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

Journal Article · Sat Oct 01 04:00:00 EDT 1994 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(94)90345-X· OSTI ID:6556612

Qing, H ^[1]; Veizer, J ^[2]

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1. Univ. of Ottawa, Ontario (Canada)
2. Univ. of Ottawa, Ontario (Canada) Ruhr Universitat, Bochum (Germany)

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 [delta][sup 18]O values with decreasing age, from a minimum of -10.5[per thousand] in the earliest Ordovician to a maximum of -1.5[per thousand] at the Ordovician/Silurian boundary. The [delta][sup 13]C values show a general positive correlation with [delta][sup 18]O and increase from -2.5[per thousand] in the earliest Ordovician to as high as +7[per thousand] at the Ordovician/Silurian boundary. The magnitudes of Ordovician excursions for both [delta][sup 18]O and [delta][sup 13]C 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 [delta][sup 18]O values in these samples, but this does not preclude their utility as a recorder of a primary [delta][sup 13]C signal. The observed [delta][sup 18]O trend for the well preserved portion of the record may reflect progressive cooling during Ordovician, perhaps complemented by a changing [delta][sup 18]O value 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 [sup 87]Sr/[sup 86]Sr.

• OSTI ID: 6556612
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 58:20; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
540310* -- Environment
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CARBON 13
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EVEN-EVEN NUCLEI
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PALEOCLIMATOLOGY
PALEONTOLOGY
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