Early to middle Eocene calcareous nannofossils of the SW Pacific: Paleobiogeography and paleoclimate

Earth’s climate underwent a long-term warming trend from the late Paleocene to early Eocene (~58–51 Ma), with global temperature reaching a sustained maximum during the Early Eocene Climatic Optimum (EECO; 53–50 Ma). Geochemical proxies indicate tropical or warm subtropical sea-surface temperature (SST) conditions in middle and high latitudes in the early Eocene, implying a very low latitudinal temperature gradient. This study investigates whether calcareous nannofossil assemblages in the southwest (SW) Pacific provide evidence of these conditions at middle latitudes in the early to middle Eocene, particularly during the EECO. Specifically, this study documents the biogeographic changes of warm- and cold-water nannofossil species along a paleolatitudinal transect through the EECO to track changes in water masses/ocean circulation at that time. Early to middle Eocene calcareous nannofossil assemblages were examined from four sites along a latitudinal transect in the SW Pacific, extending from Lord Howe Rise in the north to Campbell Plateau in the south and spanning a paleolatitude of ~46–54°S. All of the sections studied in this project span nannofossil zones NP10–16 (Martini, 1971). The data indicate up to three regional unconformities through the sections: at mid-Waipara, Deep Sea Drilling Project (DSDP) Site 207 and 277, part or all of Zone NP10 (lower Waipawan) is missing; at Sites 207 and 277 a possible hiatus occurs within NP12 (upper Waipawan–lower Mangaorapan); and at all sites part or all of Zone NP15 (lower Bortonian) is missing. Results of this study indicate that nannofossil assemblages in the SW Pacific are more similar to floras at temperate to polar sites rather than those at tropical/subtropical sites. However, variations in the relative abundance of key species in the SW Pacific are broadly consistent with the trends seen in the geochemical proxy records: an increase in warm-water taxa coincided with the EECO, corroborating geochemical evidence for a temperature maximum in the SW Pacific during this interval. The increase in the abundance and diversity of warm-water taxa and decrease in the abundance of cool-water taxa through the EECO supports previous suggestions that a warm-water mass (northward of the proto-Tasman Front) extended to ~55°S paleolatitude during this interval in response to enhanced poleward heat transport and intensification of the proto-East Australian Current. At the southernmost site, DSDP Site 277, a relatively short-lived influx of warm-water taxa at ~51 Ma suggests that warm waters expanded south at this time. However, greater diversity and abundance of warm-water taxa throughout the EECO at DSDP Site 207, suggests that the proto-East Australian Current exerted greater influence at this latitude for a longer duration than at Site 277. An increase in the abundance of cool-water taxa and decrease in diversity and abundance of warm-water taxa at all sites is recorded following the termination of the EECO. This corresponds with the contraction of the proto-Tasman Front due to weakened proto-East Australian Current flow and associated amplification of the proto-Ross Gyre. Previous estimates of SSTs from geochemical proxies in the SW Pacific during the EECO indicate that there was virtually no latitudinal temperature gradient and temperatures were tropical to subtropical (>20°C). However, nannofossil data from this study indicate warm temperate conditions (~15–20°C) during the EECO, suggesting that a reduced latitudinal gradient was maintained through this interval, which is in agreement with climate models.