ALBERT

Description: Well-preserved late Miocene through Pleistocene age radiolarian assemblages were recovered during ODP Leg 111 at Site 677, on the southern flank of the Costa Rica Rift in the eastern equatorial Pacific. Radiolarian "event" biostratigraphy (first and last morphotypic appearances) was established for Holes 677A and 677B using 21 species yielding 24 reliable datum levels. The cold upwelling waters above this site have prevented many typical tropical Pacific stratigraphic radiolarians from being useful age indicators. Biostratigraphic datum levels were assigned absolute ages based on previous work and were used to date the cores. Sedimentation rates varied from 3.7 cm/1000 yr in the late Pleistocene to 6.0 cm/1000 yr in the late Miocene. The age of the oldest sediments at this site is estimated as 5.89-6.37 Ma, which indicates that Site 677 is between magnetic anomalies 3A and 4. A total of 67 taxa were assessed for stratigraphic relevance at this site and are listed in the Appendix. One previously unknown Pliocene radiolarian stratigraphic indicator, Botryostrobus euporus (Ehrenberg), is identified.

Description: The recovery of continuous sedimentary sequences has been and is a high-priority goal of the Ocean Drilling Program, yet its success has not been determined. We test the stratigraphic continuity of continuously cored sequences from ODP Leg 111 Holes 677A and 677B and DSDP Leg 69 Hole 504. The two holes at Site 677 and Hole 504 are correlated using high-resolution biostratigraphy, tephra stratigraphy, and time series of opaline silica and calcium carbonate. The correlation coefficient between the time series is maximized through an inverse correlation procedure. Changes in slope of the mapping function that relates the two sections indicate gaps or changes in the accumulation rate between sections. We assume as a first approximation that the sedimentary sequences are identical in each hole, given their close proximity. Slope changes of the mapping functions therefore result from errors introduced by coring. This assumption is justified by the fact that most mapping function slope changes occur at core breaks. Having identified missing sections in one site, it is possible to fill these gaps with the appropriate section from the adjacent site, thereby improving continuity. This is possible only where the sediment gaps identified at core breaks in one hole do not correspond to core break levels in the other hole. Results show that in the upper 100 m of the stratigraphic column, 3.1% and 4.6% were unsampled in Holes 677A and 677B, respectively, and 15% of the Pliocene portion of Hole 504 was not sampled by the hydraulic piston corer. This indicates that even in relatively calm seas, double or triple coring is required in order to ensure that gaps and doubly sampled intervals of the stratigraphic column which occur at core breaks are recognized and sampled at least once.