ANSTO :: Browsing by Author "Wilcken, KM"

Browsing by Author "Wilcken, KM"

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15th International Conference on Accelerator Mass Spectrometry
(Australian Nuclear Science and Technology Organisation, 2021-11-15) Bertuch, F; Child, DP; Fink, D; Fülöp, RH; Hotchkis, MAC; Hua, Q; Jacobsen, GE; Jenkinson, A; Levchenko, VA; Simon, KJ; Smith, AM; Wilcken, KM; Williams, AA; Williams, ML; Yang, B; Fallon, SJ; Wallner, T
On behalf of the AMS-15 Organising committee, we would like to thank you for attending the 15th International Conference on Accelerator Mass Spectrometry. Held as an online event for the first time, the 2021 conference attracted over 300 attendees with presentations delivered by colleagues and professionals from around the globe.Applications of AMS to the world’s most pressing problems/questions: A-1 : Earth’s dynamic climate palaeoclimate studies, human impacts on climate, data for climate modelling. A-2 : Water resource sustainability groundwater dating, hydrology, water quality and management A-3 : Living landscapes soil production, carbon storage, erosion, sediment transport, geomorphology. A-4 : Catastrophic natural events volcanoes, cyclones, earthquakes, tsunamis, space weather, mass extinctions. A-5 : Advancing human health metabolic and bio-kinetic studies, bomb-pulse dating, diagnostics and bio-tracing. A-6 : Challenges of the nuclear age nuclear safeguards, nuclear forensics, nuclear waste management, nuclear site monitoring, impacts of nuclear accidents. A-7 :Understanding the human story archaeology, human evolution and migration, history, art and cultural heritage A-8 : Understanding the cosmos fundamental physics, nuclear astrophysics, nuclear physics AMS Research and Development: T-1 : Novel AMS systems, components and techniques T-2 : Suppression of isobars and other interferences T-3 : Ion sourcery T-4 : New AMS isotopes T-5 : Advances in sample preparation T-6 : Data quality and management T-7 : Facility Reports (Poster Presentation only)
26Al/10Be ratios reveal the source of river sediments in the Kimberley, NW Australia
(Wiley, 2020-02-07) Cazes, G; Fink, D; Codilean, AT; Fülöp, RH; Fujioka, T; Wilcken, KM
We use cosmogenic 10Be and 26Al in both bedrock and fluvial sediments to investigate controls on erosion rates and sediment supply to river basins at the regional scale in the Kimberley, NW Australia. The area is characterised by lithologically controlled morphologies such as cuestas, isolated mesas and extensive plateaus made of slightly dipping, extensively jointed sandstones. All sampled bedrock surfaces at plateau tops, ridgelines, and in the broader floodplain of major rivers over the region show similar slow lowering rates between 0.17 and 4.88 m.Myr-1, with a mean value of 1.0 ± 0.6 m.Myr-1 (n=15), whilst two bedrock samples collected directly within river-beds record rates that are one to two orders of magnitude higher (14.4 ± 1.5 and 20.9 ± 2.5 m.Myr-1, respectively). Bedrock 26Al/10Be ratios are all compatible with simple, continuous sub-aerial exposure histories. Modern river sediment yield lower 10Be and 26Al concentrations, apparent 10Be basin-wide denudation rates ranging between 1.8 and 7.7 m.Myr-1, with a median value of 2.6 m.Myr-1, more than double the magnitude of bedrock erosion rates. 26Al/10Be ratios of the sediment samples are lower than those obtained for bedrock samples. We propose that these depleted 26Al/10Be ratios can largely be explained by the supply of sediment to river basins from the slab fragmentation and chemical weathering of channel gorge walls and plateau escarpments that result in diluting the cosmogenic nuclide concentration in river sediments measured at the basin outlets. The results of a mass-balance model suggest that ~60–90% of river sediment in the Kimberley results from the breakdown and chemical weathering of retreating vertical sandstone rock-walls in contrast to sediment generated by bedrock weathering and erosion on the plateau tops. This study emphasises the value of analysing two or more isotopes in basin-scale studies using cosmogenic nuclides, especially in slowly eroding post-orogenic settings. © 2019 John Wiley & Sons, Ltd.
36Cl exposure-age chronology of Late Pleistocene glaciations on Mount Tymphi, Pindus Mountains, northwest Greece
(International Union for Quaternary Research (INQUA), 2019-07-27) Allard, JL; Hughes, PD; Woodward, JC; Fink, D; Simon, KJ; Wilcken, KM
The mountains of Greece and the wider Balkans were glaciated during the Pleistocene. The most extensive glaciations occurred during the Middle Pleistocene when large ice caps and glaciers formed in several ranges including the Dinaric Alps and the Pindus Mountains. The Late Pleistocene in the Balkans, however, was characterised by smaller ice masses with glaciers restricted to the highest mountains. Nevertheless, these glaciers were important in shaping much of the high mountain landscapes that we see today and they supplied many rivers with outwash sediments. Here we present preliminary results from 36Cl terrestrial cosmogenic nuclide analyses. From samples collected in summer 2017, we have calculated exposure ages from moraine boulders and glaciated bedrock in the uppermost valleys and cirques of Mount Tymphi massif (2497 m) in northwest Greece. We address both a significant geographical gap in Mediterranean glacial chronologies and the temporal gap in the glacial history of this region by targeting the previously undated Late Pleistocene glacial record. Coupled with published U-series ages from lower elevation moraines, this is the first glacial chronology in the east-central Mediterranean based on multiple dating methods. On the southwest side of the massif, 18 exposure ages obtained from 3 moraines in the cirque and mouth of the Laccos Megalon Litharion valley and the mouth of the western cirque of Tsioumako (2155 m), indicate moraine formation between 1700 m and 2050 m a.s.l. during the Late Pleistocene. This complements previous U-series ages obtained from secondary calcites in glacial sediments below 1700 m (and elsewhere in the Balkans), which demonstrate more extensive glaciations dating to the Middle Pleistocene. At ~1430 m a.s.l. in the Laccos cirque on the northeast side of the massif, 6 exposure ages (5 boulders, 1 bedrock) from a pair of end moraines represent the first dates from this side of Mount Tymphi. They suggest small cirque glaciers persisted until at least the end of the Late Pleistocene, facilitated by avalanching snow and shading from the cliffs of Goura (2467 m). Understanding where, when and why glaciers reached their maxima during the last glacial cycle is not only important for understanding the dynamics of the glacial climate in this region, but also for the timing of sediment and meltwater delivery to river systems; the dynamics of Mediterranean refugia; and has implications for understanding the environmental context of nearby Middle and Upper Palaeolithic archaeological records.
Accelerator mass spectrometry on SIRIUS: new 6MV spectrometer at ANSTO
(University of Jyväskylä, Finland, 2016-07-03) Wilcken, KM; Fink, D; Hotchkis, MAC; Garton, DB; Button, D; Mann, M; Kitchen, R
As a part of Australian Federal Government funding in 2009 to establish a centre for accelerator science a new 6 MV state of the art accelerator – SIRIUS – was purchased. The system is now commissioned and comprises ion sources and beam lines to cater for a wide variety of both IBA and AMS applications. The ion source used for AMS (MC-SNICS) is the latest incarnation followed by 45 degree spherical ESA(R=0.3 m) and double focusing injection magnet (R=1 m, ME=20) prior the accelerator. At the terminal we have a choice of 2 stripper gasses and/or stripper foils. The high-energy spectrometer for AMS consists of a 1.27 m radius analyzing magnet with ME=176, 45 degree ESA (R=3.81m), followed by a switching magnet and 3 beam lines: one with a standard multianode ionization chamber; one with an absorber cell in front of the detector; whereas the third beam line has a time-of- ight detector. Details of the instrument design and performance data for 10Be, 26Al and 36Cl will be presented. © The Authors
Accelerator mass spectrometry on SIRIUS: new 6MV spectrometer at ANSTO
(Elsevier, 2016-07-08) Wilcken, KM; Fink, D; Hotchkis, MAC; Garton, DB; Button, D; Mann, M; Kitchen, R
The Centre for Accelerator Science at ANSTO operates four tandem accelerator systems for Accelerator Mass Spectrometry (AMS) and Ion Beam Analysis (IBA). The latest addition to the fleet is SIRIUS, a 6 MV combined IBA and AMS system. Following initial ion beam testing, conditioning and debugging software and hardware, SIRIUS is now commissioned. Details of the instrument design and performance data for 10Be, 26Al and 36Cl are presented.
Actinides AMS on the VEGA accelerator
(Elsevier B. V., 2019-01-01) Hotchkis, MAC; Child, DP; Froehlich, MB; Wallner, A; Wilcken, KM; Williams, ML
The VEGA 1MV accelerator at ANSTO is designed to be a highly versatile AMS instrument. In this paper we focus on describing those aspects of the system that are designed to optimise its performance for actinides isotopic analysis, in particular the implementation of fast isotope cycling and multiple isotope detection methods to enable isotope detection across a wide range of rates and currents. Charge state yields are reported in the energy range from 0.3 to 1.0 MeV with helium gas stripping, showing that the highest yield for the 3+ charge state occurs around 1 MeV and exceeds 40%. Accuracy and precision for uranium isotope ratios are shown to approach 1% over a wide range of concentrations and isotope ratios. The ionisation efficiency for plutonium is shown to exceed 3%, leading to overall detection efficiency over 1%. In the absence of background, this leads to sub-attogram detection limits for several Pu isotopes including 244Pu. Crown Copyright © 2018 Published by Elsevier B.V.
Actinides isotopic analysis using a 1 MV AMS system
(University of Jyväskylä, Finland, 2016-07-03) Hotchkis, MAC; Child, DP; Wilcken, KM; Kitchen, R
The VEGA 1 MV AMS system at ANSTO has been custom-designed to cover analysis of a wide range of long-lived radioisotopes, including routine radiocarbon analysis and multiple-isotope analysis of actinides. The system incorporates 1.0 m radius injection and analysing magnets with o -axis cups on high and low mass sides. Following the analysing magnet, rare isotope beams pass through a 1m radius spherical electrostatic analyser and a 120 1 m radius magnet. The detector station consists of a two-anode gas detector, with o -axis options to direct isotopes to either a Faraday cup or an electron multiplier ion counter. All three analysing magnets are fitted with electrostatic bouncer systems. At the LE end, the bouncer works in the usual way for all isotope combinations of interest, including 12C-13C-14C and actinides. The HE bouncers are used to transmit a range of masses of interest for actinides analysis, for example mass 239 to 244 Pu isotopes. For uranium analysis, the less rare isotopes can be directed to o axis cups or the ion counter. Software has been implemented to enable a high degree of exibility in analysing up to 8 isotopes at a time. In this paper we present details of the system and its performance and applications. © The Authors
Antiphase dynamics between cold-based glaciers in the Dry Valleys region and ice extent in the Ross Sea, Antarctica during MIS 5
(Australian Nuclear Science and Technology Organisation, 2021-11-17) Anderson, JTH; Fink, D; Fujioka, T; Wilson, GS; Wilcken, KM; Abramov, A; Demidov, N
During interglacial conditions in Marine Isotope Stage (MIS) 5, outlet and alpine glaciers in the Dry Valleys region, Antarctica, appear to have advanced in response to increased open ocean in the Ross Sea. However, the timing and extent of antiphase behaviour between these glaciers and ice in the Ross Sea is poorly resolved. Here, we report the antiphase behaviour through retreat of a peripheral lobe of Taylor Glacier in Pearse Valley, an area that was glaciated during MIS 5. First, we measured cosmogenic ¹⁰ Be and ²⁶ Al in 3 granite cobbles from thin, patchy drift (Taylor 2 Drift) in Pearse Valley to constrain the timing of retreat of Taylor Glacier. Assuming simple continuous exposure, our exposure ages suggest Taylor Glacier had partially retreated from Pearse Valley no later than 71 ka. Timing of retreat after 71 ka, until the Last Glacial Maximum (LGM), when Taylor Glacier was at a minimum position, remains unresolved. Second, we measured paired ¹⁰ Be and ²⁶ Al depth profiles to ~3 metres in permafrost in proximity to the cobble sampling sites at Pearse Valley and neighbouring Lower Wright Valley. The ¹⁰ Be and ²⁶ Al depth profiles from both sites show no clear exponential attenuation trend which suggest that both deposits are too young for the most recent exposure (≥71 ka for Pearse Valley and LGM – early Holocene for Lower Wright Valley) to alter the profile, and measured nuclide concentrations are essentially dominated by inheritance. Using ¹⁰ Be depth profile data from Pearse Valley we calculate a maximum age of ~100 ka for surface layer permafrost formation. The cobble exposure-ages and depth profile constrain surface permafrost formation in Pearse Valley to between ~71 – 102 ka, following the retreat of the Taylor Glacier from Pearse Valley. These new data are consistent with geochronology from central Taylor Valley, and suggest changes in moisture delivery over Taylor Dome during MIS 5c and 5a appear to be associated with the extent of the Ross Ice Shelf and sea ice in the Ross Sea. Our ²⁶ Al/¹⁰ Be concentration ratios for all depth profile samples also exhibit relatively constant and suppressed ratios of 4.3 and 5.2 for Pearse and Wright valleys, respectively, indicating that prior to surface permafrost formation, these sediments experienced a significant complex prior exposure history. Assuming a simple burial scenario, the observed ²⁶ Al/¹⁰ Be ratios are equivalent to a total exposure-burial history of ~1.2 Ma, somewhat consistent with the Packard dune field in Victoria Valley. Our new data corroborate antiphase behaviour between outlet and alpine glaciers in the Dry Valleys region and ice extent in the Ross Sea, and imply a causal mechanism with cold-based glacier advance and retreat being controlled by moisture availability and drying, respectively due to ice retreat and expansion in the Ross Sea. © The Authors
Carbonate and silicate intercomparison materials for cosmogenic 36Cl measurements
(Elsevier, 2019-09-15) Mechernich, S; Dunai, TJ; Binnie, SA; Goral, T; Heinze, S; Dewald, A; Schimmelpfennig, I; Keddadouche, K; Aumaître, G; Bourlès, D; Marrero, S; Wilcken, KM; Simon, KJ; Fink, D; Phillips, FM; Caffee, M; Gregory, LC; Phillips, R; Freeman, SPHT; Shanks, R; Sarikaya, MA; Pavetich, S; Rugel, G; Merchel, S; Akçar, N; Yesilyurt, S; Ivy-Ochs, S; Vockenhuber, C
Two natural mineral separates, labeled CoCal-N and CoFsp-N, have been prepared to serve as intercomparison material (ICM) for in situ-produced cosmogenic 36Cl and natural chlorine (Clnat) analysis. The sample CoCal-N is derived from calcite crystals in a Namibian lag deposit, while the sample CoFsp-N is derived from a single crystal of alkali-feldspar from a Namibian pegmatite. The sample preparation took place at the University of Cologne and a rotating splitter was used to obtain homogeneous splits of both ICMs. Forty-five measurements of CoCal-N (between 1 and 16 per facility) and forty-four measurements of CoFsp-N (between 2 and 20 per facility) have been undertaken by ten target preparation laboratories measured by seven different AMS facilities. The internal laboratory scatter of the 36Cl concentrations indicates no overdispersion for half of the laboratories and 3.9 to 7.3% (1σ) overdispersion for the others. We show that the CoCal-N and CoFsp-N splits are homogeneous regarding their 36Cl and Clnat concentrations. The grand average (average calculated from the average of each laboratory) yields initial consensus 36Cl concentrations of (3.74 ± 0.10) × 106 at 36Cl/g (CoCal-N) and (2.93 ± 0.07) × 106 at 36Cl/g (CoFsp-N) at 95% confidence intervals. The coefficient of variation is 5.1% and 4.2% for CoCal-N and CoFsp-N, respectively. The Clnat concentration corresponds to the lower and intermediate range of typical rock samples with (0.73 ± 0.18) µg/g in CoCal-N and (73.9 ± 6.8) µg/g in CoFsp-N. We discuss the most relevant points of the sample preparation and measurement and the chlorine concentration calculation to further approach inter-laboratory comparability. We propose to use continuous measurements of the ICMs to provide a valuable quality control for future determination of 36Cl and Clnat concentrations. © 2019 Elsevier B.V.
The Centre for Accelerator Science at ANSTO
(International Atomic Energy Agency, 2014-01-14) Hotchkis, MAC; Child, DP; Cohen, DD; Dodson, JR; Fink, D; Fujioka, T; Garton, DB; Hua, Q; Ionescu, M; Jacobsen, GE; Levchenko, VA; Mifsud, C; Pastuovic, Z; Siegele, R; Smith, AM; Wilcken, KM; Williams, AG
In 2009, the Federal government provided funding of $25m to ANSTO through the Education Investment Fund, to build state-of-the-art applied accelerator science facilities, with the primary aim of providing world-leading accelerator mass spectrometry (AMS) and ion beam analysis (IBA) facilities. New buildings are now under construction and Building plans are now well advanced, and two new accelerators are on order with National Electrostatics Corporation, USA. The 1MV AMS accelerator system is designed with the capability to perform high efficiency, high precision AMS analysis across the full mass range. Large beam-optical acceptance will ensure high quality and high throughput radiocarbon measurements. High mass resolution analyzers, at low and high energy, coupled to a novel fast isotope switching system, will enable high quality analysis of actinide radioisotopes. The 6MV tandem accelerator will be instrumented with a wide range of AMS, IBA and ion irradiation facilities. The three ion sources include hydrogen and helium sources, and a MCSNICS sputter source for solid materials. The AMS facility has end stations for (i) a gasabsorber detector for 10Be analysis, (ii) a time-of-flight detector, (iii) a gas-filled magnet and(iv) a general use ionization detector suited to 36Cl and other analyses. Initially, there will be four IBA beamlines, including a new ion beam microprobe currently on order with Oxford Microbeams. The other beamlines will include an on-line ion implanter, nuclear reaction analysis and elastic recoil detection analysis facilities. The beam hall layout allows for future expansion, including the possibility of porting the beam to the existing ANTARES beam hall for simultaneous irradiation experiments.Two buildings are currently under construction, one for the new accelerators and the other for new chemistry laboratories for AMS and mass spectrometry facilities. The AMS chemistry labs are planned in two stages, with the new radiocarbon labs to come in the second phase of work.
Chemical removal of sulphur from AgCl and AgBr for 36Cl measurements at ANSTO
(Australian Nuclear Science and Technology Organisation, 2021-11-17) Simon, KJ; Wilcken, KM; Amatya Joshi, A
Measurements of 36Cl on the 6MV tandem accelerator (SIRIUS) at ANSTO began in 2016, and since then over 1000 groundwater and rock samples have been processed and measured. The challenge with the sample preparation for 36Cl is to keep the 36S rates consistently low to minimise the impact to the 36Cl ion detection. This is generally achieved by precipitating sulphate as BaSO4 before a final precipitation as AgCl. We tested a range of methods for their efficacy, ease of use and consistency in keeping the 36S rates low. For measurement, the AgCl is backed in a bed of AgBr, but the sulphur rate of commercially available AgBr can vary significantly between batches. Preparing AgBr in house can produce very low sulphur rates [1,2]. Alternatively, we have achieved similarly low 36S rates by treating commercially available AgBr in 1M HNO3 for >24 hours.
Choosing the right stripper gas from AMS and other applications with tandem accelerators at low and medium terminal voltage
(Department of Nuclear Physics Research School of Physics and Engineering, The Australian National University, 2013-04-11) Hotchkis, MAC; Child, DP; Fink, D; Levchenko, VA; Wilcken, KM
Recent experimentation with stripper gases used for Accelerator Mass Spectrometry (AMS) has seen a divergence in the practices adopted at laboratories performing AMS of high mass species (such as actinides) at low and medium terminal voltages. At low voltages (<1 MV), the Ion Beam Group at ETH Zurich has demonstrated the advantages of using helium as the stripper gas, for both radiocarbon AMS [1] and for actinides AMS [2]. Meanwhile, at ANSTO we have investigated several gases at 4 MV [3, 4] and find that a multi–atom molecular gas such as sulphur hexafluoride provides the best yield for actinides AMS. In both cases, data published 40 years ago provided clues as to the optimum gas in each situation.
A cosmogenic glacial chronology of Lake Judd, southwest Tasmania and latitudinal shifts in the Roaring Forties
(International Union for Quaternary Research (INQUA), 2019-07-27) Fink, D; Wilcken, KM; Simon, KJ; Schulmeister, J; Thackray, GD
Limited attention has been paid to the glacial history of Tasmania (40-44 S). At the northern limits of today’s Westerly storm track (the Roaring Forties), it experiences precipitation changes due to shifts in the Southern Annular Mode and ITCZ. Cosmogenic dating in southwest Tasmania (Kiernan 2010) highlights the marginal nature of its glaciation where small precipitation or temperature changes cause large changes in mass balance. This makes southwestern Tasmania an excellent location to study Westerly flow through the Last Glacial Cycle. The prevailing idea is that during glacial times, Westerlies expand northward to lower Southern Ocean latitudes, delivering increased rainfall to Tasmania, whilst during interglacials, their migration polewards reduces precipitation. Somewhat counterintuitively, from a Tasmanian perspective of increased Westerly circulation and wind strength, maxima glaciation should coincide with peak global glacials. Although well preserved, extensive last glacial cycle moraine sequences are not common, a series of more than 20 terminal moraine ridges have been mapped over 3 km concentric with the southern margin of Lake Judd, in the SW wilderness corner of Tasmania. Morphologically, the Lake Judd sequence is believed to represent three phases; pre MIS-6, MIS 5-3 and MIS 2 (LGM), representing substantially different volumes of ice accumulation. Our first field campaign collected 16 dolerite samples (36Cl) from the three innermost concentric moraines adjacent to lake edge and 4 quartzite (10Be, 26Al) samples from the most distal moraines. Our aim is to test correlation of the long-term glacial history with records of westerly fluctuations from NZ speleothems (Whittaker et al., 2011) to determine whether the latitudinal variation in Westerly winds in the Australasian region are zonal during glacial times as they are demonstrated to be in interglacial intervals (Fletcher and Moreno, 2012). Preliminary exposure age calculations on the full set of 20 samples are in poor agreement with Kiernan’s age associations. The 10Be (and 26Al) ages on the distal moraine indicate a most probable MIS 4/5 affiliation (>50 ka), significantly younger than expected. In contrast, the 16 dolerite samples from the 3 innermost proximal moraines at lake edge gave a complex set of ages that ranged from MIS 3 to MIS 4 affiliation (45-25ka), somewhat older than predicted. To add further complexity, these 16 ages show no intra or inter moraine age trends with respect to their morphologic positions and ice flow direction. These results contrast with those from other Tasmanian glacial records and reveal the need for a concretive effort to apply cosmogenic exposure dating to the numerus glacial deposits already mapped throughout Tasmania.
Cosmogenic radionuclides as signatures of past Solar storm events
(Australian National University, 2019-09-09) Smith, AM; Wilcken, KM; Simon, KJ; Dee, MW; Kuitems, M; Scifo, A; Moy, A; Curran, MAJ; Wallner, A; Fink, D; Fujioka, T
This collaborative project examines the relationship between the ‘Carrington Event’ (CE), the largest solar storm of modern times, and two recently discovered cosmic radiation events of greater magnitude, the ‘Miyake Events’ (ME). The intention is to construct cosmogenic isotope (14C, 10Be and 36Cl) profiles across the CE, so they can be compared with similar data that have already been obtained for the ME. We will use ice cores from Law Dome, East Antarctica, collected under Australian Antarctic Science awards, for the 10Be and 36Cl analyses. The large diameter DSS0506 ice core will permit high-resolution measurements at ANSTO of 10Be and 36Cl across the CE. Furthermore, we also intend to measure 10Be and 36Cl in the main DSS ice core across the ME. These measurements will complement existing data as both isotopes will be measured in the same ice core for each event for the first time and at high temporal resolution. New tree rings spanning the CE and ME, sourced from the Oxford Dendrochronology Laboratory, have been measured for 14C at the University of Groningen at mostly annual resolution. The ultimate goal of this study is to determine whether or not all three events are manifestations of the same phenomena. A secondary goal is to provide a check on the independent DSS-main ice core chronology. The CE of 1859 is known from geomagnetic data and contemporary records of the aurorae, which were observed as far south as the tropics. The event predated ground-based neutron detectors and routine cosmogenic isotope measurement, so the intensity of the incident particle radiation is still a matter of conjecture. Indeed, this question has been thrown into sharp focus recently by new discoveries in palaeoastronomy. Analyses of natural archives (tree-rings and ice-cores) have revealed that production of the cosmogenic isotopes 14C, 10Be and 36Cl spiked dramatically in the years 774-775 AD and 993-994 AD. Such anomalies could only have been generated by sudden bursts of cosmic radiation. Several sources were initially proposed for the radiation, however, the consensus now is that they were driven by solar activity. Here we discuss progress with the measurement of the cosmogenic radioisotopes and consider how the relative production rates of the cosmogenic radioisotopes may be used to substantiate a solar cause for the historical radiation events and to infer the spectral hardness of the initiating solar protons. © The Authors.
Cosmogenic radionuclides at Law Dome, East Antarctica, as signatures of past Solar storm events.
(Australian Nuclear Science and Technology Organisation, 2021-11-17) Smith, AM; Curran, MAJ; Fink, D; Dee, MW; Kuitems, M; Levchenko, VA; Moy, A; Scifo, A; Simon, KJ; Wilcken, KM
This project investigates evidence for increased atmospheric production of cosmogenic radionuclides in ice core records at Law Dome, East Antarctica, during three extreme events. These events are the Carrington Event (CE) of 1859 AD [1], the largest solar storm of modern times, and two recently discovered cosmic radiation events of even greater magnitude, the Miyake Events (ME) of 774/5 AD [2] and 993/4 AD [3]. Our intention is to determine ¹⁴ C, ¹⁰ Be and ³⁶ Cl profiles, with the highest sub-annual temporal resolution to date, across these events to determine whether or not all three events are manifestations of the same phenomena. Understanding the frequency, origin and magnitude of these events is essential for future-proofing modern communication infrastructure against such high magnitude radiation impacts from space. Identification of the events also provides an independent check on the Law Dome ice chronology. New annual Δ¹⁴ C measurements in tree rings, in combination with earlier published data, show that the ME774 and the ME993 events occurred in close proximity to the point of maximum activity of the 11-year solar cycle [4]. Although it did not leave any radiocarbon signature, the CE1859 event was already known to have occurred around the point of maximum activity of the solar cycle from sunspot records. Ice samples for ¹⁰ Be and ³⁶ Cl analysis are derived from ice cores drilled near the summit of Law Dome, East Antarctica. This is the first time these radionuclides have been measured at the same site for these events, allowing a direct comparison of ME774, ME993 and CE1859 under similar transport conditions. Both ME samples were taken from sections of core where the amount of available ice was limited, and the CE samples were taken from a section where more ice was available. AMS measurements involved some method development at ANSTO, measuring both ¹⁰ Be and ³⁶ Cl in the same samples, with sample sizes challenging for the ME samples. Preliminary ¹⁰ Be results at annual resolution spanning 30 years allowed an exact location of the events. We have clearly identified the expected ME774 and ME993 ¹⁰ Be peaks, which were ~ 4 years and ~ 2 years, respectively, within the error of when the layer-counted DSS ice core chronology had suggested. Accordingly, a further set of ¹⁰ Be samples at sub-annual seasonal resolution have been taken to better define the fine structure and amplitude of the signal but are currently not processed. We will also prepare a set of ³⁶ Cl AMS targets from the sub-annual ice core samples and the initial annual survey samples. No discernible ¹⁰ Be peak or ³⁶ Cl peak was found for CE1859 at annual resolution. © The Authors
Curious case of 26Al accelerator mass spectrometry
(Australian National University, 2019-09-09) Wilcken, KM; Rood, DH
Accelerator mass spectrometry measurement of 26Al suffers from low negative ionisation yield that often becomes the limiting factor. To counter the low Al− yield it has been recognised that AlO− produces negative ions much more efficiently and is a potential avenue to improve the measurement precision. When using AlO− for the measurement there is an additional challenge to separate the interfering isobar 26Mg and 26Al, but this can be achieved effectively with gas-filled magnet. However, this seemingly neat solution of using AlO− instead Al− for the measurement does not necessarily yield as clear cut improvements in precision as one would hope. To illustrate this point, data from conventional measurement method at ANSTO is presented and benchmarked against published data using AlO− method
A data-model comparison on the glacial thinning history of Byrd Glacier, Antarctica
(Australian Institute of Nuclear Science and Engineering, 2021-11-17) Stutz, J; Eaves, S; Moore, C; Wilcken, KM; Norton, K; Lowry, D
Response of marine based sectors of the Antarctic Ice Sheet (AIS) to projected warming remains a significant uncertainty in sea level rise projections. The aim of this project is to provide understanding of past mechanisms and feedbacks of ice sheet change, and to reduce uncertainty in projections of future change. We extend the satellite record of ice sheet change by targeting strategic locations around the margins of the Ross Ice Shelf, which is responsible for buttressing large sectors of the marine-based West AIS. At these locations, glacial sediments deposited on nunataks adjacent to dynamic ice margins record the transient evolution of ice thickness throughout the Holocene period. Here, we focus on the Byrd Glacier, which drains 10% of the East AIS by area and contributes ~20% of the total mass of the Ross Ice Shelf. At Lonewolf Nunataks along the upper Byrd Glacier, our cosmogenic surface exposure ages constrain (i) past rates of ice thinning; (ii) total magnitudes of ice elevation change; and (iii) the absolute timing of ice discharge and thinning events in these sensitive regions. In this presentation, we will review the existing knowledge of Transantarctic Mountain outlet glaciers and present new data from our 2019-20 season along the upper Byrd Glaciers. Comparing our data to recent regional-scale ice sheet model simulations of the last deglaciation, we observe a distinct time lag between modelled thinning and our data-constrained thinning history. Our new data-model comparison will inform high-resolution, sector-scale numerical glacier model experiments, in which we seek to determine drivers of ice sheet thinning and retreat. © The Authors
Defining denudation rate of carbonate rocks using cosmogenic 36Cl in the Taurus Mountain, S-SW Turkey
(Copernicus GmbH, 2021-04-19) Hashemi, K; Sarikaya, MA; Wilcken, KM
Denudation of carbonate terrains occurs by the contribution of both chemical and mechanical weathering. In-situ cosmogenic 36Cl is a robust proxy to quantify the long-term total denudation rate of carbonate rocks. In this study, we defined the steady-state denudation rate of carbonate bedrock using 36Cl for 105-106 years under the temperate Mediterranean to semi-arid climate in and around the Taurus Mountain Range, S-SW Turkey. We collected 13 samples from different lithological units; Jurassic-Cretaceous neritic limestone in the temperate western Taurus, Miocene neritic limestone in the semi-arid Central Taurus, and Mesozoic marbles in the continental part of Central Taurus. The calculated denudation rates range from 28.9 ± 1.4 mm/ka in the Mediterranean coastal range to 1.6 ± 0.1 mm/ka towards northern continental/rain shadow. We compared the denudation rates with a range of parameters such as topographic, climatic, lithologic and mechanical properties of rocks. For almost all samples denudation rate increases with elevation, with two exceptions with the highest rates despite their lower elevations. This high denudation rates could be due to their proximity to the sea. Our results showed that denudation rates decrease with increasing distance from the coast. All denudation rates showed a positive correlation with mean annual precipitation (MAP ~ 400-760 mm) as suggested by other studies worldwide. Annual temperatures (MAT ~ 6-16 °C), however, has a negative correlation with the denudation rates, i.e. the highest denudations occur in the low temperatures (MAT ~ 6-8 °C). The mechanical strength of the rocks was measured with a Schmidt hammer in the field. The high rebound values of Schmidt hammer, indicating the high mechanical strength, correlate with low denudation rates. Nevertheless, the mechanical strength of the carbonate bedrock is not as effective as precipitation or available moisture on denudation rates. This could be shown by two samples close to the Mediterranean which both have high mechanical strength nonetheless show the highest denudation rates. In conclusion, our study suggests that carbonate bedrock denudation in Taurus Mountain correlates with high elevation (~ 1900-2250 m), high precipitation (~ 700-800 mm), low temperatures (~ 6-8 °C) and short distances from the Mediterranean coast.
The distribution and fractionation of beryllium isotopes in various reactive phases of Antarctic marine sediments
(Australian Nuclear Science and Technology Organisation, 2021-11) Jeromson, MR; Fujioka, T; Fink, D; Post, A; Simon, KJ; Sánchez-Palacios, JT; Blaxell, M; Enge, TG; Wilcken, KM; White, DA
Beryllium isotopes, ¹⁰ Be and ⁹ Be, in Antarctic marine sediments are increasingly being applied as paleoenvironmental proxies and indicators of past ice shelf extent. The evidence base for interpreting meteoric-¹⁰ Be concentrations and ¹⁰ Be/⁹ Be ratios has largely been derived from examining their spatial distribution in modern depositional environments, or by correlation with other proxies in paleo-records, such as diatom abundance. Meteoric-¹⁰ Be is geochemically adsorbed onto sediment grains in the reactive phase during transport from the atmosphere to deposition on the seafloor. Unlike meteoric-¹⁰ Be, ⁹ Be is both available within the reactive phase after crustal weathering and native within mineral lattice in significant quantities. The complexity in fixing and preserving the Be isotopes onto grain surfaces leads to uncertainties in selecting the chemistry methods to consistently extract the reactive phases of ¹⁰ Be and ⁹ Be in different sediments. This gap in understanding the physical behaviour and geochemical forms of reactive Be in Antarctic sediments limits their utility in reconstruction of paleoenvironmental conditions. We conducted a sequential leach procedure on three homogenised sediment grab samples from the front of the Amery Ice Shelf that span a range of water masses. Using different chemical reagents, from very weak to very strong, five phases of Be isotope signatures were extracted sequentially, including : i) water soluble, ii) amorphous oxides leached by 0.5M HCl, iii) crystalline oxides leached by 1M NH₂ OH-HCl in 1M HCl, iv) organic leached by 0.01M HNO₃ and H₂ O₂ , and v) mineral/residual phase dissolved by HF– with the water through to organic leach making the reactive phase. We found that the amorphous and crystalline oxide phases contained the largest fraction of ¹⁰ Be, about 90% of total ¹⁰ Be, with the remaining 10% being in the mineral/residual phase. For ⁹ Be, the oxide phases contained only 10-30%, the majority of ⁹ Be being in the residual phase. The water-soluble and organic chemical treatments were inefficient in extracting any significant reactive Be. This distribution has been observed in other deep marine and continental riverine sediments. However, the proportional distribution of the two isotopes between the amorphous and crystalline oxides differed for our Antarctic sediments compared to those other studies. While reactive ⁹ Be was close to equally split across the two oxide phases, 80% of reactive ¹⁰ Be was located within the amorphous phase, with the remainder within the crystalline oxide phase. The difference in fractionation provides evidence for different sources of each isotope and different processes affecting their deposition. ⁹ Be is sourced primarily from the Earth’s crust and is likely segregated into the different fractions during the process of subglacial chemical weathering. Open water ¹⁰ Be is processed in the water column, where interaction with biogeochemical processes likely segregates it into the more labile phases. These findings inform decisions regarding the selection of procedures for efficient and reproducible extraction of meteoric-¹⁰ Be, and for understanding the processes that drive the source and distribution of different isotopes around ice shelf systems. © The Authors
Earthquake hazard assessment uncertainty reduced by fragile geologic features in coastal Central California
(Copernicus GmbH, 2020-05-04) Caklias, A; Rood, DH; Stirling, M; Madugo, C; Abrahamson, NA; Wilcken, KM; Gonzalez, T; Kottke, A; Whittaker, AK; Page, W; Stafford, P
Probabilistic seismic hazard analysis (PSHA) models typically provide estimates of ground motions for return periods that exceed historical observations. It is therefore important to develop quantitative methods to evaluate and refine ground motion estimates for long return periods, especially in proximity to major earthquake sources where estimates can be very high. Here we provide empirical constraints over 10,000s years on ground motions from onshore and offshore seismic sources in central California using the distribution, age and fragility (probability of toppling given an intensity of ground shaking) of fragile geologic features. The fragility is estimated for seven precariously balanced rocks (PBRs) formed on uplifted marine terrace palaeo-sea stacks. The site is <10 km from the Hosgri fault, a major offshore fault considered part of the San Andreas fault system. PBR 3D models were constructed using photogrammetry and used to define normalized geometric measures that could be combined with empirical models to estimate the probability of toppling (i.e., fragility), over a range of vector ground motions (PGA and PGV/PGA). Using vector hazard and the fragility, the likelihood of survival was then computed. The PGA associated with a 50 percent chance of survival varies from ~0.4-1.3 g for the selected PBRs. We obtain fragility ages (time that each PBR achieved its current geometry) using Be-10 cosmogenic surface exposure dating. Extremely low Be-10 concentrations (~5000 at/g) in modern high-stand samples demonstrates minimal inheritance and reliability of chert age estimates. Additionally, the volume of colluvium surrounding the palaeo-sea stack outcrops, determined from LiDAR, combined with alluvial fan surface dating (using Be-10 and soil profile development indices) indicates low erosion rates (~2.5 mm/ky) and long-term stability. Exposure ages that bound the fragility age by approximating the removal of surrounding blocks range ~17-95 ky. The similar age distributions of block removal events around all of features suggests that all PBRs share a common evolution, and we interpret ~21 ka as the most defensible fragility age estimate of all seven PBRs, with negligible change to their fragility between that time and now. Despite the lack of constraints on the recurrence behaviour of the Hosgri Fault, the slip rate is such that the PBRs have almost certainly experienced multiple large-magnitude, near-field earthquakes, and therefore provide rare constraints on low frequency ground motions. Each estimate output from the PSHA model is evaluated against the ground-motion corresponding to the 95% probability of survival of the most fragile PBR over the 21 ka fragility age. The logic tree branches that produce estimates inconsistent with the survival of the PBR are removed from the PSHA model. From the consistent logic tree branches a new PSHA model is produced that has reduced mean ground-motion levels and reduced uncertainty between the estimates. At the 10-4 hazard level, the mean ground motion estimate is reduced by ~30% and the range of estimated 5th-95th percentile ground motions is reduced by ~50%. © Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 Licence.

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