'Paleontological Institute at The University of Kansas'
Abstract
The Late Devonian extinctions at the Frasnian-Famennian (F-F) boundary and the Devonian-Carboniferous (D-C) boundary were investigated in the Woodford Shale of southcentral Oklahoma with organic geochemical, bulk geochemical, petrographic, and paleontologic techniques. Three sections were collected, two outcrop sections in the Arbuckle Mountains, and one measured core section from the western Arkoma basin. The ratios of extractable biomarkers including steranes, indicative of differing eukaryote input, and pristane/phytane, indicative of oxic or anoxic depositional conditions, display different responses to the F-F boundary and the D-C boundary, as do the abundances of isorenieratane, indicative of photic zone anoxia, and gammacerane, indicative of water column stratification. The ratio of C29 steranes to C27 and C28 steranes are higher in abundance around the F-F boundary and lower in abundance around the DC boundary, indicating different algal communities at each extinction. High concentrations of isorenieratane and gammacerane at the F-F boundary indicate periods of anoxia, while the absence of isorenieratane at the D-C boundary indicates oxic deposition. Similarly, microfossils from the two extinction horizons show different patterns. At the F-F boundary the abundances of the algal cyst Tasmanites are elevated, while the fossils recovered from the upper Woodford Shale by this study and previous authors show an increase in diversity of brown-algae-type microfossils and low diversity benthic faunas dominated by scolecodonts and agglutinated foraminifera. These combined microfossil data and biomarker data suggest a top-down mode of anoxia maintenance during F-F extinctions and a period oxygen-poor waters caused by upwelling during the Hangenburg event. Thus, unlike previous scenarios explaining the F-F and D-C extinctions as a result of a single cause these data suggest that the extinctions are likely results of different processes. Fourier transform infrared (FTIR) microspectroscopy is a chemical characterization technique that can be applied to fossils. In this study, select scolecodont and conodont microfossils from the Woodford Shale were analyzed with FTIR microspectroscopy to reveal different characteristic chemical structure and composition. Conodont FTIR spectra show a predominance of phosphate and carbonate stretching modes with minor aliphatic, olefinic, and carbonyl stretching modes. Scolecodont FTIR spectra are dominated by organic stretching and deformation modes with prominent aliphatic, olefinic, carbonyl, and ether bands with little evidence for inorganic minerals, and also show similarities to modern chitin, albeit with a noted absence of amide bonds. Considering that not a single analysis of extant polychaete jaws has returned significant values of chitin, scolecodont FTIR spectra are probably representative of a scleroprotein material. These data reveal that scolecodont elements can easily be distinguished from conodont elements with FTIR microspectroscopy as scolecodonts are often nearly to completely organic and conodont spectra display weak aliphatic carbon bands, and are dominated by a strong phosphate and carbonate stretching and overtone bands. This provides future fossil workers with a viable method to independently identify enigmatic tooth like microfossils that cannot be confidently assigned to either scolecodont or conodont groups by morphology alone particularly in basal assemblages
