Astronomy and Astrophysics – Astronomy
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..os32a03s&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #OS32A-03
Astronomy and Astrophysics
Astronomy
1520 Magnetostratigraphy, 3030 Micropaleontology
Scientific paper
The Middle Miocene comprises the Langhian and Serravallian Stages of European stratigraphy. Boundary stratotypes for the base of the Langian, the base of the Serravallian and the top of the Serravallian (base of the Tortonian) have not yet been formally defined but in round figures the age interval is from 15.5 Ma to 10.5 Ma with the intervening boundary at about 13.5 Ma. In the deep sea this time interval is particularly difficult to deal with because the carbonate compensation depth shallowed more than once, with the effect of generating condensed sections devoid of critical microfossils. In addition true hiatuses exist in many deep ocean sections that may have been induced either/both by instability caused by carbonate dissolution or by changes in deep currents. ODP Leg 154 had the good fortune to core two sites (925 and 926) that both contain a complete record of the Middle Miocene. Site 929 also preserves a more-or-less complete record but the record is intensely dissolved and includes several turbidites. Astronomical tuning of the earlier Middle Miocene time scale proved quite difficult on the basis only of shipboard data but with scanning XRF data for several critical cores we have completed a robust time scale. The base of the Tortonian is generally characterised by the first occurrence of Neogloboquadrina acostoensis, which has an open-ocean age around 10 Ma. Additional work is needed to characterise this boundary, but the astronomical time scale for a number of open-ocean nannofossil biostratigraphic events is secure. During the Serravallian intense 40-ka dissolution cycles cover the interval 12.4 to 11.4 Ma after which the CCD gradually deepens and precession cycles start to dominate the record again. If the base of the Serravallian is recognised by the LO of Sphenolithus heteromorphus, this coincides with the first of a group of three strong 100-ka eccentricity cycles within which there is a vigorous precession response. Both the biostratigraphic event and the astronomical age (13.54 Ma) are secure. The base of the Langian is transgressive and post-dates the first appearance of P. sicana in European sections. It is often approximated in open ocean records by the LO of Helicosphaera ampliaperta (15.8 Ma published age) or by the FO of Discoaster signus (which often coincides with the end of an abundance acme of D. deflandrei; 16.2 Ma published age). The boundary is generally supposed to be close to C5Cn, although the normally magnetised material in the Moria Section must represent C5Dn if the radiometric dates have any credibility. In Leg 154 material the LO of Helicosphaera ampliaperta has an age close to 15.0 Ma; the FO of Discoaster signus is at about 15.6 Ma, and the FO of P. sicana older than 17 Ma making this a very insecurely calibrated boundary. We have correlated the record of DSDP holes 521 and 521A, which have an excellent magnetostratigraphy for C5Cn to C5Bn, to the ODP Leg 154 material, enabling us to estimate the age of the young end of C5Cn as about 15.9 Ma (cf 16.014 in CK95) and the old end of C5Bn as about 15.1 Ma (cf 15.155 in CK95). This interval is characterised by a strong response to eccentricity cycles (both in ODP Leg 154 sites and in DSDP521). After about 14.6 Ma there are very strongly expressed 40-ka dissolution cycles associated with a very shallow CCD until about 14.0 Ma. Published stable isotope data suggest that there was a significant cooling and/or antarctic ice build up at about 14.0 Ma.
Raffi Isabella
Roehl U.
Shackleton Nicholas J.
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