Reconstruction of a climate record for the past 140 kyr based on diatom valve flux data from Lake Biwa, Japan

Abstract

To reconstruct the pattern of past climate change in central Japan during the last 140 kyr, total planktonic diatom valve concentrations (valves g−1) and fluxes (valves cm−2 year−1) of total planktonic diatoms flux (PVF) and individual species were examined using a 140-m core taken from Lake Biwa, Shiga Prefecture. Most records had a sample resolution between approximately 150 and 300 yr. Based on characteristics of past and modern diatom responses to possible climate variables, we interpreted changes in Stephanodiscus suzukii flux (SVF) to reflect changes in phosphorus levels, which reflect, in turn, summer precipitation levels; changes in Aulacoseiva nipponica flux (AVF) reflect winter vertical lake-water mixing induced by winter temperatures and snowfall levels. Thus, changes in total planktonic diatom flux reflect a combination of summer precipitation, winter temperature, and snowfall values. During the 140–101 ka interval, changes in S. suzukii productivity at a millennial timescale may correspond to changes in summer rainfall in central Japan. The disappearance of A. nipponica during the same period could indicate weaker vertical mixing, possibly caused by increased temperatures and decreased snowfall levels in winter. During the 101–70 ka interval, the AVF record shows levels near or above those observed in present times, indicating that winter water temperatures fell within the optimal range for A. nipponica to prosper. Generally low AVF values during the 70–7 ka interval indicate weak winter vertical mixing and cold winters. The many intervals with low PVF values during the same period suggest decreased summer precipitation levels. Between 7 and 0 ka, PVF, SVF, and AVF records show levels near or above those of the present, suggesting winter temperatures favorable for A. nipponica growth, and snowfall and summer precipitation levels probably similar to or above those currently recorded.