BIOTIC AND ABIOTIC EVENTS OF THE LATE CRETACEOUS ARCTIC BIOGEOGRAPHIC REALM

Abstract

The marine biota of the Arctic paleobiogeographic realm differed from the biota of the Boreal realm in being taxonomically depauperated rather than in having endemics of a high rank (Superrealm). The biota of the West Siberian province is typical of the Arctic. The Upper Cretaceous composite section in northern West Siberia contains traces of biotic and abiotic events restricted to the boundaries of stages or substages: Cenomanian-Turonian, Middle-Upper Turonian, Turonian-Coniacian, Lower-Upper Coniacian, Coniacian-Santonian, Santonian-Campanian, and Maastrichtian-Danian. There are two levels at which the maximum diversity of bottom mollusks, foraminifers, and dinocysts coincides and the taxonomic composition of the same groups becomes most even: at the Cenomanian-Turonian boundary and in the Late Santonian. They coincide with the warmest episodes in the Late Cretaceous of northern Siberia. The two bioevents correspond in time to accumulation of "black shales", associated with the eustatic rise of a sea level in the late Cenomanian-early Turonian and vast Boreal transgression in the Santonian-Early Campanian. No significant extinction has been recorded at the Cenomanian-Turonian boundary. The extinction of invertebrates in the Maastrichtian of the West Siberian province proceeded step by step. By the end of the Late Cretaceous, the Siberian biota became less diverse as a result of general cooling in northern Eurasia.