passive continental margins. Haq et al. (1988) provided a recent summary of the apparent eustatic curve compiled from many well-dated seismic stratigraphic studies. The Cenozoic is represented by parts A and B of the Tejas megasequence and includes seven second-order supercycles. Each of these supercycles spans about 10 Ma and terminates at a sequence boundary of major magnitude. Shackleton et al. (1988), Williams (1988), Christie-Blick et al. (1990), and Miller et al. (1991) have carefully considered the relationships between isotopic and seismic studies as they bear on the history of Cenozoic climate. Potential pitfalls for the seismic method are confounding the effects of local sedimentary load or local tectonism with more global eustatic effects. For these reasons, we focus on the isotopic record as the more reliable signal of global climatic change during the Cenozoic. It should be noted, however, that for many intervals, there is substantial correlation between the seismic and isotopic approaches.
Shackleton and Opdyke (1977) showed that during the late Pleistocene, oxygen isotope ratios covaried between tropical planktic and benthonic foraminifera. They interpreted the simultaneous changes in surface and bottom-dwelling foraminifera as evidence of growth and decay of ice sheets on the continents. Therefore, the d18O record represents change in both ocean temperature and ice volume. The problem has been to partition these two effects. Miller et al. (1991) recently extended the covarying isotopic studies to sediments of mid-Tertiary age, recognizing key intervals of ice buildup by covariant increases in heavy oxygen ratios in Miocene and even Oligocene cores.
Prentice and Matthews (1988) have attempted to monitor Cenozoic sea-level change by analyzing oxygen isotope ratios in planktic foraminifera from equatorial regions. The efficacy of this approach depends on the assumption that equatorial sea-surface temperatures (away from upwellings) have not changed during the Cenozoic. Thus, they reason that observed isotopic changes wholly reflect waxing and waning of glacial ice. Figure 11.2 juxtaposes North American land mammal immigration episodes with the trace of Cenozoic oxygen isotope ratios based on planktic forams from equatorial regions. We discuss each of the continental first-order immigration episodes in relation to the marine record.
The earliest pair of first-order immigration episodes are the Clarkforkian and early Wasatchian, straddling the Paleocene-Eocene boundary. Although they fall generally within the warmest climatic interval of the Cenozoic, they correlate with small cooling events in the isotope curve at about 59 and 56 Ma. The Clarkforkian immigration episode correlates particularly well with the abrupt cooling episode at the end of the Paleocene (59 Ma), demonstrated by Kennett and Stott (Chapter 5, this volume) in high-resolution data based on planktonic forams from the southern ocean. Miller et al. (1987) postulated a sea-level drop at this time, and Haq et al. (1988) recognized a second-order drop in sea-level (TA 222-24) in the Thanetian.
The very large Wasatchian land mammal immigration episode reflects plate tectonic effects in the North Atlantic that established a very broad, low latitude corridor across the Thulean route and thus produced extremely close faunal resemblance between North America and Europe as discussed above.
The next first-order episode occurs in the early Duchesnean (Late Eocene) at about 40 Ma (Emry, 1981; Krishtalka et al., 1987). This episode corresponds well to a number of global Late Eocene events. Miller et al. (1987) show a major oxygen isotope increase at about 40 Ma based on benthic forams, and this event is also seen in planktonic forams from equatorial cores presented by Prentice and Matthews (1988). These isotopic events correlate with the beginning of the major sea-level drop TA4 (Priabonian) of Haq et al. (1988). According to Hallam (1984) and others cited therein, this latest Eocene sea-level drop is greater than that of the Late Oligocene. The profound global climatic shift of the Late Eocene correlates with a strong increase in the Earth's thermal gradient due to cooling in the southern ocean (Kennett and Barker, 1990).
During the Neogene, land mammal immigration episodes increased markedly in North America. Because of their importance and frequency, these Miocene and Pliocene immigrations were subjected to detailed analysis by Tedford et al. (1987). In Figure 11.3, we juxtapose the full array of Neogene land mammal immigration episodes (including third-order episodes) with the d18O excursions numbered by Miller et al. (1991). As suggested by Opdyke (1990) the episodes correlate remarkably closely with oxygen isotope events in the marine record. Only one isotope event (namely Miocene 3) fails to correlate with an immigration episode in North America.
On the other hand, several immigration episodes in the Miocene of North America fail to correlate with any of the positive isotope excursion numbered by Miller et al. (1991). One first-order immigration episode (Arikareean 2 at 20 Ma) fails this correspondence test; as noted below it does correlate with a small (unnumbered) positive isotope excursion. Blancan 1 (at 4.8 Ma) also appears to be unrequited, but in fact it may correspond with Pliocene 1 of Miller et al. (1991). Other lesser immigration episodes at 14.5, 7.0, and 6.0 Ma do not correspond to numbered isotope excursions.
The Early Miocene records the largest set of generic immigrations in the history of the North American land mammal fauna. Three land mammal dispersal episodes fall near the boundary between Arikareean and Hemingfordian: they begin with a second-order episode at 21 Ma