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Glacioeustatic changes in the early and middle Eocene (51–42 Ma): Shallow-water stratigraphy from ODP Leg 189 Site 1171 (South Tasman Rise) and deep-sea ¹⁸O records

Stephen F. Pekar^,1, Audrey Hucks^,2, Michael Fuller^,3 and Shawna Li^#,4

¹ Queens College, School of Earth and Environmental Sciences, 65-30 Kissena Boulevard, Flushing, New York 11367, USA, and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA
² The Pennsylvania State University, Department of Geoscience, 303 Deike Building, University Park, Pennsylvania 16802-2712, USA
³ University of Hawaii at Manoa, Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, 1618 East-West Road, POST Building, Honolulu, Hawaii 96822, USA
⁴ Columbia University, Department of Earth and Environmental Engineering, New York, New York 10027, USA

Sequence boundary ages determined in shallow-water sediments obtained from ODP (Ocean Drilling Program) Leg 189 Site 1171 (South Tasman Rise) compare well with other stratigraphic records (New Jersey, United States, and northwestern Europe) and ¹⁸O increases from deep-sea records, indicating that significant (>10 m) eustatic changes occurred during the early to middle Eocene (51–42 Ma). Sequence boundaries were identified and dated using lithology, bio- and magnetostratigraphy, water-depth changes, CaCO₃ content, and physical properties (e.g., photospectrometry). They are characterized by a sharp bioturbated surface, low CaCO₃ content, and an abrupt increase in glauconite above the surface. Foraminiferal biofacies and planktonic/benthic foraminiferal ratios were used to estimate water-depth changes. Ages of six sequence boundaries (50.9, 49.2, 48.5–47.8, 47.1, 44.5, and 42.6 Ma) from Site 1171 correlate well to the timings of ¹⁸O increases and sequence boundaries identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and ¹⁸O increases indicates a global control and that glacioeustasy was operating in this supposedly ice-free world. This is supported by previous modeling studies and atmospheric CO₂ estimates showing that the first time CO₂ levels decreased below a threshold that would support the development of an Antarctic ice sheet occurred at ca. 51 Ma. Estimates of sea-level amplitudes range from 20 m for the early Eocene (51–49 Ma) and 25 m to 45 m for the middle Eocene (48–42 Ma) using constraints established for Oligocene ¹⁸O records.

Key Words: Eocene • eustasy • Ocean Drilling Program • Leg 189 • stratigraphy • foraminifer • sea level • Australia • Site 1171

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