An Austral radiolarian biozonation for the Paleogene

Abstract

ABSTRACT: We have integrated southern mid- and high-latitude (Austral) radiolarian biozonations with the well-established low-latitude (Tropical) biozonation using new biostratigraphic and magnetostratigraphic constraints on radiolarian bioevents in the Southwest (SW) Pacific, Southeast (SE) Indian and Northwest (NW) Atlantic Oceans. Our primary study sites include Mead Stream, New Zealand, and DSDP Sites 277 and 207 (SW Pacific; 45-54 degrees South at 50 Ma), ODP Site 752 and IODP Site U1514 (SE Indian; 50 degrees South at 50 Ma), and IODP Site U1403 (NW Atlantic; 30 degrees North at 50 Ma). The Austral and Tropical zonal schemes have been calibrated to GPTS2020. We introduce new zonal codes to rectify current confusion surrounding use of "RP" zones. Austral zones are codified as "RPA" zones and Tropical zones are codified as "RPT". Our study finds that radiolarian datums are generally isochronous within the mid-latitude SW Pacific and SE Indian Oceans from Paleocene to middle Eocene and are also isochronous in the high-latitude Southern Ocean (>60 degrees South paleolatitude) over the late middle Eocene to Oligocene interval of overlap. Older radiolarian assemblages are not known from the Southern Ocean. Early to middle Paleocene radiolarian assemblages in the SE Indian Ocean (zones RPA2-RPA5) differ from coeval SW Pacific assemblages by lacking significant numbers of Cretaceous survivors. The reasons for this difference are uncertain. Although the late Paleocene to Eocene radiolarian assemblages in the SW Pacific and SE Indian Ocean lack many low-latitude index species, the timing of Indian Ocean bioevents agrees better with low-latitude biozonations than the SW Pacific, suggesting a stronger connection with low-latitude watermasses. Assemblages from NW Atlantic IODP Site U1403 include numerous low-latitude index species and can be correlated with zones RPT6-RPT13. Many of the species transitions in biostratigraphically important Eocene lineages, however, occur later than in lower latitudes.