Paired carbon isotopic analysis of Ordovician bulk carbonate (δ13Ccarb) and organic matter (δ13Corg) spanning the Great Ordovician Biodiversification Event

Paired measurements of bulk carbonate (delta C-13(carb)), organic matter (delta C-13(org)), and their difference (Delta C-13) can be used to estimate changes in isotopic fractionation through time as a function of O-2/CO2 in the atmosphere. However, because local scale processes can also affect Delta C-13, it is essential to compare sections from widely separated water masses. Here we present new delta C-13(org) data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published delta C-13(carb) records from these sections and paired delta C-13 values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician delta C-13(carb) excursions and now provide a composite Ordovician 613Corg record. New Lower Ordovician (Tremadocian Stage) delta C-13(org) data range from ca. -26 to -28%., decreasing throughout the Lower-Middle Ordovician (Floian-Dapingian Stages) to ca. -29 to -31 parts per thousand delta C-13(org) values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (SandbianKatian) delta C-13(org) data from North America. delta C-13 values from well-preserved intervals generally vary between +26 to +28%o throughout the Lower to Middle Ordovician (Tremadocian to early Darriwilian), but increase to +31%o during the mid-late Darriwilian and mid Sandbian, similar to published data from younger Late Ordovician positive delta C-13 excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Delta C-13 trend shows a-3%o increase throughout the Early-Middle Ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms. Modeling studies predict that pCO(2) decreased during this time, suggesting that the effect of pCO(2) on Delta C-13 may have been overwhelmed by other controls, such as an in increase in pO(2) or a higher O-2/CO2 ratio during this biodiversification event. (C) 2015 Elsevier B.V. All rights reserved.