Central Arctic Ocean paleoceanographic interpretations based on Late Cenozoic calcareous dinoflagellates

Four calcareous dinoflagellate morphotypes, interpreted to represent a single species, Thoracosphaera arctica n. sp., occur in upper Miocene to Holocene sediments of the central Arctic Ocean. The calcareous fossils generally are rare except during one interval, that of the latest Pliocene to late Pleistocene. During this interval the fossil dinoflagellates were alternately abundant and rare in approximate coordination with greater and lesser times of foraminifera productivity. Dinoflagellates and foraminifera generally are rare during times of accumulation of abundant coarse textured ice-rafted sediment during this interval.The life cycle of the dinoflagellate that produces the Arctic Ocean calcareous fossil is unknown but the fossils probably are either resting cysts, motile vegetative cells or hypnozygotes. A vegetative cell origin better explains the co-occurrence of foraminifera as well as the negative correlation with ice-rafting. This tentative interpretation leads to the conclusion that during the interval 2.0 to 0.7 m.y. b.p., the Arctic Ocean had relatively productive surface waters with very thin or even periodic absence of pack-ice alternating with conditions more like that of the present. During periods before 2.0 m.y. and since 0.7 m.y., the Arctic Ocean probably was ice-covered, with oceanographic conditions similar to that of the modern ocean. These conclusions differ from previous interpretations that the Arctic was continually ice-covered, had no surface productivity or was covered by a giant, Antarctic-like ice-cap.