Testing the potential of Serpulidae tubes as an indicator of past relative sea level at higher latitude coast using shored wave dissipating blocks along the northern Pacific coast of Japan

Mr Kai Leggett1, Yusuke Yokoyama1, Yosuke Miyairi1, Kosuke Ota1, Naoto Fukuyo1, Yoshiki Shirahama2

1The University of Tokyo, Atmosphere and Ocean Research Institute, , Japan, 2Advanced Industrial Science and Technology, Geological Survey of Japan, Research Institute of Earthquake and Volcano Geology, Active Fault Research Group, , Japan

Determining the relative sea-level (RSL) is necessary to understand long-term crustal movement, because elevation of paleo-shorelines can indicate vertical displacement during tectonic uplift (eg., Tam and Yokoyama, 2021). Defining the sea-level range represented by each proxy is required to accurately determine RSL. High-resolution dating of the proxy is also required to assess long term RSL change. For high-resolution dating, a suitable sample must be used to minimize the age errors.

In western Japan, dating has been conducted using Pomatoleios krausii (PK), a type annelid which lives around ambient sea-level, as a RSL indicator (Shishikura et al., 2008). However, ¹⁴C dating using PK has not been conducted along the Tohoku Pacific coast, northern Japan, due to sample scarcity caused by lower sea surface temperature (SST) despite frequent occurrence of earthquake. Therefore, using alternative proxies is necessary for reconstructing long-term crustal movements along the Tohoku Pacific coast. Hydroides ezoensis (HE) is a calcium carbonate tube forming annelid which lives around ambient sea-level and is widespread along the Tohoku Pacific coast. The reliability of HE as a RSL indicator, especially as a dating sample, must be confirmed for use in reconstructing long-term crustal movements.

Around the Tohoku Pacific coast, vertical mixing between the warm Kuroshio current (KC) and the cold Oyashio current (OC) occurs, where the OC flows underneath the KC. This fluctuation depends on the season, as the KC is dominant at the surface during the summer, and the OC is affective to the water mass near the surface during the winter. The OC is a deep water sourced current and has a relatively low ¹⁴C values. Therefore, dating samples influenced by low ¹⁴C values from the OC, results in large age uncertainties due to the local reservoir effect. From previous ecological research, HE is known to grow only during the summer (Miura and Kajita, 1983).
Through analysis and comparison of the HE Δ¹⁴C with previous research, we examine whether it retains 14C value of the ocean surface during the Tohoku Pacific coast summer. We collected HE calcium carbonate tubes attached to Tetrapods shored at Iwate Prefecture, Tohoku. A total of 18 samples were collected and analyzed for Δ¹⁴C using a single-stage accelerator mass spectrometer (AMS) at the Atmosphere and Ocean Research Institute, the University of Tokyo.

Aerial photographs showed that sampled Tetrapods were installed in the 2000s and shored between 2014 and 2016, suggesting that HE became attached to the Tetrapods during this period. The average of Δ¹⁴C was compared with previous data during the same period. The results showed that samples were uninfluenced by the OC, indicating that HE retains ¹⁴C value only from the ocean surface in summer when the KC prevails. This suggests that HE is suitable for high-resolution ¹⁴C dating to reconstruct past RSL, and for understanding long-term crustal movements in Tohoku.

References
Tam, E., & Yokoyama, Y. 2021. Earth System Science Data, 13(4), 1477-1497.
Shishikura et al., 2008. Ann Rep Active Fault and Paleoearthquake Res., 8, 267-280.
Miura T., & Kajiwara T., 1983, Benthos Res., 25,40-45


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Date

Nov 08 - 19 2021