In 2000, ODP Leg 189 recovered cores at five sites on the slopes of Tasmania and on the East Tasman Rise and South Tasman Rise, in 2,463 to 3,568 m water depths. Over 4,500 m of core were extracted with excellent overall recovery of 89%. From the sediments and microfossils recovered, high-resolution records were developed that greatly improved our understanding of Southern Ocean evolution and its relation with Antarctic climatic development. The relatively shallow region off Tasmania is one of the few places where well-preserved and almost-complete marine Cenozoic carbonate-rich sequences can be drilled in present-day latitudes of 40°-50°S and paleolatitudes of up to 70°S. In fact, sedimentological and paleontological evidence indicate that during the middle Eocene, the site of Site 1171 existed in a mainly neritic environment (<200 meters of water depth), with good to excellent preservation of the calcareous microfossils.

This figure is a reconstruction of the area between Australia and Antarctica circa 40 Ma (PLATES/UTIG, 2003). Shown are ODP Leg 189 Sites 1168-1172 and other DSDP and ODP sites in this area. Note the close proximity of the south Tasman Rise and site 1171 to Antarctica during the Eocene. In fact, the foraminiferal study of the Eocene strata recovered from Site 1171, represents the most southerly study of shallow marine foraminifers obtained from cores.

I was selected as one of the on board sedimentologists for ODP Leg 189. One of the research projects I worked on was developing high-resolution lithologic and foraminiferal biofacies records for the early to middle Eocene (51-42 Ma) section of Site 1171. During initial lithologic description on board ship, I identified lithologic cycles that were subsequently placed into a sequence stratigraphic framework. Post cruise studies developed high-resolution sedimentological and foraminiferal biofacies records that permitted identification of sequences their bounding unconformable surfaces and systems tracts within (LINK Pekar et al 2005.pdf>. Sequence boundaries were identified and dated using lithology, bio- and magnetostratigraphy, water-depth changes, CaCO3 content and physical properties (e.g., photospectrometry). They were often 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 place constraints on water-depth changes.

Summary figure modified from Pekar et al. (2005) for Site 1171 between 700 and 400 mbsf. Systems tracts terminology based on Posamentier et al. (1988). Increased redness in the sediment (more positive a*) generally indicates lower nannofossil content, while more greenness (more negative a*) indicates higher nannofossil percent. Increased green can also indicate higher glauconite content. Higher CaCO3 content is used as a proxy for higher nannofossil abundance. Calcium carbonate percents, photospectrometry, and nannofossil and glauconite percent (from smear slides) are from Exon, Kennett, Malone et al. (2001).

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 oxygen isotope increases (inferred ice volume increase) and sequence boundaries (inferred sea level lowering) identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and oxygen increases indicates a global control and that glacioeustasy was operating in this supposedly ice free world. 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 oxygen isotope records.

Figure modified from Pekar et al. (2005). Comparison of sequence development at ODP Leg 189 Site 1171, deep-sea oxygen isotope records and events, and sequence boundaries identified in New Jersey (Browning et al., 1996) and Europe (de Graciansky et al., 1998). Yp3-5 and Lu1-4 are Eocene sequences identified in Site 1171 and E4-E9 are Eocene sequences identified in New Jersey. Shaded boxes indicate time represented by sediments and curved lines represent hiatuses. Horizontal wavy lines show ages of sequence boundaries of de Graciansky et al. (1998). Composite d18O record from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites from Zachos et al. (2001). Oxygen isotope events (dashed gray lines) represent inferred ice growth and were defined on the basis of substantial global increases (>0.5 per mil) in benthic oxygen isotope values and coeval shifts in western equatorial planktonic oxygen isotope records (Bralower et al., 1995). Each event corresponds with a sequence boundary (inferred base-level lowering) at Site 1171. Additional peaks evident in these records were not recognized globally in benthic foraminiferal oxygen isotope data (i.e., at more than one site) or have not been identified in western tropical planktonic oxygen isotope data. These peaks may represent local variability (i.e., bottom-water temperature changes) or perhaps higher frequency events not recognized previously. In all of the isotope records, the potential exists for aliasing of obliquity and precessional Milankovitch cycles.