Systematic Age Gap of Radiocarbon Dates between Organic Bulk Sediment Samples and Fossil Samples of Animal and Plant in Holocene Core Sediments and Their Causes

Abstract

Rradiocarbon dates of organic bulk sediment samples, as well as animal and plant fossil samples, from Holocene coastal lowland sediments are compared to clarify characteristic differences between the dates of both samples and estimate the true age of sedimentation. The samples were collected from drilled cores obtained from two sites. An OSN-Br core was drilled at a river mouth in Kuji City, Iwate Prefecture, and consisted of marine sediments showing sea-level rise during the Holocene. Two KYD-Br cores were drilled at a small valley in Yamada Town, Iwate Prefecture, and consisted of peaty sediment and interbedded event deposits （e.g., tephra and tsunami deposits） . Based on a comparison with the dates of samples collected from the same horizon or ages of tephras, radiocarbon dates of organic bulk sediment samples of the OSNBr core showed a systematic age difference between the dates of animal and plant fossil samples and tephras. These organic bulk sediment samples were interpreted to include old carbon derived from the hinterland, particularly from Paleogene to Cretaceous sedimentary rocks. On the other hand, radiocarbon dates of organic bulk sediment samples of the KYD-Br cores agree with dates of in-situ plant fossil samples （seed and leaf） and are consistent with the age of tephras. Therefore, it can be assumed that the dates of organic bulk sediment samples represent the true age of sedimentation.

Re-examination of the age of historical and paleo-tsunami deposits at Koyadori on the Sanriku Coast, Northeast Japan

“…Ishimura and Miyauchi (2015) excavated the KYD trench (a 12-m-long, 3-m-wide, and 2-m-deep trench) and used block samples (50-cm-long, 10-cm-wide, and 5-cm-deep) from the three trench walls: grid S-1.95 m on the south wall, grid E-11.75 m on the east wall, and grid W-2.75 m on the west wall (Additional file 1: Figure S1). According to the results of Ishimura et al (2016), organic sediment samples from the block samples were dated. Sediments having a thickness of 1-2 cm were resampled for radiocarbon dating in the horizons that were nearly identical to those where Ishimura and Miyauchi (2015) sampled (Fig.…”

“…Thus, Ishimura et al (2016) examined a kind of material that shows the true sediment age within the marsh sediments at Koyadori using drilled core samples (Ishimura et al 2014), because appropriate materials for age estimations (e.g., seeds, fruits, and leaves) could not be found from all horizons in the limited samples. Ishimura et al (2016) compared the radiocarbon dates of various materials (e.g., plant fragments, seeds, leaves, and organic sediments) with the age of a tephra. Consequently, all plant fragments showed the same or younger dates than those of the organic sediments, fragile organic materials (leaves and seeds), and age of a tephra.…”

Progress in tsunami sedimentology

“…In steep slope settings, as the tsunami breaks nearshore (d), suspended sediments have not reached saturated levels (e), and larger amounts of onshore sediments can be suspended (f). Gouramanis et al, 2017;Takeda et al, 2018 , Sawai andNasu, 2005;Nakamura et al, 2012;, , Fletcher et al, 2017, 2016, Shore et al, 1995, , 2016Ishimura, 2017 and Fink, D., 2017, AMS radiocarbon dating of pollen concentrates in a karstic lake system. Quat.…”