Diatoms from Lake Kushu: A pilot study to test the potential of a Late Quaternary palaeoenvironmental archive from Rebun Island (Hokkaido Region, Japan)

Abstract

Rebun Island is a key research area for the Baikal-Hokkaido Archaeology Project to better understand the dynamics of the Neolithic hunter–gatherers in the NW Pacific region. Hence, the ca. 19.5m sediment core RK12 spanning the last ca. 16.6cal. kyr BP was obtained from Lake Kushu. Our aim is to test its potential as a high-resolution multi-proxy archive. Here, we used diatoms to investigate the modern ecosystem of Lake Kushu and its surrounding area on Rebun Island and of Hime-numa Pond on Rishiri Island and selected core samples for comparison. Modern diatom and stable isotope analyses show well-mixed freshwater bodies with eutrophic, alkaline conditions. The fossil diatom and geochemical sediment analyses display three phases that represent major changes in the lake development: (i) a marshy phase (ca. 16.6–10cal. kyr BP); (ii) a brackish water lagoon phase (ca. 10–6.6cal. kyr BP); and (iii) a freshwater lake phase (since ca. 6.6cal. kyr BP). This shows the major role of the post-glacial climate amelioration, global sea-level rise and marine transgression in the development of this landscape. Further analyses will provide a palaeolimnological record at (sub-)decadal resolution that will facilitate the interpretation of the hunter–gatherer dynamics.