[M-IS28] 東アジア‐北西太平洋域高解像度古気候観測網

[MIS28-18] ブランチGDGT組成に基づく過去28万年間の琵琶湖の湖水pHと 水温の変動

*味岡 拓¹、山本 正伸²、竹村 恵二³、林田 明⁴、北川 浩之⁵ (1.産業技術総合研究所地質情報研究部門、2.北海道大学大学院地球環境科学研究院、3.京都大学大学院理学研究科付属地球熱学研究施設、4.同志社大学理工学部、5.名古屋大学大学院環境学研究科)

キーワード：琵琶湖, 湖沼堆積物, 湖水pH, GDGT, MBT, CBT

We generated a 280,000 yr record of water pH and temperature in Lake Biwa, central Japan, by analysing the methylation index (MBT') and cyclisation ratio (CBT) of branched tetraethers in sediments from piston and borehole cores. Our aim was to understand the responses of precipitation and air temperature in central Japan to the East Asian monsoon variability on orbital timescales. Because the water pH in Lake Biwa is determined by phosphorus and alkali cation inputs, the record of water pH should indicate the changes in precipitation and temperature in central Japan. Comparison with a pollen assemblage in a Lake Biwa core suggests that lake water pH was determined by summer temperature in the low-eccentricity period before 55 ka, while it was determined by summer precipitation in the high-eccentricity period after 55 ka. From 130 to 55 ka, the variation in lake pH (summer precipitation) lagged behind that in summer temperature by several thousand years. This perspective is consistent with the conclusions of previous studies (Igarashi and Oba, 2006; Yamamoto, 2009), in that the temperature variation preceded the precipitation variation in central Japan.