Oxygen and carbon isotopic composition of Ordovician brachiopods: Implications for coeval seawater

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 δ18O 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 δ13C values show a general positive correlation with δ18O 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 δ18O and δ13C 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 δ18O values in these samples, but this does not preclude their utility as a recorder of a primary δ13C signal. The observed δ18O trend for the well preserved portion of the record may reflect progressive cooling during Ordovician, perhaps complemented by a changing δ18O 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 87Sr86Sr. Superimposed on the above long-term δ18O 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 δ13C 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