Age Assessment of Eocene–Pliocene Drill Cores Recovered During the SHALDRIL II Expedition, Antarctic Peninsula Steven M. Bohaty,1,2 Denise K. Kulhanek,3,4 Sherwood W. Wise Jr.,3 Kelly Jemison,3 Sophie Warny,5 and Charlotte Sjunneskog6 Pre-Quaternary strata were recovered from four sites on the continental shelf of the eastern Antarctic Peninsula during the SHALDRIL II cruise, NBP0602A (March–April 2006). Fully marine shelf sediments characterize these short cores and contain a mixture of opaline, carbonate-walled, and organic-walled microfossils, suitable for both biostratigraphic and paleoen- vironmental studies. Here we compile biostratigraphic information and pro- vide age assessments for the Eocene–Pliocene intervals of these cores, based primarily on diatom biostratigraphy with additional constraints from calcar- eous nannofossil and dinoflagellate cyst biostratigraphy and strontium isotope dating. The Eocene and Oligocene diatom floras are illustrated in nine figures. A late Eocene age (~37–34 Ma) is assigned to strata recovered in Hole 3C, and a late Oligocene age (~28.4–23.3 Ma) is determined for strata recovered in Hole 12A. Middle Miocene (~12.8–11.7 Ma) and early Pliocene (~5.1–4.3 Ma) ages are assigned to the sequence recovered in holes 5C and 5D, and an early Pliocene age (~5.1–3.8 Ma) is interpreted for cores recovered in holes 6C and 6D. These ages provide chronostratigraphic ground truthing for the thick sequences of Paleogene and Neogene strata present on the northwestern edge of the James Ross Basin and on the northeastern side of the Joinville Plateau, as interpreted from a network of seismic stratigraphic survey lines in the drilling areas. Although representing a coarse-resolution sampling of the complete sedimentary package, the well-constrained ages for these cores also 1Earth and Planetary Sciences Department, University of California, Santa Cruz, California, USA. 2Now at School of Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton, UK. 3Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, Florida, USA. 4Now at Department of Paleontology, GNS Science, Lower Hutt, New Zealand. 5Department of Geology and Geophysics and Museum of Natural Science, Louisiana State University, Baton Rouge, Louisiana, USA. 6Antarctic Research Facility, Florida State University, Tallahassee, Florida, USA. Tectonic, Climatic, and Cryospheric Evolution of the Antarctic Peninsula Special Publication 063 Copyright 2011 by the American Geophysical Union. 10.1029/2010SP001049 63 64 AGE ASSESSMENT OF EOCENE–PLIOCENE DRILL CORES allow for the broad reconstruction of marine and terrestrial paleoenviron- ments in the Antarctic Peninsula for the late Eocene-to-early Pliocene time interval. 1. INTRODUCTION The Antarctic Peninsula is a highly sensitive region to global climate change. Over the past ~50 years, instrumental temperature records from multiple stations show a higher degree of warming in the Antarctic Peninsula relative to other areas of Antarctica and the global average [Vaughan et al., 2003; Turner et al., 2005]. Given the modern sensitivity of this area to atmospheric temper- ature change, a pressing question is the future stability and sensitivity of ice sheets to climate change in the Antarctica Peninsula region. Critical insight into this question can be obtained from reconstructing the past behavior of ice sheets and climatic history of the region. Ice-core records from the Antarctic Peninsula currently extend only back to ~1200 cal yr BP [Mosley-Thompson and Thompson, 2003]. Extension of the glacial and paleoclimatic history of the region to millennial and million-year timescales must therefore rely on the geological record. Most geological information concerning the paleoclimatic history of the Antarctic Peninsula region is confinedtolatePleistocene–Holocene time intervals. Marine sediment cores, for example, have been effectively used to reconstruct ice-sheet retreat history since the Last Glacial Maximum [e.g., Heroy and Anderson, 2007] and a detailed paleoceanographic history through the Holocene [e.g., Domack et al., 2003]. The pre-Quaternary record is much less known. Knowledge of the Eocene–Pliocene history has been pieced together primarily from drill cores on the continental rise on the western side of the Antarctic Peninsula (Ocean Drilling Program (ODP) Leg 178) [Barker and Camerlenghi, 2002] and outcrop records from Brabant, James Ross, Seymour, Cockburn, Alexander, and King George islands [e.g., Pirrie et al., 1997; Troedson and Smellie, 2002; Birkenmajer et al., 2005; Hambrey et al., 2008; Smellie et al., 2009]. Obtaining additional records and further exploitation of existing records, however, is necessary to answer key paleoclimatic questions for the region, such as ice sheet presence/stability in warm intervals of the Pliocene [Smellie et al., 2009] and the timing, extent, and character of initial ice-sheet development in the region during the Paleogene [Anderson et al., 2006]. Drilling in high-latitude regions such as the Antarctic Peninsula presents a number of chal- lenges. Most areas along the continental shelves are subject to sea ice cover, drifting icebergs, and rapidly changing weather conditions. In addition, both the coarse-grained nature of proximal glacimarine sediments and the erosive nature of glaciers further hinder the recovery and paleocli- matic interpretation of Antarctic drill cores. In ice-proximal areas, these challenges have been overcome in a variety of ways, including strategic use of large drilling ships by international ocean drilling expeditions (e.g., Deep Sea Drilling Project (DSDP) Leg 28, ODP Legs 178 and 188, and Integrated Ocean Drilling Program Expedition 318) and sea ice/ice shelf-based drilling projects (e.g., Cape Roberts Project and ANDRILL). Nevertheless, drilling in shallow continental shelf areas of the Antarctic Peninsula is not feasible using either a large, non-ice-strengthened drill ship or an ice-based platform. In order to overcome these obstacles, the Shallow Drilling project (SHALDRIL) was designed around the idea of mounting a medium-sized drill rig on an ice- breaking research vessel in order to allow quick drilling of shallow targets in ice-infested areas. Testing of the SHALDRIL strategy was undertaken during two cruises in the austral fall seasons of 2005 and 2006. The primary drilling target of these cruises was a section of thick, seaward dipping strata on the eastern (Weddell Sea) side of the Antarctic Peninsula: an untapped rock sequence ideal for reconstructing the Eocene-to-Pleistocene paleoclimatic and glacial history of the region [Anderson, 1999]. BOHATY ET AL. 65 The two SHALDRIL cruises utilized the research vessel icebreaker (RVIB) Nathaniel B. Palmer, temporarily mounting a drill rig over a moon pool in the aft section of the ship. During the first season of drilling (SHALDRIL Cruise NBP05-02, March–April 2005), a long Holocene succession was cored at a site along the South Shetland Islands [Milliken et al., 2009], as well as a number of technical holes aimed at testing the drill rig (Shipboard Scientific Party, SHALDRIL 2005 cruise report, 2005, http://www.arf.fsu.edu/projects/documents/shaldril2005.pdf ). After modification of the drilling tools and techniques, 12 sites were drilled during the second cruise (SHALDRIL II Cruise NBP0602A, March–April 2006) in the northwestern Weddell Sea, off the eastern tip of the Antarctic Peninsula (Shipboard Scientific Party, SHALDRIL II 2006 NBP0602A cruise report, 2006, http://www.arf.fsu.edu/projects/documents/Shaldril_2_Report. pdf, hereinafter Shipboard Scientific Party, 2006). The major successes of this cruise were the recovery of a long Holocene record from the Firth of Tay near Joinville Island [Michalchuk et al., 2009] and penetration into thick, pre-Quaternary strata on the continental shelf at four of the sites: NBP0602A-3, NBP0602A-5, NBP0602A-6, and NBP0602A-12 (Figure 1 and Table 1). Sedi- ments of Eocene age were recovered from a stratigraphic succession within the James Ross Basin at Site 3, and Oligocene–Pliocene age sediments were cored on the northeastern edge of the Joinville Plateau at sites 5, 6, and 12. In this chapter, we compile and revise the biostratigraphic age constraints for the pre-Quater- nary cores recovered on the SHALDRIL II cruise at sites NBP0602A-3, NBP0602A-5, NBP0602A-6, and NBP0602A-12, updating the initial shipboard biostratigraphic report (Ship- board Scientific Party, 2006). We also report on two strontium isotope ages that were generated after the cruise, which provide supporting age information. The presence of reworked sedimen- tary clasts of Paleocene and Cretaceous age within upper Quaternary sediments recovered at Site NBP0602A-9 has been reported separately by Kulhanek [2007]. Although the short cores recovered on the SHALDRIL cruise do not individually span long intervals of time, the age estimates provide valuable constraints for interpreting many different aspects of the geologic and paleoclimate history of the Antarctic Peninsula. Important outcomes of the integrated results include derivation of the erosional and glacial history of the eastern Antarctic Peninsula from the seismic architecture of the James Ross Basin and Joinville Plateau [Anderson, 1999; Smith and Figure 1. Location map of sites drilled during the SHALDRIL II expedition that recovered pre-Quaternary strata. Drill core sections of Eocene-to-Pliocene age were recovered on the Weddell Sea side of the