Expansion and diversification of high-latitude radiolarian assemblages in the late Eocene linked to a cooling event in the Southwest Pacific

Abstract

Abstract. The Eocene was characterised by "greenhouse" climate conditions that were gradually terminated by a long-term cooling trend through the middle and late Eocene. This long-term trend was determined by several large-scale climate perturbations that culminated in a shift to "ice-house" climates at the Eocene–Oligocene Transition. Geochemical and micropaleontological proxies suggest that tropical-to-subtropical sea-surface temperatures persisted into the late Eocene in the high-latitude Southwest Pacific Ocean. Here, we present radiolarian microfossil assemblage and foraminiferal oxygen and carbon stable isotope data from Deep Sea Drilling Project (DSDP) Sites 277, 280, 281 and 283 from the middle Eocene to early Oligocene (~ 40–33 Ma) to identify oceanographic changes in the Southwest Pacific across this major transition in Earth's climate history. The Middle Eocene Climatic Optimum at ~ 40 Ma is characterised by a negative shift in foraminiferal oxygen isotope values and a radiolarian assemblage consisting of about 5 % of low latitude taxa Amphicraspedum prolixum group and Amphymenium murrayanum. In the early late Eocene at ~ 37 Ma, a positive oxygen isotope shift can be correlated to the Priabonian Oxygen Isotope Maximum (PrOM) event – a short-lived cooling event recognized throughout the Southern Ocean. Radiolarian abundance, diversity, and preservation increase during the middle of this event at Site 277 at the same time as diatoms. The PrOM and latest Eocene radiolarian assemblages are characterised by abundant high-latitude taxa. These high-latitude taxa also increase in abundance during the late Eocene and early Oligocene at DSDP Sites 280, 281 and 283 and are associated with very high diatom abundance. We therefore infer a~northward expansion of high-latitude radiolarian taxa onto the Campbell Plateau towards the end of the late Eocene. In the early Oligocene (~ 33 Ma) there is an overall decrease in radiolarian abundance and diversity at Site 277, and diatoms are absent. These data indicate that, once the Tasman Gateway was fully open in the early Oligocene, a frontal system similar to the present day was established, with nutrient-depleted subantarctic waters bathing the area around DSDP Site 277, resulting in a more oligotrophic siliceous plankton assemblage.

Variation of minerals and clay minerals recorded in the Neo-Tethys (central Turkey): new evidence of climatic changes during the middle Eocene

“…The global warming event that took place during the middle Eocene, also known as MECO, occurred at approximately 40.5 Myr ago and lasted for almost 500 ka (BOHATY;ZACHOS, 2003;BOHATY et al, 2009). Despite its clear stable isotopic signature, which was found in the Atlantic, Pacific, and Indian Oceans (BOHATY et al, 2009;EDGAR et al, 2010;PASCHER et al, 2015), as well as in the Tethys (JOVANE et al, 2007;LUCIANI et al, 2010;SPOFFORTH et al, 2010;DAWBER et al, 2011;Savian et al, 2014), the climatic mechanism responsible for this event are still not well understood. Computer model projections, when compared to observed data, fail to explain the long duration of the warming conditions, as well as their abrupt end (SLUIJS et al, 2013).…”

A Song of Ice and a Warm Southern Ocean

“…To a large extent, our knowledge on the development of Paleogene Southern Ocean surface circulation is built on biogeographic patterns in organic-walled dino agellate cyst assemblages (Wrenn and Beckman, 1982;Bijl et al, 2011) as well as supporting information from other microfossil groups (Pascher et al, 2015), both backed up by numerical model simulations (e.g. Huber et al, 2004;Sijp et al, 2014).…”