Preprints
https://doi.org/10.5194/egusphere-2024-713
https://doi.org/10.5194/egusphere-2024-713
20 Mar 2024
 | 20 Mar 2024

Technical note: Optimizing the in situ cosmogenic 36Cl extraction and measurement workflow for geologic applications

Alia Lesnek, Joseph M. Licciardi, Alan J. Hidy, and Tyler S. Anderson

Abstract. In situ cosmogenic 36Cl analysis by accelerator mass spectrometry (AMS) is routinely employed to date Quaternary surfaces and assess rates of landscape evolution. However, standard laboratory preparation procedures for 36Cl dating require the addition of large amounts of isotopically enriched chlorine spike solution; these solutions are expensive and increasingly difficult to acquire from commercial sources. In addition, the typical workflow for 36Cl dating involves measuring both 35Cl/37Cl and 36Cl/Cl concurrently on the high-energy (post-accelerator) end of the AMS system, but 35Cl/37Cl determinations using this technique can be complicated by isotope fractionation and system memory during measurement. The traditional workflow also does not provide 36Cl extraction laboratories with the data needed to calculate native Cl concentrations in advance of 36Cl/Cl measurements. In light of these concerns, we present an improved workflow for extracting and measuring chlorine in geologic materials. Our initial step is to characterize 35Cl/37Cl on up to ~1 g sample aliquots prepared in Ag(Cl, Br) matrices, which greatly reduces the amount of isotopically enriched spike solution required to measure native Cl content in each sample. To avoid potential issues with isotope fractionation through the accelerator, 35Cl/37Cl is measured on the low-energy, pre-accelerator end of the AMS line. Then, for 36Cl/Cl measurements, we extract Cl as AgCl or Ag(Cl, Br) in analytical batches with a consistent total Cl load across all samples; this step is intended to minimize source memory effects during 36Cl/Cl measurements and allows for preparation of AMS standards that are customized to match known Cl contents in the samples. To assess the efficacy of this extraction and measurement workflow, we compare chlorine isotope ratio measurements on seven geologic samples prepared using standard procedures and the updated workflow. Measurements of 35Cl/37Cl and 36Cl/Cl are consistent between the two workflows, and 35Cl/37Cl measured using our methods have considerably higher precision than those measured following standard protocols. The chemical preparation and measurement workflow presented here (1) reduces the amount of isotopically enriched chlorine spike used per rock sample by up to 95 %, (2) identifies rocks with high native Cl concentrations, which may be lower priority for 36Cl surface exposure dating, at an early stage of analysis, and (3) allows laboratory users to maintain control over the total chlorine content within and across analytical batches. These methods can be incorporated into existing laboratory and AMS protocols for 36Cl analyses and will increase the accessibility of 36Cl dating for geologic applications.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Alia Lesnek, Joseph M. Licciardi, Alan J. Hidy, and Tyler S. Anderson

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-713', Shasta Marrero, 30 Apr 2024
    • RC2: 'Reply on RC1', Shasta Marrero, 01 May 2024
      • AC1: 'Reply on RC3', Alia J. Lesnek, 16 Jun 2024
    • AC1: 'Reply on RC3', Alia J. Lesnek, 16 Jun 2024
  • RC3: 'Comment on egusphere-2024-713', Irene Schimmelpfennig, 02 May 2024
    • AC1: 'Reply on RC3', Alia J. Lesnek, 16 Jun 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-713', Shasta Marrero, 30 Apr 2024
    • RC2: 'Reply on RC1', Shasta Marrero, 01 May 2024
      • AC1: 'Reply on RC3', Alia J. Lesnek, 16 Jun 2024
    • AC1: 'Reply on RC3', Alia J. Lesnek, 16 Jun 2024
  • RC3: 'Comment on egusphere-2024-713', Irene Schimmelpfennig, 02 May 2024
    • AC1: 'Reply on RC3', Alia J. Lesnek, 16 Jun 2024
Alia Lesnek, Joseph M. Licciardi, Alan J. Hidy, and Tyler S. Anderson
Alia Lesnek, Joseph M. Licciardi, Alan J. Hidy, and Tyler S. Anderson

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Short summary
We present an improved workflow for extracting and measuring chlorine isotopes in rocks and minerals. Experiments on seven geologic samples demonstrate that our workflow provides reliable results while offering several distinct advantages over traditional methods. Most notably, our workflow reduces the amount of isotopically enriched chlorine spike used per rock sample by up to 95 %, which will allow researchers to analyze more samples using their existing laboratory supplies.