East Kilbride, United Kingdom
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Roberts D.H.,Durham University | Rea B.R.,University of Aberdeen | Lane T.P.,Durham University | Schnabel C.,NERC Cosmogenic Isotope Analysis Facility | Rodes A.,NERC Cosmogenic Isotope Analysis Facility
Journal of Geophysical Research: Earth Surface | Year: 2013

This paper presents the first assessment of the Uummannaq ice stream system (UISS) in West Greenland. The UISS drained ~6% of the Greenland ice sheet (GrIS) at the Last Glacial Maximum (LGM). The onset of the UISS is a function of a convergent network of fjords which feed a geologically controlled trough system running offshore to the shelf break. Mapping, cosmogenic radiogenic nuclide (CRN) dating, and model output reveal that glacially scoured surfaces up to 1266 m above sea level (asl) in fjord-head areas were produced by warm-based ice moving offshore during the LGM, with the elevation of warm-based ice dropping westwards to ~700 m asl as the ice stream trunk zone developed. Marginal plateaux with allochthonous blockfields suggest that warm-based ice produced till and erratics up to ~1200 m asl, but CRN ages and weathering pits suggest this was pre-LGM, with only cold-based ice operating during the LGM. Deglaciation began on the outer shelf at ~14.8 cal. kyrs B.P., with Ubekendt Ejland becoming ice free at ~12.4 ka. The UISS then collapsed with over 100 km of retreat by ~11.4 ka-10.8 cal. kyrs B.P., a rapid and complex response to bathymetric deepening, trough widening, and sea-level rise coinciding with rapidly increasing air temperatures and solar radiation, but which occurred prior to ocean warming at ~8.4 cal. kyrs B.P. Local fjord constriction temporarily stabilized the unzipped UISS margins at the start of the Holocene before ice retreat inland of the current margin at ~8.7 ka. Key Points Topograpy and geology control ice stream evolution Ice streams control regional landscape evolution through thermal partitioning Rapid deglaciation accelerated by bathymetric overdeepening + sea-level rise ©2013. American Geophysical Union. All Rights Reserved.


Vermeesch P.,University of London | Fenton C.R.,NERC Cosmogenic Isotope Analysis Facility | Kober F.,ETH Zurich | Wiggs G.F.S.,University of Oxford | And 2 more authors.
Nature Geoscience | Year: 2010

The Namib Sand Sea is one of the world's oldest and largest sand deserts1, yet little is known about the source of the sand in this, or other large deserts2. In particular, it is unclear whether the sand is derived from local sediment or comes from remote sources. The relatively uniform appearance of dune sands and low compositional variability within dune fields3 make it difficult to address this question. Here we combine cosmogenic-nuclide measurements and geochronological techniques to assess the provenance and migration history of sand grains in the Namib Sand Sea. We use U-Pb geochronology of detrital zircons to show that the primary source of sand is the Orange River at the southern edge of the Namib desert. Our burial ages obtained from measurements of the cosmogenic nuclides 10Be, 26Al and 21Ne suggest that the residence time of sand within the sand sea is at least one million years. We therefore conclude that, despite large climatic changes in the Namib region associated with Quaternary -interglacial cycles4,5, the area currently occupied by the Namib Sand Sea has never been entirely devoid of sand during the past million years. © 2010 Macmillan Publishers Limited. All rights reserved.


Walker R.T.,University of Oxford | Khatib M.M.,Birjand University | Bahroudi A.,Geological Survey of Iran | Bahroudi A.,University of Tehran | And 6 more authors.
Geomorphology | Year: 2013

We describe the seismicity and late Cenozoic deformation associated with a blind thrust fault at Tabas-e-Golshan (hereafter referred to as Tabas), eastern Iran, which generated a devastating Mw 7.3 earthquake on the 16th September 1978. Measurements from a structural transect through the Sardar anticline segment indicate fault-propagation folding above a gently ~ 20° eastward-dipping blind thrust fault. The thrust flattens into a horizontal detachment at a depth of only ~ 2 km. Tightening of the fold forelimb is accommodated by flexural slip along numerous bedding planes, with many of the slip surfaces showing fresh striations with a large component of right-lateral strike-slip. A steeply dipping fault zone showing almost pure strike-slip is also developed within the forelimb of the fold. Our field observations are consistent with the source parameters of the 1978 Tabas earthquake, and additional events in 1979 and 1980, which all involved slip on a shallowly-dipping thrust with a significant component of right-lateral slip. The surface of an alluvial fan, which is likely to have been abandoned at 8-10 ka, has been folded as it crosses the Sardar anticline. The age constraints, combined with topographic profiles along the deformed fan surface and constraints on the dip of the fault at depth, provide an approximate rate of horizontal shortening of ~ 1.5 mm/yr. Shortening at Tabas appears to result from transpressional bending at the north end of the Nayband strike-slip fault. A northward continuation of the Nayband Fault, which may be slipping at rates of > 2 mm/yr, is identified along the base of the Shotori Mountains ~ 10-20 km east of the Tabas thrust. The range-front fault did not move in 1978 and constitutes an additional threat to local populations. © 2013.


Ballantyne C.K.,University of St. Andrews | Wilson P.,University of Ulster | Gheorghiu D.,NERC Cosmogenic Isotope Analysis Facility | Rodes A.,NERC Cosmogenic Isotope Analysis Facility
Earth Surface Processes and Landforms | Year: 2014

The temporal pattern of rock-slope failures (RSFs) following Late Pleistocene deglaciation on tectonically stable terrains is controversial: previous studies variously suggest (1) a rapid response due to removal of supporting ice ('debuttressing'), (2) a progressive decline in RSF frequency, and (3) a millennial-scale delay before peak RSF activity. We test these competing models through beryllium-10 (10Be) exposure dating of five closely-spaced quartzite RSFs on the Isle of Jura, Scotland, to establish the relationship between timing of failure and those of deglaciation, episodes of rapid warming and periods of rapid glacio-isostatic uplift. All five dated RSFs occurred at least 720-2240 years after deglaciation, with the probability of failure peaking ~2 ka after deglaciation, consistent with millennial-scale delay model (3). This excludes debuttressing as an immediate cause of failure, though it is likely that time-dependent stress release due to deglacial unloading resulted in progressive development of failure planes within the rock. Thaw of permafrost ice in joints is unlikely to have been a prime trigger of failure as some RSFs occurred several centuries after the onset of interstadial warming. Conversely, the timespan of the RSFs coincides with the period of maximum glacio-isostatic crustal uplift, suggesting that failure was triggered by uplift-driven seismic events acting on fractured rock masses. Implications of this and related research are: (1) that retreat of the last Pleistocene ice sheets across tectonically-stable mountainous terrains was succeeded by a period of enhanced rock-slope failure due to deglacial unloading and probably uplift-driven seismicity; (2) that the great majority of RSFs in the British Isles outside the limits of Loch Lomond Stadial (= Younger Dryas) glaciation are of Lateglacial (pre-Holocene) age; and (3) numerous RSFs must also have occurred inside Loch Lomond Stadial (LLS) glacial limits, but that runout debris was removed by LLS glaciers. © 2013 John Wiley & Sons, Ltd.


Ballantyne C.K.,University of St. Andrews | Rinterknecht V.,University of St. Andrews | Gheorghiu D.M.,NERC Cosmogenic Isotope Analysis Facility
Journal of Quaternary Science | Year: 2013

During the last glacial maximum, the Galloway Hills in southwest Scotland acted as a major centre of ice dispersion within the last British-Irish Ice Sheet (BIIS). Six new or recalibrated 10Be exposure ages for samples obtained from boulders near the former ice divide yielded uncertainty-weighted mean ages of 15.15±0.72 ka (Lm scaling), or 15.33±0.74 ka (Du scaling). These ages indicate that the former ice dome centred over the Galloway Hills had almost (or completely) disappeared by ca. 15 ka, imply prior deglaciation of all of southwest Scotland and refute suggestions that ice cover persisted in this area during the Lateglacial Interstade. They strongly support recent models advocating extensive deglaciation of all areas occupied by the last BIIS (apart from the Scottish Highlands) prior to warming at the onset of the Lateglacial Interstade (ca. 14.7 ka). Three samples obtained from boulders on a large latero-terminal moraine (Tauchers moraine) near the former ice divide yielded a weighted mean age of 11.91±0.77 ka (Lm scaling) or 12.01±0.78 ka (Du scaling), confirming that the moraine is of Loch Lomond (Younger Dryas) Stadial age, and suggesting that the moraine was deposited 200-500 years before the end of the stade. © 2013 John Wiley & Sons, Ltd.


Gregory L.C.,University of Oxford | Thomas A.L.,University of Oxford | Thomas A.L.,University of Edinburgh | Walker R.T.,University of Oxford | And 11 more authors.
Quaternary Geochronology | Year: 2014

Time-averaged fault slip-rates can be established by reliably dating the abandonment of an alluvial deposit that has been displaced by Quaternary movement along a cross-cutting fault. Unfortunately, many Quaternary dating techniques are hindered by uncertainties inherent to individual geochronometers. Such uncertainties can be minimised by combining multiple independent techniques. In this study, we combine 10Be exposure dating of boulder tops and U-series dating of layered pedogenic carbonate cements accumulated on the underside of clasts from two separate alluvial surfaces. These surfaces are both displaced by the active Ölgiy strike-slip fault in the Mongolian Altay Mountains. We date individual layers of pedogenic carbonate, and for the first time apply a Bayesian statistical analysis to the results to develop a history of carbonate accumulation. Our approach to the U-series dating provides an age of initiation of carbonate cement formation and avoids the problem of averaging contributions from younger layers within the carbonate. The U-series ages make it possible to distinguish 10Be samples that have anomalously young exposure ages and have hence been subject to the effects of post-depositional erosion or exhumation. The combination of 10Be and U-series dating methods provides better constrained age estimates than using either method in isolation and allows us to bracket the abandonment ages of the two surfaces as 18.0-28.1kyr and 38.4-76.4kyr. Our ages, combined with measurements of the displacement of the surfaces, yield a right-lateral slip-rate for the Ölgiy fault of 0.3-1.3mmyr-1, showing that it is a relatively important structure within the active tectonics of Mongolia and that it constitutes a substantial hazard to local populations. © 2014 Elsevier B.V.


Wilcken K.M.,Scottish Enterprise | Freeman S.P.H.T.,Scottish Enterprise | Dougans A.,Scottish Enterprise | Xu S.,Scottish Enterprise | And 2 more authors.
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2010

Small changes to our ion source and gas ionization detector have significantly improved Cl measurement by reducing source memory and increasing interference rejection. Gas stripped low energy 30 MeV 36Cl5+ ions are still efficiently transported to the detector but 36S vs. 36Cl separation is improved by an order of magnitude. Accordingly 36Cl/Cl background is <10-15 before additional interference correction that is also newly automated. 35Cl- currents have increased to 30 μA and 36Cl/Cl inter-cathode repeatability is 3%. © 2009 Elsevier B.V. All rights reserved.


Wilcken K.M.,Scottish Enterprise | Freeman S.P.H.T.,Scottish Enterprise | Schnabel C.,NERC Cosmogenic Isotope Analysis Facility | Binnie S.A.,University of Edinburgh | And 2 more authors.
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2013

Cosmogenic 36Cl analysis by accelerator mass spectrometry (AMS) is a valuable environmental and geological sciences research tool. Overcoming the stable nuclide 36S isobar interfering with measurement is challenging, however. Traditionally this has required large accelerators, but following recent technical advances it is now possible with ∼30 MeV ion energies. Consequently 5 MV or even smaller modern bespoke spectrometers are now 36Cl-capable, increasing accessibility and promoting wider and more varied 36Cl use. However, the technical ability to identify 36Cl ions is quite distinct from demonstrated high-performance AMS. Such is the theme of this paper. We present a systematic analysis of the accurate measurement of sample radioisotope relative to the stable chlorine, the normalisation of the measured ratio and correction for remaining 36S interference, all combined with the use of stable-isotope dilution to determine sample Cl concentration to begin with. We conclude by showing that repeated analyses support our claims for routine 3% 36Cl-AMS data. Accordingly, the modest SUERC spectrometer well competes with the performance of larger longer-established instruments, and the results may be quite generic for modern bespoke instruments. © 2012 Elsevier B.V. All rights reserved.

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