Age Offsets between Radiocarbon Sample Materials, and Sedimentation Rate Based Holocene Sea-level Change Captured in Ocean Sediment Cores from Ube City, Yamaguchi Prefecture, Japan

Mr Evan Tam1,2,3, Dr. Yusuke Yokoyama1,2,4,5,6, Dr. Yukari Miyashita3, Dr. Yosuke Miyairi1, and Milne., G.7

1Atmosphere and Ocean Research Institute, University Of Tokyo, Kashiwa-shi, Japan, 2Graduate Program on Environmental Sciences, Graduate School of Science, University of Tokyo, Meguro-ku, Japan, 3Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba-shi, Japan, 4Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Bunkyo-ku, Japan, 5Biogeochemistry Program, Japan Agency for Marine-Earth Science and Technology, Yokosuka-shi, Japan, 6Research School of Physics, The Australian National University, Canberra, Australia, 7Department of Earth and Environmental Science, University of Ottawa, Ottawa, K1N 6N5, Canada

Radiocarbon dating can be applied to many sample materials, but ages achieved from varying sample types from a contemporaneous depositional setting can show age differences of up to several thousand years. The offset between these ages (here after called δage) is well documented, especially for bulk sediment ¹⁴C dating (e.g. Raymond and Bauer, 2001), but limited studies have investigated the causes for down core variability in δage. Studies thus far have illustrated that changes in δage can shed light on processes that may alter depositional settings, such as changes in past climate, sea level variability (Ishiwa et al., 2021; Yokoyama et al., 2019), and intensity of monsoon events (Nakamura et al., 2016).

This study examined two shallow ocean sediment cores sourced from the Seto Inland Sea off the coast of Ube city, Yamaguchi prefecture, Japan, sampled within 50m of each other. Both cores were comprised of predominantly silt to silty clay, with limited sections of clay- and sand-based facies. The cores were subsampled for bulk sediment and shell specimens, which were utilized in radiocarbon dating and CNS chemical ratio analyses. Shell based ¹⁴C ages from upper ~1.25m of both cores exhibited relatively rapid changes in age from early to late Holocene age (1.2 ka cal AD to 8.2 ka cal BP), whereas downcore sections displayed slower changes in ages from both shell based (7.1 ka cal BP to 8.3 ka cal BP) and bulk sediment based (8.8 ka cal BP to 10.4 ka cal BP) ¹⁴C ages. δage values calculated between shell and bulk sediment based ¹⁴C ages ranged between 0.7 ka to 2.2 ka, and C/N ratios varied from ~9 to 21.

Relatively slow changes in ¹⁴C ages revealed slower sedimentation rates downcore in both cores, in contrast to increased sedimentation rates revealed by relatively rapid changes in upcore ¹⁴C ages. Sedimentation rates and their corresponding ¹⁴C ages align with sea-level rise and fall associated with the Holocene High Stand (from between ~7 ka to 4 ka; Yokoyama & Purcell, 2021). δage between shell and bulk sediment ¹⁴C ages vary in downcore sections, with changes caused by relative increases in ¹⁴C sediment ages. These shifts align with increased C/N ratio results, which combined with C/N ratio vs δ¹³C analyses suggest that changes in δage correlate to periods of increased land-sourced sedimentation. Increases in upcore ages from shell based ¹⁴C dating are interpreted as sediment reworking due to shallow ocean bottom erosion. In addition to locally constraining the relative timing of sea transgression and regression during the Holocene High Stand, this study highlights the need to understand changes in the depositional setting to accurately represent ages of deposition.

P.A. Raymond and J.E. Bauer (2001), Nature, 497-500 (2001).
T. Ishiwa et al. (2021), Quat. Sci. Rev., 266, 107079.
Y. Yokoyama et al. (2019), Quat. Sci. Rev., 206, 150-161.
Nakamura et al. (2016), Quat Int., 397, 349-359.
Y. Yokoyama and T. Purcell (2021), Geosci. Lett., 8, 1-19.


Ph.D Candidate at the University of Tokyo, Graduate Program on Environmental Sciences. My research utilizes radiocarbon dating, CNS analysis, Glacio-Isostatic Adjustment (GIA) modeling, and tectonics to investigate quaternary changes in sea-level in and around Japan.

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Nov 08 - 19 2021