Publications
131 results found
Gonzalez VS, Bierman PR, Nichols KK, et al., 2016, Long-term erosion rates of Panamanian drainage basins determined using in situ 10Be, Geomorphology, Vol: 275, Pages: 1-15, ISSN: 1872-695X
Erosion rates of tropical landscapes are poorly known. Using measurements of in situ-produced 10Be in quartz extracted from river and landslide sediment samples, we calculate long-term erosion rates for many physiographic regions of Panama. We collected river sediment samples from a wide variety of watersheds (n = 35), and then quantified 24 landscape-scale variables (physiographic, climatic, seismic, geologic, and land-use proxies) for each watershed before determining the relationship between these variables and long-term erosion rates using linear regression, multiple regression, and analysis of variance (ANOVA). We also used grain-size-specific 10Be analysis to infer the effect of landslides on the concentration of 10Be in fluvial sediment and thus on erosion rates.Cosmogenic 10Be-inferred, background erosion rates in Panama range from 26 to 595 m My− 1, with an arithmetic average of 201 m My− 1, and an area-weighted average of 144 m My− 1. The strongest and most significant relationship in the dataset was between erosion rate and silicate weathering rate, the mass of material leaving the basin in solution. None of the topographic variables showed a significant relationship with erosion rate at the 95% significance level; we observed weak but significant correlation between erosion rates and several climatic variables related to precipitation and temperature. On average, erosion rates in Panama are higher than other cosmogenically-derived erosion rates in tropical climates including those from Puerto Rico, Madagascar, Australia and Sri Lanka, likely the result of Panama's active tectonic setting and thus high rates of seismicity and uplift. Contemporary sediment yield and cosmogenically-derived erosion rates for three of the rivers we studied are similar, suggesting that human activities are not increasing sediment yield above long-term erosion rate averages in Panama.10Be concentration is inversely proportional to grain size in landslide and f
Corbett LB, Bierman PR, Rood DH, 2016, Constraining multi-stage exposure-burial scenarios for boulders preserved beneath cold-based glacial ice in Thule, northwest Greenland, Earth and Planetary Science Letters, Vol: 440, Pages: 147-157, ISSN: 0012-821X
Boulders and landscapes preserved beneath cold-based, nonerosiveglacial ice violate assumptions associated with simple cosmogenicexposure dating. In such a setting, simple single isotope exposure agesover estimate the latest period of surface exposure; hence, alternateapproaches are required to constrain the multi-stage exposure/burialhistories of such samples. Here, we report 28 paired analyses of 10Be and26Al in boulder samples from Thule, northwest Greenland. We use numericalmodels of exposure and burial as well as Monte Carlo simulations toconstrain glacial chronology and infer process in this Arctic regiondominated by cold-based ice. We investigate three specific cases that canarise with paired nuclide data: (1) exposure ages that are coeval withdeglaciation and 26Al/10Be ratios consistent with constant exposure; (2)exposure ages that pre-date deglaciation and 26Al/10Be ratios consistentwith burial following initial exposure; and (3) exposure ages that predatedeglaciation and 26Al/10Be ratios consistent with constant exposure.Most glacially-transported boulders in Thule have complex histories; somewere exposed for tens of thousands of years and buried for at leasthundreds of thousands of years, while others underwent only limitedburial. These boulders were recycled through different generations oftill over multiple glacial/interglacial cycles, likely experiencingpartial or complete shielding during interglacial periods due to rotationor shallow burial by sediments. Our work demonstrates that the landscapein Thule, like many high-latitude landscapes, was shaped over long timedurations and multiple glacial and interglacial periods throughout theQuaternary.
Corbett LB, Bierman PR, Rood DH, 2016, An approach for optimizing in SITU cosmogenic 10BE sample preparation, Quaternary Geochronology, Vol: 33, Pages: 24-34, ISSN: 1871-1014
Optimizing sample preparation for the isotopic measurement of 10Be extracted from quartz mineral separates has a direct positive effect on the accuracy and precision of isotopic analysis. Here, we demonstrate the value of tracing Be throughout the extraction process (both after dissolution and after processing), producing pure Be (by optimizing ion exchange chromatography methods and quantifying quartz mineral separate and final Be fraction purity), and minimizing backgrounds (through reducing both laboratory process blanks and 10B isobaric interference). These optimization strategies increase the amount of 10Be available for analysis during accelerator mass spectrometry (AMS), while simultaneously decreasing interference and contamination, and ensuring that sample performance matches standard performance during analysis. After optimization of our laboratory's extraction methodology, 9Be3+ ion beam currents measured during AMS analysis, a metric for sample purity and Be yield through the extraction process, matched the 9Be3+ beam currents of AMS standards analyzed at the same time considering nearly 800 samples. Optimization of laboratory procedures leads to purer samples that perform better, more consistently, and more similarly to standards during AMS analysis, allowing for improved precision and accuracy of measurements used for dating and quantification of Earth surface processes.
Portenga EW, Rood DH, Bishop P, et al., 2016, A late Holocene onset of Aboriginal burning in southeastern Australia, Geology, Vol: 44, Pages: 131-134, ISSN: 0091-7613
The extent to which Aboriginal Australians used fire to modify their environment has been debated for decades and is generally based on charcoal and pollen records rather than landscape responses to land-use change. Here we investigate the sensitivity of in-situ–produced 10Be, an isotope commonly used in geomorphological contexts, to anthropogenic perturbations in the southeastern Australian Tablelands. Comparing 10Be-derived erosion rates from fluvial sediment (8.7 ± 0.9 mm k.y.–1; 1 standard error, SE; n = 11) and rock outcrops (5.3 ± 1.4 mm k.y.–1; 1 SE; n = 6) confirms that landscape lowering rates integrating over 104–105 yr are consistent with rates previously derived from studies integrating over 104 to >107 yr. We then model an expected 10Be inventory in fluvial sediment if background erosion rates were perturbed by a low-intensity, high-frequency Aboriginal burning regime. When we run the model using the average erosion rate derived from 10Be in fluvial sediment (8.7 mm k.y.–1), measured and modeled 10Be concentrations overlap between ca. 3 ka and 1 ka. Our modeling is consistent with intensified Aboriginal use of fire in the late Holocene, a time when Aboriginal population growth is widely recognized.
Bender AM, Amos CB, Bierman P, et al., 2016, Differential uplift and incision of the Yakima River terraces, central Washington State, Journal of Geophysical Research: Solid Earth, Vol: 121, Pages: 365-384, ISSN: 2169-9356
The fault-related Yakima folds deform Miocene basalts and younger deposits of the Columbia Plateau in central Washington State. Geodesy implies ~2 mm/yr of NNE directed shortening across the folds, but until now the distribution and rates of Quaternary deformation among individual structures has been unclear. South of Ellensburg, Washington, the Yakima River cuts a ~600 m deep canyon across several Yakima folds, preserving gravel-mantled strath terraces that record progressive bedrock incision and related rock uplift. Here we integrate cosmogenic isochron burial dating of the strath terrace gravels with lidar analysis and field mapping to quantify rates of Quaternary differential incision and rock uplift across two folds transected by the Yakima River: Manastash and Umtanum Ridge. Isochron burial ages from in situ produced 26Al and 10Be at seven sites across the folds date episodes of strath terrace formation over the past ~2.9 Ma. Average bedrock incision rates across the Manastash (~88 m/Myr) and Umtanum Ridge (~46 m/Myr) anticlines are roughly 4 to 8 times higher than rates in the intervening syncline (~14 m/Myr) and outside the canyon (~10 m/Myr). These contrasting rates demonstrate differential bedrock incision driven by ongoing Quaternary rock uplift across the folds at rates corresponding to ~0.13 and ~0.06 mm/yr shortening across postulated master faults dipping 30 ± 10°S beneath the Manastash and Umtanum Ridge anticlines, respectively. The reported Quaternary shortening across the anticlines accounts for ~10% of the ~2 mm/yr geodetic budget, suggesting that other Yakima structures actively accommodate the remaining contemporary deformation.
McPhillips D, Hoke GD, Liu-Zeng J, et al., 2016, Dating the incision of the Yangtze River gorge at the First Bend using three-nuclide burial ages, Geophysical Research Letters, Vol: 43, Pages: 101-110, ISSN: 1944-8007
Incision of the Yangtze River gorge is widely interpreted as evidence for lower crustal flow beneath the southeast margin of the Tibetan Plateau. Previous work focused on the onset of incision, but the duration of incision remains unknown. Here we present cosmogenic nuclide burial ages of sediments collected from caves on the walls of the gorge that show the gorge was incised ~1 km sometime between 18 and 9 Ma. Thereafter, incision slowed substantially. We resolve middle Miocene burial ages by using three nuclides and accounting for in situ muogenic production. This approach explains the absolute concentrations of 10Be, 26Al, and 21Ne, as well as 26Al/10Be and 21Ne/10Be ratios. A declining incision rate challenges existing geodynamic interpretations by suggesting that either (1) surface uplift has ceased immediately south of the plateau margin or (2) gorge incision is not a useful proxy for the timing of surface uplift.
Xu S, Freeman SPHT, Rood DH, et al., 2015, Decadal <sup>10</sup>Be, <sup>26</sup>Al and <sup>36</sup>Cl QA measurements on the SUERC 5 MV accelerator mass spectrometer, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, Vol: 361, Pages: 39-42, ISSN: 0168-583X
We quantify the routine performance and uncertainties of cosmogenic 10Be, 26Al and 36Cl QA measurements made on the SUERC 5 MV accelerator mass spectrometer since 2004. Our analysis compiles data from primary (NIST SRM4325 for 10Be, Purdue Z92-0222 for 26Al and Purdue Z93-0005 for 36Cl) and secondary (Nishiizumi's series for 10Be, 26Al and 36Cl) reference samples with 10Be/9Be, 26Al/27Al and 36Cl/Cl ratios ranging between 10-11 and 10-13. Our decadal datasets indicate that the 10Be, 26Al and 36Cl secondary standard samples have average standard deviations 1.1%-2.4%, 1.1%-2.8% and 3.0%-3.1%, respectively. The average statistical uncertainties based on counting statistics are 1.0%-1.8%, 0.9%-2.8% and 2.5%-2.7% for 10Be, 26Al and 36Cl, respectively. These indicate additional uncertainties (0.6%-1.6% for 10Be, 0.5%-2.4% for 26Al and 1.4%-1.7% for 36Cl) above those calculated from counting statistics alone. The average differences between the measured and the nominal values are within ±1% in 13 of 14 secondary samples.
Shanks RP, Ascough PL, Dougans A, et al., 2015, Performance of the rebuilt SUERC single-stage accelerator mass spectrometer, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, Vol: 361, Pages: 76-79, ISSN: 0168-583X
The SUERC bipolar single-stage accelerator mass spectrometer (SSAMS) has been dismantled and rebuilt to accommodate an additional rotatable pre-accelerator electrostatic spherical analyser (ESA) and a second ion source injector. This is for the attachment of an experimental positive-ion electron cyclotron resonance (ECR) ion source in addition to a Cs-sputter source. The ESA significantly suppresses oxygen interference to radiocarbon detection, and remaining measurement interference is now thought to be from 13C injected as 13CH molecule scattering off the plates of a second original pre-detector ESA.
Gold PO, Behr WM, Rood D, et al., 2015, Holocene geologic slip rate for the Banning strand of the southern San Andreas Fault, southern California, Journal of Geophysical Research. Solid Earth, Vol: 120, Pages: 5639-5663, ISSN: 2169-9313
Northwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates but is poorly understood. In this paper, we report the first slip rate measured for the Banning fault strand. We constrain the depositional age of an alluvial fan offset 25 ± 5 m from its source by the Banning strand to between 5.1 ± 0.4 ka (95% confidence interval (CI)) and 6.4 + 3.7/−2.1 ka (95% CI) using U-series dating of pedogenic carbonate clast coatings and 10Be cosmogenic nuclide exposure dating of surface clasts. We calculate a Holocene geologic slip rate for the Banning strand of 3.9 + 2.3/−1.6 mm/yr (median, 95% CI) to 4.9 + 1.0/−0.9 mm/yr (median, 95% CI). This rate represents only 25–35% of the total slip accommodated by this section of the southern San Andreas Fault, suggesting a model in which slip is less concentrated on the Banning strand than previously thought. In rejecting the possibility that the Banning strand is the dominant structure, our results highlight an even greater need for slip rate and paleoseismic measurements along faults in the northwestern Coachella Valley in order to test the validity of current earthquake hazard models. In addition, our comparison of ages measured with U-series and 10Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms.
Dejong BD, Bierman PR, Newell WL, et al., 2015, Pleistocene relative sea levels in the Chesapeake Bay region and their implications for the next century, GSA Today, Vol: 25, Pages: 4-10, ISSN: 1052-5173
Today, relative sea-level rise (3.4 mm/yr) is faster in the Chesapeake Bay region than any other location on the Atlantic coast of North America, and twice the global average eustatic rate (1.7 mm/yr). Dated interglacial deposits suggest that relative sea levels in the Chesapeake Bay region deviate from global trends over a range of timescales. Glacio-isostatic adjustment of the land surface from loading and unloading of continental ice is likely responsible for these deviations, but our understanding of the scale and timeframe over which isostatic response operates in this region remains incomplete because dated sea-level proxies are mostly limited to the Holocene and to deposits 80 ka or older. To better understand glacio-isostatic control over past and present relative sea level, we applied a suite of dating methods to the stratigraphy of the Blackwater National Wildlife Refuge, one of the most rapidly subsiding and lowest-elevation surfaces bordering Chesapeake Bay. Data indicate that the region was submerged at least for portions of marine isotope stage (MIS) 3 (ca. 60-30 ka), although multiple proxies suggest that global sea level was 40-80 m lower than present. Today MIS 3 deposits are above sea level because they were raised by the Last Glacial Maximum forebulge, but decay of that same forebulge is causing ongoing subsidence. These results suggest that glacio-isostasy controlled relative sea level in the mid-Atlantic region for tens of thousands of years following retreat of the Laurentide Ice Sheet and continues to influence relative sea level in the region. Thus, isostatically driven subsidence of the Chesapeake Bay region will continue for millennia, exacerbating the effects of global sea-level rise and impacting the region's large population centers and valuable coastal natural resources.
Winsor K, Carlson AE, Caffee MW, et al., 2015, Rapid last-deglacial thinning and retreat of the marine-terminating southwestern Greenland ice sheet, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 426, Pages: 1-12, ISSN: 0012-821X
Marine-terminating outlet glaciers are a major source of modern ice loss from the Greenland Ice Sheet (GrIS), but their role in GrIS retreat during the last deglaciation is not well constrained. Here, we develop deglacial outlet glacier retreat chronologies for four regions in southwest and south Greenland to improve understanding of spatial variations in centennial- to millennial-scale ice loss under a warming climate. We calculate 10Be surface exposure ages of boulders located in fjords near the towns of Qaqortoq, Paamiut, Nuuk, and Sisimiut. Our northernmost study site, Sisimiut, deglaciated earliest at ∼18 ka to ∼15 ka with an average thinning rate of 0.1–0.3 m yr−1. Inland retreat from Sisimiut to the modern ice margin took ∼7 ka at an average retreat rate of 15–20 m yr−1. A 10Be-dated moraine ∼25 km from the modern GrIS margin deposited at ∼8 ka suggests a possible ice-margin still-stand, but this does not change overall retreat rates. After retreat from the small coastal Sisimiut fjords, the GrIS margin was mainly land-terminating in this region. In contrast, earliest exposure occurred at ∼12 ka near Qaqortoq, and 11–10 ka near Nuuk and Paamiut, with ice thinning at rates of 0.2–0.3 m yr−1 to instantaneous within measurement uncertainty. Ice retreat inland through the extensive Nuuk, Paamiut, and Qaqortoq fjord systems to near modern ice margins occurred in <1 ka, resulting in minimum retreat rates of 25–65 m yr−1 and maximum retreat rates of ∼95 m yr−1 to instantaneous within the uncertainty of our measurements. This rapid thinning and retreat of marine-terminating southwest GrIS margins is contemporaneous with an incursion of relatively warm ocean waters into the Labrador Sea and toward the southwest Greenland coast, suggesting that a warming ocean may have contributed to the more rapid retreat of marine GrIS termini in the Nuuk, Paa
Pupim FDN, Bierman PR, Assine ML, et al., 2015, Erosion rates and landscape evolution of the lowlands of the Upper Paraguay river basin (Brazil) from cosmogenic <SUP>10</SUP>Be, GEOMORPHOLOGY, Vol: 234, Pages: 151-160, ISSN: 0169-555X
- Author Web Link
- Cite
- Citations: 13
Wyshnytzky CE, Ouimet WB, McCarthy J, et al., 2015, Meteoric <SUP>10</SUP>Be, clay, and extractable iron depth profiles in the Colorado Front Range: Implications for understanding soil mixing and erosion, CATENA, Vol: 127, Pages: 32-45, ISSN: 0341-8162
- Author Web Link
- Cite
- Citations: 16
Portenga EW, Bierman PR, Duncan C, et al., 2015, Erosion rates of the Bhutanese Himalaya determined using <i>in situ</i>-produced <SUP>10</SUP>Be, GEOMORPHOLOGY, Vol: 233, Pages: 112-126, ISSN: 0169-555X
- Author Web Link
- Cite
- Citations: 48
Reusser L, Bierman P, Rood D, 2015, Quantifying human impacts on rates of erosion and sediment transport at a landscape scale, Geology (Boulder), Vol: 43, Pages: 171-174, ISSN: 0091-7613
Establishing background (geologic) rates of erosion is prerequisite to quantifying the impact of human activities on Earth’s surface. Here, we present 10Be estimates of background erosion rates for ten large (10,000–100,000 km2) river basins in the southeastern United States, an area that was cleared of native forest and used intensively for agriculture. These 10Be-based rates are indicative of the pace at which the North American passive-margin landscape eroded before European settlement (∼8 m/m.y.). Comparing these background rates to both rates of post-settlement hillslope erosion and to river sediment yields for the same basins, we find that following peak disturbance (late 1800s and early 1900s), rates of hillslope erosion (∼950 m/m.y.) exceeded 10Be-determined background rates more than one-hundred fold. Although large-basin sediment yields during peak disturbance increased 5–10× above pre-settlement norms, rivers at the time were transporting only ∼6% of the eroded material; work by others suggests that the bulk of historically eroded material remained and still remains as legacy sediment stored at the base of hillslopes and along valley bottoms. Because background erosion rates, such as we present here, reflect the rate at which soil is generated over millennial time scales, they can inform and enhance landscape-management strategies.
Corbett LB, Bierman PR, Lasher GE, et al., 2015, Landscape chronology and glacial history in Thule, northwest Greenland, QUATERNARY SCIENCE REVIEWS, Vol: 109, Pages: 57-67, ISSN: 0277-3791
- Author Web Link
- Cite
- Citations: 22
Ouimet W, Dethier D, Bierman P, et al., 2015, Spatial and temporal variations in meteoric <SUP>10</SUP>Be inventories and long-term deposition rates, Colorado Front Range, QUATERNARY SCIENCE REVIEWS, Vol: 109, Pages: 1-12, ISSN: 0277-3791
- Author Web Link
- Cite
- Citations: 12
McPhillips D, Bierman PR, Rood DH, 2014, Millennial-scale record of landslides in the Andes consistent with earthquake trigger, NATURE GEOSCIENCE, Vol: 7, Pages: 925-930, ISSN: 1752-0894
- Author Web Link
- Cite
- Citations: 41
Fogwill CJ, Turney CSM, Golledge NR, et al., 2014, Drivers of abrupt Holocene shifts in West Antarctic ice stream direction determined from combined ice sheet modelling and geologic signatures, Antarctic Science, Vol: 26, Pages: 674-686, ISSN: 1365-2079
Determining the millennial-scale behaviour of marine-based sectors of the West Antarctic Ice Sheet (WAIS) is critical to improve predictions of the future contribution of Antarctica to sea level rise. Here high-resolution ice sheet modelling was combined with new terrestrial geological constraints (in situ14C and 10Be analysis) to reconstruct the evolution of two major ice streams entering the Weddell Sea over 20 000 years. The results demonstrate how marked differences in ice flux at the marine margin of the expanded Antarctic ice sheet led to a major reorganization of ice streams in the Weddell Sea during the last deglaciation, resulting in the eastward migration of the Institute Ice Stream, triggering a significant regional change in ice sheet mass balance during the early to mid Holocene. The findings highlight how spatial variability in ice flow can cause marked changes in the pattern, flux and flow direction of ice streams on millennial timescales in this marine ice sheet setting. Given that this sector of the WAIS is assumed to be sensitive to ocean-forced instability and may be influenced by predicted twenty-first century ocean warming, our ability to model and predict abrupt and extensive ice stream diversions is key to a realistic assessment of future ice sheet sensitivity.
Nichols KK, Bierman PR, Rood DH, 2014, <sup>10</sup>Be constrains the sediment sources and sediment yields to the Great Barrier Reef from the tropical Barron River catchment, Queensland, Australia, Geomorphology, Vol: 224, Pages: 102-110, ISSN: 0169-555X
Estimates of long-term, background sediment generation rates place current and future sediment fluxes to the Great Barrier Reef in context. Without reliable estimates of sediment generation rates and without identification of the sources of sediment delivered to the reef prior to European settlement (c. 1850), determining the necessity and effectiveness of contemporary landscape management efforts is difficult. Here, using the ~2100-km2 Barron River catchment in Queensland, Australia, as a test case, we use in situ-produced 10Be to derive sediment generation rate estimates and use in situ and meteoric 10Be to identify the source of that sediment, which enters the Coral Sea near Cairns. Previous model-based calculations suggested that background sediment yields were up to an order of magnitude lower than contemporary sediment yields. In contrast, in situ 10Be data indicate that background (43 t km-2 y-1) and contemporary sediment yields (~45 t km-2 y-1) for the Barron River are similar. These data suggest that the reef became established in a sediment flux similar to what it receives today. Since western agricultural practices increased erosion rates, large amounts of sediment mobilized from hillslopes during the last century are probably stored in Queensland catchments and will eventually be transported to the coast, most likely in flows triggered by rare but powerful tropical cyclones that were more common before European settlement and may increase in strength as climate change warms the south Pacific Ocean. In situ and meteoric 10Be concentrations of Coral Sea beach sand near Cairns are similar to those in rivers on the Atherton Tablelands, suggesting that most sediment is derived from the extensive, low-gradient uplands rather than the steep, more rapidly eroding but beach proximal escarpment. © 2014 .
Bierman PR, Coppersmith R, Hanson K, et al., 2014, A cosmogenic view of erosion, relief generation, and the age of faulting in southern Africa, GSA Today, Vol: 24, Pages: 4-11, ISSN: 1052-5173
Southernmost Africa, with extensive upland geomorphic surfaces, deep canyons, and numerous faults, has long interested geoscientists. A paucity of dates and low rates of background seismicity make it challenging to quantify the pace of landscape change and determine the likelihood and timing of fault movement that could raise and lower parts of the landscape and create associated geohazards. To infer regional rates of denudation, we measured10Be in river sediment samples and found that south-central South Africa is eroding ∼5 m m.y.-1, a slow erosion rate consistent with those measured in other non-tectonically active areas, including much of southern Africa. To estimate the rate at which extensive, fossil, upland, silcrete-mantled pediment surfaces erode, we measured10Be and26Al in exposed quartzite samples. Undeformed upland surfaces are little changed since the Pliocene; some have minimum exposure ages exceeding 2.5 m.y. (median, 1.3 m.y.) and maximum erosion rates of <0.2 m m.y.-1(median, 0.34 m m.y.-1), consistent with no Quaternary movement on faults that displace the underlying quartzite but not the silcrete cover. We directly dated a recent displacement event on the only recognized Quaternary-active fault in South Africa, a fault that displaces both silcrete and the underlying quartzite. The concentrations of10Be in exposed fault scarp samples are consistent with a 1.5 m displacement occurring ca. 25 ka. Samples from this offset upland surface have lower minimum limiting exposure ages and higher maximum erosion rates than those from undeformed pediment surfaces, consistent with Pleistocene earthquakes and deformation reducing overall landscape stability proximal to the fault zone. Rates of landscape change on the extensive, stable, silcretized, upland pediment surfaces are an order of magnitude lower than basin-average erosion rates. As isostatic response to regional denudation uplifts the entire landscape at several meters per million years, valleys
Winsor K, Carlson AE, Rood DH, 2014, <sup>10</sup>Be dating of the Narsarsuaq moraine in southernmost Greenland: Evidence for a late-Holocene ice advance exceeding the Little Ice Age maximum, Quaternary Science Reviews, Vol: 98, Pages: 135-143, ISSN: 0277-3791
In southernmost Greenland near Narsarsuaq, the terminal Narsarsuaq moraine was deposited well outside of a historical Little Ice Age (LIA) moraine adjacent to the modern ice margin. Using 10Be surface exposure dating, we determine Narsarsuaq moraine abandonment at 1.51±0.11ka. A second set of 10Be ages from a more ice-proximal position shows that ice has been within or at its historical (i.e., LIA) extent since 1.34±0.15ka. Notably, Narsarsuaq moraine abandonment was coincident with climate amelioration in southern Greenland. Southern Greenland warming at ~1.5ka was also concurrent with the end of the Roman Warm Period as climate along the northern North Atlantic sector of Europe cooled into the Dark Ages. The warming of southern Greenland and retreat of ice from the Narsarsuaq moraine is consistent with studies suggesting possible anti-phase centennial-scale climate variability between northwestern Europe and southern Greenland. Other southernmost Greenland ice-margin records do not preclude a pre-LIA ice-margin maximum, potentially concurrent with a Narsarsuaq advance prior to ~1.51ka, but also lack sufficient ice-margin control to confirm such a correlation. We conclude that there is a clear need to further determine whether a late-Holocene pre-LIA maximum was a local phenomenon or a regional southern Greenland ice maximum, and if this advance and retreat reflects a regional fluctuation in climate. © 2014.
Xu S, Freeman SPHT, Rood DH, et al., 2014, <sup>26</sup>Al interferences in accelerator mass spectrometry measurements, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, Vol: 333, Pages: 42-45, ISSN: 0168-583X
The identification of interferences to 26Al was conducted with a 5 MV tandem accelerator mass spectrometer. In addition to 9Be 1+, 17O2+ and 35Cl4+ ions observed previously, this study confirmed existence of the most significant interference 37Cl4+ continuum ion to 16 MeV 26Al3+ by measuring primary standard mixed with Cl with various 37Cl/35Cl ratios. The 37Cl- ions were formed by 37Cl16O- molecular-dissociation before the injection magnet, resulting in -0.7% of 26Al- magnetic rigidity. Subsequently, the 37Cl4+ ions have ME/q2 value that differ from 26Al3+ by -0.1%. These allow the 37Cl - and 37Cl4+ to simultaneously pass through injection magnet, analytical magnet and high-energy analyser, and finally reach the detector with 26Al3+. Further investigations on high charge states (26Al5+ and 26Al7+) indicate that the problem of interferences is generic. That is, interferences closest to 24 MeV 26Al5+ ions include 10B 2+, 16O3+, 35Cl7+ and 37Cl7+ ions, while 32 MeV 26Al7+ ions may be interfered by 7Li2+, 16O 4+, 18O5+, 35Cl9+ and 37Cl9+. However, it remains unclear that 37Cl continuum events observed in 26Al3+-AMS do not exist in 26Al5+ and 26Al7+-AMS operations. © 2014 Elsevier B.V. All rights reserved.
Carlson AE, Winsor K, Ullman DJ, et al., 2014, Earliest Holocene south Greenland ice sheet retreat within its late Holocene extent, Geophysical Research Letters, Vol: 41, Pages: 5514-5521, ISSN: 0094-8276
Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.
Nelson AH, Bierman PR, Shakun JD, et al., 2014, Using in situ cosmogenic <sup>10</sup>Be to identify the source of sediment leaving Greenland, Earth Surface Processes and Landforms, Vol: 39, Pages: 1087-1100, ISSN: 0197-9337
We use the concentration of in situ 10Be in quartz isolated from fluvial and morainal sand to trace sediment sources and to determine the relative contribution of glacerized and deglaciated terrain to Greenland's sediment budget. We sampled along the western, eastern, and southern margins of the Greenland Ice Sheet, and collected sediment sourced from glacerized (n=19) and non-glacerized terrain (n=10), from channels where sediment from glacerized and non-glacerized terrain is mixed (n=28), from Holocene glacial-fluvial terraces (n=4), and from one sand dune. In situ 10Be concentrations in sediment range from 1600 to 34 000 atoms g-1. The concentration of in situ 10Be in sediment sourced from non-glacerized terrain is significantly higher than in sediment sourced from glacerized areas, in mixed channel sediment, and in terrace sediment that was deposited during the Holocene. To constrain the timing of landscape exposure for the deglaciated portion of the Narsarsuaq field area in southern Greenland, we measured in situ 10Be concentration in bedrock (n=5) and boulder (n=6) samples. Paired bedrock and boulder ages are indistinguishable at 1σ uncertainty and indicate rapid exposure of the upland slopes at ~10.5ka.The isotope concentration in sediment sourced from non-glacerized terrain is higher than in sediment sourced from glacerized terrain because the non-glacerized landscape has been exposed to cosmic radiation since early Holocene deglaciation. Sediment from glacerized areas contains a low, but measurable concentration of 10Be that probably accumulated at depth during a prolonged period of exposure, probably before the establishment of the Greenland Ice Sheet. The concentration of 10Be in mixed fluvial sediment and in terrace sediment is low, and similar to the concentration in sediment from glacerized areas, which indicates that the Greenland Ice Sheet is the dominant source of sediment moving through the landscape outside the glacial margin in the areas we
Reusche M, Winsor K, Carlson AE, et al., 2014, <sup>10</sup>Be surface exposure ages on the late-Pleistocene and Holocene history of Linnébreen on Svalbard, Quaternary Science Reviews, Vol: 89, Pages: 5-12, ISSN: 0277-3791
Arctic glaciers were sensitive to past changes in high-latitude winter precipitation and summer temperature. Here we develop a late-Pleistocene to Holocene history for Linnébreen (Linné Glacier) in western Svalbard using 10Be surface exposure ages on isolated erratic and moraine boulders. We show that Linnébreen had separated from the larger ice sheet over Svalbard and was retreating up valley around the start of the Younger Dryas cold period. We attribute this retreat during a cold period on Svalbard to moisture starvation of Linnébreen from advanced sea ice and/or elevated shortwave boreal summer insolation that overwhelmed any reduction in sensible heat. After an ice-free period during the early to middle Holocene, Linnébreen reformed sometime after 4.6±0.2ka, and was at a position roughly equivalent to its Little Ice Age (LIA) maximum extent before it began to retreat at 1.6±0.2ka. Comparison with calibrated 14C dates from three other glaciers could suggest that this period of ice retreat at ~1.6ka could be regional in extent. Linnébreen occupied the pre-LIA moraine when there was an increased ratio of cold Arctic-sourced relative to warm Atlantic-sourced waters around Svalbard and advanced sea ice. The retreat of Linnébreen at ~1.6ka was concurrent with the increased presence of warm Atlantic waters around Svalbard and attendant sea-ice retreat. These coincident changes in ocean temperatures, sea-ice extent, and Linnébreen moraine age could imply a climatic forcing of the pre-LIA advance and retreat of Linnébreen. Summer temperatures, rather than changes in precipitation, would then be dominant in driving ice retreat, although the possibility of stochastic glacier-margin variability cannot be excluded. Our data therefore suggest that Linnébreen may have responded differently to past changes in sea-ice extent that could depend on the background climate state (deglacial climate vs
Johnson JS, Bentley MJ, Smith JA, et al., 2014, Rapid thinning of pine island glacier in the early holocene, Science, Vol: 343, Pages: 999-1001, ISSN: 0036-8075
Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet, has been undergoing rapid thinning and retreat for the past two decades. We demonstrate, using glacial-geological and geochronological data, that Pine Island Glacier (PIG) also experienced rapid thinning during the early Holocene, around 8000 years ago. Cosmogenic (10)Be concentrations in glacially transported rocks show that this thinning was sustained for decades to centuries at an average rate of more than 100 centimeters per year, which is comparable with contemporary thinning rates. The most likely mechanism was a reduction in ice shelf buttressing. Our findings reveal that PIG has experienced rapid thinning at least once in the past and that, once set in motion, rapid ice sheet changes in this region can persist for centuries.
Dethier DP, Ouimet W, Bierman PR, et al., 2014, Basins and bedrock: Spatial variation in 10Be erosion rates and increasing relief in the southern Rocky Mountains, USA, Geology, Vol: 42, Pages: 167-170, ISSN: 0091-7613
© 2014 Geological Society of America. We used measurements of cosmogenic 10Be in alluvium to estimate erosion rates on a 103-104 yr time scale for small (0.01-47 km2), unglaciated basins in northern Colorado, southern Wyoming, and adjacent western Nebraska (western United States). Basins formed in Proterozoic cores of Laramide ranges are eroding more slowly (23 ± 7 mm k.y.-1, n = 19) than adjacent basins draining weakly lithified Cenozoic sedimentary rocks (75 ± 36 mm k.y. -1, n = 20). Erosion rates show a relationship to rock resistance and, for granitic rocks, to basin slope, but not to mean annual precipitation. We estimated longer-term (> 105 yr time scale) erosion rates for the granitic core of the Front Range by measuring the concentration of 10Be and 26Al produced mainly by muon interactions at depths 1.7-10 m below the surface. Concentrations imply erosion rates of 9-31 mm k.y. -1, similar to shorter-term erosion rates inferred from alluvial sediment. The spatial distribution of erosion rates and stratigraphic evidence imply that relief in the southern Rocky Mountains increased in the late Cenozoic; modern relief probably dates from post-middle Miocene time.
Davis M, Matmon A, Placzek CJ, et al., 2014, Cosmogenic nuclides in buried sediments from the hyperarid Atacama Desert, Chile, Quaternary Geochronology, Vol: 19, Pages: 117-126, ISSN: 1871-1014
The evolution of Terrestrial Cosmogenic Nuclides (TCN) from an alluvial section in the Atacama Desert is examined. We reconstruct a burial history for the last ~10Ma using 40Ar/39Ar dating of volcanic ash layers interbedded with alluvial sediments; this independent dating allows us to distinguish between the effects of erosion, post-burial subsurface production, and radioactive decay during burial on TCN concentrations. Our TCN results show significant post-burial production, which is the result of the extremely slow sedimentation rate (~3m/Ma) and the old age of the sediments. Although distinct differences in TCN concentrations are apparent between the lower and upper parts of the sedimentary section, we show that these differences are most likely related to post-burial production and age, and not to changes in bedrock erosion rates or changes in elevation due to tectonic activity. Our approach provides a test to the applicability of the two-isotope cosmogenic burial dating system (26Al-10Be) in regions of extremely slow sedimentation rates. Our results reveal geomorphic stability in terms of erosion and sedimentation rates for the late Miocene-Pliocene in the Atacama Desert. © 2013 Elsevier B.V.
Bierman PR, Corbett LB, Graly JA, et al., 2014, Preservation of a preglacial landscape under the center of the greenland ice sheet, Science, Vol: 344, Pages: 402-405, ISSN: 0036-8075
Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been nonerosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.