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

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.