Diatom Biostratigraphy of the Middle- and High-Latitude North Atlantic Ocean, Deep Sea Drilling Project Leg 94

Moderately well-preserved diatoms are present in lower Miocene to Quaternary sediment recovered during DSDP Leg 94, except for the upper middle Miocene to upper Miocene, which is typically barren of diatoms. The diatom species are characteristic of a warm-temperate assemblage similar to that described by Baldauf (1985a) from the Rockall Plateau region of the North Atlantic Ocean. The diatom zonation of Baldauf (1985a) is recognized for the upper Miocene to Quaternary. The diatom zonation of Barron (1983, 1985a) from the Pacific Ocean is utilized in the lower and middle Miocene. The recognition of these zonations over a wide geographic region indicates that diatoms are useful for biostratigraphic control of specific time-intervals in the North Atlantic Ocean. Pliocene and Quaternary diatom datums are correlated directly to the paleomagnetic stratigraphy of Leg 94, and absolute ages are estimated for the first or last occurrence of species. Several biostratigraphic events are synchronous in the midto high-latitude North Atlantic Ocean but diachronous between the North Atlantic and North Pacific oceans; others are synchronous between both oceans. Denticulopsis seminae and D. seminae var. fossilis are observed at several Leg 94 sites. The occurrence of this species within the middle Quaternary of the North Atlantic Ocean suggests that an open communication may have existed between the North Atlantic Ocean and the Bering Sea. INTRODUCTION Because few diatom biostratigraphic or paleoceanographic studies existed for the middleto high-latitude North Atlantic Ocean (see Baldauf, 1985a, for discussion), a middle Miocene to Holocene diatom zonation was developed by Baldauf (1985a) from material from Deep Sea Drilling Project Leg 81, within the Rockall Plateau region of the North Atlantic Ocean. The late Miocene to Holocene part of this zonation is based on the occurrence in the North Atlantic of a warm-temperate diatom assemblage similar to that recorded from the eastern equatorial Pacific by Burckle (1972, 1977), Baldauf (1985b), and Barron (1985a). The middle Miocene diatom zones proposed by Baldauf (1985a) for the Rockall Plateau region utilize both warm-temperate species and species more characteristic of the cool, high latitudes. Problems in using low-latitude marker species within the high-latitude North Atlantic Ocean are discussed by Backman et al. (1985). The major difficulty in using diatoms for biostratigraphic control in the North Atlantic Ocean is the sporadic preservation of silica in the sediment. In Leg 81 material, diatoms are generally concentrated within specific stratigraphic intervals. For example, they are normally absent in the upper middle Miocene to lower upper Miocene but are present in the lower middle Miocene. In the upper Pliocene-Pleistocene of Hole 552A (Fig. 1), diatom abundance and preservation also fluctuate, probably in response to changing oceanographic conditions affect70°N Ruddiman, W. E, Kidd, R. B. , Thomas, E. , et al., Init. Repts. DSDP, 94: Washington (US. Govt. Printing Office). 2 Present address: Ocean Drilling Program, Texas A&M University, College Station, TX 77843. 60