Trace-element study of brachiopod shells from the Upper Ordovician Ellis Bay Formation (Anticosti Island, Quebec, Canada) shows that they underwent only a slight (< or =20%) diagenetic equilibration with meteoric waters despite approximately 450 million years of postdepositional history. This, in contrast to large trace-element repartitioning observed in their enclosing rocks, attests to the relative stability of low-Mg calcite even within the realm of meteoric waters. Paleozoic brachiopods apparently exerted a biological control on the distribution of Na and Mg in their low-Mg calcitic shells, preferentially incorporating the former and discriminating against the latter trace constituent.--Journal abstract.

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