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Wilson et al. (2002) use 40Ar/39Ar data from the Cape Roberts Project CRP-2A core to assign absolute ages to polarity chrons in the vicinity of the Oligocene-Miocene (O-M) boundary, and estimate an age of 24.0 Ma for the O-M boundary. The identification of polarity chron 6Cn.2n, marking the O-M boundary, relies on biostratigraphy and 87Sr/86Sr data.

The last occurrence of Dictyococcites bisectus correlates to C6Cn and is believed to be a reliable datum even at high southerly latitudes. In the nanno-fossil range chart given by Watkins and Villa (2000) for core CRP-2A, the highest stratigraphic occurrence of (rare) Dictyococcites bisectus is at 144.44 m below seafloor (mbsf). This horizon is overlain by barren samples at 140.22 and 136.63 mbsf (top of studied section). As noted by Watkins and Villa (2000), barren samples above the 144.44 mbsf level indicate the possibility that the datum may not represent a “true” last occurrence. The last occurrence of the diatom Lisitzinia ornata in the 256.9–259.2 mbsf interval of core CRP-2A is cited as evidence that this level corresponds to C6C, based on observations at Ocean Drilling Program (ODP) Holes 747A and 748B. At Hole 747A, the occurrence of Lisitzinia ornata at the end of its range is rare and sporadic, and the magnetic stratigraphy at both holes, particularly Hole 748B, is difficult to interpret (see Wise et al., 1992).

The 87Sr/86Sr ages given for core CRP-2A (Lavelle, 2000; Wilson et al., 2002) are based on the 87Sr/86Sr age tables of McArthur et al. (2001). Direct correlation of 87Sr/86Sr data to polarity subchrons close to the O-M boundary is possible at ODP Hole 747A (Oslick et al., 1994), Deep Sea Drilling Program (DSDP) Site 522 (Reilly et al., 2002), and ODP Site 1090 (Channell et al., 2003). The 87Sr/86Sr curve derived from these locations is flat immediately prior to the O-M boundary (Fig. 1). We conclude that it is not possible to resolve ages in the 23.5 to 25 Ma (C7–C6C) interval of the CK95 time scale (Cande and Kent, 1995) using strontium isotopes.

The biostratigraphic and 87Sr/86Sr data do not rule out an interpretation in which the normal polarity zone(s) of sequences 10 and 11 in core CRP-2A is (are) one (both) of the normal polarity subzones of C7n, rather than C6Cn.3n as interpreted by Wilson et al. (2002). If the normal polarity zone of sequence 10 corresponds to C7n.1n and the normal polarity zone of sequence 11 corresponds to C7n.2n, then the 40Ar/39Ar ages from core CRP-2A are consistent with polarity chron ages 0.88 m.y. younger than CK95, consistent with the Shackleton et al. (2000) age for the O-M boundary at 22.92 Ma.