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 δ18O records

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 δ 1 8 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 3 content, and physical properties (e.g., photospectrometry). They are characterized by a sharp bioturbated surface, low CaCO 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 δ 1 8 O increases and sequence boundaries identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and δ 1 8 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 ρCO 2 estimates showing that the first time ρCO 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 ∼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 δ 1 8 O records.