Biostratigraphically-controlled Darriwilian (Middle Ordovician) δ13C excursions in North China: Implications for correlation and climate change

A continuous, 545 m thick carbonate succession through the Darriwilian Bei'anzhuang and Machiakou (Majiagou) formations in the Tao-112 drillcore in the Ordos Basin, North China was sampled for an integrated study of conodont biostratigraphy (48 samples) and carbon isotope chemostratigraphy (510 samples). Two internationally correlatable conodont biozones were recognized in the drillcore, namely the Histiodella holodentata-Tangshanodus tangshanensis Biozone and the Plectodina onychodonta Biozone in stratigraphically ascending order. Constrained by the conodont biostratigraphy, two carbon isotopic anomalies can be distinguished, including the short-term LDNICE (Lower Darriwilian Negative Isotopic Carbon Excursion) in the lower part of the Bei'anzhuang Formation, and the long-lasting MDICE (Middle Darriwilian Isotopic Carbon Excursion) spanning the upper part of the Bei'anzhuang Formation and much of the Machiakou Formation. Moreover, the present high-resolution carbon isotope record allows the recognition of two peaks of the MDICE, the stratigraphically lower, small peak MDICE-1 and the follow-up, long-lived peak MDICE-2. The Darriwilian carbon isotope excursions in North China can be comparable to the δ13Ccarb trends described in coeval successions in Baltoscandia, North America, South Korea, and other regions of China, suggesting their significance for global correlations and providing an integrated and high-resolution chemo-biostratigraphic framework for sedimentary records of Darriwilian age in North China. Based on widely accepted driving mechanisms of global carbon-cycle perturbations, here we put forth an inference on the climatic pattern of the Darriwilian that a short warm period causing the LDNICE in the early Darriwilian was followed in turn by a moderate global cooling event resulting in the MDICE during the middle and late Darriwilian. Carbon cycle processes of the Middle and Late Ordovician indicate that there was a clear pre-Hirnantian cooling step, with gradually enlarged alternating relative warmer and rather cooler interludes, culminating in the 'Hirnantian' ice age.