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

The Phanerozoic seawater strontium isotope (⁸⁷Sr/⁸⁶Sr) curve is constructed using a variety of geologic materials, which include skeletal grains, carbonate mud, and phosphatic bioclasts and nodules. Brachiopods are particularly useful in preserving seawater ⁸⁷Sr/⁸⁶Sr values because 1) evidence for recrystallization of prismatic shell material can be confirmed with microscopy, and 2) brachiopod shells are made with low-Mg calcite, which is the most stable CaCO₃ polymorph and resistant to diagenetic recrystallization. Brachiopods, however, are susceptible to preservational biases, especially in environments when unfavorable living conditions were present. When brachiopods are absent or poorly preserved, one must use other materials to estimate seawater ⁸⁷Sr/⁸⁶Sr, such as bulk carbonate rock or other fossils.

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.