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

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Bulletin of the Geological Society of America





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The present study on bulk carbonate 87Sr/86Sr stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater 87Sr/86Sr curve for the Tremadocian-KatianStages (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 87Sr/86Sr 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 87Sr/86Sr and its utility for 87Sr/86Sr chemostratigraphy. A high-resolution 87Sr/86Sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87Sr/86Sr (87Sr/86Srseawater). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for 87Sr/86Sr in order to determine the conditions under which 87Sr/86Srseawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the 87Sr/86Srseawater trend, but that its 87Sr/86Sr 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 87Sr/86Sr (Δ87Sr/86Sr) was determined by taking the difference between 87Sr/86Sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87Sr/86Sr values. Cross plots comparing Δ87Sr/86Sr 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 87Sr/86Srseawater (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 oC. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield 87Sr/86Srseawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater 87Sr/86Sr can be preserved in bulk carbonates with low [Sr] if pore water:rock ratios are low (<10-100) or if pore fluid 87Sr/86Sr is similar to the seawater 87Sr/86Sr value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of 87Sr/86Srseawater, with a sample variation on par with fossil materials (<10 × 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.

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