COSMOGENIC CHLORINE-36 DATING OF EARTH SURFACE CHANGE: RECENT PROGRESS IN LABORATORY METHODS AND NEW APPLICATIONS (Invited Presentation)

Cosmogenic ³⁶Cl dating is widely used within the geosciences to determine the duration of surface exposure of Quaternary features such as glacial deposits, lava flows, landslides, terraces, and fault scarps. However, standard laboratory preparation procedures for ³⁶Cl dating require the addition of large amounts of expensive spike solution, and native Cl concentrations are typically not calculated in advance of ³⁶Cl/Cl measurements. Here, we present an updated 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 major advantages over traditional methods, including: (1) an up to 95% reduction in the amount of isotopically enriched chlorine spike used per rock sample, which will allow researchers to analyze more samples using their existing laboratory supplies, and (2) early and cost-effective identification of rocks with high native Cl concentrations, which may be lower priority for ³⁶Cl dating. With these methodological improvements in mind, we will briefly discuss how we are using ³⁶Cl dating of nearshore lava flows to reconstruct volcanic activity and postglacial sea-level change in Southeast Alaska. ³⁶Cl ages from four samples collected along a transect from above the marine limit to the modern shore, along with published constraints on relative sea-level change (from ¹⁴C on marine shells) and eruption timing (from K-Ar), demonstrate the utility of ³⁶Cl dating for reconstructing sea level through time. This approach opens up new possibilities for sea-level change studies in areas lacking organic material for ¹⁴C dating, and highlights the broad capabilities of cosmogenic nuclides for understanding Earth surface processes.