A QUANTITATIVE ANALYSIS OF CALCAREOUS NANNOFOSSILS ACROSS A LATE OLIGOCENE PALEOLATIUDE TRANSECT OF THE NORTH ATLANTIC OCEAN

Samples from ODP Sites 926, 628, 563, U1406, 647, and 918, were analyzed quantitatively across a paleolatitude transect of the North Atlantic Ocean to determine the paleolatitudinal distribution of calcareous nannofossils in the Late Oligocene and the effects of that distribution on biostratigraphic resolution. Detrended Correspondence Analysis (DCA), a Temperature index (TI), and the Shannon Diversity Index (H), were used to examine the paleoenvironmental gradients which exerted the most control over the distribution of species and their abundances. The temperature index correlates significantly to the first axis of the DCA, suggesting that thermal controls were the most important factor in the distribution of Late Oligocene nannofossils in the North Atlantic. Shannon Diversity and the percent abundance of the taxon Reticulofenestra minuta correlates significantly to DCA 2, suggesting that surface water mass fertility was an important secondary controlling factor during the Late Oligocene. A biochronological analysis utilizing Unitary Associations (UA) divided the assemblage into 6 UAs, correlating with existing biostratigraphic schemes. UA 3 and UA 4, defined by the base of Sphenolithus calyculus, was not used consistently in current biostratigraphy, and offers an additional bioevent for the Upper Oligocene. The absence of UA 5 at Hole 926B reveals a disconformity that was not identified previously. Current zonation schemes cannot be accurately applied to the North Atlantic Ocean north of 53° latitude. The best biostratigraphic resolution in the Late Oligocene North Atlantic is restricted to the upper mid latitudes Sites 628, 563, and U1406. These latitudes, from 27-40^°, represent the warm gyre center and the temperate western gyre perimeter.