NEEDS MORE SPICE: REVISITING THE STEPTOEAN POSITIVE ISOTOPIC CARBON EXCURSION IN WESTERN LAURENTIA

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a conspicuous chemostratigraphic feature of the Paibian Stage and may record a global perturbation to the carbon cycle. The onset of the excursion is placed at the Marjumiid-Pterochephaliid (M-P) faunal turnover and is used as a chronostratigraphic correlation tool.

Recent studies integrating Chemical Abrasion - Isotope Dilution Thermal Ionization Mass Spectrometry (CA-IDTIMS) U-Pb detrital zircon (DZ) maximum depositional ages (MDA) with Cambrian trilobite biostratigraphy have revealed asynchrony in faunal turnovers, and are refining the Geologic Timescale. Likewise, integrative studies document asynchroneity of the M--P biomere and the time-transgressive nature of the SPICE event itself. These decoupled signals speak to the complex nature of the environmental and biotic changes in the Steptoean, and motivate further bio-, chemo-, and chronostratigraphic investigations.

The SPICE event in Northern Utah occurs over the fossiliferous, mixed carbonate-siliciclastic uppermost Nounan Formation and overlying Worm Creek Member of the St. Charles Formation, spanning the Crepicephalus–Elvina biozones and the Sauk II–III megasequence transition. Several orthoquartzites punctuate this carbonate succession allowing a powerful, multi-technique approach to constrain the SPICE event through trilobite biostratigraphy, chemostratigraphy, and high-precision CA-IDTIMS DZ MDAs. We use this section as a laboratory to investigate the tempo and time-transgressive nature of the SPICE as well as test current biostratigraphic models, with the eventual goal of refining the Geologic Timescale.

Here we build upon prior work by 1) enhancing the δ¹³C record, 2) pairing this with a new δ¹⁸O record, and 3) augmenting trilobite biostratigraphy. We integrate these data with new CA ID-TIMS DZ MDA coincident with the falling limb of the SPICE (Elvinia biozone); our preliminary results document numerical ages for these events significantly younger than previously thought—a result consistent with other recent chronostratigraphic studies of Cambrian sequences of southwestern Laurentia.