Browsing by Author "Rood, DH"
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- ItemCurious case of 26Al accelerator mass spectrometry(Australian National University, 2019-09-09) Wilcken, KM; Rood, DHAccelerator 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
- ItemDating lacustrine sediments in the central Jordan Valley, Israel: Implications for cosmogenic burial dating(18th INQUA Congress, 2011-07-21) Davis, M; Matmon, A; Ron, H; Fink, D; Niedermann, S; Rood, DHWe apply the cosmogenic two isotope burial method to Pliocene to Pleistocene lacustrine sediments (Erk-El-Ahmar (EEA) formation) in the central Jordan Valley, Israel, in the attempt to extend the applicability of the method to common geological settings in which exposure-burial histories are not well constrained. Previous estimates attributed an age of ~1.8 Ma to the bottom of the EEA formation. 26Al and 10Be concentrations were measured in 11 samples collected from the 170 m tectonically-tilted section. 21Ne concentrations were measured in two of these samples. All samples yielded burial ages that range between 3.5 and 5.3 Ma, much older than the previously estimated age of the EEA formation. All three isotopic pairs (26Al/10Be, 26Al/21Ne, and 10Be/21Ne) yielded similar burial ages within 1?. Samples of identical age that were collected from a specific horizon are presently located at decreasing depths below the surface due to the tectonic tilting. Nevertheless, they yielded identical burial ages suggesting rapid recent erosion in this tectonically active region and insignificant production of cosmogenic isotopes at depth by muons. All samples were found to contain two distinct populations of grains (chert and quartz) mixed at various ratios. These two grain-types were eroded from different sources with significantly different pre-burial exposure histories. The cosmogenic nuclide concentrations in the samples are in accordance with those expected for the mixing of two such sources. Calculations of two-source mixing show that initial 26Al/10Be ratios may be lower relative to the expected surface ratios and result in burial ages overestimated by as much as 500 ka. Our burial ages suggest that the water body that deposited the EEA sediments was contained within the initial topographic depression that formed along the central Jordan Valley segment of the Dead Sea rift. Copyright (c) 2011 INQUA 18
- ItemEarthquake 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, PProbabilistic 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.
- ItemMulti-objective optimisation of a rock coast evolution model with cosmogenic 10Be analysis for the quantification of long-term cliff retreat rates(American Geophysical Union (AGU), 2021-12-15) Shadrick, J; Hurst, MD; Rood, DH; Piggott, MD; Wilcken, KM; Seal, A; Hebditch, BGUnderstanding the antecedent trajectory of rock coast evolution is central to the development of predictive models of cliff retreat that account for a changing climate. Moreover, the unison between high-precision, measured datasets and coastal evolution modelling is essential in order to understand long-term behaviours of real-world coastal sites. We present a methodology that uses site-specific topographic and cosmogenic 10Be data to perform multi-objective model optimisation of a coupled rock coast evolution and cosmogenic radionuclide production model. This new capability allows for a time-series of cliff retreat rates to be quantified for rock coast sites over millennial timescales. This is the first study that has 1) applied a process-based coastal evolution model to quantify long-term cliff retreat rates for real, rock coast sites, and 2) coupled cosmogenic radionuclide analysis with a process-based model. Process-based coastal evolution models simplify erosional processes and, as a result, often have equifinality properties, for example, that similar topography develops via different evolutionary trajectories. Here, we show that coupling modelled topography with modelled 10Be concentrations can reduce equifinality in model outputs. Furthermore, these results reveal that multi-objective optimisation is essential in limiting model equifinality caused by parameter correlation to constrain best-fit model results for real-world sites. Our results show that in order for modelled platform morphology and 10Be concentrations to match both measured datasets simultaneously, the model requires negligible weathering and lowering of the shore platform, suggesting that erosion of these rock coasts is wave-dominated. Furthermore, results from two UK sites indicate that the rates of cliff retreat over millennial timescales are primarily driven by the rates of relative sea level rise. This implies that future increases in the rate of relative sea level rise could accelerate cliff retreat, even at these rock coast sites that currently exhibit minimal erosion. These findings provide strong motivation for further studies that investigate the effect of past and future relative sea level rise on cliff retreat at other rock coast sites globally. Plain-language Summary Here we use topographic and 10Be concentration data to optimise a coastal evolution model. Model results strongly suggest the erosion at the coast sites investigated are wave-dominated, as minimal weathering rates are needed to match model results to the measured datasets. Cliff retreat rates are calculated for two UK sites for the past 8000 years, and for the first time, highlight a strong link between the rate of sea level rise and long-term cliff retreat rates. This method enables us to study past cliff response to sea level rise, and so, to greatly improve forecasts of future responses to accelerations in sea level rise that will result from climate change.
- ItemNew 10Be exposure ages improve Holocene ice sheet thinning history near the grounding line of Pope Glacier, Antarctica(European Geosciences Union, 2022-12-06) Adams, JR; Johnson, JS; Roberts, SJ; Mason, PJ; Nichols, KA; Venturelli, RY; Wilcken, KM; Balco, G; Goehring, B; Hall, B; Woodward, J; Rood, DHEvidence for the timing and pace of past grounding line retreat of the Thwaites Glacier system in the Amundsen Sea embayment (ASE) of Antarctica provides constraints for models that are used to predict the future trajectory of the West Antarctic Ice Sheet (WAIS). Existing cosmogenic nuclide surface exposure ages suggest that Pope Glacier, a former tributary of Thwaites Glacier, experienced rapid thinning in the early to mid-Holocene. There are relatively few exposure ages from the lower ice-free sections of Mt. Murphy (<300 m a.s.l.; metres above sea level) that are uncomplicated by either nuclide inheritance or scatter due to localised topographic complexities; this makes the trajectory for the latter stages of deglaciation uncertain. This paper presents 12 new 10Be exposure ages from erratic cobbles collected from the western flank of Mt. Murphy, within 160 m of the modern ice surface and 1 km from the present grounding line. The ages comprise two tightly clustered populations with mean deglaciation ages of 7.1 ± 0.1 and 6.4 ± 0.1 ka (1 SE). Linear regression analysis applied to the age–elevation array of all available exposure ages from Mt. Murphy indicates that the median rate of thinning of Pope Glacier was 0.27 m yr−1 between 8.1–6.3 ka, occurring 1.5 times faster than previously thought. Furthermore, this analysis better constrains the uncertainty (95 % confidence interval) in the timing of deglaciation at the base of the Mt. Murphy vertical profile (∼ 80 m above the modern ice surface), shifting it to earlier in the Holocene (from 5.2 ± 0.7 to 6.3 ± 0.4 ka). Taken together, the results presented here suggest that early- to mid-Holocene thinning of Pope Glacier occurred over a shorter interval than previously assumed and permit a longer duration over which subsequent late Holocene re-thickening could have occurred. © Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 License.
- ItemQuality assurance and control data for cosmogenic Be-10 sample preparation in the CosmIC laboratory at Imperial College London(Australian Nuclear Science and Technology Organisation, 2021-11-17) Rood, DH; Nichols, KA; Rood, A; Seal, A; Wilcken, KMQuality assurance and control (QA/QC) are sacrosanct for accelerator mass spectrometry (AMS) sample preparation laboratories. However, quantitative data necessary to assess QA/QC is rarely published for cosmogenic nuclide sample preparation laboratories around the world. Here, we compile and report QA/QC data for the first 5 years of Be-10 sample preparation operations in the CosmIC laboratory at Imperial College London, United Kingdom. Our QA/QC data include: 1) inductively coupled plasma - optical emission spectroscopy (ICP-OES) assay data for mineral purity, carrier concentration, beryllium yield, and beryllium purity; 2) AMS data for ion beam current, carrier blanks, laboratory process blanks, and measured isotope ratio versus precision; and 3) nuclide concentration data for replicate samples. These results serve to verify and benchmark our current Be-10 sample preparation methods and data quality. In turn, our QA/QC data set the bar for future development and improvements in the CosmIC laboratory and other cosmogenic isotope sample preparation laboratories globally.
- ItemTectonically-dominated Quaternary landscape evolution of the Ventura basin, southern California, quantified using cosmogenic isotopes and topographic analyses(Copernicus GmbH, 2020-05-04) Rood, DH; Hughes, A; Whittaker, AC; Bell, RE; Wilcken, KM; Corbett, A; Bierman, P; DeVecchio, DE; Rockwell, TKSpatial and temporal variations in fault activity informs models of seismic hazards and can affect local patterns of relief generation and channel morphology. Therefore, the quantification of rates of fault activity has important applications for understanding natural hazards and landscape evolution. Here, we quantify the complex interplay among tectonic uplift, topographic development, and channel erosion recorded in the hanging walls of several seismically-active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating to construct a basin-wide geochronology for the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our geochronology of the Saugus Formation is used to calculate tectonically-driven rock uplift rates and reduce uncertainties in fault-slip rates. In addition, we calculate 10Be catchment-averaged erosion rates, characterise patterns of catchment relief and channel steepness indices, and analyse river long-profiles in fault hanging walls to compare with patterns of fault displacement rates averaged over various temporal scales. The results of the burial dating confirm that the Saugus Formation is time-transgressive with ages for the top of the exposed Saugus Formation of ~0.4 Ma in the western Ventura basin and ~2.5 Ma in the eastern Ventura basin. The burial ages for the base of shallow marine sands, which underlie the Saugus Formation throughout the basin, are ~0.6 Ma in the western Ventura basin and ~3.3 Ma in the eastern Ventura basin. The results of the landscape analysis indicate that relief, channel steepness, and erosion rates are still adjusting to tectonic boundary conditions imposed by different tectonic perturbations that have occurred at various times since ~1.5 Ma, which include fault initiation and fault linkage. The data presented here suggest that, for transient landscapes in sedimentary basins up to 2500 km2, where climate can be considered uniform, fault activity is the primary control on patterns of relief generation and channel morphology over periods of 104 to 106 years. © Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 Licence.
- ItemUnraveling rift margin evolution and escarpment development ages along the Dead Sea fault using cosmogenic burial ages(Elsevier Inc., 2014-07-01) Matmon, A; Fink, D; Davis, M; Niedermann, S; Rood, DH; Frumkin, AThe Dead Sea fault (DSF) is one of the most active plate boundaries in the world. Understanding the Quaternary history and sediments of the DSF requires investigation into the Neogene development of this plate boundary. DSF lateral motion preceded significant extension and rift morphology by -10 Ma. Sediments of the Sedom Formation, dated here between 5.0 0.5 Ma and 6.2 +/- 451 Ma, yielded extremely lowl Be concentrations and 26A1 is absent. These reflect the antiquity of the sediments, deposited in the Sedom Lagoon, which evolved in a subdued landscape and was connected to the Mediterranean Sea. The base of the overlying Amora Formation, deposited in the terminal Amora Lake which developed under increasing relief that promoted escarpment incision, was dated at 3.311 Ma. Burial ages of fluvial sediments within caves (3.4 +/- 0.2 Ma and 3.6 +/- 0.4 Ma) represent the timing of initial incision. Initial DSF topography coincides with the earliest Red Sea MORB's and the East Anatolian fault initiation. These suggest a change in the relative Arabian-African plate motion. This change introduced the rifting component to the DSF followed by a significant subsidence, margin uplift, and a reorganization of relief and drainage pattern in the region resulting in the topographic framework observed today. (C) 2014 University of Washington. © 2014, Elsevier Inc.