GM2.12

EDI
Novel applications and technique developments of cosmogenic nuclides

The introduction of terrestrial cosmogenic nuclides as a geochronology technique provided an important quantitative tool that spurred large developments in geomorphology and it continues to be an essential tool in this field. Cosmogenic nuclides are produced primarily in the top two meters of Earth’s surface, meaning they can be used to provide important information on the exposure ages of features at the surface, like river terraces, fault scarps or glacial moraines, burial ages of deep deposits, as well as quantitative information on the rates of surface processes, like erosion or weathering. Continued technique development and creative applications both continue to expand the ways we can use cosmogenic nuclides.

This session explores both technique developments and novel applications of cosmogenic nuclides, inviting projects at any stage from early development to well-established methods applied to novel situations. We invite any type of cosmogenic nuclide technique developments, including new measurement methods, laboratory techniques, modelling, or theoretical advancements. All cosmogenic nuclide applications in any field are welcome, but we especially encourage contributions using multiple nuclides, combinations with other geochronology techniques, and other creative applications.

Public information:
The introduction of terrestrial cosmogenic nuclides as a geochronology technique provided an important quantitative tool that spurred large developments in geomorphology and it continues to be an essential tool in this field. Cosmogenic nuclides are produced primarily in the top two meters of Earth’s surface, meaning they can be used to provide important information on the exposure ages of features at the surface, like river terraces, fault scarps or glacial moraines, burial ages of deep deposits, as well as quantitative information on the rates of surface processes, like erosion or weathering. Continued technique development and creative applications both continue to expand the ways we can use cosmogenic nuclides.

This session explores both technique developments and novel applications of cosmogenic nuclides, inviting projects at any stage from early development to well-established methods applied to novel situations. We invite any type of cosmogenic nuclide technique developments, including new measurement methods, laboratory techniques, modelling, or theoretical advancements. All cosmogenic nuclide applications in any field are welcome, but we especially encourage contributions using multiple nuclides, combinations with other geochronology techniques, and other creative applications.
Convener: Shasta MarreroECSECS | Co-conveners: Steven Binnie, Andrew Hein, Susan Ivy-Ochs, Angel Rodes
vPICO presentations
| Wed, 28 Apr, 13:30–14:15 (CEST)
Public information:
The introduction of terrestrial cosmogenic nuclides as a geochronology technique provided an important quantitative tool that spurred large developments in geomorphology and it continues to be an essential tool in this field. Cosmogenic nuclides are produced primarily in the top two meters of Earth’s surface, meaning they can be used to provide important information on the exposure ages of features at the surface, like river terraces, fault scarps or glacial moraines, burial ages of deep deposits, as well as quantitative information on the rates of surface processes, like erosion or weathering. Continued technique development and creative applications both continue to expand the ways we can use cosmogenic nuclides.

This session explores both technique developments and novel applications of cosmogenic nuclides, inviting projects at any stage from early development to well-established methods applied to novel situations. We invite any type of cosmogenic nuclide technique developments, including new measurement methods, laboratory techniques, modelling, or theoretical advancements. All cosmogenic nuclide applications in any field are welcome, but we especially encourage contributions using multiple nuclides, combinations with other geochronology techniques, and other creative applications.

Session assets

Session materials

vPICO presentations: Wed, 28 Apr

Chairpersons: Shasta Marrero, Steven Binnie, Andrew Hein
13:30–13:35
13:35–13:37
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EGU21-5346
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ECS
Sebastien J.P. Lenard, Maarten Lupker, Irene Schimmelpfennig, Vincent Godard, Clement Desormeaux, Negar Haghipour, Lukas Wacker, Georges Aumaître, Karim Keddadouche, and Didier L. Bourles

Denudation rates are routinely derived from concentrations of terrestrial in situ produced cosmogenic nuclides (TCN), particularly from 10Be concentrations in river sand. Denudation rates are calculated assuming that they remain steady throughout the integration time scale of the TCN. However, such an assumption is possibly unverified in settings with negligible tectonics, where rates typically range from 10 to 100 mm/ky. In those settings, the TCN conveys a signal that integrates denudation over a time span longer than a few thousand years. The signal may include periods when anthropogenic and climatic forcing on denudation was distinct from modern times. For instance, agricultural practices were limited before 6,000 years B.P. and climatic conditions were colder and drier before 10,000 years B.P. A variable forcing may produce variable and transient denudation rates. In that case, the assumption of steady denudation rates is invalid, and their derivation may introduce a bias.

To detect transient landscapes and resolve such a bias, we can take advantage of the different sensitivity of the 14C and 10Be TCNs to recent and short-term changes in surface denudation. In situ 14C is more sensitive than 10Be to such changes, because of a shorter half-life (5,700 y compared to 1.4 My). This potential application of coupled 14C - 10Be measurements has recently been discussed in several theoretical studies (Hippe, 2017; Mudd, 2017; Skov et al., 2019). Despite the improvement of 14C extraction lines and measurement facilities (Hippe et al., 2009; Lupker et al., 2019), sensitivity tests remain limited on natural cases (Hippe et al., 2012).

Here, we propose assessing this new application by in situ 14C - 10Be measurements on river sand from the Cevennes and the Monts Margeride within the Variscan Massif Central in France. With an average elevation of ~700 m, this mountain range presents an asymmetrical topography, composed of a low-relief surface reaching 1,700 m, and bordered by a gently sloping flank to the west and a steep escarpment to the southeast, along the Cevennes fault. This escarpment receives frequent and seasonal extreme precipitation events (300-700 mm in 48h) on its southeast flank.

The range is subject to very limited seismic activity and appears relevant for an application of the 14C-10Be couple. Basins are rich in quartz and have homogeneous lithology. The recent paleoclimatic context is well constrained, with substantial climatic variations but with limited Pleistocene glaciations (e.g. Fauquette et al., 1999; Magny et al., 2003; Mayewski et al., 2004). The Massif Central is subject to active erosion processes, without major contribution from stochastic events such as landslides. Denudation rates are in the range of the theoretical study of Skov et al. 2019 (Schaller et al. 2001; Molliex et al. 2016; Olivetti et al. 2016; Desormeaux et al., 2021) and several studies have suggested transient denudation patterns (Schaller et al. 2001; Olivetti et al. 2016). With our new measurements, we will verify whether the 14C-10Be couple has sufficient resolution to detect such transience in natural cases.

How to cite: Lenard, S. J. P., Lupker, M., Schimmelpfennig, I., Godard, V., Desormeaux, C., Haghipour, N., Wacker, L., Aumaître, G., Keddadouche, K., and Bourles, D. L.: Can 14C-10Be detect transient patterns of denudation ? Application to the French Massif Central., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5346, https://doi.org/10.5194/egusphere-egu21-5346, 2021.

13:37–13:39
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EGU21-10051
Vincent Godard, Andre Salgado, Lionel Siame, Jules Fleury, and Aster Team

Transient evolution and adjustment to changing tectonic and climatic boundary conditions is an essential attribute of landscapes. We present a new approach to detect and quantify transience in slow erosion landscapes over 100 ka timescales. We compare curvature and cosmogenic nuclides measurements (10Be and 26Al) at hilltop sites with predictions of hillslope diffusion theory, in the slowly evolving quartzitic Serra do Cipó range in SE Brazil, and we observe a distinctive signature of an acceleration of denudation. The timing of this increase cannot be unequivocally associated with a single climatic event but is consistent with climatically-modulated important fluctuations in precipitation and erosion in this area during Middle and Late Pleistocene.

How to cite: Godard, V., Salgado, A., Siame, L., Fleury, J., and Team, A.: Detecting transience in slow evolution landscapes using cosmogenic nuclides and high resolution morphometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10051, https://doi.org/10.5194/egusphere-egu21-10051, 2021.

13:39–13:41
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EGU21-5643
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ECS
Catharina Dieleman, Marcus Christl, Christof Vockenhuber, Philip Gautschi, and Naki Akçar

Deckenschotter (Cover Gravels in German) are Quaternary glacio-fluvial gravels, which unconformably overlie Tertiary Molasse or Mesozoic bedrock in the Northern Alpine Foreland. They comprise also the evidence of the Early Pleistocene glaciations. A significant phase of incision separated them into Höhere Deckenschotter (HDS: Higher Cover Gravels) and Tiefere Deckenschotter (TDS: Lower Cover Gravels) based on their topography. How the landscape evolved during Deckenschotter times is still not fully understood. The new cosmogenic nuclide chronology suggests that HDS deposited around 2 Ma and TDS around 1 Ma. In addition, 2 Ma old Deckenschotter are located at the same topographic elevation as the 1 Ma ones at Irchel (Canton of Zurich). This, indeed, points to cut-and-fill sequences and challenges the chronology based on the morphostratigraphy.

The aim of this study is to reconstruct the drainage patterns, base level changes, and thus the landscape evolution in the northern Alpine Foreland during the Early Pleistocene. Therefore, we focused on three Deckenschotter sites at Irchel and one in the area around Lake Constance. Sediments at these sites were analysed in detail to reveal their provenance, transport mechanism, depositional environment, and paleoflow regimes. Their chronology was established by isochron-burial dating. Our results indicate that the analysed sediments were transported from the Central and eastern Central Alps as well as from the Molasse to the foreland first by glaciers and then by rivers. They are deposited in a glacio-fluvial environment in the vicinity of a glacier. Based on the reconstructed chronology in this study and published cosmogenic nuclide ages, we propose that Deckenschotter are cut-and-fill sequences accumulated in three pulses between 2.5 Ma and 1 Ma. This cut-and-fill system implies that the regional base level was relatively constant during the Early Pleistocene. In addition, the depositional environment of Deckenschotter shows the presence of glaciers in the foreland. The 2.5 Ma old gravels, therefore, document the first advance of glaciers onto the Alpine Foreland. This seems to be synchronous with a first onset of glaciations on the northern hemisphere, which is assumed to occur at around 2.7 Ma.

How to cite: Dieleman, C., Christl, M., Vockenhuber, C., Gautschi, P., and Akçar, N.: Early Pleistocene complex cut-and-fill sequences in the Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5643, https://doi.org/10.5194/egusphere-egu21-5643, 2021.

13:41–13:43
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EGU21-12374
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ECS
Ewelina Broś, Florian Kober, Susan Ivy-Ochs, Reto Grischott, Marcus Christl, Christof Vockenhuber, Colin Maden, and Hans-Arno Synal

The oldest Quaternary deposits of the Swiss Northern Alpine Foreland are found on numerous hilltops, up to 300 m above the current valley bottoms. These Deckenschotter deposits consist mainly of glaciofluvial sediments intercalated with glacial sediments. Traditionally, the Deckenschotter are divided into two units: Höhere Deckenschotter (HDS – Higher Deckenschotter) and Tiefere Deckenschotter (TDS – Lower Deckenschotter). Elevation differences between the two suggest a phase of 100-150 m of incision (Graf, 2009).

Knowledge of their age of deposition is necessary for understanding the long-term landscape evolution as well as for assessing the long-term safety of the planned deep geological repository for nuclear waste in northern Switzerland (NTB 14-01, 2014). In this study, the method of isochron-burial dating was implemented to address the question of the age of the Deckenschotter. We aim to reconstruct the chronology of the alternating deposition and incision of the gravel units in the Northern Alpine Foreland. Our focus is placed on similar and complementary Deckenschotter sites located in the Northern Alpine Foreland in crucial locations in order to establish sound long-term landscape evolution scenarios. One of these is a former gravel pit, Feusi, situated in the southern slope of the hill chain called ‘Egg’ or ‘Schliniker Platten’, north of the village Oberweningen. The outcrop comprises several gravel units intercalated with glacigenic diamict layer in the upper part. Previous age estimates with the isochron-burial dating method indicate an age of 1.1 ± 0.2 Ma for the diamict layer (NAB 19-025, 2020). Knudsen et al. (2020) reported an age of 0.93 ± 0.13 Ma for the same layer based on a slightly different age calculation approach.

We sampled the lowermost accessible horizon, the Egg Schotter, of the Feusi outcrop at an altitude of ~580 m a.s.l. This horizon is located close to the base of the outcrop, just a few meters above the contact with the underlying Molasse and in a clear stratigraphic position, 20 m below the previously dated diamict. Study of the lowermost unit will allow us to temporally examine the earliest phases of Deckenschotter accumulation. Weathering horizons in the gravel layers overlying the Egg Schotter suggests periodic subaerial exposure. Therefore, the total time contained in the sediment package is difficult to estimate. Having two horizons dated at different depth in the same outcrop may provide insight into the timespan hidden between the deposition and weathering of different gravel layers. Indications of the timespan of HDS activity could be further gleaned by comparing to the age from the glacigenic sediment. In order to achieve this, eight clast samples of quartz-rich lithologies, of various shapes and sizes were collected in the Egg Schotter and processed for isochron-burial dating. The cosmogenic nuclides 10Be and 26Al were extracted and measured with the new MILEA accelerator at the accelerator mass spectrometry facility, ETH Zurich. The first results of this study will be presented.

 

Graf, H.R. 2009: Quaternary Science Journal 58, 12–53

Nagra, NTB 14-01, 2014

Nagra, NAB 19-025, 2020

Knudsen, M.F. et al. 2020. Earth and Planetary Science Letters, 549, 116491

How to cite: Broś, E., Kober, F., Ivy-Ochs, S., Grischott, R., Christl, M., Vockenhuber, C., Maden, C., and Synal, H.-A.: Delving deeper into cosmogenic 26Al-10Be isochron-burial dating of Swiss Deckenschotter deposits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12374, https://doi.org/10.5194/egusphere-egu21-12374, 2021.

13:43–13:45
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EGU21-14848
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ECS
Jesper Nørgaard, John Jansen, Stephanie Neuhuber, Zsófia Ruszkizcay-Rüdiger, Sandra Braumann, Markus Feibig, Philipp Häuselmann, and Mads Faurschou Knudsen

For sediment-burial dating with a cosmogenic nuclide pair, the isochron burial method performs well provided that the sediment source has undergone (1) steady erosion and (2) continuous exposure to cosmic rays. These conditions exert important limitations on applications of the method. And yet, in mountainous fluvial and glacial landscapes, it is commonly found that the source area has experienced landsliding or glacial quarrying (i.e., non-steady erosion), and/or intermittent sediment storage or burial beneath glaciers (i.e., discontinuous exposure). As well as breaching the assumptions of the isochron method, such processes tend to yield low nuclide concentrations in the sample, which further limits its workability.

Here we present a more flexible method that accommodates complex, non-steady pre-burial erosion and exposure histories: conditions that exclude the isochron burial method. P-PINI (Particle Pathway Inversion of Nuclide Inventories) is a Monte Carlo-based inversion model that employs a source-to-sink approach for estimating the depositional age of fluvial and glaciogenic sediments. This method has been successfully applied to the Deckenschotter in the northern Alpine foreland (see Knudsen et al. 2020, Earth & Planetary Science Letters 549, 116491). As with the isochron burial method, P-PINI exploits an ensemble of paired nuclide (e.g., 10Be-26Al) concentrations measured in different samples from the same depth in a sedimentary sequence. But unlike the isochron method, P-PINI applies a stochastic approach to simulate a wide range of possible pre-depositional exposure and erosion histories for each individual sample. These different pre-burial histories (unique to each sample) are then integrated with the constraint that all samples share a common burial history at the sink. Where cosmogenic nuclide data (or other chronometric data, e.g., OSL) are available for multiple sites, Bayesian inference modelling can impose a priori relative age constraints, or estimates on the maximum duration of sediment storage.

In this presentation, we extend P-PINI to explore how sediment storage and reworking (i.e., a range of burial depths and durations) between source and sink affects burial age estimates. Significant intermediate storage is characteristic of large river systems, such as the Danube River. Using cosmogenic 10Be-26Al concentrations measured in fluvial gravels at Gänserndorf and Schlosshof, two terraces along the Danube River in the Vienna Basin (Braumann et al., 2019. Quat. Int. 509. 87-102), we examine how the burial ages at these two sites are a function of the pre-burial history experienced by the samples.  

How to cite: Nørgaard, J., Jansen, J., Neuhuber, S., Ruszkizcay-Rüdiger, Z., Braumann, S., Feibig, M., Häuselmann, P., and Faurschou Knudsen, M.: P-PINI: a new inversion method for sediment-burial dating, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14848, https://doi.org/10.5194/egusphere-egu21-14848, 2021.

13:45–13:47
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EGU21-13808
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ECS
Applicability of the authigenic 10 Be/ 9 Be dating to deltaic deposits: Preliminary results from the Slanicul de Buzau section, Pliocene, Romania.
(withdrawn)
Kishan Aherwar, Michal Šujan, Katarína Šarinová, Régis Braucher, Arjan de Leeuw, Andrej Chyba, and Aster Team
13:47–13:49
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EGU21-12406
Michal Šujan, Régis Braucher, Michal Kováč, Kishan Aherwar, Imre Magyar, Zsófia Ruszkiczay-Rüdiger, Andrej Chyba, Orsolya Sztanó, Dániel Botka, Klement Fordinál, Matúš Tibenský, Peter Joniak, Samuel Rybár, and Bernhard Salcher and the AsterTeam

Bourlès et al. (1989: Geochim. Cosmochim. Acta) suggested that authigenic 10Be/9Be ratio could provide a geochronological tool to date deposition of clay-bearing sediment settled in a water column up to 14 Ma old. It is based on ratio of atmospheric cosmogenic radionuclide 10Be delivered to depositional environments by precipitation and stable 9Be extracted from rock massifs by chemical weathering. Determination of the initial 10Be/9Be ratio is essential for efficient application of the dating and may vary spatially as well as in time due to changes in drainage basins, depositional environments, climate, and other factors. The potential of the authigenic 10Be/9Be dating was evaluated during last years in the Pannonian Basin realm, located in Central Europe. This contribution summarizes successful applications as well as discovered problems and challenges, which motivate the ongoing research.

Two initial 10Be/9Be ratios were established from Holocene alluvial and lacustrine clays in the Danube Basin (Šujan et al., 2016: Glob. Planet. Change). The dating was applied to shallow to deep-water sediments deposited in Lake Pannon within the Danube Basin, and helped to constrain paleogeographic changes in the age range of 11.6–3 Ma. Application of the method to the post-rift alluvial succession with high subsidence rates of 50–400 m/Ma in the range of ~9.5–6.0 Ma yielded data consistent with other geochronological proxies (Šujan et al., 2020: Sed. Geol.; Joniak et al., 2020: Palaeo3). The fast accumulation and tectonic quiescence likely provided stable environmental conditions favorable for the dating method applicability.

Lacustrine and deltaic deposits of Lake Pannon were analyzed from cores of Paks boreholes in the central part of the Pannonian Basin. The resulting authigenic 10Be/9Be ages are generally in agreement with magnetostratigraphic age constraints correlated using seismic stratigraphy (Magyar et al., 2019: Földt. Közl.). Outliers with relative enrichment of 10Be appear in most distal facies, where low terrestrial 9Be input is expected.

A study of turbidite deposits from the Transylvanian Basin allowed to compare the established lacustrine initial 10Be/9Be with a ratio independently calculated from Ar/Ar dated horizon (Botka et al., 2019: Austrian J. Earth. Sci.). Majority of samples provided a good fit with other age proxies, while one sedimentary interval exhibits twofold increase of 10Be/9Be probably indicating variability in the environmental conditions (Baranyi et al., 2021: Rev. Palaeobot. Palyn.).

An order of magnitude higher authigenic 10Be/9Be comparing to the established initial ratios were obtained from supposed early Pleistocene sediments from the locality Sollenau in the Vienna Basin. The visual appearance implies, that secondary pedogenic processes might be responsible for a post-depositional input of 10Be (Willenbring, von Blanckenburg, 2010: Earth. Sci. Rev.). Another case of high 10Be/9Be preventing age calculation was observed in a Pleistocene alluvial environment with intense loess input.

An ongoing research aims to determine the effects of changes in depositional process, sediment source proximity and provenance on the applicability of the dating method. This research was financially supported by the Slovak Research and Development Agency under contract APVV-16-0121 and by the Hungarian National Research, Development and Innovation Office under contract NKFIH-116618.

How to cite: Šujan, M., Braucher, R., Kováč, M., Aherwar, K., Magyar, I., Ruszkiczay-Rüdiger, Z., Chyba, A., Sztanó, O., Botka, D., Fordinál, K., Tibenský, M., Joniak, P., Rybár, S., and Salcher, B. and the AsterTeam: Applicability and challenges for the authigenic 10Be/9Be dating as revealed by studies from the Pannonian Basin realm, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12406, https://doi.org/10.5194/egusphere-egu21-12406, 2021.

13:49–13:51
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EGU21-14132
David Fink, Philip Hughes, Reka Fulop, Klaus Wilcken, Patrick Adams, Craig Woodward, James Shulmeister, Toshiyuki Fujioka, and Peter Ryan

Cosmogenic production rates (PRs) are the essential conversion factor between AMS cosmogenic concentrations and absolute exposure ages. The accuracy of cosmogenic glacial chronologies and reliability in their comparison to other paleoclimate systems  is largely contingent on the precision and accuracy of the adopted production rate. This is particularly critical in determining past glacial geochronologies at the scale of millennial temporal resolution. Most PR calibrations are carried out at deglaciation sites where radiocarbon provides the independent chronometric control usually based on 14C ages in basal sediments or varves from lake or bog cores which is assumed to represent the minimum age for glacial retreat. Under these conditions and hence provide PRs as maximum values. Given that today most AMS facilities can deliver 10-Be, 26-Al and 36-Cl data with total analytical errors less than 2% ( for 10 ka exposure), the precision of a PR remains largely dependent  on the error in the independent chronology and accuracy of AMS standards. The history over the past 20 years of the ever-decreasing value of  SLHL 10-Be cosmogenic spallation PRs   from initial estimates of about 7 atoms/g/a to the current  ‘accepted‘ (global average) values of ~4 atoms/g/a,   is an interesting story in itself and demonstrates the complexity in such determinations.  

Over the past few years new web-based calculators are now available to calculate uniformly new production rates from either new data or combinations of any set of published data (CRONUS-Earth, CRONUS-UW, CosmoCalc, ICE-D, CREp). This delivers a means by which new production rates can be seamlessly integrated and compared using identical constants, methods and statistics that were used to generate (currently accepted) global average or regional production rates.

 For the British Isles, there are a number of 10-Be reference sites that give PRs (Lm scheme) between 3.89±3%  atoms/g/a  (Putnam, QG, v50, 2019) to 4.20±1% atoms/g/a (Small, JQS, v30, 2015) which convert to 3.95 and 4.28, respectively, using datasets in the ICE-D calculator). This difference in 10-Be spallation PRs has recently raised some debate and challenges for the timing of the local-LGM and demise of the British Ice Sheet. This work provides a new  British Isles site specific 10Be PR from the  Arenig Mountains in North Wales where radiocarbon dating of basal sediments from a bog core associated with a series of nearby cirque moraines provides independent age control.  Similarly in the South Island of New Zealand, the current accepted 10Be PR is 3.76±2% (Putnam, QG 2009; converts to 3.94±1% using ICE-D) and is the only available PR that is used for these southern hemispheric glacial sites. This work provides a new Australasian site specific 10Be PR from Arthurs Pass retreat moraines where radiocarbon dating of basal sediments from three cores extracted from a bog impounded by the moraine provides independent age control. 

How to cite: Fink, D., Hughes, P., Fulop, R., Wilcken, K., Adams, P., Woodward, C., Shulmeister, J., Fujioka, T., and Ryan, P.: A tale of two bogs - new 10Be production rates from UK and NZ calibrated by basal 14C ages , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14132, https://doi.org/10.5194/egusphere-egu21-14132, 2021.

13:51–13:53
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EGU21-16090
Ana Carracedo Plumed, Derek Fabel, and Richard Shanks

With the present AMS 10Be uncertainties (~2% best case scenario) and the increasing need for more precise cosmogenic 10Be data it has become imperative to improve AMS measurements. Precision depends on counting statistics which in turn depend on ion beam current stability and sample longevity. The ion beam currents are dependent on the metal matrix in which BeO is dispersed; the matrix:BeO ratio; homogeneity of the mixture and the packing of the AMS cathode. We aim to understand the effect of cathode homogeneity in generating stable beam currents. We have performed a series of experiments using different metal matrices (Nb, Ag, Fe) in different forms (solid and in solution). The metals have been added to different stages of the sample precipitation process and both BeO and Be(OH)2 have been pressed into AMS cathodes and analysed at SUERC. We will discuss results of these experiments and introduce an innovative use of polyoxometalates (molibdanate and niobate) to create a homogeneous compound that has the potential to generate stable ion beam currents from sputter ion sources.

How to cite: Carracedo Plumed, A., Fabel, D., and Shanks, R.: Current stability to improve AMS precision for cosmogenic 10Be applications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16090, https://doi.org/10.5194/egusphere-egu21-16090, 2021.

13:53–13:55
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EGU21-10247
Olga Yakubovich, Finlay Stuart, Alexander Mochalov, and Roman Palamarchuk

The application of cosmogenic noble gases (3He and 21Ne) in detrital grains to quantify sediment transport rates and storage timescales is largely undeveloped. We have previously shown that cosmogenic He can be measured in single grains (Yakubovich et al. 2019). The low He diffusion rate and the chemical and/or physical robustness of metal alloys (eg Au, Ag, Cu, Pt, Fe) means the technique has potential to determine how critical metals accumulate at the Earth surface.  In an effort to determine how long detrital Pt-alloy grains spend travelling to placer deposits we have measured cosmogenic3He concentrations in 60 (0.5–7 mg) grains of isoferroplatinum (Pt3Fe) from the world largest alluvial Pt placer deposits in the Kondyor-Uorgalan rivers in Khabarovsk region, and the Is-Turinsk and Nyas’ma river systems in Middle Urals, Russia.

In both systems, there are no significant cosmogenic 3He in the grains from the low order streams that drain the source rock. 3Hecosconcentrations in Pt grains from distant placers (30 km) varies in order of magnitude from 0.3 to 30 x 107at g-1in grains in Uorgalan river, 5 to 40 x 107at g-1in Glubokinskoe placer deposit (Is river) and 20 to 70 x 107at g-1in the Generalka deposit (Nyas’ma river). Converting this to surface residence times (P = 25 at g y-1, Yakubovich et al., 2019) yields model exposure durations of0.1­–10, 3–20 and 13–38 Myr for grains from the Uorgalan, Glubokinskoe and Generalka deposits respectively. Assuming that all grains were extracted from shielded locations this exposure ages indicate the total time of transport and residence of the grains on river bed surface prior to final deposition.

Pt grains are added to the river system constantly. If we assume that the average transport conditions are essentially identical for all grains, then the difference between the maximum and minimum exposure ages within the same location indicates the total duration of placer supply from the source rock. For the Urals it is equal for 17–25 Myr, while for Kondyor-Uorgalan placer it is 10 Myr. This is in a good agreement with geological observations. Within Kondyor-Uorgalan placer deposit the age of the alluvium varies from Neogene to Quaternary, while in the Urals Pt-bearing Jurassic, Neogene and Quaternary fluvial sediments are distinguished within the placers.

Paleo peneplain surfaces are established nearby the Kondyor massif and pre-date the most productive platinum sands. Several stages of peneplenisation are evident in the Urals from thick weathering crusts. The exposure ages indicate10’s million-year scale of transport/deposition histories of detrital Pt alloys, which might result of multiple redeposition of material during long term accumulation of cosmogenic 3He during peneplenisation.  

Yakubovich O., Stuart F.M., Nesterenok A. and Carracedo A. (2019). Chem. Geol. 517, 22-33.

How to cite: Yakubovich, O., Stuart, F., Mochalov, A., and Palamarchuk, R.: Cosmogenic 3He in detrital Pt-alloy grains: tracing the accumulation of critical metals, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10247, https://doi.org/10.5194/egusphere-egu21-10247, 2021.

13:55–13:57
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EGU21-15013
Adrian M. Hall, Fin Stuart, Kalle Kirsimae, and Peeter Somelar

Stable cosmogenic Ne isotopes are widely used to determine the erosion rate of slowly-eroding land surfaces through the Cenozoic. Constraining erosion and surface exposure back in Earth history remains a challenge largely due to the presence of Ne isotopes generated by natural decay processes over the lifetime of rocks.  Prospects are best when cosmogenic nuclide production has been significant and nucleogenic Ne production is low and can be quantified.  We have explored the limits of palaeo-cosmogenic Ne in one of the Earth’s most extensive erosion surfaces, the late Precambrian Great Unconformity in Estonia. Here deep kaolinitic saprolites formed on Baltica prior to the deposition of Late Ediacaran quartz sandstones. On the basis of geochemical mass balance the duration of saprolite development is estimated to be of the order of a few Myr.

Borehole F163 samples a section through still-buried weathered unconformity that includes a saprolite surface consistent with negligible erosion during the marine transgression. Samples from the unconformity have 21Ne concentrations (>108 atoms/g) that are significantly higher than shielded samples from >20 m below the unconformity. This difference is borne out by Ne isotope composition, and leads to the tanatalising prospect that Precambrian cosmogenic Ne is present in the saprolite. Using modern 21Ne production rates the palaeosols appear to record a few million years irradiation. This is broadly consistent with geochemical estimates of saprolite development.  Samples from the uppermost preserved part of the weathering profile in borehole F231 have low 21Ne concentrations that are indistinguishable from deeper in the rock profile. This would require profile truncation or the redeposition of weathered material.  The borehole is located on the western flank of an uplifted basement block rising ca 130 m above the typical Precambrian basement level in the area and likely that the thick regolith contains material eroded from the uplifted basement units. Clearly these are early days and quantifying surface exposure in deep time will require effort in field as well as the lab.

How to cite: Hall, A. M., Stuart, F., Kirsimae, K., and Somelar, P.: Understanding 21Ne inventories in Precambrian basement below the Great Unconformity in Estonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15013, https://doi.org/10.5194/egusphere-egu21-15013, 2021.

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