Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u11a..12s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #U11A-12
Mathematics
Logic
1035 Geochronology
Scientific paper
Even without Shackleton's collaboration it is highly probable that an important paper by Hays and Imbrie relating to the astronomical theory of the Pleistocene ice ages would have appeared in the mid 1970's with the important data being Hays' radiolarian counts. Given Hays' expertise in the stratigraphic correlation of cored sequences it is also likely that soon after, attempts would have been made to develop an astronomically calibrated time scale even in the absence of stable isotope records. In fact for sequences older than about 2.5 Ma stable isotope data have played a relatively minor part in facilitating astronomical time scale development. The prime requirement has been an ability to handle and interpret complex arrays of stratigraphic data together with the existence of geological deposits that display lithological cyclicity. The Middle Miocene has proved to be a challenging interval of time to calibrate to an astronomical template. Few continuous sequences are available; good biostratigraphic datums are sparse; and the underlying climatic responses were probably almost as complex as they were in the late Pleistocene. ODP Site 925 on Ceara Rise contains a good sequence from which we have benthic stable isotope data, proxies for percent carbonate, proxies for carbonate dissolution and precise nannofossil biostratigraphy, all in an orbitally calibrated record. In this interval the oxygen isotope record almost certainly records variability in (Antarctic) ice volume as well as temperature, while percent carbonate certainly records variability in all of terrigenous input, carbonate production and carbonate dissolution. The beauty of an astronomical time scale is that even with data from only a single site it becomes possible to begin to tease apart such a complex array of responses, just as Hays and colleagues were able to learn so much about the Pleistocene from a single location in the Southern Ocean. The Middle Miocene also anticipates the greatest challenge facing the astronomical theory of the Pleistocene ice ages: the 100,000 year cycle. The Middle Miocene data imply 100,000 year cycles in Antarctic ice volume, with increased terrigenous input at this period being associated with glacio-eustatic low stands. However neither the stable isotope signal nor the dissolution proxies have significant precessional variability so that precessional variability in terrigenous content is probably driven by varying river transport. Unlike the Pleistocene, glacial maxima are associated with high eccentricity.
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