Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.107..349h&link_type=abstract
Earth and Planetary Science Letters, Volume 107, Issue 2, p. 349-368.
Astronomy and Astrophysics
Astronomy
119
Scientific paper
The early Pleistocene to late Pliocene astronomically calibrated time scale of Shackleton et al. [1] and Hilgen [2] is extended to the Miocene/Pliocene boundary. This is done by correlating the detailed record of CaCO3 cycles in the Trubi and the lower part of the overlying Narbone Formation (Rossello composite section; Sicily) to the astronomical record, using (1) inferred phase relations between these CaCO3 cycles and the corresponding orbital cycles of precession and eccentricity, and (2) calibration points provided by the previously established astronomical calibration of sapropelitic layers which occur in the topmost part of the CaCO3 record [2].
This correlation allows all small-scale CaCO3 cycles in the Rossello composite section to be coded after the correlative precession peak and to be dated astronomically with an accuracy of 1 ky. In combination with the available high-resolution magnetostratigraphy, it further allows the construction of an astronomically calibrated Geomagnetic Polarity Time Scale for the major part of the Gilbert and Gauss Chrons. This new time scale gives ages for the reversal boundaries of the Kaena (3.04-3.11 Ma) and Mammoth (3.22-3.33 Ma), the Gilbert/Gauss Chron boundary (3.58 Ma), and the reversal boundaries of the Cochiti (4.18-4.29 Ma), Nunivak (4.48-4.62 Ma), Sidufjall (4.80-4.89 Ma) and Thvera (4.98-5.23 Ma). These ages are significantly older than the ages given by conventional time scales. Age discrepancies gradually increase with increasing age: the age of 5.32 Ma for the Miocene/Pliocene boundary, as defined by the base of the Trubi on Sicily, is almost 0.5 m.y. (9.5%) older than the presently accepted age of 4.86 Ma [3].
These considerable discrepancies with the conventional time scale can either be explained by an (consistent) error in the decay constants used in K/Ar radiometric dating or, more likely, by diffusional loss of radiogenic argon.
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