The Paleogene Pacific Ocean was connected to the other oceans differently than the modern Pacific (Fig. F1). Before 25 Ma the major passages to the Indian and Atlantic Oceans were found in the tropics rather than through the high southern latitudes. The Panama Gateway between the tropical Atlantic and Pacific did not disappear completely until ~3 Ma (Coates and Obando, 1996), whereas the tropical Indonesian Passage between the Indian and Pacific Oceans became significantly restricted at ~11.5 Ma (Kennett et al., 1985). The Southern Ocean passages began opening in the middle Eocene and opened deeply at the end of the Eocene and during the Oligocene. It is now widely accepted that the Tasman Passage became a significant connection between the Indian and Pacific Oceans at ~35.5 Ma (Stickley et al., 2004). In contrast, the timing of the opening of Drake Passage is a subject of ongoing debate. Drake Passage may have formed deepwater connections between the Atlantic and Pacific as early as 34–31 Ma (Latimer and Filippelli, 2002; Lawver and Gahagan, 1998, 2003; Livermore et al., 2005) or as late as 24–17 Ma (Barker, 2001; Pfuhl and McCave, 2005). Recently, Scher and Martin (2006) suggest that significant connections through the Drake Passage were established as early as 41 Ma.
Modern mountain belts around the Pacific, which steer winds and define important loci of air-sea interactions, were markedly different in height and position at the beginning of the Cenozoic (Harrison et al., 1998; Chase et al., 1998). Nevertheless, Earth's energy balance at the top of the atmosphere appears to have been roughly the same at the beginning of the Cenozoic as it is today, and periodic changes in insolation were driven by changes in Earth's orbit and orientation with respect to the sun (Laskar et al., 2004; Pälike et al., 2004). Tectonic reorganizations by themselves were not the dominant cause of the different climates of the Cenozoic. Instead, the tectonic reorganizations affected climate by controlling the processes responsible for transporting or trapping solar heat and maintaining greenhouse gas levels in the atmosphere. The different climates of the Cenozoic provide natural experiments to understand interactions among the crust, oceans, atmosphere, cryosphere, and biosphere.
One result from modeling studies is that the basic structure of ocean circulation (i.e., the distribution of gyres and upwelling systems) seems to be recognizably similar to modern patterns (Huber and Caballero, 2003; Huber et al., 2003, 2004). The major exception is the disappearance of the Antarctic Pacific and Indian Ocean subpolar gyres with the Paleogene opening of the Southern Ocean gateways (Tasman Gateway and Drake Passage) (Huber et al., 2004). Another notable difference between the Paleogene and modern Pacific is a stronger interaction of the South Pacific subtropical gyre with that of the Indian Ocean. Although the general features of the Pacific Ocean are relatively constant, the position of fronts and other boundaries of circulation regimes may have moved by >5° in latitude over the Cenozoic (Huber and Sloan, 2001). In addition, the average temperature of the individual gyres is thought to have changed along with the cooling of the planet (Huber et al., 2004; Huber and Nof, 2006). Such changes in sea-surface temperature (SST) and position of these ocean heat reservoirs are important to the atmospheric redistribution of heat and precipitation (Huber and Sloan, 1999).