Details

Autor(en) / Beteiligte

• Okuda, Masaaki
• Nakagawa, Takeshi
• Takemura, Keiji

Titel

Present status of pollen-based palaeoclimate reconstruction from Lake Biwa sediment

Ist Teil von

• The Quaternary Research (Daiyonki-Kenkyu), 2010/06/01, Vol.49(3), pp.133-146

Ort / Verlag

Japan Association for Quaternary Research

Erscheinungsjahr

2010

Link zum Volltext

Quelle

Free E-Journal (出版社公開部分のみ）

Beschreibungen/Notizen

• The Lake Biwa sediment, with its clayey deposit 250 m thick covering the past 430,000 years, is suitable for orbital-scale palaeoclimate reconstruction because of abundant tephras as well as quick and stable sediment accumulation. A fossil pollen record from the 250 m muds (Miyoshi et al., 1999) contains five glacialinterglacial cycles correlating with MIS 1 to MIS 12, based on the 780 kyr pollen stratigraphy constructed by connecting it with an older pollen record obtained from the Choshi district, east-central Japan. This article demonstrates two important findings recovered from the Lake Biwa pollen record. One is about age control, showing that the pollen ratio between temperate conifers (Cryptomeria, Sciadopitys and T-C-C) and boreal conifers (Abies, Picea, Tsuga and Pinus) correlates with marine δ18O records during the past 430 kyr. Coupled with climate preferences of these pollen taxa, we propose that the temperate/boreal conifer pollen ratio serves as an indicator for the glacial-interglacial cycle, at least in central Japan, providing a time control method for terrestrial deposits of middle Pleistocene age. Another finding from Lake Biwa pollen data is the result of an application of modern analogue technique (MAT), which can quantitatively extract palaeoclimate information from fossil pollen data. The significance of MAT application is that it provides a new finding concerning a monsoon circulation mechanism. The reconstructed palaeotemperature variations correlate well with the above-mentioned temperate/boreal conifer ratio, and show a significant 100 kyr cycle similarly to marine δ18O. By contrast, the reconstructed palaeoprecipitation variations (particularly Psum) are dominated by a strong 19-23 kyr periodicity, revealing that the summer monsoon variations are almost free from the glacial-interglacial cycle. These observations can be explained as follows. Despite the significant influence of the glacial-interglacial cycle on terrestrial temperatures, it affects the Pacific air mass and the Siberian air mass almost equally, and so is cancelled out from the land-ocean temperature difference (i.e., Tvar). By contrast, insolation forcing affects the marine and continental air masses differentially, and so it is never can-celled out from the land-ocean temperature difference, providing a strong 19-23 kyr periodicity for monsoon variations. This mechanism is probably shared by monsoon regions of different continents. The paleoclimate reconstruction resulted in new findings, but also some new problems. This article describes two current problems and some resolutions in the pollen-based palaeoclimate reconstruction from Lake Biwa sediment.