UPPER DEVONIAN (LATE FRASNIAN-EARLY FAMENNIAN) CONODONT BIOSTRATIGRAPHY AND ∂18OAPATITE ISOTOPIC AND SUBTROPICAL NORTH AMERICAN SST RECORDS SPANNING THE KELLWASSER EXTINCTION EVENTS

Upper Devonian strata in the Iowa Basin (central United States) provide one of the best-documented records in North America of Late Frasnian stepped Kellwasser Extinction Bioevents within a detailed conodont biostratigraphic framework. To assess the role of climate change as a driver for the Kellwasser Extinction bioevents, δ¹⁸O_apatite records from Polygnathus decorosus and P. unicornis were done to estimate changes in Sea Surface Temperatures (SST) in offshore and onshore sections spanning the LKE and UKE intervals in north-central and eastern Iowa.

Fifty-nine samples collected through the type section of the Sweetland Creek Shale in Muscatine County consisting of a condensed deep ramp hemipelagic shales and mudstones of the Sweetland Creek and lower Grassy Creek Shale. These deposits span Frasnian Zone 11 through the Palmatolepis d. platys Zone with clearly identified LKE and UKE intervals. Thirty-nine large samples from subtidal inner-middle shelf facies of the upper part of the Cerro Gordo and Owen members of the Lime Creek Formation at the Bruns Quarry in Butler County comprise expanded conodont and megafossil records through Frasnian Zone 12 (upper part) and Subzone 13a and document the LKE and post-LKE recovery intervals.

The offshore Sweetland Creek data record 6-7 δ¹⁸O excursions of up to +1‰ from base of the LKE interval into the earliest Famennian, and appear to correspond closely to short eccentricity signals documented from the same interval in southeastern Iowa. The LKE interval spans the descending limb of C° excursion 1 reflecting a calculated SST warming of 4.5 C° (Kolodney). The UKE interval spans a cooling event of at least 3 C° calculated from δ¹⁸O values in excursion interval 6. Calculated offshore SSTs ranged from 26 to 31 C°. Shallow platform δ¹⁸O data from the upper Lime Creek Formation represent expanded records of excursions 1 to 3 (up to +1.3 ‰), with a calculated LKE warming of 6 C° with SSTs ranging from 23-30.7 C°.