Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic

Abstract

Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic. The age of 66 Ma for the Cretaceous/Paleogene (K/P) boundary used for calibration in the geomagnetic polarity timescale of Cande and Kent [1992] (hereinafter referred to as CK92) was supported by high precision laser fusion Ar/Ar sanidine single crystal dates from nonmarine strata in Montana. However, these age determinations are now considered to be anomalously old due to problems with sample preparation [Swisher et al., 1992, 1993]. A consensus is developing for an age of 65 Ma for the K/P boundary as recorded in marine [Swisher et al., 1992; Dalryrnple et al., 1993] and nonmarine [Swisher et al., 1993] sediments, and the 65 Ma age has been adopted, for example, as an anchor point in the Mesozoic timescale of Gradstein et al. [1994]. Astrochronologic control for the geomagnetic polarity timescale has been developed by Shackleton et al. [1990] and Hilgen [1991] for the Pleistocene and Pliocene to the base of the Thvera polarity subchron (subchron C3n.4n) and has been confirmed to about 3.3 Ma using high-precision At/At dating [Renne et al., 1993]. The astronochronologic estimates for the Brunhes/Matuyama (0.78 Ma) and Matuyama/Gauss (2.60 Ma) boundaries were already used for calibration in CK92; thus the good agreement of CK92 with the astronomical timescale to the older end of chron C2A (Gauss/Gilbert boundary) is not unexpected. An appreciable discrepancy, however, emerges in the early Pliocene where the astronomical timescale gives ages for the constituent polarity intervals of chron C3n (C3n. ln, C3n.2n, C3n.3n, and C3n.4n, or Cochiti, Nunivak, Sidufjall, and Thvera subchrons, respectively) that are systematically 150 to 180 kyr older than the interpolated ages in CK92. Wilson [1993] showed that the astrochronology gives a more consistent seafloor spreading history when applied to his revised spacings of anomalies on several Pacific spreading ridges. This points to the magnetic anomaly spacings for this interval used for interpolation by Cande and Kent [1992] as the likely source of the discrepancy and suggests that the available astronochronology provides reliable ages for polarity chrons through the Pliocene (see also Renne et al., 1994).Copyfight 1995 by tho Amorican Ooophysical Union.

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

“…The astronomical chronology has subsequently been used to recalibrate the MPTS in the vicinity of the Oligocene/Miocene boundary 36 , and indicates that this boundary is ,900 kyr younger than that reported in ref. 33. Because both age models for site 929 are based on a fundamental 40-kyr frequency in the magnetic susceptibility data, the durations of geomagnetic chrons and the frequency of the oxygen isotope signal is not affected by changes to absolute ages of the MPTS.…”

“…One is based on a nannofossil and planktonic foraminiferal biostratigraphy and, like our CRP-2/2A core data, is calibrated to the MPTS of ref. 33. Durations for intervals between biostratigraphic datums were estimated from inferred ,40-kyr frequencies in magnetic susceptibility records.…”

New dating of the Homo erectus cranium from Lantian (Gongwangling), China

“…Three normal (N1, N2 and N3) and three reversed (R1, R2 and R3) polarity zones were recognized, as at other Chinese loess sections (Heller and Liu, 1982;Liu, 1985;Kukla and An, 1989;Rutter et al, 1991;Zheng et al, 1992;Ding et al, 2002). By comparison with the GPTS (Valet and Meynadier, 1993;Cande and Kent, 1995;Channell et al, 2002), N1 corresponds to the Brunhes chron, and the B/M boundary (0.78 Ma) is situated within horizons S7 to L8. N2 corresponds to the Jaramillo subchron (0.99e1.07 Ma), and lies between horizons S10 and S12.…”

“…Nevertheless, this estimate has subsequently been widely cited as its most probable age. A slightly older age estimate has been proposed in the light of the revised GPTS (Cande and Kent, 1995) in which the ages of the B/M boundary, the Jaramillo subchron and the Olduvai subchron are 0.78, 0.99e1.07 and 1.77e1.95 Ma BP, respectively. Subsequently, Heslop et al (2000) and Ding et al (2002) proposed two new timescales for the Chinese loess sequence, in which the age of L15 was estimated as between 1.190e1.223 Ma and 1.240e1.263 Ma, respectively (see Table 1).…”

Reconstructing plate-motion changes in the presence of finite-rotations noise

“…It is known, however, that these measurements feature substantial noise, which is the unknown finite-rotation fraction unrelated to actual plate motions. Noise arises mainly from the challenge of identifying precisely magnetic lineations of the ocean-floor, often from insufficiently long segments 11 of slowly spreading ridges, and partly from the calibration accuracy of geomagnetic reversal timescales 12,13 . In theory one would need a statistically significant number of repeated measurements of each finite rotation to minimise the associated noise.…”