Changes in marine productivity and redox conditions during the Late Ordovician Hirnantian glaciation

Changes in marine productivity and redox conditions during the end-Ordovician (Himantian) glaciation and Ordovician-Silurian transition were investigated through Mo-isotope and major- and trace-element analyses of the Wangjiawan (Hubei Province) and Nanbazi (Guizhou Province) sections from the Yangtze Platform of South China. Katian shales of the Wufeng Formation, which yield the graptolites Dicellograptus complanatus, Dicellograptus complexus, and Paraorthograptus pacificus, were deposited under euxinic conditions at both localities, as shown by high Mo-EF, U-EF, and delta Mo-98 values. A major sea-level regression during the Himantian glaciation resulted in shallowing and a shift toward better-oxygenated conditions within the Yangtze Sea, as well as deposition of thin-bedded siliceous sediments, calcirudite debris flows, and limestone turbidites of the Kuanyinchiao Formation, the base of which correlates with the first phase of the end-Ordovician mass extinction. The termination of the Himantian glaciation at the top of the Kuanyinchiao Formation was associated with a major sea-level transgression, a rapid expansion of euxinia in the Yangtze Sea (as documented by a return of high Mo-EF, U-EF and delta Mo-98 values), and the second phase of the mass extinction, during which the cool-adapted Hirnantian Fauna went extinct. The long-term cooling trend of the Middle and Late Ordovician, which culminated in the Hirnantian glaciation, was driven by enhanced burial of organic carbon, as documented by delta C-13(carb), delta C-13(org), and delta S-34(py) records. Increased organic carbon burial was linked to high rates of marine productivity, as shown by high TOC and biogenic Ba concentrations especially at the deeper, less-restricted Wangjiawan locale, producing the HICE (Himantian Isotopic Curve Excursion) delta C-13(carb) excursion. The locus of organic carbon burial during the Hirnantian crisis shifted to deeper-water environments that were located outside the study region. The relatively rapid onset and termination of the Himantian glaciation were probably due to crossing of tipping points in the late Ordovician climatic-oceanic system. (C) 2014 Elsevier B.V. All rights reserved.