Abstract:
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 delta (super 18) O 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, CaCO (sub 3) content, and physical properties (e.g., photospectrometry). They are characterized by a sharp bioturbated surface, low CaCO (sub 3) 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 delta (super 18) O increases and sequence boundaries identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and delta (super 18) O 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 rho CO (sub 2) estimates showing that the first time rho CO (sub 2) 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 approximately 20 m for the early Eocene (51-49 Ma) and approximately 25 m to approximately 45 m for the middle Eocene (48-42 Ma) using constraints established for Oligocene delta (super 18) O records.