Neodymium isotopic composition of Ordovician-early Silurian conodonts
Date
1995
Authors
Wright, Cynthia Anne
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Abstract
Neodymium (Nd) and samarium (Sm) isotopic ratios were determined in conodont apatite to attain a seawater proxy to test recent paleogeographic reconstructions, and by association, other reconstructions involving paleobiogeography and paleoceanography. Conodont samples were chosen from the major cratons and microplates to achieve a global picture of the Ordovician and Early Silurian paleogeography. Fourty-five samples yielded satisfactory εNd values for a few time slices in this interval.
The results of the analysis reveal that conodonts are useful proxy tools. They have Nd and Sm isotopic abundances that are within accurate and reproducible analytical limits. The pattern of values reveals a consistent picture of the changes in the Ordovician to Early Silurian oceans. The εNd(T) values for Laurentia are strongly negative in the Early Ordovician and this is in marked contrast to other cratons and microplates. The evolution of the signature over time reveals an increasingly radiogenic component for Laurentia (-22 to -18 changing to -13 to -5). The isotopic values and the secular variation curve are a function of both regional and global tectonic processes, the most obvious being the Taconic Orogeny and the closure of the Iapetus Ocean with the northward drift of Baltica and Avalonia.
In this study, both global and regional scale paleogeographic models of other authors are considered. The εNd data help define ancient ocean masses and are relevant to the patterns of conodont biogeography. However, there are not precise correlations with the various realms and provinces. Finally, the data are used to test paleoceanographic circulation models and the data generally support large scale circulation patterns based on Ekman transport.
The use of Nd isotopic signatures representing ancient oceans provides an independent method to test recent paleogeographic reconstruction models. The isotopic signals can integrated into the construction such models so that a more coherent picture of ancient environments can emerge.