AN ALTERNATIVE INTERNATIONAL CORRELATION OF THE LATE ORDOVICIAN CARBON ISOTOPE CHEMOSTRATIGRAPHIC RECORD: IMPLICATIONS FOR LATEST ORDOVICIAN TEMPERATURE HISTORY

Excursions in the carbon isotopic records through the late Katian and Hirnantian differ substantially among sites around the globe in the magnitude of the excursions, the duration of the events and the precise timing of the peak or minimum values. These differences reflect the interaction of local and global drivers of the carbon system and require precise independent calibrations of the δ¹³C chemostratigraphy for use in global correlation and paleoenvironmental inference. New eNd data from late Katian and Hirnantian strata in Nevada and Yukon lead us to reconsider correlations in this interval. The post-Eureka Ordovician succession in Nevada comprises six stratigraphic sequences (01-06). Graptolite and chemostratigraphic evidence indicates that O5-O6 encompass the Hirnantian HICE excursion, which in this region exhibits a strong double peak in δ¹³C org values: one in the basal Hirnantian, O5 strata (early M. extraordinarius Zone) and a second in mid Hirnantian, O6 strata (early to mid M. persculptus Zone). These relations fit precisely with those recently demonstrated at the well known Mirny Creek section in Siberia, which also exhibits two distinct HICE peaks, and with the glacio-eustatic history inferred at both tropical sites, such as Anticosti Island, and peri-glacial sites in North Africa. In the Anticosti succession, as at Dob's Linn in Scotland, the early Hirnantian (M. extraordinarius Zone - Anceps Band E) peak is weakly developed and the main HICE peak, which is present in the Laframboise Mbr and upper Hartfell Shale (respectively), represents only the mid Hirnantian-M. persculptus Zone peak. The early Hirnantian peak appears to be manifested in the Laurentian midcontinent by the Elkhorn δ¹³C excursion. This excursion is either missing or is weakly developed within the B. gamachiana Zone strata at the top of the Pirgu Stage in the Baltic succession, based on chitinozoan correlations. These relations also suggest that the prominent O4 δ¹³C excursion (upper D. ornatus/early D. anceps Zone) seen in the Nevada sections corresponds to the Whitewater excursion and the Paroveja excursion in Baltica. Sea surface temperatures inferred recently from carbonate clumped isotope paleothermometry suggest separate minima in the early and mid Hirnantian when replotted based on our revised correlations.