 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; November 2002; v. 30; no. 11; p. 1043–1046; 2 figures; 1 table. 1043 Integrated chronostratigraphic calibration of the Oligocene-Miocene boundary at 24.0  0.1 Ma from the CRP-2A drill core, Ross Sea, Antarctica Gary S. Wilson Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand Mark Lavelle British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK, and Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK William C. McIntosh New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA Andrew P. Roberts School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK David M. Harwood David K. Watkins    Department of Geosciences, University of Nebraska, Lincoln, Nebraska 68588-0340, USA Giuliana Villa Dipartimento di Scienze della Terra, Universita ` di Parma, Parco Area delle Scienze 157A, 43100 Parma, Italy Steven M. Bohaty Earth Sciences Department, University of California, 1156 High Street, Santa Cruz, California 95064, USA Chris R. Fielding Department of Earth Sciences, University of Queensland, Brisbane, QLD 4072, Australia Fabio Florindo Leonardo Sagnotti    Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, I-00143 Rome, Italy Timothy R. Naish Institute of Geological and Nuclear Sciences Ltd, P.O. Box 30-368, Lower Hutt, New Zealand Reed P. Scherer Department of Geology and Environmental Geosciences, Northern Illinois University, De Kalb, Illinois 60115, USA Kenneth L. Verosub Department of Geology, University of California, Davis, California 95616, USA ABSTRACT An expanded Oligocene-Miocene boundary interval recovered in the Cape Roberts Project CRP-2A core from beneath the Ross Sea, Antarctica, has yielded a high-resolution integrated chrono- stratigraphy that has, in turn, enabled a new, more direct, calibra- tion of magnetic polarity and biostratigraphic events. The Oligocene- Miocene boundary interval in the CRP-2A core comprises three 60-m-thick, rapidly deposited (0.5 m/k.y.) sedimentary se- quences (sequences 9, 10, and 11). In sequences 10 and 11, single- crystal, laser-fusion 40 Ar/ 39 Ar analyses of anorthoclase phenocrysts from two tephra horizons independently calibrate the CRP-2A magnetic-polarity stratigraphy and age model. Sequences 10 and 11 encompass subchron C6Cn.3n, which is dated as 24.3  0.1 to 24.16  0.1 Ma. Sequence 9 is interpreted to encompass subchron C6Cn.2n and the Oligocene-Miocene boundary, which is dated as 24.0  0.1 Ma. These ages are 0.2 m.y. older than those of the geomagnetic polarity time scale calibrated from seafloor-spreading ridges and 0.9–1.3 m.y. older than the newly proposed astronom- ically calibrated ages. We contend that the discrepancy with the astronomically calibrated ages arises from a mismatch of three 406 k.y. eccentricity cycles or a 1.2 m.y. modulation of obliquity am- plitude in the astronomical calibration of the Oligocene–Miocene time scale. Keywords: chronostratigraphy, isotope dating, Antarctica, biostratig- raphy, Cape Roberts Project. INTRODUCTION Dating and correlation of the Oligocene-Miocene boundary in stratigraphic sequences has proved historically difficult and has been compounded by changes in the criteria for identification of the Oli- gocene-Miocene boundary. Berggren et al. (1985) defined it at the last occurrence (LO) of the nannofossil Dictyococcites bisectus in Deep Sea Drilling Project (DSDP) Site 522, where it occurs in the middle of chron C6Cn. When Cande and Kent (1992) revised the geomagnetic polarity time scale (GPTS), they assigned the Oligocene-Miocene boundary to the base of subchron C6Cn.2n, which slightly preceded the LO of D. bisectus. Shackleton et al. (2000) proposed an astronomical calibration of the Oligocene-Miocene boundary in cores from Ocean Drilling Pro- gram (ODP) Site 929 and DSDP Site 522, where it is 0.9 m.y. younger than the boundary of Cande and Kent (1995). Despite the lack of a reliable magnetic polarity stratigraphy for Site 929, Shackleton et al. (1999) were able to match the composite magnetic susceptibility record with the orbital data of Laskar et al. (1993) by relying on the 406 k.y. and 100 k.y. eccentricity cycles. The result suggested that Oligocene biostratigraphic datums in Site 929 cores were much younger than their previously accepted ages (e.g., Berggren et al., 1995). Shackleton et al. (2000) used key biostratigraphic and carbon isotopic data in the vicinity of the Oligocene-Miocene boundary to correlate Sites 929 and 522 and to retune the Site 522 magnetostratigraphy (Tauxe et al., 1983) to the Site 929 astronomical time scale. The resulting age model sug- gested that the paleomagnetic subchrons within chrons C6 and C7 at Site 522 were also 0.9 m.y. younger than the conventionally calibrated GPTS (Cande and Kent, 1995). CRP-2A CORE In the Austral spring of 1998, an Oligocene-Miocene succession was recovered in the Cape Roberts Project CRP-2A core from offshore Cape Roberts, Antarctica (Fig. 1; Cape Roberts Science Team, 1999). Fielding et al. (2000) subdivided the CRP-2A succession into strati- graphic sequences on the basis of recurrent sedimentary lithofacies that are interpreted to represent cycles of glacial advance and retreat with associated changes in relative sea level. Between 130.27 and 306.65 mbsf (below seafloor) in the core, sequences 9, 10, and 11 represent an expanded Oligocene to Miocene succession with sediment accu- mulation rates of 1000 m/m.y. The chronology of the succession is well defined by diatom (Scherer et al., 2000) and calcareous nanno- fossil (Watkins and Villa, 2000) biostratigraphy and by 87 Sr/ 86 Sr (Lav- elle, 2000) and 40 Ar/ 39 Ar (McIntosh, 2000) ages on mollusk fragments and ash horizons and volcanic clasts, respectively (Fig. 2). A magnetic polarity stratigraphy correlation (Wilson et al., 2000a; Fig. 2) and de- velopment of an age model demonstrate sedimentary sequence dura- tions comparable to orbital frequencies of obliquity (40 k.y.) and ec- centricity (100 k.y.; Naish et al., 2001). CHRONOSTRATIGRAPHIC DATA Diatom and Calcareous Nannofossil Biostratigraphy Scherer et al. (2000) defined three diatom zones within sequences 9, 10, and 11 of the CRP-2A core as part of a new Antarctic conti-