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