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

The present study on bulk carbonate Sr-87/Sr-86 stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater Sr-87/Sr-86 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 Sr-87/Sr-86 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 Sr-87/Sr-86 and its utility for Sr-87/Sr-86 chemostratigraphy. A high-resolution Sr-87/Sr-86 curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater Sr-87/Sr-86 (Sr-87/Sr-86 seawater). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for Sr-87/Sr-86 in order to determine the conditions under which Sr-87/Sr-86 seawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the Sr-87/Sr-86 seawater trend, but that its Sr-87/Sr-86 values are commonly more variable than those of conodont apatite. On average, bulk carbonate samples of the same age vary by 10-20 x 10(-5), compared to 5-10 x 10(-5) for conodont apatite. The amount of isotopic alteration of bulk carbonate from seawater Sr-87/Sr-86 (Delta Sr-87/Sr-86) was determined by taking the difference between Sr-87/Sr-86 values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont Sr-87/Sr-86 values. Cross plots comparing Delta Sr-87/Sr-86 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 Sr-87/Sr-86 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 similar to 150 degrees C. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield Sr-87/Sr-86 seawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater Sr-87/Sr-86 can be preserved in bulk carbonates with low [Sr] if pore water: rock ratios are low (<10-100) or if pore fluid Sr-87/Sr-86 is similar to the seawater Sr-87/Sr-86 value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of Sr-87/Sr-86 seawater, with a sample variation on par with fossil materials (<10 x 10(-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.