CORRELATING MIDDLE–LATE ORDOVICIAN CARBONATE STRATA ACROSS THE IAPETUS USING STRONTIUM ISOTOPES (87SR/86SR) CALIBRATED TO CONODONT BIOSTRATIGRAPHY

A global carbonate stratigraphic framework is needed to test models of paleoenvironmental change, but provinciality of age-diagnostic fossils and local controls on chemostratigraphic tools such as marine carbon isotope ratios (δ¹³C) have historically made long-distance correlation a difficult task. Radiogenic strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of marine carbonates and bioapatite are globally homogenous and change steadily throughout the Middle–Late Ordovician, therefore representing a promising tool for correlation. We present a calibration of Middle–Late Ordovician (465–453 Ma) seawater ⁸⁷Sr/⁸⁶Sr to high-resolution conodont biostratigraphy from two classic carbonate sections in central Sweden (Fjäcka and Kårgärde). Each Sr sample (N=10) was isolated from >3 mg of well-preserved conodonts (Conodont Alteration Index (CAI) = 1–2) from a single bed, and ⁸⁷Sr/⁸⁶Sr measured with a Thermo Fisher Triton Plus thermal ionization mass spectrometer at Ohio State University. ⁸⁷Sr/⁸⁶Sr is compared to published measurements of a cosmopolitan conodont assemblage from a Laurentian reference section (Antelope Range, central Nevada) which includes the index taxa P. serra, P. anserinus, and B. gerdae of the North Atlantic fauna, allowing for trans-Iapetus correlation of these zonal boundaries. Between 465–460 Ma, Baltic values align well with the Laurentian reference section, showing a baseline of 0.7088 before an inflection to lower ⁸⁷Sr/⁸⁶Sr at ~462 Ma. Between 460–453 Ma, Baltic ⁸⁷Sr/⁸⁶Sr is as much as 2 x 10⁻⁴ greater than Laurentian values. No significant change in the clay content of host rock, Sr concentration of conodonts, or CAI are observed at this divergence from the Laurentian trend, suggesting this offset may represent 1–2 Myr diachroneity of North Atlantic fauna first appearance datums between Laurentia and Baltica rather than diagenetic overprinting. Ongoing work will attempt to constrain the degree of diagenetic alteration of Baltic conodonts by measurement of elemental ratios in conodonts and petrographic analysis of host rock, as well as expand the stratigraphic coverage and resolution of our Baltic ⁸⁷Sr/⁸⁶Sr record with additional measurements from Swedish sections and integration of unpublished conodont ⁸⁷Sr/⁸⁶Sr measurements from the Middle Ordovician of Estonia (Uuga Cliff; N=6).