CARBON ISOTOPE SHIFTS AND GLACIATION IN THE CARBONIFEROUS: AN UPDATE FROM THE ARROW CANYON RANGE IN SOUTHERN NEVADA

The Carboniferous System is continuously exposed in the Arrow Canyon Range of Nevada and has been the subject of detailed biostratigraphic investigation over the past 40 years. Carbonate d¹³C analysis of this 1.3 km marine succession was undertaken to provide a reference standard for d¹³C in western North America that can be used to evaluate global changes in carbon cycling. The results of ~700 analyses reveal a complex d¹³C curve that does not support the notion of gradually increasing values throughout the Carboniferous. Rather, a large +7‰ anomaly found in the lowermost Mississippian stage (Kinderhookian) stands far above the next highest peaks of +3-4‰ in the Meramecian, Atokan and Missourian. This Kinderhookian shift is apparently global, based on comparison with numerous sections in North America and on data from Belgium spanning the time-equivalent Hun and Yvoir Members of the du Bocq Formation. Viewed in its entirety, the d¹³C curve at Arrow Canyon is similar to the midcontinent United States brachiopod calcite curve of Mii et al. (1999), and supports their finding that heavy (+6‰) values characterizing the Upper Carboniferous in Russia (Bruckschen et al. 1999) and other western paleo-Tethys localities are not representative of global seawater.

Because the magnitudes of d¹³C fluctuations are similar throughout the post-Kinderhookian Carboniferous at Arrow Canyon, it appears that massive organic carbon burial in coal swamps during the Pennsylvanian did not greatly increase global organic C burial rates and oceanic d¹³C. This is consistent with the notion that the Carboniferous trough in atmospheric CO₂ was chiefly a response to enhanced silicate weathering related to land plant evolution (Berner, 1998), but also raises the possibility that CO₂ values were higher than current models indicate. The sea level record at Arrow Canyon shows a major draw down in the Chesterian associated with falling d¹³C values, and if eustatic, this further emphasizes the complex link between organic carbon burial and the main episode of Carboniferous glaciation.