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 deltaO-18 values with decreasing age, from a minimum of -10.5parts per-thousand in the earliest Ordovician to a maximum of -1.5parts-per-thousand at the Ordovician/Silurian boundary. The deltaC-13 values show a general positive correlation with deltaO-18 and increase from -2.5parts-per-thousand in the earliest Ordovician to as high as +7parts-per-thousand at the Ordovician/Silurian boundary. The magnitudes of Ordovician excursions for both deltaO-18 and deltaC-13 are similar to those reported previously for the entire Phanerozoic. Different genera of brachiopods from the same stratigraphic 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 deltaO-18 values in these samples, but this does not preclude their utility as a recorder of a primary deltaC-13 signal. The observed deltaO-18 trend for the well preserved portion of the record may reflect progressive cooling during Ordovician, perhaps complemented by a changing deltaO-18 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 Sr-87/Sr-86. Superimposed on the above long-term deltaO-18 trend is a positive excursion of about 2parts-per-thousand 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 deltaC-13 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.