Radiocarbon ages and tsunami deposits in laminated mud layers from the Lake Harutori, Pacific coast of Hokkaido, Northeast Japan

Full text: In order to identify the chronological changes of the marine reservoir effect (MRE) in the Northeast Japan, the radiocarbon ages of terrestrial plants and marine shells were measured from two sediment cores obtained from the Lake Harutori, Northeast Japan [1-2]. This lake is located where the one of the greatest MRE were detected in East Asia, which faces on the Oyashio cold current. This drilling site locates in a tectonic active region in the southwestern Kuril arc associated by the oblique subduction of the Pacific Plate under the Okhotsk Plate [3]. Based on analyses of lithology, molluscan and diatom assemblages, and radiocarbon dating, we interpreted five sedimentary units in order of decreasing age: fluvial sediments, transgressive tidal flat, transgressive lagoon, deltaic lagoon sediments and artificial fills. The lagoon sediments consisted of laminated mud layers with terrestrial plant and marine shell fragments. They were intercalating 28 event deposits by 22 sand layers of tsunamis and 6 volcanic ashes [1-2]. The structure was clearly imaged by a core scanning of a computed tomography (CT) and X-ray fluorescence (ITRAX). The reservoir ages were obtained from the transgressive tidal flat to deltaic lagoon sediments in the last 9,000 years. The chronological change of MRE was correlated with the patterns from the other coastal area in Southwest Japan [4-7] and South Korea [8-10]. The core scanning of CT and ITRAX was performed under the cooperative research program of Center for Advanced Marine Core Research, Kochi University (Accept No. 18B061, 19A056, 20A063). This study was partially funded by the Japan Society for the Promotion of Science Kakenhi grant number JP18H01310.