OCEAN CHEMISTRY DURING THE ORDOVICIAN RADIATION: HIGH-RESOLUTION C- AND S-ISOTOPE PROFILES THROUGH THE YANGTZE PLATFORM, HUBEI PROVINCE, CENTRAL CHINA

Systematic paleontological investigation of Middle to Late Ordovician strata of the adjacent Yangtze Platform, Jiangnan Slope, and Zhujiang Basin has demonstrated that the Great Ordovician Biodiversification Event (GOBE), which represents the greatest sustained increase in marine animal diversity in Earth history, is both temporally and spatially heterogeneous. Whereas onshore graptolites and brachiopods reach their maximum diversity in the Floian, offshore graptolites and trilobites do not reach maximum diversity until the Darwillian and Sandbian, respectively. Although the causes of the GOBE remain elusive, its occurrence during a prolonged period of greenhouse climate conditions that may have experienced globally extensive explosive volcanism, stagnant oceanographic conditions, and extensive deep-ocean anoxia suggest that the variable availability of essential bionutrients may have played an important role in determining patterns of biodiversification.

Here we present new C- and S-isotope data from the Middle Ordovician of the Yangtze Platform, Hubei Province, Central China. On the Yangtze Platform, Middle Ordovician strata are represented by interbedded argillaceous to nodular limestone and calcareous mudstone of the Dawan Formation (Floian-Dapingian-Darwillian) and massively bedded limestone of the Kuniutan Formation (Darwillian). C-isotopes recorded in these strata fall within the same range (-0.8 to +0.6‰) as those preserved globally at this time. Such isotopic stability has been attributed to reduced productivity driven by stagnant oceans, deep-water anoxia, and resultant phosphorous limitation (Saltzman 2005). S-isotope data from Argentina and Newfoundland (Thompson & Kah 2008) support a hypothesis of nutrient limitation. Here, stratigraphically short-term isotopic shifts of up to 6‰ dominate mid-shelf strata and are interpreted to reflect transient changes in the extent of oceanic bottom-water anoxia and associated changes in redox cycling. New, high-resolution S-isotope data from stratigraphically condensed, deeper-water strata of the Yangtze Platform are critical to investigating the extent to which deep-ocean anoxia in the Middle Ordovician may have controlled organic carbon supply, sulfate reduction, and pyrite burial during this time.