Palaeoceanography

During the early Palaeogene, Antarctica was less glacierized than it is now, or not glacierized at all (e.g. Zachos et al., 2001; Pagani et al., 2005; see above). South America and Australia were still not fully separated from the Antarctic continent (e.g. Livermore et al., 2005; Brown et al., 2006), which

• Endemics dominant (> 50 %) © Endemics present (5-50%) O No or rare endemics (< 5 %)

Figure 8.11: Generalized (Middle Eocene) dinocyst geographical distribution map overlain with the ocean circulation pattern inferred from GCM results. Maps derived from the Ocean Drilling Stratigraphic Network (ODSN). Shaded areas indicate mostly submerged (continental) blocks (e.g., Brown et al., 2006). Abbreviations: TA-SW, Trans-Antarctic Seaway (hypothetical; see Wrenn and Beckmann, 1982); TSA-SW, Trans-South American Seaway (hypothetical; see Kohn et al., 2004); EAC, East Australian Current; p-LC, proto-Leeuwin Current; p-RG, proto-Ross Gyre;

TC, Tasman Current.

prevented the development of a (proto-) Antarctic Circumpolar Current (ACC). It was hypothesized that during the early Palaeogene, warm ocean currents from lower latitudes could reach and warm Antarctica. The opening and subsequent deepening of critical conduits (i.e. Drake Passage and the Tasmanian Gateway) towards the end of the Eocene have long been thought to have played a central role in ACC establishment and Antarctic cooling (e.g. Kennett et al., 1975; Kennett, 1977, 1978; Murphy and Kennett, 1975). However, recent advances through ocean drilling (e.g. ODP Leg 189) and coupled Global Circulation Model (GCM) experiments suggest that Eocene Southern Ocean surface circulation patterns were fundamentally different than previously thought, and that the opening and deepening of oceanic gateways were of little climatic consequence (Sloan and Huber, 2001; Huber et al., 2004; Warnaar, 2006). Instead, it is nowadays argued that the ''greenhouse-icehouse'' transition was caused by changes in greenhouse gas concentrations, rather than oceanographic changes (e.g. DeConto and Pollard, 2003a,b; Huber et al., 2004; Pagani et al., 2005).

The high degree of endemism in the circum-Antarctic marine microfossil associations denies the existence of a southward-bound, warm, proto-East Australian Current as proposed by Kennett et al. (1975) and Exon et al. (2004), according to Huber et al. (2004) (Fig. 8.11). For example, a colder northward flowing western boundary current, designated the ''Tasman Current'' (see Huber et al., 2004) existed off southeast Australia. GCM experiments indicate that the Eocene Southern Ocean, including the southern Pacific, was dominated by clockwise gyres (Sloan and Huber, 2001; Huber et al., 2004). Moreover, several studies show that the Tasmanian Gateway had already been open to neritic water depths (i.e. <200m) since at least the Middle Eocene (Stickley et al., 2004). Deepening to bathyal water depths (i.e. 200-4,000 m) occurred during the early Late Eocene (~35.5 Ma). The Drake Passage had possibly been open to (upper) bathyal water depths by the Middle Eocene (Eagles et al., 2006; Scher and Martin, 2006). Both tectonic events thus seem to have occurred too early to be related to the Antarctic glaciation in the Early Oligocene (i.e. the Oi-1 stable-isotope event (33.3 Ma), e.g. Miller et al., 1998; Zachos et al., 2001).

Given the similar continent-ocean configuration, a corollary of the GCM experiments is that the Palaeogene circum-Antarctic surface circulation should not have been fundamentally different from the Cretaceous situation. If this were the case, and as long as substantial equator-pole temperature gradients existed, then it may be expected that, throughout the Late Cretaceous to early Palaeogene time interval, circum-Antarctic waters were consistently dominated by endemic biota, particularly in environments influenced by the proposed western boundary currents. Both hypotheses were recently tested by Warnaar (2006) by mapping distribution patterns of circum-Antarctic dinocysts through Palaeogene times and comparing them with coupled GCM results.

Warnaar (2006) conceived a model termed the ''refrigerator trap" wherein it is hypothesized that cosmopolitan and endemic dinoflagellates were taken through the cold and darkness along the Antarctic continent, transported by the proto-Ross Gyre. Conceivably, taxa normally living in warmer waters (e.g. the East Australian Current) that were trapped in the gyre were unable to survive such conditions. It is conceivable that the endemic taxa (notably taxa of the ''Transantarctic Flora'' and bi-polar Phthanoperidinium echinatum group) were specifically adapted to tolerate cold conditions (4°C to the freezing point), prolonged darkness and possibly seasonal sea ice.

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