Neogen diatom biostratigraphy of ODP Leg 138 sites

Ocean Drilling Program (ODP) Leg 138 recovered more than 5500 m of Quaternary to middle Miocene (~17 Ma) sediments from 11 sites in the eastern equatorial Pacific Ocean. These sediments represent the most complete stratigraphic sequence recovered since the start of scientific ocean drilling by the Deep Sea Drilling Project (DSDP) and ODP. The diatoms observed generally are common to abundant and well-preserved throughout the samples examined. The assemblages are characterized by species typical of low-latitudes and regions of high surface-water productivity and are dominated by Thalassiothrix longissima, Thalassionema nitzschioides, Azpeitia nodulifer, and numerous species of Thalassiosira and Nitzschia. Fifty-six biostratigraphic events were identified at Sites 844 through 852, allowing us, in part, to use the diatom zonation of Barron (1985a). This zonation was modified by replacing the Rhizosolenia preabergonii Zone and the upper portion of the Nitzschia jouseae Zone, as used by Barron (1985a), with the Nitzschia marina and Nitzschia jouseae zones, as used by Baldauf (1984, 1987). Twenty-nine biostratigraphic events have been correlated to the Leg 138 paleomagnetic stratigraphy of Schneider (this volume). Nineteen of these events are well constrained to permit recalibration. Diatoms were rare or absent in samples examined from Sites 853 and 854. As such, these sites are not included in the following discussion