Recovery from the Late Permian mass extinction was slowed by continued environmental perturbations during the Early Triassic. Rapid fluctuations of the Early Triassic marine carbonate carbon isotope record indicate instability in the global carbon cycle, and recent d 18 O apatite studies link elevated temperatures to the prolonged biotic recovery. High temperatures potentially caused enhanced continental weathering that was detrimental to marine ecosystems, but linking weathering rates to temperature has proven difficult. One proxy for weathering is the 87 Sr/ 86 Sr of marine carbonate; we present here an 87 Sr/ 86 Sr record from an upper Permian-lower Triassic succession near Zal, Iran, that is coupled to a d 13 C carbonate record. An increase in the rate of 87 Sr/ 86 Sr rise from the Dienerian to the Smithian may be linked to elevated continental weathering rates caused by warming during the Smithian.
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 (<10-100) or if pore fluid 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 (<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 ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.