A positive trend in seawater 87Sr/86Sr values over the Early–Middle Frasnian boundary (Late Devonian) recorded in well-preserved conodont elements from the Holy Cross Mountains, Poland

Over the Early–Middle Frasnian boundary interval (Late Devonian), the oceans experienced major changes in global carbon cycling and nutrient dynamics with a prominent positive carbon isotope shift at the base of the Palmatolepis punctata Zone (Middle Frasnian). The aim of this study was to construct seawater 87Sr/86Sr ratio curves for the Pa. transitans–Pa. hassi zonal interval using well-preserved conodont apatite from two sections in the Holy Cross Mountains, south-central Poland. The final curves suggest that seawater 87Sr/86Sr values were steady at ~ 0.70794–0.70799 during the transitans Zone but began to rise in the early punctata Zone towards a value of ~ 0.70814 in the late punctata Zone (an increase of ~ 0.0002). The increase in seawater 87Sr/86Sr values over the Early–Middle Frasnian boundary appears to be the second of two prominent short-term rises in 87Sr/86Sr values, which led to an overall shift in values from a Givetian plateau of between ~ 0.70780 and 0.70785 to a Late Frasnian plateau of between 0.7080 and 0.7082. Correlation with a regional positive δ13C anomaly and evidence for eutrophication in the marine environment suggests that the causes of these phenomena may be linked and that the positive 87Sr/86Sr shift was caused by an increase in the continental flux of Sr to the oceans, i.e., the weathering flux. A possible explanation for an increase in the weathering flux is the denudation of a new Acadian–Eovariscan orogen, which would have been mostly weathered in the humid, warm tropical/equatorial region.