STRONTIUM ISOTOPE (87SR/86SR) STRATIGRAPHY OF LOWER DEVONIAN CARBONATE ROCKS FROM THE GREAT BASIN REGION: TESTING SR ISOTOPE METHODS USING CONODONT APATITE AND BULK CARBONATE
This study tests the hypothesis that Lower Devonian conodont (bioapatite) and bulk carbonate samples can be used to provide reasonable estimates for seawater 87Sr/86Sr in the absence of brachiopods. A ~150-m-thick succession of Lower Devonian carbonate rocks were sampled for bulk carbonate and conodont fossils from the Roberts Mountains of the Great Basin region of central Nevada. This offers an opportunity to compare the published brachiopod-based 87Sr/86Sr curve with 87Sr/86Sr values measured from 19 bulk carbonate and conodont samples, which may serve as a proxy for seawater 87Sr/86Sr with minimal alteration. Preliminary bulk carbonate data show that 87Sr/86Sr values preserve a decreasing trend from 0.7101 to 0.7088 from the base to top of the section, similar to brachiopod-based values that decrease from 0.7087 to 0.7081. This similarity suggests that though the bulk rock-based curve preserves a general seawater 87Sr/86Sr trend, it is not a reliable indicator for seawater 87Sr/86Sr values. Further analysis will show whether a conodont-based 87Sr/86Sr curve for the Early Devonian can be used in parallel to brachiopod-based data, analogous to a published conodont-based curve for the Ordovician, or if brachiopods are the ideal geologic material to use for late Paleozoic 87Sr/86Sr study.