[U-03] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U03-P01] Abalone Shells as an Archive for Oceanographic Changes off Sanriku Coast, Northeastern Japan, during the Anthropocene

*Huang Zihan^1,2, Yusuke Yokoyama^1,2,3, Yosuke Miyairi¹, Kotaro Shirai¹, Hideki Fukuda¹, Kiyoshi Tanaka¹, Hayakawa Jun¹, Ota Kosuke¹, Shoko Hirabayashi¹ (1.Atmosphere and Ocean Research Institute, the University of Tokyo, 2.Graduate School of Arts and Sciences, The University of Tokyo, 3.Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo)

Keywords:Abalone, Dissolved inorganic carbon, Radiocarbon, accelerator mass spectrometry (AMS), stable isotopes, Sanriku

In the dynamic marine environment off Sanriku Coast in Northeast Japan, the interplay of Oyashio and Kuroshio currents significantly shapes regional ocean circulation and ecosystems. Amidst the uncertainties of the impact of 21^st-century global warming on these currents and the marine environment, this research embarks on a journey to unravel the intricate story of these oceanographic transformations in the recent decade from 2012 to 2022 utilizing abalone shells as archives. Abalone shell as a climate proxy is still a new approach in paleoclimatology and paleoceanography studies. However, with their distinctive growth patterns and isotopic compositions, abalones emerge as sentinel organisms capable of providing a nuanced perspective on the historical shifts in their marine habitat in low latitudes.
This study employs principles of radiocarbon (Δ¹⁴C), stable oxygen isotope (δ¹⁸O), and stable carbon isotope (δ¹³C) analysis. Information about ocean mixing is archived in Δ¹⁴C, making it a frequently utilized tracer in studies on water mass mixing. However, owefHothethethe application of Δ¹⁴C in high-latitude waters has been constrained by the absence of corals. The reconstruction of seawater temperature frequently relies on δ¹⁸O, while δ¹³C offers insights into the metabolic and kinetic processes of the organism. Through this multifaceted approach, we intend to test whether abalone shells are good archives for oceanographic changes by deciphering the isotopic information encapsulated within abalone shells. We also aim to shed light on critical aspects such as the accuracy of age determination, the accuracy of seawater temperature reconstruction, Oyashio intrusion into Otsuchi Bay, and the interplay of ocean mixing and Δ¹⁴C signatures.
This study unveils several key findings regarding abalone shells as environmental archives. Firstly, the δ¹⁸O in abalone shells emerges as a robust indicator that excellently records temperature variation without cessation in winter, offering enhanced accuracy in age determination. Secondly, the Δ¹⁴C analysis of seawater demonstrates the thorough mixing of surface seawater in the bay mouth of Otsuchi Bay within the top 100 meters. Thirdly, discrepancies in Δ¹⁴C values between the surface calcite layer and the pearl aragonite layer of abalone shells emphasize the limitation of bulk analysis used in previous studies on abalone Δ¹⁴C. Fourthly, the seasonal variations in seawater and abalone shell Δ¹⁴C provide valuable insights into the dynamic ocean mixing processes. Last but not least, inter-shell variations highlight a declining mean Δ¹⁴C with age, attributed to the diminishing bomb Δ¹⁴C signal over time. In summary, these findings underscore the potential of abalone shells as paleoclimate archives, enriching our understanding of past environmental conditions.