Late Middle Pleistocene sequences in the lower Ob' and Irtysh (West Siberia) and new multi-proxy records of terrestrial environmental change

The Middle Pleistocene environmental history of the north of West Siberia is very incomplete. Only a few poorly preserved paleoarchives are known so far in this huge part of Eurasia encompassing up to one million square kilometers. Here we present new data (micromammals, insects, plant macrofossils and luminescence dating) from the lower reaches of the Ob' and Irtysh Rivers. We reconstruct the circumstances of the formation of the fossil-bearing sites Bolshaya Ob 440 km - Khashgort (BOB440) and Gornopravdinsk 2 (GP2) and compare the results with the current state of knowledge of the environmental change in this area. The combined evidence suggests that the fossil-bearing unit known as the Khashgort diagonal sands at BOB440 yields 1) a late Middle Pleistocene complex of fauna and flora of a cold terrestrial environment with tundra-like vegetation, 2) a Middle Pleistocene flora of interglacial environment with temperate vegetation, and 3) reworked fossils form the Late Cretaceous - Oligocene. Geochronometric data suggest that the Khashgort sand unit at BOB440 might have been formed during the Middle/Late Pleistocene transition. However, because of a high variation in results obtained on repeated samples from the same strata, some uncertainty of the luminescence dating remains, which requires further verification. The fossil-bearing fluvial sequence at GP2 represents the lower part of the Chembakchino Formation and encompasses the lower part of the Tobolian Horizon (correlated with Holsteinian). The micromammal complex comprises Dicrostonyx simplicior (intermediate morphological stage S2 sensu Smirnov et al., 1997), Lasiopodomys gregalis, Microtus nivaloides lidiae and other taxa, which are characteristic of the early evolutionary stage of late Middle Pleistocene faunas in the north of West Siberia. The paleoecological analysis of plant and animal remains at GP2 suggests an environmental change from cold, tundra-like landscapes at the Shaitanian/Tobolian transition (MIS 12 - MIS 11) to non-analogous open patchy landscapes with highly productive moss and herbaceous vegetation during the early Tobolian time.