increase linearly with modern precipitation rates. Based on our landscape analyses, we suggest that this relationship arises because intense precipitation triggers landslides, particularly on slopes that have been steepened by glacial erosion before or during the Last Glacial Maximum. We conclude that the high modern interglacial denudation rates we observe in the Washington Cascades are driven by a disequilibrium between the inherited topography and the current spatial distribution of erosional processes that makes this range particularly sensitive to spatial variations in climate. © 2011 Macmillan Publishers Limited. All rights reserved.
Merchel S, Bremser W, Alfimov V, et al., 2011, Ultra-trace analysis of <sup>36</sup>Cl by accelerator mass spectrometry: An interlaboratory study, Analytical and Bioanalytical Chemistry, Vol: 400, Pages: 3125-3132, ISSN: 1618-2642
A first international 36Cl interlaboratory comparison has been initiated. Evaluation of the final results of the eight participating accelerator mass spectrometry (AMS) laboratories on three synthetic AgCl samples with 36Cl/Cl ratios at the 10-11, 10-12, and 10-13 level shows no difference in the sense of simple statistical significance. However, more detailed statistical analyses demonstrate certain interlaboratory bias and underestimation of uncertainties by some laboratories. Following subsequent remeasurement and reanalysis of the data from some AMS facilities, the round-robin data indicate that 36Cl/Cl data from two individual AMS laboratories can differ by up to 17%. Thus, the demand for further work on harmonising the 36Cl-system on a worldwide scale and enlarging the improvement of measurements is obvious. © 2011 Springer-Verlag.
West N, Kirby E, Bierman P, et al., 2011, Preliminary estimates of regolith generation and mobility in the Susquehanna Shale Hills Critical Zone Observatory, Pennsylvania, using meteoric <sup>10</sup>Be, Applied Geochemistry, Vol: 26, ISSN: 0883-2927
This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric 10Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (~21ka) and deforestation during early colonial land-use. Bulk soil samples (n=16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples (n=14) were also collected from a 24m deep core drilled into the northern ridge top. All meteoric 10Be concentration profiles show a declining trend with depth, with most of the 10Be retained in the uppermost decimeters of the soil. Meteoric 10Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71±0.02×1010at/cm2 and 3.69±0.02×1010at/cm2, than at the ridge top site (1.90±0.01×1010at/cm2). The 10Be inventory at the convex ridge top site implies a minimum residence time of ~10.6ka, or if erosion is steady, an erosion rate of 19.4±0.2m/My. © 2011 Elsevier Ltd.
Balco G, Purvance MD, Rood DH, 2011, Exposure dating of precariously balanced rocks, Quaternary Geochronology, Vol: 6, Pages: 295-303, ISSN: 1871-1014
Precariously balanced rocks (PBRs) are freestanding boulders that are precarious or fragile in the sense that they could be toppled by relatively low-amplitude earthquake ground motion. They are important in paleoseismology because their continued existence limits the amplitude of ground motion experienced at their location during their lifetime. In order to make quantitative use of PBRs for seismic hazard studies, one must determine when they attained their present state of fragility, that is, the point in time when the contact between the rocks and the pedestals on which they rest was exhumed from surrounding soil and the rock became vulnerable to earthquake ground motions. Cosmogenic-nuclide exposure dating can be used for this purpose, but is complicated because nuclide production occurs throughout exhumation of the PBR, so the apparent exposure age of any part of the rock surface exceeds the time that the rock has actually been precariously balanced. Here we describe a method for determining the length of time that a PBR has been fragile by measuring cosmogenic-nuclide concentrations at several locations on the PBR surface, and linking them together with a forward model that accounts for nuclide production before, during, and after exhumation of the PBR. Fitting model to data yields the rate and timing of rock exhumation and thus the length of time the rock has been fragile. We use this method to show that an example PBR in southern California has been fragile for 18.7 ± 2.8 ka. © 2011 Elsevier B.V.
Corbett LB, Young NE, Bierman PR, et al., 2011, Paired bedrock and boulder <sup>10</sup>Be concentrations resulting from early Holocene ice retreat near Jakobshavn Isfjord, western Greenland, Quaternary Science Reviews, Vol: 30, Pages: 1739-1749, ISSN: 0277-3791
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. © 2011 Elsevier Ltd.
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