We (Wilson et al., 2002) reported an integrated chronostratigraphic data set from the CRP-2A core, Ross Sea, Antarctica. We used biostratigraphic and strontium isotope data to identify polarity chrons C6Cn.1r–C6Cn.3n (sequences 9, 10, and 11) and the Oligocene-Miocene boundary in the CRP-2A core. Chron C6Cn.3n (sequences 10 and 11) was, in turn, independently dated by single crystal laser fusion 40Ar/39Ar ages on anorthoclase phenocrysts from two discrete tephra horizons. This new calibration indicated that the geomagnetic polarity time scale in the vicinity of the Oligocene-Miocene boundary was ~0.2 m.y. older than the conventional calibration of the geomagnetic polarity time scale (Cande and Kent, 1995) and 0.9–1.3 m.y. older than the astronomical calibration of Shackleton et al. (2000). We argued that the discrepancy between our new calibration and the astronomical calibration of Shackleton et al. (2000) arose from a mismatch of three 406 k.y. eccentricity cycles or a 1.2 m.y. modulation of obliquity amplitude in the astronomical calibration.
Channell and Martin point out some limitations in the biostratigraphic data set used and contend that it is not possible to rule out correlation of the normal magnetic polarity of sequences 10 and 11 with chrons C7n.1n and/or C7n.2n, a correlation which would be consistent with the 22.92 Ma age of the Oligocene-Miocene boundary proposed by Shackleton et al. (2000).
In Figure 1, we demonstrate that the data presented by us (Wilson et al., 2002) allow a unique correlation of the normal magnetic polarity of sequences 10 and 11 of the CRP-2A core with chron C6Cn.3n of the geomagnetic polarity time scale. We accept that the last occurrence of Dictyococcites bisectus in the CRP-2A core may well not be a true last occurrence. However, it still affords a minimum age for strata at ~148 m below seafloor (mbsf) within sequence 9 and thus was plotted with an arrow (indicating a possibly older age) in Wilson et al. (2002, their Fig. 2). Combined with the strontium data from two well-preserved in situ articulated bivalves at 194.89 mbsf and 246.97 mbsf, the two normal polarity intervals within sequences 9, 10, and 11 are constrained to chrons C6Cn.2n and C6Cn.3n, respectively, and not to older normal polarity subchrons as suggested by Channell and Martin. Unfortunately, Channell and Martin (their Fig. 1) misplotted some of the strontium data from the CRP-2A core. We have included the correct values in Table 1 and point out that only two of the ages are from in situ material and the other two ages are from material of uncertain origin and indicate maximum ages only, and thus were plotted with arrows on the error bars in Wilson et al. (2002, their Fig. 2). The new Sr data from Channell et al. (2002) do not provide detailed coverage of the interval between 24 and 26 Ma. Thus, we have plotted our Sr data (Fig. 1) against the LOWESS fit to the marine Sr curve of McArthur et al. (2001, Fig. 1), which incorporates the data set of Oslick et al. (1994).
We conclude that the coincidence of diatom and nannofossil datums with 87Sr/86Sr and 40Ar/39Ar ages adequately constrains the magnetostratigraphy from the CRP-2A core and offers an opportunity to independently calibrate the Oligocene-Miocene boundary interval of the geomagnetic polarity time scale. Further testing of our calibration will require high-resolution chronologies based on independent age determinations such as the 40Ar/39Ar data from the CRP-2A core (Wilson et al., 2002; McIntosh, 2000).