Radiocarbon age offsets of Plants and Shells in Holocene sediments from the Sukumo plain, Southwest Coast of Shikoku, Southwest Japan

Abstract

To identify chronological changes in the marine reservoir effect in southwest Japan, the radiocarbon ages of eight pairs of marine shells and terrestrial plants were measured from the same horizons of one core of Holocene sediments. This core was obtained from the northern part of the Sukumo Plain in southwestern Shikoku Island, which faces the warm Kuroshio Current. The drilling site is located in an area of subsidence associated with the convergence of the Philippine Sea Plate and the Eurasian Plate. On analyses of lithology, molluscan assemblages, and radiocarbon dating, we interpreted, from oldest to youngest, nine units: basement rock, a braided river channel, a meandering river channel, an estuary, a transgressive inner bay, Kikai-Akahoya volcanic ash, a deltaic inner bay, a tidal flat, and artificial soil. Changes in the depositional environment were mainly associated with sea-level rise after the end of the last glaciation. The reservoir ages of the eight pairs, 330 ± 70 yr, were obtained from a transgressive inner bay to deltaic inner bay sediments, formed during 4100–9200 cal BP. The chronological change in the reservoir effect allows us to correlate the Sukumo Core sediments with previous results from southwestern Japan, the Korean Peninsula, and Taiwan Island.

Radiocarbon age offsets of Plants and Bioclasts in the Holocene Sediments from the Miyazaki Plain, Southeast Coast of Kyushu, Southwest Japan

“…The results for the MIK core were approximately 0 to 200 years larger than the previous values obtained from Hakata Bay, the Oita plain, the Sukumo Plain, and the South China Sea for 7000 to 9000 cal BP (Figure 4, Nakanishi et al 2017aNakanishi et al , 2017cNakanishi et al , 2019Yu et al 2010). However, the values were approximately 0 to 400 years larger than that those obtained from the Korean Peninsula and off of Taiwan for 7000 to 9000 cal BP (Figure 4, Nakanishi et al 2013Nakanishi et al , 2015Nakanishi et al , 2017bYang et al 2018).…”

“…The sand bed at the base of the K-Ah was identified as a tsunami deposit generated by the K-Ah eruption (Maeno et al 2006;Maeno and Imamura 2007). This tsunami deposit was also reported at the Oita area (Fujiwara et al 2010;Nakanishi et al 2017c), north of the drilling site, at the Sukumo area (Tsuji et al 2018;Nakanishi et al 2019), northeast of the drilling site ( Figure 1B). The parallel-laminated sandy mud implies that it was formed in an intertidal environment.…”

“…The K-Ah tephra and tsunami deposit was identified at the elevation of 4.38 m. This is significantly higher than on the Oita Plain with an elevation of -36 m, 147 km north of the drilling site of the MIK core (Nakanishi et al 2017c), and the Sukumo Plain with an elevation of -19 m, 167 km northeast of the MIK coring site (Nakanishi et al 2019). These horizons were identified at the uppermost part of the transgressive inner bay or prodelta sediments, and then they had been covered by deltaic inner bay or deltaic sediments.…”

“…Email: whong@kigam.re.kr. (Nakanishi et al 2013(Nakanishi et al , 2015(Nakanishi et al , 2017a(Nakanishi et al , 2017b(Nakanishi et al , 2017c(Nakanishi et al , 2019Yang et al 2018). The reservoir ages of these areas from 100 cal BP to 10,200 cal BP were found to range between 60 and 1100 years.…”

“…To identify the chronological changes in the reservoir effect and paleoenvironment of the Miyazaki Plain southeast of Kyushu Island (Figure 1A), the 14 C ages of marine bioclasts and terrestrial plants from the same horizons of one core were measured. The results were then compared to values from the adjacent alluvial plains and inner bay (Nakanishi et al 2017a(Nakanishi et al , 2017c(Nakanishi et al , 2019, the Korean Peninsula (Nakanishi et al 2013(Nakanishi et al , 2015(Nakanishi et al , 2017b, the South China Sea (Yu et al 2010), and Taiwan (Yang et al 2018).…”