Biogenic silica record of the Lake Baikal response to climatic forcing during the Brunhes

This work presents a detailed, orbitally tuned biogenic silica record of co ntinental paleoclimate change during the Brunhes chron, The Brunhes/Matuyam a boundary lies within the warm isotopic stage 19 in Baikal, and the bounda ries between eight lithological cycles correspond to terminations in the ma rine oxygen isotope record, The high amplitude and resolution of climatical ly driven changes in BioSi content in Lake Baikal sediments permits tuning of almost every precessional cycle during the Brunhes and reveals the struc ture of interglacial stages, For example, the last three interglacial stage s (MIS 5, 7, and 9) clearly consist of five substages (a, b, c, d, e) corre sponding to precessional insolation peaks. Abrupt and intense regional glac iations in Siberia during substages 5d and 7d were driven by extreme insola tion minima. During substage 9d cooling was more gradual in response to mor e moderate forcing. The impact of strong glaciation is also observed in the middle of stage 15, where full glacial conditions appear to have lasted fo r over 30,000 yr during substages 15d, 15c, and 15b, Marine oxygen isotopic stage 11 appears to be the warmest period during the Brunhes in the Lake B aikal record, with at least three substages, A new hypothesis is presented regarding the response of the Lake Baikal Bio Si record to insolation forcing, Based on the mechanism controlling modern diatom blooms, biogenic silica production is hypothesized to be dependent o n changes in the heat balance of the lake and consequently on changes in th e thermal structure of the water column. This mechanism is also sensitive t o short-term sub-Milankovich cooling events, such as the mid-Eemian cooling , the Montaigu event during substage 5c, and a cooling which appears to be analogous to the Montaigu event during substage 9c, The continuity of the L ake Baikal paleoclimate record, its sensitivity to orbital forcing, and its high resolution make it an excellent candidate for a new "paleoclimatic st ratotype" section for continental Asia. (C) 2001 University of Washington.