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

Abstract

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 δ 13C 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.