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Mareike Schmidt, BHAP PhD student, presents at 13th International Paleolimnology Symposium (IPS2105)

August 25, 2015

Mareike Schmidt, BHAP PhD student, recently presented a paper at the 13th International Paleolimnology Symposium (IPS2105) held in Lanzhou, China from August 4-7, 2015.

Title: A 17 kyr history of Lake Kushu from Rebun Island in northern Japan

1*Schmidt, M., 1Tarasov, P.E., 2Hoelzmann, P., 3Meyer, H.

1Institute of Geological Sciences, Palaeontology, Freie Universität Berlin, Malteserstr. 74-100, Building D, 12249 Berlin, Germany; 2Institute of Geographical Sciences, Physical Geography, Freie Universität Berlin, Malteserstr. 74-100, Building B, 12249 Berlin, Germany ; 3Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Research Unit Potsdam, Telegrafenberg A43, 14473 Potsdam, Germany

*Email contact: mareike.schmidt@fu-berlin.de

Abstract: Lake Kushu is the only noticeable freshwater body on Rebun Island in the Sea of Japan. This relatively small landmass known for its abundant archaeological sites representing Jomon, Okhotsk and Ainu cultures has been selected as a key research area by the international Baikal-Hokkaido Archaeology Project (BHAP: http://bhap.artsrn.ualberta.ca/) aiming in better understanding the dynamics of the hunter-gatherer population in northern Japan during the Holocene, including the role of various environmental factors, i.e. climate change and sea level fluctuations. A ca. 19.5 m bottom sediment core from Lake Kushu was extracted in February 2012 as a high-resolution multidisciplinary archive, for pollen, diatom, and geochemical analyses. The age model is based on 57 radiocarbon dates indicating a continuous environmental archive spanning the last ca. 17 kyr.

In August 2014, a set of 19 surface sediment, surface water, plant, and rainwater samples was collected from Lake Kushu environment. On site, we measured pH, conductivity, temperature and surface water oxygen content. Lake Kushu (4 m a.s.l.) spans an area of 0.5 km2 with a maximum water depth of 5.8 m and a total shoreline of 3 km. Proximity to the Sea of Japan (400 m) and low elevation allows the assumption that the Holocene sea level fluctuations and earthquake induced tsunamis played an important role in the post-glacial history of Lake Kushu and left recognizable traces in diatom, isotope and geochemical composition of lake sediments.

Modern Lake Kushu shows highly eutrophic, alkaline freshwater conditions. Surface water samples are mainly composed by planktonic Aulacoseira granulata (10-20%) and benthic Nitzschia palea (13-38%), Staurosirella pinnata (5-22%), and Staurosirella berolinensis (11-19%). Sediment surface samples are dominated by benthic S. pinnata (7-80%), S. berolinensis (3-21%), Amphora indistincta (25%), and Pseudostaurosira brevistriata (22%), while planktonic A. granulata reaches 5-21%. The oxygen isotope values of the surface water samples (all about -8‰) represent a well-mixed water body and indicate the absence of major evaporation processes. To test the potential of the sediment core RK12-02 from Lake Kushu as a high-resolution environmental archive, 10 randomly selected sediment samples, spanning an interval between 300 and 17,200 cal. yr BP, have been analysed for diatoms and selected elements (e.g. Ca, Fe, Mg, Mn, P, S, Sr, Ti) . The pilot results indicate three different environmental phases with prominent changes in salinity and in relative lake level: (i) a marshy phase (ca. 16,600 to 10,000 yr BP), with little amount of diatom valves (ii) brackish water lagoon phase (ca. 10,000 to 6,600 yr BP), suggested by presence of brackish/ marine Chaetoceros species, and (iii) a freshwater lake phase (6,600 to 300 yr BP) shown by the dominance of planktonic freshwater Aulacoseira species. This pilot study suggests a very interesting history of Lake Kushu especially regarding marine influences. Further high-resolution diatom and geochemical analyses will provide an excellent record to detect even short-time events, such as tsunami or strong storm impact. In addition, the reconstruction of the lake history will serve for better understanding the dynamics of the hunter-gatherers throughout the late glacial and Holocene interval.

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