Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.v32c1041k&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #V32C-1041
Mathematics
Logic
4860 Radioactivity And Radioisotopes, 8404 Ash Deposits, 9604 Cenozoic
Scientific paper
The standard geological time scale of Berggren et al. (1995) and Cande and Kent (1995) is calibrated with different absolute dating techniques, i.e. the Plio - Pleistocene relies on astronomical tuning, and older parts of the time scale are based on radio-isotopic (40Ar/39Ar and U/Pb) calibration methods. In the new edition of the standard geological timescale (Lourens et al., to be published in 2004) the entire Neogene will rely on astronomical dating. Therefore, it is of crucial importance that all dating methods produce equivalent absolute ages when the same geological event is dated. The Mediterranean Neogene provides an excellent opportunity to compare different dating methods by isotopic dating (40Ar/39Ar, U/Pb) of volcanic ash layers intercalated in astronomically dated sediments. Here we will show that in spite of potential errors in all methods, we succeeded to intercalibrate the 40Ar/39Ar and astronomical methods, arriving at astronomically calibrated age of 28.24 +/- 0.01 Ma for the in 40Ar/39Ar geochronology commonly used standard FCT sanidine. The advantage of an astronomically calibrated FCT above a K/Ar calibrated standard is a smaller error in the absolute age due to the lack of uncertainties related to 40K and radiogenic 40Ar contents in the primary standard and a decreasing influence of errors in the decay constant (branching ratio is not required). In addition to an astronomically calibrated FCT age we propose to introduce an astronomically dated standard. A direct astronomically dated standard can be regarded as a "primary" standard and does not require intercalibration with other standards, thus reducing analytical (and geological) uncertainties. Ash layers intercalated in sedimentary sequences in the Melilla Basin, Morocco appear to be the most suitable for this purpose. A reliable astronomical time control is available and intercalated ash layers contain sanidine phenocrysts up to 2 mm. Four ash layers are not or barely affected by xenocrystic contamination. Further, two of these ash layers (Mes-9 and Mes-4) can easily be recognized in the Messadit section due to their thickness of respectively 0.6 m and 5.0 m. The other two can also easily be recognized, but require inspection of the cyclostratigraphic framework. To avoid any confusion about the location of the ash layers we propose either Mes-9 or Mes-4 as potential 40Ar/39Ar dating standards. Before this standard can be introduced additional work (e.g. microprobe analyses to confirm homogeneity of the populations) will be done in the near future to decide on the "best" standard. Berggren, W.A., Kent, D.V., Swisher, C.C., III and Aubry, M.P. (1995) A revised Cenozoic geochronology and chronostratigraphy. SEPM Special Publication 54: 129-212.
Cande, S.C. and Kent, D.V. (1995) Revised calibration of the geomagnetic timescale for the Late Cretaceous and Cenozoic. Journal of geophysical research, 100(B4): 6093-6095.
Lourens, L.J., Hilgen, F.J., Laskar, J., Shackleton, N.J. and Wilson, D. (2004) The Neogene Period. In: A Geological Timescale 2004, edited by Felix Gradstein, Jim Ogg and Alan Smith (in press).
Hilgen Frits
Krijgsman Wout
Kuiper Klaudia
Wijbrans Jan
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