Middle Eocene to Late Oligocene Antarctic glaciation/deglaciation and Southern Ocean productivity Giuliana Villa 1 , Chiara Fioroni 2 , Davide Persico 1 , Andrew P. Roberts 3 , and Fabio Florindo 4 1 Dipartimento di Fisica e Scienze della Terra “Macedonio Melloni”, Università di Parma, Parma, Italy, 2 Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia, Modena, Italy, 3 Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory, Australia, 4 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Abstract During the Eocene-Oligocene transition, Earth cooled significantly from a greenhouse to an icehouse climate. Nannofossil assemblages from Southern Ocean sites enable evaluation of paleoceanographic changes and, hence, of the oceanic response to Antarctic ice sheet evolution during the Eocene and Oligocene. A combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes. A cooling trend, started in the Middle Eocene, was interrupted by warming during the Middle Eocene Climatic optimum and by short cooling episodes. The cooling episode at 39.6 Ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from ~39.1 to ~36.2 Ma. We suggest that oligotrophic conditions were associated with increased water mass stratification, low nutrient contents, and high efficiency of the oceanic biological pump that, in turn, promoted sequestration of carbon from surface waters, which favored cooling. After 36.2 Ma, we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance. This turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient Antarctic ice sheet and/or to iron delivery to the sea surface. Eutrophic conditions were maintained throughout the Oligocene, which was characterized by a nannofossil assemblage shift toward cool conditions at the Eocene-Oligocene transition. Finally, a warm nannofossil assemblage in the Late Oligocene indicates a warming phase. 1. Introduction Earth’ s climate underwent major change during the Middle Eocene to Early Oligocene that led to a turnover from a greenhouse to an icehouse world, the causes of which are still debated. The transition, which culmi- nated with expansion of a continental-scale Antarctic ice sheet, is recorded by an abrupt benthic foraminif- eral δ 18 O shift in the earliest Oligocene, the Oi-1 event at ca 34 Ma [Miller et al., 1991; Zachos et al., 2001], which was associated with a global deepening of the carbonate compensation depth by about 1000 m [van Andel, 1975; Coxall et al., 2005; Pälike et al., 2012]. During this period, high latitude deep and surface ocean waters cooled significantly [Zachos et al., 2001; Villa et al., 2008; Liu et al., 2009; Bohaty et al., 2012], with in- creasing evidence that tropical/subtropical temperatures also decreased [Lear et al., 2008; Wade et al., 2012]. The Late Eocene, before the major climate change at the Eocene-Oligocene transition (EOT) [Pearson et al., 2008], was a period of climatic deterioration. The importance of cooling at this time and whether it was sufficient to trigger formation of small Antarctic ice sheets and associated sea ice [DeConto et al., 2007] is debated actively. The Southern Ocean (SO) is a key area for reconstructing paleoclimatic conditions during this period because Antarctic ice sheet formation influenced global climate by enhancing global albedo and modifying global ocean currents and heat transport. High latitude climate deterioration in the Late Eocene can be assessed by quantifying changes in calcareous nannofossil assemblages that lived in the upper photic zone (the uppermost 100 m of the water column) of the SO. Modern coccolithophorids are sensitive to sea surface temperature (SST), salinity, and nutrient supplies, both at mid-low latitudes [Okada and Honjo, 1973; Winter and Siesser, 1994] and in the SO [Villa et al., 2005]. Nannofossil assemblages can be used to decipher past environmental variables including productivity, temperature, VILLA ET AL. ©2013. American Geophysical Union. All Rights Reserved. 1 PUBLICATION S Paleoceanography RESEARCH ARTICLE 10.1002/2013PA002518 Key Points: • Southern Ocean nannofossil paleo- ecology, of mid Eocene-late Oligocene • Eocene-Oligocene nannofossils and magnetotactic bacterial assemblages • Nannofossils, carbon cycle, productivity, atmospheric CO 2 as climatic feedbacks Supporting Information: • Readme • Figure S1 • Figure S2 Correspondence to: G. Villa, giuliana.villa@unipr.it Citation: Villa, G., C. Fioroni, D. Persico, A. P. Roberts, and F. Florindo (2014), Middle Eocene to Late Oligocene Antarctic gla- ciation/deglaciation and Southern Ocean productivity, Paleoceanography, 29, doi:10.1002/2013PA002518. Received 31 MAY 2013 Accepted 17 DEC 2013 Accepted article online 20 DEC 2013