Strontium isotope (87Sr/86Sr) stratigraphy of Ordovician bulk carbonate: Implications for preservation of primary seawater values

The present study on bulk carbonate 87 Sr/ 86 Sr stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater 87 Sr/ 86 Sr curve for the Tremadocian–Katian Stages (485–445 Ma). Here, we directly compare the curve based on conodont apatite (including some new data not published in earlier work) with a new curve based on 87 Sr/ 86 Sr results from bulk carbonate from the Tremadocian–Sandbian Stages. We sampled eight Lower to Upper Ordovician carbonate successions in North America to assess the reliability of bulk carbonate to preserve seawater 87 Sr/ 86 Sr and its utility for 87 Sr/ 86 Sr chemostratigraphy. A high-resolution 87 Sr/ 86 Sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 Sr/ 86 Sr ( 87 Sr/ 86 Sr seawater ). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for 87 Sr/ 86 Sr in order to determine the conditions under which 87 Sr/ 86 Sr seawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the 87 Sr/ 86 Sr seawater trend, but that its 87 Sr/ 86 Sr values are commonly more variable than those of conodont apatite. On average, bulk carbonate samples of the same age vary by 10–20 × 10 −5 , compared to 5–10 × 10 −5 for conodont apatite. The amount of isotopic alteration of bulk carbonate from seawater 87 Sr/ 86 Sr (Δ 87 Sr/ 86 Sr) was determined by taking the difference between 87 Sr/ 86 Sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 Sr/ 86 Sr values. Cross plots comparing Δ 87 Sr/ 86 Sr values to bulk carbonate Sr concentration ([Sr]) and conodont color alteration indices (CAI; an estimate of the thermal history of a rock body) indicate that bulk carbonate is most likely to preserve 87 Sr/ 86 Sr seawater (minimally altered) when either: (1) bulk carbonate [Sr] is greater than 300 ppm, or (2) carbonate rocks experienced minimal thermal alteration, with burial temperatures less than ~150 °C. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield 87 Sr/ 86 Sr seawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater 87 Sr/ 86 Sr can be preserved in bulk carbonates with low [Sr] if pore water:rock ratios are low ( 87 Sr/ 86 Sr is similar to the seawater 87 Sr/ 86 Sr value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of 87 Sr/ 86 Sr seawater , with a sample variation on par with fossil materials ( −5 ), particularly for successions where well-preserved fossil material (i.e., conodonts or brachiopods) is not available, such as Precambrian strata, sequences recording mass extinction events, or otherwise fossil-barren facies. These criteria and model predictions based on bulk carbonate [Sr] must be considered in the context of whether a limestone accumulated under calcite seas (e.g., Ordovician), with relatively high seawater Sr/Ca, or aragonite seas, in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non-seawater Sr.