1000 years of climate variability in central Asia: assessing the evidence using Lake Baikal (Russia) diatom assemblages and the application of a diatom-inferred model of snow cover on the lake

The mainly endemic phytoplankton record of Lake Baikal has been used in this study to help interpret climate variability during the last 1000 years in central Asia. The diatom record was derived from a short core taken from the south basin and has been shown to be free from any sedimentary heterogeneities. We employ here a diatom-based inference model of snow accumulation on the frozen lake for the first time (r²_boot=0.709; RMSEP=0.120 log cm). However, palaeoenvironmental reconstructions have been improved by the use of correction factors, specifically developed for the dominant phytoplankton (Aulacoseira baicalensis, Aulacoseira skvortzowii, Cyclotella minuta, Stephanodiscus meyerii and Synedra acus) in the south basin of Lake Baikal. Cluster analysis identifies three significant zones in the core, zone 1 (c. 880 AD–c. 1180 AD), zone 2 (c. 1180–1840 AD) and zone 3 (c. 1840–1994 AD), coincident with the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the period of recent warming, respectively. Our results indicate that S. acus dominated the diatom phytoplankton within zone 1 coincident with the MWP. S. acus is an opportunistic species that is able to increase its net growth when A. baicalensis does not. During this period, conditions are likely to have been unfavourable for the net increases in A. baicalensis growth due to the persistence of warm water in the lake, together with an increased length of summer stratification and delay in timing of the autumnal overturn. In zone 2, spring diatom crops blooming under the ice declined in abundances due in part to increased winter severity and snow cover on the lake. Accumulating snow on the lake is likely to have arisen from increased anticyclonic activity, resulting in prolonged winters expressed during the LIA. Thick, accumulating snow cover inhibits light penetration through the ice, thereby having negative effects on cell division rate and extent of turbulence underneath the ice. Consequently, only taxa whose net growth occurs during autumn overturn (C. minuta) predominate in the lake at this time. Diatom census data and reconstructions of snow accumulation suggest that warming in the Lake Baikal region started as early as c. 1750 AD, with a shift from taxa that bloom during autumn overturn to assemblages that begin to grow underneath the frozen lake in spring. Very recent increases and subsequent decline of S. acus in the surface sediments of the lake mirror monitoring records of this species over the last 50 years. Our study confirms that, over the last 1000 years, physical processes are important in determining planktonic diatom populations in the lake and highlights the value of integrated plankton, trap, and sediment studies for improving quantitative palaeoenvironmental reconstructions from fossil material.