T199 High-Resolution Terrestrial and Marine Geochemical Proxies of Global Change: Progress, Problems, and Utility II

Tuesday, 20 October 2009
1:30 PM - 5:30 PM
Portland Ballrooms 251/258

OXYGEN ISOTOPE TRENDS AND INTERPRETED PALEOCLIMATE CHANGES ACROSS THE LATE CAMBRIAN POSITIVE CARBON ISOTOPE EXCURSION (SPICE EVENT)

Paper number: T199-5
Program: Technical Sessions
Time: 2:30 PM – 2:45 PM

Authors

Abstract

The Late Cambrian Steptoean positive carbon isotope excursion or SPICE event is characterized by a global ~5 ‰ C-isotope excursion which spans about 3-4 My. The excursion begins at the Crepicephalus-Aphelaspis trilobite zones, peaks in the Dunderbergia and ends in the Elvinia Zone. The excursion onset coincides with a global trilobite mass extinction event (base of Pterocephaliid biomere) and the excursion peak coincides with a maximum in long-term global sea-level fall. The origin of the SPICE event is not well understood, but appears to be related to an oceanic anoxic event that enhanced the burial of organic matter.

We sampled a range of phosphatic inarticulate brachiopod genera for oxygen isotopes across the SPICE event in the House Range of western Utah (Little Horse Canyon, Lawson Cove) to evaluate potential paleoclimate changes (seawater temperature and/or ice-volume effects) associated with the δ13C excursion and sea-level fall. δ18O trends show a steady 2.4‰ increase from the late Crepicephalus to mid Aphelaspis, followed by a 2.5‰ decrease to minimum values in the Dunderbergia; the lowest δ18O values coincide with the peak in δ13C values. The low δ18O values gradually increase and plateau into the Elvinia Zone and beyond. These preliminary results suggest that the mass extinction event is associated with peak cooling and/or maximum ice volumes, and the peak of the SPICE event and regression are associated with maximum seawater warming and/or glacial melting. We are testing to see if these δ18O trends are similar across Laurentia by analyzing phosphatic brachiopods from coeval SPICE deposits in central Iowa. If the δ18O trends are similar, then this suggests that the extinction event may be related to widespread cooling, and the hypothesis of increased carbon burial during ocean anoxia is supported by maximum seawater warming (and reduced dissolved oxygen content) during the peak SPICE event.