CARBON ISOTOPE STRATIGRAPHY OF THE LATE IBEXIAN (FLOIAN) TO EARLY WHITEROCKIAN (DAPINGIAN) OF THE POGONIP GROUP, GREAT BASIN, USA: EVIDENCE FOR OXYGENATION DURING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT (GOBE)

The Ibexian-Whiterockian interval (Early-Middle Ordovician) is an important time in the fossil record as it marks one of the first major pulses of faunal radiation of the Great Ordovician Biodiversification Event (GOBE). Biodiversity and shell bed abundance increases markedly from the late Ibexian (Floian) to the Whiterockian (Dapingian-Darriwilian). The Ibexian is interpreted to be a time of cooling of shallow seas and increases in oxygen and carbonate ion availability. The δ¹³C record can provide a test of these hypotheses. This study focuses on rocks deposited during the late Ibexian (Acodus deltatus/Oneotodus costatus North American Midcontinent conodont biozone) and the Whiterockian (Histiodella sinuosa conodont biozone). Carbonate rocks collected from the Pogonip Group at Shingle Pass, Nevada and the Ibex Hills, UT are interpreted to have accumulated on a shallow subtidal ramp with mixed siliciclastic and carbonate sedimentation. Micritic samples were measured for δ¹³C. Results show δ¹³C increased gradually from -2.0‰ to -0.5‰ (V-PDB), an increase that is similar in timing and magnitude recorded in sections deposited in other basins during the Floian (Argentine Precordillera and Newfoundland). Above the Ibexian-Whiterockian contact a gradual return to values of -3.5‰ is observed, a δ¹³C shift encompassing a time span of ~10 million years during the Floian-Dapingian.

A long-lived δ¹³C increase during the late Ibexian is consistent with an increase in the rate of organic carbon burial. Burial of organic matter removes carbon dioxide from the ocean-atmosphere reservoir and increases the amount of free oxygen. The timing of this carbon burial and oxygenation also occurs at roughly the same time as the onset of some of the earliest pulses of biodiversification and increased abundance of skeletal carbonate. Enhanced organic carbon burial is also consistent with conodont oxygen isotope studies showing that the Early Ordovician may have been a time of progressive cooling, although the precise timing of events remains uncertain.