The Paleocene/Eocene boundary is situated in the lower part of polarity chron C24r, but its precise position and age have been contentious. Dates range from about 54.8 (Berg-gren et al., 1995b) to 55.8 million years ago (Gradstein et al., 2004) in various reports over the past decade or so, most centering around 55.0 million years ago. The debate here, as for the Eocene/Oligocene boundary, stems partly from the difficulty of correlating mammal-bearing continental beds with discontiguous marine strata on which much of Cenozoic geochronology is based. As a result, the Paleocene/Eocene boundary has varied relative to the Thanetian/ Ypresian Stage/Age boundary in Europe and the Clarkforkian/Wasatchian Land-Mammal Age boundary in North America. For example, different authors have considered the Clarkforkian to be entirely Paleocene, or all or partly of early Eocene age, and the Wasatchian to be entirely Eocene or to have begun during the late Paleocene. In Europe, a stratigraphic gap was found between the Thanetian and the Ypresian, further complicating matters and making precise placement of the boundary uncertain.
This dilemma has been largely resolved by the recent decision to place the beginning of the Eocene at the onset of the isochronous, worldwide Carbon Isotope Excursion (CIE), a major perturbation in the global carbon cycle reflected by a negative excursion in 813C (Kennett and Stott, 1991; Dupuis et al., 2003). The ultimate cause of this sudden input of massive amounts of carbon into the atmosphere is controversial (volcanism or comet impact are just two hypotheses; Bralower et al., 1997; Kent et al., 2003), but most authorities agree that it can be traced to the release of methane gas on the ocean floor (Dickens et al., 1995; Katz et al., 1999; Norris and Röhl, 1999; Svensen et al., 2004). The CIE coincided with a brief period of global warming, the Initial Eocene Thermal Maximum (also called the Paleocene-Eocene Thermal Maximum; Sloan and Thomas, 1998; Aubry et al., 2003) and has been recognized in both marine and terrestrial sediments globally. Its onset also coincides with the beginning of the Wasatchian Land-Mammal Age in North America and the beginning of the Ypresian Stage in Europe, which are characterized by substantial faunal turnover, including the abrupt appearance of perissodactyls, artio-dactyls, euprimates, and hyaenodontid creodonts. By this convention, the Clarkforkian is entirely of Paleocene age. In northern Europe, cores now fill the former stratigraphic gap and show that the CIE is situated near the base of the "gap," just above the Thanetian (Steurbaut et al., 2003).
Although there is now agreement on exactly where to place the Paleocene/Eocene boundary, controversy persists over its calibration, because no absolute (radiometric) dates are known for this event. Consequently its age has been interpolated based on radiometric dates tied to the geomagnetic polarity time scale, together with data from astronomical cycle stratigraphy. Thus Aubry et al. (2003) dated the start of the CIE at about 55.5 million years ago (but allowed that it could be closer to 55.0 Ma), whereas many other authors place it at 55.0 million years ago (e.g., Bowen et al., 2002; Gingerich, 2003; Koch et al., 2003). However, Röhl et al. (2003: 586) noted that their earlier estimate of 54.98 million years ago (Norris and Röhl, 1999) was "likely to be too young by several 100 k.y." because of inaccuracies in the calibration points used, which suggests that the estimate by Aubry et al. was closer. The most recent time scale placed the Paleocene/Eocene boundary at 55.8 ± 0.2 million years ago (Gradstein et al., 2004).
Aubry et al. (2003) proposed that the name "Sparnacian" be used as a new earliest Eocene stage/age to encompass the time represented by the hiatus between classical Thanetian and Ypresian. The term "Sparnacian" was already applied to early Ypresian faunas by some paleomammalogists (e.g., Savage and Russell, 1983), but the Sparnacian stratotype, as well as some classic Sparnacian assemblages, may not be of
of the International Commission on Stratigraphy (Gradstein et al., 2004) differs in relatively minor by 0.2-1.0 million years. The largest difference is the Paleocene/Eocene boundary, placed at 55.8 Ma
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