5:15 PM - 6:45 PM
[HQR04-P02] Diatom fossil assemblages of Meike-pond, Oki Island for the last 1 thousand years
Keywords:Paleolimnology, diatom, coastal lake
Diatom fossils were analyzed on a sediment core Meike-pond, Oki Island, Shimane Pref. for reconstructing environmental changes. We also aimed to find tsunami event deposits as this fresh water pond faces outer ocean (Sea of Japan) by gravel bar. The core is 148cm long and consisted of homogeneous organic clay. Radiocarbon datings showed that it deposited for the last 1 thousand years. Analysis was carried out at 2cm intervals except for the surface 30cm which was analyzed at 1cm intervals.
Diatom taxa were mainly Fragilarioid taxa (e.g. Staurosira, Pseudostaurosira) and Aulacoseira, and no brackish or marine water indicator taxa were observed in the whole sequence. Lowest part of the core, corresponding to 11-12th century, contained planktonic Asterionella formosa indicating deeper lake environment relative to the present. In the middle part of the core at 14-15th century, large shift of dominant taxon occurred from Aulacoseira ambigua to Aulacoseira granulata and Cyclotella radiosa. This indicates the eutrophication of the lake possibly induced by human settlement. Uppermost 20cm depths showed gradual decrease of water pH, and its decrease likely accelerated at the surface 6cm where chrysophycean cysts and humic water indicator taxa increased notably. This might be caused by sediment infilling of the pond basin and prevalence of littoral water plants. However, falling of air-borne dust coming from surrounding area is attributable since remarkable increase of ash particles and heavy metals produced by fossil fuel combustion was observed in core sediment of Oike-pond, a neighboring pond of the Meike-pond. This study showed that each genus, species and variety within Fragilarioid taxa showed different share trends, represented by a unique increase of Staurosira construens var. construens at 14th century, so their separate identification might bring additional paleoenvironmental information.
Diatom taxa were mainly Fragilarioid taxa (e.g. Staurosira, Pseudostaurosira) and Aulacoseira, and no brackish or marine water indicator taxa were observed in the whole sequence. Lowest part of the core, corresponding to 11-12th century, contained planktonic Asterionella formosa indicating deeper lake environment relative to the present. In the middle part of the core at 14-15th century, large shift of dominant taxon occurred from Aulacoseira ambigua to Aulacoseira granulata and Cyclotella radiosa. This indicates the eutrophication of the lake possibly induced by human settlement. Uppermost 20cm depths showed gradual decrease of water pH, and its decrease likely accelerated at the surface 6cm where chrysophycean cysts and humic water indicator taxa increased notably. This might be caused by sediment infilling of the pond basin and prevalence of littoral water plants. However, falling of air-borne dust coming from surrounding area is attributable since remarkable increase of ash particles and heavy metals produced by fossil fuel combustion was observed in core sediment of Oike-pond, a neighboring pond of the Meike-pond. This study showed that each genus, species and variety within Fragilarioid taxa showed different share trends, represented by a unique increase of Staurosira construens var. construens at 14th century, so their separate identification might bring additional paleoenvironmental information.