Astronomical calibration of late middle Eocene radiolarian bioevents from ODP Site 1260 (equatorial Atlantic, Leg 207) and refinement of the global tropical radiolarian biozonation

Abstract

Abstract. The middle Eocene sedimentary sequence drilled at Ocean Drilling Program Site 1260 (Leg 207), Demerara Rise, western equatorial Atlantic, yielded a rich and diverse radiolarian fauna. The exceptionally expanded and complete sedimentary record of this site, as well as the existence of an orbital chronological framework, allowed us to study a number of radiolarian bioevents with a very fine temporal resolution. We compiled a well-resolved succession of 71 radiolarian bioevents (first occurrences, last occurrences, and evolutionary transitions) and provided calibrations to the geomagnetic polarity timescale and astronomical timescale. Comparison of the radiolarian successions at ODP Site 1260A with the northwestern Atlantic IODP Site U1403 and the IODP Sites U1331, U1332, and 1333, situated in eastern equatorial Pacific, allowed the demonstration of the synchroneity of primary radiolarian bioevents that underpin the middle Eocene zonal scheme. Several secondary bioevents were also found to be synchronous between the two oceans and were therefore used to define seven new subzones for the low-latitudinal middle Eocene sequences: Siphocampe ? amygdala interval subzone (RP13a), Coccolarnacium periphaenoides interval subzone (RP13b), Artostrobus quadriporus interval subzone (RP14a), Sethochytris triconiscus interval subzone (RP14b), Podocyrtis (Podocyrtopsis) apeza interval subzone (RP14c), Artobotrys biaurita interval subzone (RP15a), and Thyrsocyrtis (Pentalocorys) krooni interval subzone (RP15b). This refined radiolarian biozonation has significantly improved stratigraphic resolution and age control for the late middle Eocene interval (an average of two subzones per 1.5 million years). A substantial diachronism was also found in 20 secondary radiolarian bioevents between the two oceans. The majority of radiolarian species appear to have evolved first in the equatorial Atlantic Ocean and subsequently in the equatorial Pacific. However, the reasons for this pattern of diachroneity currently remain uncertain and would require a greater sampling coverage to be elucidated.