The Eocene-Oligocene transition (EOT) marked the initiation of large-scale Antarctic glaciation. This fundamental change in Cenozoic climate state is recorded in deep-sea sediments by a rapid benthic foraminiferal δ 18 O 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 water environments, particularly at middle to high latitudes. Here we present new stratigraphic records of the C13r/C13n magnetochron boundary and the EOT in the clay-rich Blanche Point Formation, South Australia. The Blanche Point Formation was deposited in a shallow shelf setting (water depths of <100 m) at a paleolatitude of~51°S. We present high-resolution δ 18 O, δ 13 C, and Mg/Ca records of environmental change from well-preserved benthic foraminifera of latest Eocene age at this site. A marked, negative δ 13 C excursion occurs immediately before EOT Step 1 and may be a globally representative signal. An~2°C cooling of shallow shelf seawater is evident from benthic foraminiferal Mg/Ca across Step 1. This cooling signal is both sufficient to account fully for the δ 18 O increase in our data and is of similar amplitude to that documented in published records for shallow shelf and upper water column open ocean settings, which suggests no obvious polar amplification of this cooling signal. Our results strengthen the evidence base for attributing EOT Step 1 to global cooling with little contribution from ice volume growth and contradict the mechanism suggested to explain the inferred northward migration of the intertropical convergence zone in the contemporaneous equatorial Pacific Ocean.
Key Points:• We present a detailed stable isotopic and Mg/Ca record for the early Eocene-Oligocene transition from southern Australia at~51°S • Shallow shelf (<100 m deep) seawater temperatures fell 2°C across this part of the transition • Surface seawater δ18O values were probably closer to positive one than negative one within this embayment in the latest Eocene, indicating that a source of relatively saline water was present at this time
Supporting Information:• Supporting Information S1
