Middle Eocene to early Oligocene paleoceanography of the Southern Ocean: Critical events in the greenhouse to icehouse transition

A fundamental shift from "greenhouse" to "icehouse" climate modes occurred through the Eocene-Oligocene time interval (∼55 to 23 Ma), exemplified by a dramatic cooling of deep waters and the first appearance of large Antarctic ice sheets. Although these long-term trends have been well established, very few high-resolution paleoceanographic records have documented short-term climatic variability through this transition. To this end, I have constructed high-resolution foraminiferal stable isotope and Mg/Ca temperature records at multiple deep-sea drill sites, with a primary focus on Southern Ocean records that span the late middle Eocene-to-earliest Oligocene interval (∼43 and 32 Ma). When considered together, these new paleoceanographic records provide evidence of dynamic climate variability in the transition from greenhouse to icehouse climate states.;In Chapter 1, a transient warming event was identified within delta 18O records spanning the late middle Eocene interval. This event, designated as the Middle Eocene Climatic Optimum (MECO), is characterized by surface and deep-water warming of ∼4°C in the Southern Ocean and represents a major reversal in the long-term cooling trend through the middle and late Eocene. Further documentation of this event in Chapter 2 demonstrates that warming during the MECO occurred at both mid-latitude and high-latitude locations. Peak warming is also associated with a decrease in carbonate accumulation at sites below 3000 m water depth, indicating that a transient shoaling of the carbonate compensation depth (CCD) occurred in conjunction with the event. These observations make a convincing argument that the dynamic climate history of the late middle Eocene is due to variability in atmospheric pCO 2 levels.;In Chapter 3, the nature of the deep-sea delta18O signal through the Eocene-Oligocene transition was investigated at Southern Ocean drill sites. Planktic foraminiferal Mg/Ca records indicate that Southern Ocean waters cooled ∼2-3°C in tandem with the prominent increase in delta 18O values near the Eocene-Oligocene boundary (i.e. the Oi-1 event). Based on these results, the Oi-1 delta18O shift is interpreted to represent a combination of cooling and an increase in ice volume. If the latest Eocene was predominantly ice free, an ice-volume estimate of ∼2.0E+07 km3 is calculated for the earliest Oligocene interval, equivalent to ∼80% of the modern East Antarctic Ice Sheet volume. This revised estimate is significantly lower than for scenarios in which no high-latitude or deep-water cooling is inferred.