PLIOPLEISTOCENE NANNOFOSSIL BIOSTRATIGRAPHY AND CALIBRATION TO OXYGEN-ISOTOPE STRATIGRAPHIES FROM DEEP-SEA DRILLING PROJECT SITE-607 AND OCEAN DRILLING PROGRAM SITE-677

Evolutionary events within early Pleistocene Gephyrocapsa assemblages provide at least four biostratigraphically useful events occurring in the Matuyama Chron after the Olduvai Subchron. We establish an informal taxonomy based on three size groups (<4.0 mum, 4.0-5.5 mum, >5.5 mum) resulting in four reproducible biostratigraphic events that reflect episodes of adaptive radiation within the Gephyrocapsa lineage. Four additional, late Pliocene-early Pleistocene events are also investigated. The successful astronomical calibration of oxygen isotope stratigraphies from Deep Sea Drilling Project site 607 and Ocean Drilling Program site 677 in the Matuyama Chron permits calibration of the biostratigraphic events to these uniquely resolved isotope chronologies. Shackleton et al.'s [1990] time scale gives the following age estimates (in million years): (reemG) reentrance of medium-sized Gephyrocapsa (1.028,0.957) (677, 607); (tlG) top large Gephyrocapsa (1.240, 1.227); (blG) base large Gephyrocapsa (1.457,1.479); (bmG) base medium-sized Gephyrocapsa (1.670, 1 .700); (tHS) top H. sellii (1.470, 1.24 1); (tCM) top C. macintyrei (1.597, 1.640); (tDB) top D. brouweri (1.950) (607 only); (baDT) base acme D. triradiatus (2.153) (607 only). Further comparison with other, less resolved, partial derivative O-18 stratigraphies indicate that event reemG is diachronous by 71 kyr, consistently being younger in the mid-latitudes. Event tlG appears synchronous, clustering in stage 37 in all sites. Event blG is slightly older (22 kyr) in the mid-latitudes than in the equatorial Pacific. Also, event bmG is older in the mid-latitudes (30 kyr). Event tHS shows the largest difference, occurring 229 kyr later at site 607, corresponding to 12 isotope stages. The critical paleoisotherm separating ''young'' from ''old'' last occurrences of H. sellii was probably located between approximately 11-degrees-N and 18-degrees-N in the Atlantic Ocean. Event tCM is clearly time transgressive, occuring 43 kyr earlier at site 607. Event tDB occurred a few kiloyears within basal Olduvai at site 607.