Midlatitude Southern Hemisphere Temperature Change at the End of the Eocene Greenhouse Shortly Before Dawn of the Oligocene Icehouse

• Published in: Paleoceanography and Paleoclimatology
• Main Authors: Haiblen, Anna, Opdyke, Bradley, Roberts, Andrew P., Heslop, David, Wilson, Paul A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley-Blackwell Publishing, Inc.
• Subjects: Eocene‐Oligocene transition; paleotemperature; laser ablation mass spectrometry; foraminiferal Mg/Ca; stable isotopes; stratigraphy; Pacific; Blanche; Blanche Point; Antarc*
• Online Access: http://hdl.handle.net/1885/219038; https://doi.org/10.1029/2019PA003679; https://openresearch-repository.anu.edu.au/bitstream/1885/219038/3/01_Haiblen_Midlatitude_Southern_2019.pdf.jpg

The Eocene‐Oligocene transition (EOT) marked the initiation of large‐scale Antarcticglaciation. This fundamental change in Cenozoic climate state is recorded in deep‐sea sediments by arapid benthic foraminiferalδ18O increase and appearance of ice‐rafted debris in the Southern Ocean.However, we know little about the magnitude of cooling associated with the EOT in shallow waterenvironments, particularly at middle to high latitudes. Here we present new stratigraphic records of theC13r/C13n magnetochron boundary and the EOT in the clay‐rich Blanche Point Formation, SouthAustralia. The Blanche Point Formation was deposited in a shallow shelf setting (water depths of <100 m) ata paleolatitude of ~51°S. We present high‐resolutionδ18O,δ13C, and Mg/Ca records of environmentalchange from well‐preserved benthic foraminifera of latest Eocene age at this site. A marked, negativeδ13Cexcursion occurs immediately before EOT Step 1 and may be a globally representative signal. An ~2°C Ccooling of shallow shelf seawater is evident from benthic foraminiferal Mg/Ca across Step 1. This coolingsignal is both sufficient to account fully for theδ18O increase in our data and is of similar amplitude to thatdocumented in published records for shallow shelf and upper water column open ocean settings, whichsuggests no obvious polar amplification of this cooling signal. Our results strengthen the evidence base forattributing EOT Step 1 to global cooling with little contribution from ice volume growth and contradict themechanism suggested to explain the inferred northward migration of the intertropical convergence zone inthe contemporaneous equatorial Pacific Ocean.