Early Holocene climatic instability in Japan: organic geochemical evidence in sediment cores from Lake Biwa, Lake Kizaki and the Japan Sea

Abstract

Sediment cores from Lake Biwa (Shiga Prefecture) and Lake Kizaki (Nagano Prefecture) provide evidence of variations in the climate of Japan subsequent to the end of the last glacial period. Concentrations of organic carbon increase in sediments deposited after the Pleistocene–Holocene boundary, as wetter climates washed in more soil nutrients and aquatic productivity increased. Organic matter C/N ratios simultaneously decrease, indicating smaller proportions of land-derived plant matter. Lake Biwa C/N ratios and organic carbon isotope ratios fluctuate markedly between 10 and 4 Kya, suggesting that both delivery of land-derived organic matter and production of lake-derived organic matter were variable during this period of time. Similar variability in biotic paleoceanographic indicators has been reported in sediments from the Oki Ridge in the Japan Sea and has been interpreted to record early Holocene fluctuations in the intensity of the Tsushima Current. Such fluctuations in this warm-water current are likely to have caused concordant fluctuations in climate on Honshu, a relationship that explains the variability in rainfall evident in the organic matter of the lake sediments.