A reassessment on the timing and potential drivers of the major seawater 87Sr/86Sr drop in the Ordovician Period: New evidence from conodonts in China

Abstract

The major drop in marine 87Sr/86Sr records across the Middle-Late Ordovician boundary (Darriwilian-Sandbian) was one of the most dramatic perturbations of the global Sr cycle in Phanerozoic. However, the precise timing of this isotopic event has not been well constrained, partially making it difficult to further clarify the possible drivers of this isotopic event. For this study, we present multi-continental conodont apatite-based Sr-isotope datasets from three well-studied Ordovician carbonate successions in China, i.e., the Huanghuachang section in South China, the Kalpin Shuinichang-Dawangou and the Nanyigou sections in Tarim Basin, northwestern China. Our new 87Sr/86Sr records generally accord with global seawater 87Sr/86Sr secular variation curve and are supposed to be a reflection of coeval primary seawater signatures. Constrained with detailed conodont biostratigraphic and high-resolution δ13C chemostratigraphic framework, the onset of the sharp decline in 87Sr/86Sr ratios was placed within the Histiodella holodentata zone-Eoplacognathus suecicus zone, and approximately synchronous with the initiation of the Middle Darriwilian Carbon Isotope Excursion (MDICE). Numerical modeling reveals that this major 87Sr/86Sr drop could have been generated by a sustainable increase of oceanic hydrothermal (mantle-derived) Sr flux and/or a progressive decrease of continental-derived Sr flux. Multi-proxy analyses further demonstrate that the impact of enhanced oceanic hydrothermal activities accompanying with the accelerating opening of the Rheic and Paleo-Tethys oceans, may have played a central role in driving the major 87Sr/86Sr drop during the Middle-Late Ordovician transition.