We studied fossil diatom assemblages from the uppermost part of the Gamo Formation of the Plio-Pleistocene Kobiwako Group, and reconstructed its shallow lake paleoenvironment at 1.8–1.9 Ma. Samples were collected from 11 horizons of the strata at a field site in Taga Town, eastern Shiga Prefecture, Japan. At least 495 diatom valves were counted and identified from each sample using optical microscopy. The dominant species was usually Staurosira construens or other species belonging to Fragilaria sensu lato, although there was one horizon dominated by Aulacoseira ambigua. Therefore, we inferred that the waterbody was shallow and rich in aquatic plants, but it became slightly deeper when A. ambigua was dominant. We estimated pH and total phosphorus concentration based on the diatom count data by weighted averaging, which indicated that the water was meso-to eutrophic and slightly alkaline throughout the study period. Although, specific estimates were not confident, and especially estimates of total phosphorus concentration varied substantially depending on the literature used for reference. The paleoenvironment reconstructed using fossil diatom assemblages was consistent with sedimentological findings and fossil plant evidence from the same strata.
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