Steptoean positive carbon isotope excursion

The Steptoean positive carbon isotope excursion (SPICE) is a global chemostratigraphic event which occurred during the upper Cambrian period between 497 and 494 million years ago.

[2] The general signature of the SPICE event is a positive δ13C excursion, characterized by a 4 to 6 ‰ (per mille) shift in δ13C values[1][2] within carbonate successions around the world.

[3] In both these studies, the SPICE excursion was identified and trends were observed within Cambrian formations of the Great Basin of the western United States.

[2] The SPICE event is expressed globally with known formations in 11 countries: United States, China, Australia, South Korea, Argentina, Canada, France, Kazakhstan, Scotland and Sweden (ordered by greatest to least number of localities).

The most common lithology for SPICE intervals are micritic limestones, or carbonate shales, generally interbedded with thin layers of calcareous mudstone.

[2] SPICE intervals are also highly variable when it comes to facies, with examples for shallow, intermediate and deep water settings (see map in the localities section).

[1][2] Despite being a global event, the magnitude of δ13C values observed within a SPICE interval appear to be highly affected by a variety of local conditions.

[1] Initially, these conditions would have spread slowly, limited to deep environments and having small impacts on the global carbon system.

This increase in pressure likely triggered the second wave of the End-Marjuman Biomere Extinction,[1] resulting in the disappearance of many shallow-water trilobite and brachiopod species from the fossil record at this time.

This combined with an increase in burial from expanding anoxic conditions and less bioturbation from now extinct ocean floor dwelling organisms would likely cause δ13C values to sharply rise.

[2] Finally, moving out of the End-Marjuman Biomere Extinction and into the falling SPICE, oceans likely experience significant recovery in biodiversity.

Some research suggests that the turnover in trilobite and brachiopod species that occurred during the SPICE may have a direct correlation with these subsequent events.

[2] Similar to the SPICE, the HICE event is linked to changes in the climate and falls in global sea level, resulting in anoxic conditions and an increase in organic carbon burial.

[41] This early Silurian (431 Ma) δ13C excursion also shows similarities to the SPICE event, with positive max δ13C values around 4.5‰.

Furthermore, the Ireviken event also has global occurrences, but their expression is highly influenced by local facies characteristics, similar to SPICE.

A map of SPICE localities during the upper Cambrian (500 Ma). Modified from the map published by Pulsipher et al., 2021.
A map of SPICE localities during the upper Cambrian (500 Ma). Points on the map denote the locations of formations and the color indicates the paleo-water depth. Modified image is originally sourced from Pulsipher et al., 2021. [ 2 ]
Figure showing the six stages of the SPICE: pre-SPICE, early SPICE, rising SPICE, plateau, falling SPICE, and post SPICE. Graph line shows how values of δ13C shift through each stage.
The six stages of the SPICE. The red line indicates the δ13C value. Plot modified from Pulsipher et al., 2021 [ 2 ] and Zhang et al., 2023. [ 1 ]