A meta-analysis of the Steptoean Positive Carbon Isotope Excursion: The SPICEraq database

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent chemostratigraphic feature in the Lower Paleozoic. It has been used to correlate Upper Cambrian carbonate strata globally, and is cited as intimately linked to the Crepicephalus-Aphelaspis trilobite extinction event and the Sauk II-Sauk III megasequence transition. Despite the global nature of the SPICE event, regional/local conditions serve as a control on the expression of the SPICE event in the rock record. In light of this, and to better understand how reliable the SPICE event is as a chemostratigraphic tool for correlation, we have created the “SPICEraq,” a database comprising 78 SPICE-bearing sections containing 6669 individual δ¹³C analyses. In this study, we quantitatively evaluate the variability in SPICE records, and document that, while the excursion is a global signature, its stratigraphic expression is influenced by such conditions as paleolatitude, paleocontinent, water depth, and facies. While the magnitude of the SPICE excursion is generally consistent (an ~4‰ increase), the peak δ¹³C values are quite variable (ranging from +0.35 to +5.87‰). Specifically, sections located between 30 and 60°S paleolatitude ca. 500 Ma record δ¹³C values ~1 to 2‰ lower than those from lower paleolatitudes. Sections deposited in shallow water depths and facies also record lower δ¹³C values than intermediate and deep-water facies; the deep-water facies exhibit the most ¹³C-enriched carbonates at the peak of the SPICE and post-excursion. The stratigraphic thickness of the excursion varies widely, ranging from <3 to ~884 m, and is significantly impacted by all categorical variables investigated in this study. This study documents that the rising limb of the SPICE is immediately preceded by a small negative δ¹³C excursion in 75% of sections with data collected pre-SPICE. While 32% of sections record a δ¹³C plateau during peak SPICE, its presence in the δ¹³C record does not appear to be influenced by any environmental conditions investigated herein. Altogether, these analyses indicate that regional/local conditions impact the stratigraphic expression of δ¹³C records, and thus care should be taken to use robust, quantitative measures to compare and correlate excursions.