[A-CC26] Ice cores and paleoenvironmental modeling

[ACC26-P09] Anthropogenic ³⁶Cl from 1950 to 1970 CE in a high-resolution ice core from the Dronning Maud Land, East Antarctica

*Kazuho Horiuchi¹, Kento Sato², Riko Yagihashi¹, Kimikazu Sasa³, Masumi Matsumura³, Tsutomu Takahashi³, Hideaki Motoyama⁴ (1.Graduate School of Science and Technology, Hirosaki University, 2.Faculty of Science and Technology, Hirosaki University, 3.Accelerator Mass Spectrometry Group, Tandem Accelerator Complex, University of Tsukuba, 4.National Institute of Polar Research)

Keywords:Chlorine-36, Nuclear Bomb Testing, Southern Hemisphere, Antarctica, Anthropocene, Accelerator Mass Spectrometry

Chlorine-36 (³⁶Cl) is a radioisotope of chlorine with a half-life of 301 kyr. Prior to the Atomic Era, ³⁶Cl in the environment was largely produced by spallation reactions between cosmic ray particles and target atoms (i.e. cosmogenic production). However, with the advent of atomic bomb testing, anthropogenic ³⁶Cl began to be produced by thermal neutron activation of the ³⁵Cl of sea salts. In particular, extensive marine testing in the 1950s and '60s is thought to have produced a large bomb peak of ³⁶Cl reported from a dozen ice cores worldwide (Elmore et al., 1982; Heikkilä et al., 2009; Pivot et al., 2019). In this presentation, we present an ice core record of ³⁶Cl covering the period 1950 to 1970 CE from the H15 site (69°04'10"S, 40°44'51"E) in Dronning Maud Land, East Antarctica. The ³⁶Cl was analyzed at the University of Tsukuba Tandem Accelerator Complex (UTTAC) using a common procedure for ice core ³⁶Cl analysis (e.g., Sasa et al., 2010; Kanzawa et al., 2021). The ³⁶Cl concentrations ranged from 1.1 × 10⁴ to 2.6 × 10⁵ atoms/g. It increased dramatically in the middle '50s to the most pronounced peak in the late '50s. After that, the concentration decreased rather gradually until the end of the record (1970 CE), with small peaks in the middle and the late '60s. These ³⁶Cl variations appear to be consistent with the history of explosive yields from marine nuclear bomb tests. In addition to the major features described above, there are some minor variations to be found in our record at the seasonal level of resolution. This suggests that both the insignificance of post-depositional ³⁶Cl mobility and some seasonality of ³⁶Cl deposition at the H15 site.