CARBON ISOTOPE STRATIGRAPHY OF THE LATE IBEXIAN (FLOIAN) TO EARLY WHITEROCKIAN (DAPINGIAN) AT SHINGLE PASS, SOUTH EGAN RANGE, NEVADA, USA

The Ibexian-Whiterockian interval (Early-Middle Ordovician) is an important time in the geologic record as it marks one of the first major pulses of macrofaunal radiation of the Great Ordovician Biodiversification Event (GOBE). It has been proposed that the Ibexian is characterized by physical and chemical changes in the oceans that include cooling, an increase in oxygen and associated carbonate ion availability. The δ¹³C record may potentially provide a test of these hypotheses. This study focuses on rocks deposited between the Floian (Early Ordovician; Acodus deltatus/Oneotodus costatus North American Midcontinent conodont biozone) and Darriwilian (Middle Ordovician; Histiodella sinuosa conodont biozone). Carbonate mudstones and wackestones collected at Shingle Pass, east-central Nevada are interpreted to have been deposited on a shallow subtidal mixed siliciclastic-carbonate ramp, which comprise the Parker Spring Formation, Shingle Limestone, and Kanosh Shale. Micritic samples were powdered using a drill and measured for their δ¹³C_carb values. Results show δ¹³C values increased gradually from -2.0‰ to -0.5‰ (V-PDB), an increase that is similar in timing and in magnitude to that of the late Ibexian (Floian) of the San Juan Formation from the Argentine Precordillera. Near the Ibexian-Whiterockian (Floian-Dapingian) contact a gradual return to more negative values of -2.5‰ is observed, a δ¹³C shift encompassing a time span of roughly 10 million years. A δ¹³C increase throughout the late Ibexian is consistent with an increase in the rate of organic carbon burial. The burial of organic matter removes carbon dioxide from the ocean-atmosphere reservoir and increases oxygen. The data are consistent with previous work showing the Floian was a time of cooling oceans and increasing oxygenation, which may have allowed metazoans to expand into new environments and niches. This slowing of organic carbon burial rates near the Ibexian-Whiterockian contact could indicate that a decrease in cooling rates may have occurred a few million years earlier than previously reported, depending on any changes in silicate weathering rates though this same time interval.