SILICEOUS MICROFOSSIL SUCCESSION IN THE RECENT HISTORY OF 2 BASINS INLAKE BAIKAL, SIBERIA

As part of the international cooperative Baikal Drilling Project, sili ceous microfossil assemblage succession was analyzed in two short (sim ilar to 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55 degrees 15'N, 109 degrees 30'E), a second from between the central and southern basins (Core 316, 52 deg rees 28'N, 106 degrees 5'E). The northern core had higher amounts of b iogenic silica (40 g SiO2 per 100 g dry weight sediment) compared to t he southern core, and increased deposition in the more recent sediment s. Weight percent biogenic silica was lower in the southern core, rang ing from approximately 20-30 g SiO2 per 100 g dry weight sediment thro ughout the entire core. Trends in absolute microfossil abundance minor those of biogenic silica, with generally greater abundance in the nor thern core (86-275 x 10(6) microfossils g(-1) dry sediment) compared t o the southern core (94-163 x 10(6) microfossils g(-1) dry sediment). Cluster analyses using relative abundance of the dominant diatom and c hrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be refl ecting shifts in nutrient supply and cycling driven by climatic change s. The most recent sediments in the northern basin (Zone 1, c. 1890's- 1991 A.D.) were characterized by an increased abundance of Aulacoseira baicalensis and Aulacoseira 'spore'. Zone 3 (c. 1630's-1830's A.D.) w as dominated by the endemic Cyclotella spp. and reduced abundance of t he Aulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zo nes 1 and 3 (Zone 2, c. 1830's-1890's A.D.) and below Zone 3 (Zone 4, c. 830's-1430's A.D.) are similar to one another suggesting they repre sent transitional intervals between warm and cold periods. Southern ba sin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g. Stephanodisc us binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late 1700's) suggests eviden ce for anthropogenic induced changes in southern Lake Baikal.