Computer Science
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.4429q&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 20, pp.4429-4442
Computer Science
8
Scientific paper
One hundred and two articulate brachiopods and twenty-one marine cements from Ordovician strata were analyzed for oxygen and carbon isotopes. These, together with 200 previously published analyses of Ordovician brachiopods (mostly of Upper Ordovician age), define a general trend of increasing 18 O values with decreasing age, from a minimum of - 10.5%. in the earliest Ordovician to a maximum of -1.5%. at the Ordovician /Silurian boundary. The 13 C values show a general positive correlation with 18 O and increase from -2.5%. in the earliest Ordovician to as high as +7%. at the Ordovician/Silurian boundary. The magnitudes of Ordovician excursions for both 18 O and 13 C are similar to those reported previously for the entire Phanerozoic. Different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions, suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts. Petrographic examination and trace element analyses indicate that brachiopod shells from Llandeilo to Ashgill strata are well preserved and thus, may have retained their primary isotopic signatures. The same criteria for the Tremadoc, Arenig and some Llanvirn brachiopods suggest partial diagenetic alteration of 18 O values in these samples, but this does not preclude their utility as a recorder of a primary 13 C signal. The observed 18 O trend for the well preserved portion of the record may reflect progressive cooling during Ordovician, perhaps complemented by a changing 18 O values of seawater. The latter could have been a consequence of a net increase in the rate of high-temperature water/rock interaction relative to its low-temperature counterpart, as indicated also by the large coeval decrease of marine 87 Sr / 86 Sr . Superimposed on the above long-term 18 O trend is a positive excursion of about 2%. at the end of the Ordovician, probably reflecting a large expansion of polar ice caps. The magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the Quaternary glacial episodes. The secular 13 C trend may reflect a progressive increase in marine organic productivity and/or enhanced organic deposition in the Ordovician oceans, particularly noticeable at the time of the terminal Ordovician glaciation
Qing Hairuo
Veizer Ján
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