Calcareous nannofossil abundance and datums of ODP Holes 171B-1051A and 171B-1052A

During Ocean Drilling Program Leg 171B, a thick sequence of lower to middle Eocene sediments was recovered from Sites 1051 and 1052 at Blake Nose in the North Atlantic Ocean. Calcareous nannofossils are moderately well preserved in the upper to middle Eocene sediments but are moderate to poorly preserved in the lower Eocene sediments. Calcareous nannofossils are diverse throughout the recovered sequence, which extends from nannofossil Zone CP8 to Subzone CP15a. The nannofossil biostratigraphy of these sites indicates the presence of a hiatus in Subzone CP12a in the middle Eocene, in which the major nannofossil assemblage changes dramatically from Toweius to reticulofenestrid; however, no major change in the nannoflora was observed across the Eocene/Paleocene boundary. Coccolith size evolution patterns were recognized. Coccolithus, Reticulofenestra, and Cribrocentrum specimens may suggest a trend of increasing size upward through the sedimentary sequence, but Dictyococcites does not show a similar simple trend. Most traditional zonal markers are present. The reworking of Discoaster sublodoensis and overgrowth of Tribrachiatus in the lower Eocene makes zonal subdivision of this part of the sequence difficult. For this reason, tentative nannofossil zonation is given for the lower Eocene.