STRONTIUM ISOTOPE (87SR/86SR) STRATIGRAPHY OF ORDOVICIAN CONODONT APATITE AND CARBONATE BULK ROCK: IMPLICATIONS FOR PRESERVATION OF PRIMARY SEAWATER CHEMISTRY

Strontium isotopes (⁸⁷Sr/⁸⁶Sr) assist in chemostratigraphic correlations of strata, especially in sections lacking index fossils or between different biogeographic realms. Since primary ancient ocean chemistry cannot be directly measured, well-preserved minerals that formed in equilibrium with seawater are used to acquire Sr proxy data. Low-Mg calcite of brachiopod shells is regarded as the ideal material in the preservation of seawater ⁸⁷Sr/⁸⁶Sr. However, while brachiopods are not always preserved, other materials like fine-grained calcite micrite (bulk carbonate) and biogenic phosphate (apatite) can be sampled continuously to obtain seawater ⁸⁷Sr/⁸⁶Sr. Conodonts are common in and easily isolated from Ordovician carbonate rocks, and they offer superior biostratigraphic resolution compared to brachiopods, thus providing excellent potential for high-resolution bio-chemostratigraphic studies aimed at improving and calibrating the global Sr isotopic record of ancient seas. Micrite can be a good choice but criteria to recognize well-preserved samples are difficult to define.

This study compares ⁸⁷Sr/⁸⁶Sr values of 126 bulk carbonate rock samples and 87 conodont apatite samples from six Ordovician carbonate successions in Nevada, Oklahoma, Maryland, and Virginia. Both bulk carbonate and conodont apatite data yield ⁸⁷Sr/⁸⁶Sr trends similar to the global ⁸⁷Sr/⁸⁶Sr curve measured from brachiopod calcite, but conodont apatite is consistently less scattered and generally less radiogenic (likely less altered) than bulk carbonate. On average, conodonts are 5 to 10 x 10^-5 less radiogenic than the least altered bulk carbonate samples, but in some sections bulk carbonate can be as much as 500 to 1000 x 10^-5 more radiogenic than conodonts. Variability of ⁸⁷Sr/⁸⁶Sr between closely spaced conodont samples in all sections is less than 10 x 10^-5 whereas bulk carbonate can vary by as much as 150 x 10^-5 or more, indicating conodont apatite is more predictable than bulk rock in approximating seawater ⁸⁷Sr/⁸⁶Sr. Our results, which included samples with a wide range of conodont elements with variable conodont alteration indices (CAI), suggest that conodonts collected from carbonate lithologies are as reliable as brachiopods in reconstructing seawater Sr isotope curves.