Siliceous microfossil succession in the recent history of two basins in Lake Baikal, Siberia

As part of the international cooperative Baikal Drilling Project, siliceous microfossil assemblage succession was analyzed in two short (~ 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55°15~N, 109°30~E), a second from between the central and southern basins (Core 316, 52°28'N, 106°5rE). The northern core had higher amounts of biogenic silica (40 g SiO2 per 100 g dry weight sediment) compared to the southern core, and increased deposition in the more recent sediments. Weight percent biogenic silica was lower in the southern core, ranging from approximately 20-30 g SiO2 per 100 g dry weight sediment throughout the entire core. Trends in absolute microfossil abundance mirror those of biogenic silica, with generally greater abundance in the northern core (86-275 x 106 microfossils g-1 dry sediment) compared to the southern core (94-163 x 106 microfossils g-1 dry sediment). Cluster analyses using relative abundance of the dominant diatom and chrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be reflecting shifts in nutrient supply and cycling driven by climatic changes. 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.) was dominated by the endemic CycloteIla spp. and reduced abundance of the Aulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zones 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 represent transitional intervals between warm and cold periods. Southern basin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g. Stephanodiscus binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late t 700's) suggests evidence for anthropogenic induced changes in southern Lake Baikal.