THE RELIABILITY OF THE SPICE AS A GLOBAL CORRELATION TOOL: CASE STUDY FROM NORTHERN UTAH

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent +4-5 ‰ shift in the Cambrian δ¹³C record used for global chronostratigraphy. The onset of this excursion is traditionally placed at the base of the Pterocephaliid biomere (base of Furongian Series). Recent studies on the SPICE suggest this correlation may not be reliable, as the record of the SPICE onset is apparently diachronous at biozone scales. Likewise, it is possible the onset of the SPICE is temporally incongruent with the base of the Pterocephaliid biomere and asynchrony in faunal turnovers has been demonstrated. Arguably these disparities may not be important on a geologic time scale; however, high precision U-Pb Chemical Abrasion-Isotope Dilution Thermal Ionization Mass Spectrometry (CA-IDTIMS) from zircon allows for ages with analytical error that may be less than the duration of a single biozone or chemostratigraphic perturbation, thus making 10⁶ years significant in the calibration of the contemporary Geologic Time Scale.

The SPICE in N. Utah occurs over the fossiliferous, carbonate-siliciclastic upper Nounan Fm and overlying Worm Creek Mbr of the St. Charles Fm, spanning the Crepicephalus–Elvinia biozones. This heterolithic succession allows for a robust chronostratigraphic constraint of the SPICE through trilobite biostratigraphy, chemostratigraphy, and high-precision CA-IDTIMS DZ (detrital zircon) MDAs (maximum depositional ages).

We build upon prior work by integrating δ¹³C and biostratigraphy with CA ID-TIMS DZ MDAs coincident with the onset and falling limb of the SPICE. This provides the only numerical age for the onset of the SPICE. These data further invalidate the correlation of the SPICE and the base of the Pterocephaliid biomere. These new MDAs also provide numerical constraints on the Laurentian biozone schema as well as requiring the revision of the late Cambrian Geologic Time Scale boundaries by 10⁵-10⁶ years.

While we are not arguing an inherent issue with the SPICE itself, rather its correlation, we charge stratigraphers using chemostratigraphic features as correlation tools to recognize the caveats associated with these correlations especially in context of recent paradigm shifting perspectives.