CLIMATE AND VEGETATION DYNAMICS AS FACTORS OF BOG FORMATION AND PEAT ACCUMULATION IN WEST SIBERIA

Detailed paleobotanical studies, 14C dating and paleoclimate reconstruction have been performed on three key sites from three main subzones of the taiga zone in West Siberia: southern taiga (Zhukovskoe), middle taiga (Nizhnevartovsk) and northern taiga (Noyabrsk). During the Allerod dominant were larch woodlands (with birch and spruce), cryophilic shrubs and steppe communities with periglacial steppe elements and heliophytes. The Younger Dryas was distinct for dominance of steppe communities; forests and open woodlands were confined to river valleys. Forests and open woodlands of birch, spruce and larch expanded again during Preboreal. In the Boreal period birch forests expanded over the whole area of modern forest zone. Spruce and larch were of some importance as forest-forming species only within the limits of the modern taiga subzone. During the Atlantic period dark coniferous forests gained in area. The boundaries of all subzones at the Atlantic were shifted northwards with reference to their modern position by a whole span of a subzone. The time interval about 6000-5000 BP was the warmest part of the Holocene. Drilling data show that the thickness of peat layers in the taiga zone decreased northward because the phytomass production decreased northward following the decline of heat supply.

The age of peat initiation is close for all key sites (Zhukovskoe - ~8,600 BP, Nizhnevartovsk - ~9200 BP, Noyabrsk - ~10,000 BP). The peat initiation started about the same time over the forest zone with the onset of the early Holocene global warming. Field data reveal an essential difference in the peat accumulation dynamics between the zone of frozen peat plateaux and the area of thawed peatlands. The curves of peat accumulation rate on the thawed peatlands generally show the linear trend of the accumulation. A distinct decrease in the accumulation rate is recorded in the middle Holocene. Thus, from the peat initiation to 6.5-6 ka BP the peat accumulation rate is close to 0.1 cm/year at Zhukovskoe, and ~0.05 cm/year at Nizhnevartovsk. In the middle Holocene, between 6 and 4-5.5 ka BP, the rate at those sites decreases to 0.075 cm/year and 0.03-0.04 cm/year, respectively. A somewhat higher rate has been recorded in the late Holocene (to 0.08 cm/year in Zhukovskoe and to 0.06 cm/year in Nizhnevartovsk.