GM2.5

Cosmogenic noble gas isotope ²¹Ne in terrestrial rocks has been used primarily to determine timing and rates of the Earth’s surface evolution. Here we explore the ability of detrital ²¹Ne as a provenance tracer, considering that Ne isotopes produced in source rocks could be preserved in minerals over geological time and might be predominant in total Ne inventory of sediments sunk in basins. This ability is predicated on potential source terranes of a given stratigraphic section with distinct neon isotopic signatures. Here we analyze neon isotopes of a well-dated Miocene–Pleistocene sedimentary archives in Kuqa foreland basin of southern Tianshan. The data suggest that the neon isotopic signature, which is expressed as ε_Ne and defined in this work as the excess ²¹Ne/²⁰Ne-ratio relative to atmospheric ratio, is stratigraphically sensitive to changes in local source terranes. This result is compatible with U/Pb provenance analysis and also supported by evidences from sandstone petrography and heavy mineral analysis. Influence of other non-source related ²¹Ne components in sedimentary archives on sensitivity of ε_Ne has proven to be negligible. Furthermore, the integrated stratigraphic signatures of neon isotope and U/Pb age permit the detection of differential erosion in drainage basin, by which the tectonic or climatic effects on geomorphic evolution could be deciphered.

How to cite: Ma, Y., Zheng, D., Zhang, H., and Pang, J.: Neon isotopic signature applied to detrital provenance assignment in foreland basins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3670, https://doi.org/10.5194/egusphere-egu22-3670, 2022.

Soil production and denudation are important and opposing processes that ultimately determine soil formation trajectories and, thus, the landscape. High mountainous areas are geomorphologically active environments and are strongly shaped by the redistribution of sediments and soils. With global warming and the subsequent retreat of glaciers, these processes become increasingly important. New areas having fresh and mostly unweathered glacial till is exposed and soils start to form. The dynamics of soil production and denudation in these high-mountainous landscapes are, however, not yet fully understood. We therefore aimed at exploring the relationship between soil production and denudation at different stages of soil development. This was done by comparing a calcareous and a siliceous soil chronosequence in the central Swiss Alps over the last about 14 kyr. We calculated element mass balances to determine weathering rates and measured short- and long-term erosion rates based on meteoric ^239+240Puand¹⁰Be. In both chronosequences, the erosion rates were highest in the young soils (on average 5−10 t ha^-1 a^-1 soil loss). Erosion rates decreased markedly after 3−5 ka of soil development (on average 1−2.5 t ha^-1a^-1 soil loss) to reach a more or less stable situation after 10−14 ka (on average 0.3–2 t ha^-1a^-1). Chemical weathering and soil production rates also decreased over time, particularly on the calcareous soil sequence.

Depending on the relief and vegetational development, it takes up to 10 ka to reach soil and slope stability. Despite the very high erosion rates, particularly at the start of soil formation, mineral dissolution and transformation reactions are detected and a high rate of organic matter accumulation is measured. Soil production rates reach under such conditions extreme values. In the early stages of soil development, the parent material mainly drives soil formation while at later stages the vegetation becomes more dominant as it promotes surface stability, complex hydrological pathways and chemical weathering that determine water drainage and retention dynamics.

How to cite: Egli, M. and Musso, A.: Soil dynamics at its extremes: insights from cosmogenic and fallout radioactive nuclides, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7018, https://doi.org/10.5194/egusphere-egu22-7018, 2022.

The ability to measure cosmogenic ³He from individual detrital mineral grains [1] provides the potential to tease out details of sediment storage and transport that are unavailable from bulk sample analysis, and may, for instance, shed light on the conditions necessary to form economic alluvial placer deposits. While extremely long exposure histories have been measured in detrital grains from unglaciated regions [1], the effect of repeated glacial cycles in removing economically valuable detrital minerals is unknown. Here we report the cosmogenic ³He content of 36 (2-50 mg) native gold grains from the beds of 8 streams in upland Scotland in order to determine their ability to survive glaciation.

Measured ⁴He concentrations vary from 4 to 299 x 10¹³ atoms/g these variation on the ⁴He concentrations may be related to the presence of U and Th in the mineral lattice or U- and Th-rich mineral inclusions. Based on measured Li contents (<1 ppb) the nucleogenic ³He contribution in all samples is negligible. Minimum cosmogenic ³He exposure ages have been determined using production rate of 25 atoms/g/year and assuming no shielding.  33 grains yield exposure ages that are consistent with survival of detrital gold from before the Last Glacial Maximum (i.e > 20 Ka). These grains yield ages up to 4 Ma.   This implies that a significant proportion of the detrital gold has survived several glacial cycles and may have implications for long-term preservation of economic minerals in glaciated regions.

[1] O. Yakubovich, F.M. Stuart, A. Nesterenok & A. Carracedo (2019). Chemical Geology 517, 22-33.

How to cite: Carracedo Plumed, A., di Nicola, L., Olive, V., and Stuart, F.: Long-term survival of detrital gold in glaciated landscape based on cosmogenic 3He in detrital grains from Scotland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13085, https://doi.org/10.5194/egusphere-egu22-13085, 2022.

Sequences of Holocene marine terraces are widely used in paleoseismic research to understand the timing and magnitude of earthquakes along tectonically active coastal margins worldwide. But the potential for marine terraces to be destroyed through erosion after uplift can result in incomplete records of paleoseismicity as derived from terrace chronologies, leading to misinterpretations of the paleoseismic history of a region. Here, we present measurements across a unique set of exposed bedrock marine terraces in the north-eastern South Island, NZ, to quantify the ages and erosion rates of the surfaces and produce a new chronology for paleoseismic interpretation. Surface exposure dating and multi-nuclide approaches offer the potential to quantify marine terrace preservation and destruction, potentially elucidating where terraces may be missing or removed from a sequence. Needles Point, Marlborough, NZ exhibits three well defined bare rock marine terraces and a gravel covered shore platform which was recently uplifted ~2.5m in the M_W 7.8 2016 Kaikōura earthquake. ¹⁰Be-derived ages for the platform surface and terrace 1 (T1) align with known ground surface rupturing earthquakes on the Kekerengu fault. T2 and T3 preserve older events not previously identified, potentially extending the earthquake record in this region. However, other known ground surface rupturing earthquakes on the Kekerengu fault are not preserved as terraces at Needles indicating that the preserved terraces at Needles Point do not therefore represent a full record of local paleoseismicity. As such, estimates of fault throw derived from these terraces would over-estimate earthquake magnitude.

How to cite: McLean, A., Dickson, M., Norton, K., Wilcken, K., Stephenson, W., and Litchfield, N.: Cosmogenic 10Be from uplifted bedrock marine terraces indicates revised Holocene earthquake intensity for northeast South Island, New Zealand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11019, https://doi.org/10.5194/egusphere-egu22-11019, 2022.

Cosmogenic Ne is used to record timing and rates of surface processes on the Earth and Moon and the time of meteorite release from parent bodies. Precise determination of neon isotopes in rocks and minerals has improved in the last ten years largely as a consequence of developments in mass spectrometry and associated electronics.  In this presentation we will report the performance of an ARGUS VI mass spectrometer tuned for cosmogenic Ne determinations from both extra-terrestrial and terrestrial material. The instrument is connected to an automated laser gas extraction and purification system and has several advantages over off-the-shelf instrumentation. The remote operation of sample heating, gas purification, separation and isotope analysis increases sample throughput, and the exact repetition of the procedures and overnight determination of calibrations and blank measurements improves data quality, facilitates determination of isobaric interferences and eases trouble shooting. The low static volume results in high sensitivity, while the stable electronics and multi-collection allows high precision Ne isotope determinations in terrestrial and extra-terrestrial samples that are significantly smaller than typically analysed to date.

Two analytical protocols are applied depending on sample Ne concentration. Multi-collection Faraday mode is used for extra-terrestrial material. This yields a 4-fold improvement in the overall uncertainty of the Ne isotope ratios (0.5%) compared to that obtained using 5-10x larger samples in peak-jumping mode on our workhorse instrument. Cosmogenic Ne determinations in 20-30 mg of terrestrial material are made using the compact discrete dynode detector in peak-jumping.  Replicate analysis of CREU-1 quartz yields reproducibility of ±3.7% (1σ), comparable to the data quality for 5-10x more material.  In addition to instrument performance characterisation, we will summarise data from studies of terrestrial and extra-terrestrial material that demonstrate routine capability.

How to cite: Di Nicola, L., Györe, D., Hamilton, D., and Stuart, F.: Improved cosmogenic Ne measurements using ThermoFisher ARGUS VI, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5671, https://doi.org/10.5194/egusphere-egu22-5671, 2022.

Applications of in situ produced cosmogenic ¹⁰Be and ²⁶Al for age determination of rock surfaces and sediments use quartz containing lithologies, because these nuclides are produced and retained within the crystal lattice of this mineral. To be able to calculate reliable ages, it is essential to work with pure quartz grains. The most problematic contaminant is the mineral feldspar for three reasons: 1) ¹⁰Be and ²⁶Al can also be produced in feldspar but at a different production rate 2) feldspar contains copious amounts of Al and can thus contribute stable Al (²⁷Al) in high amounts 3) it is ubiquitous and often intergrown with quartz. This leads to problems during sample processing and during Accelerator Mass Spectrometry  measurement where the rare nuclide ²⁶Al may reach its detection limit.

During sample processing the crushed and sieved rock samples are subject of physical (magnetic-, shape- and density separation) and chemical (leaching in acids) cleaning steps in order to remove all minerals (e.g. feldspars, mica, amphiboles, etc.) and have the quartz grains concentrated and purified. If the sample is sufficiently clean (i.e. the Al content is below 200 ppm) the purified quartz can enter total dissolution as the next step of nuclide extraction.

Several methods are used to check the mineralogical composition and the quality of the purified quartz sample. 1) Optical investigation using a binocular microscope, 2) X-ray diffraction (XRD) analysis of mineralogy, 3) Al content analysis after digestion and element detection as high Al content is indicative of the presence of Al containing minerals like feldspars or mica within the pure quartz fraction.

Each of those methods has individual limitations. Optical investigation may be hindered by the etched surface and quartz may look surprisingly similar to feldspar after etching. XRD has its detection limit at c. 5%, and with respect to Al content analysis, some quartz-types may naturally contain larger amounts of Al. Besides, chemical analysis as well as XRD – if not available in house – may have a long waiting time and high costs.

Here we report first results of exploring the potential of luminescence as a fast and cost-efficient alternative method to determine the content of contaminant feldspars in quartz as infrared stimulation only excites feldspars, but not quartz. The experimental setup consists of a parallel analysis of the samples using XRD to detect the bulk mineralogy, element analysis by ICP-OES analysis, and luminescence analyses, in order to develop a short, efficient luminescence measurement sequence reliably detecting feldspar contamination in samples for cosmogenic dating. While some results are promising for individual samples, the results for other samples are still ambiguous. The respective data will be presented at the conference.

This study was supported by the following projects: OMAA 105ou4, NKFIH 124807.

How to cite: Lüthgens, C., Neuhuber, S., Novothny, Á., and Ruszkiczay-Rüdiger, Z.: Development of a new rapid method for quartz purity control by luminescence for cosmogenic nuclide dating, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12050, https://doi.org/10.5194/egusphere-egu22-12050, 2022.