Development of radiolarians in the Northwest Pacific during the Paleogene

Abstract

The analysis of the taxonomic composition of radiolarians from Paleogene sections of the North-west Pacific and its surrounding continental structures from the southern Japan Archipelago to southern Koryak Highland revealed that their Paleocene-Eocene assemblages from high and low latitudes were characterized by higher similarity as compared with their Oligocene or Neogene counterparts, which indicates the lack of sharp meridional gradients between different water masses at that time. In the Paleocene-Eocene, water temperatures in high latitudes were likely substantially higher than now and the subtropical belt extended approximately up to 60° N. Two paleobiogeographic regions with radiolarian assemblages differing in a number of genera and species in common with their low-latitude communities are outlined within the latter: northern and southern. Development of radiolarians in this region demonstrates several stages, which practically correspond to their similar evolutionary stages in the tropical belt. A sharp reorganization in the radiolarian community occurred near the Eocene-Oligocene transition, when the “typical boreal” fauna differing in the taxonomic composition from that of the tropical belt was formed in high latitudes. This indicates a substantial decrease of water temperatures in high latitudes, which resulted in the development of upwellings and, presumably, the formation of the Subpolar hydrological front.