CAMBRIAN SPICE (STEPTOEAN POSITIVE CARBON ISOTOPE EXCURSION) AS A MODEL FOR COMPARABLE PROTEROZOIC HIGH-AMPLITUDE ISOTOPIC EVENTS

The Steptoean Positive Carbon Isotope Excursion (SPICE) was discovered independently at Caltech (Ripperdan and Kirschvink), Oxford (Brasier), and UCLA (Saltzman and Runnegar) in the early 1990s. It is one of the most widespread oceanic isotopic events known from the rock record, occurs at both high and low paleolatitudes, and is expressed as co-varying secular changes in carbonate carbon, organic carbon, and CAS (δ³⁴S). SPICE is well-dated globally by cosmopolitan short-lived trilobite species and its onset is used in conjunction with the FAD of Glyptagnostus reticulatus to define both the Furongian Series and Paibian Stage of the Cambrian Period. In large datasets, there is no correlation between δ¹³C and δ¹⁸O, and the extraordinarily positive carbonates at the peak of the event have withstood substantial diagenetic alteration and even Mississippi Valley-style mineralization. This is an event that stands out from background at the broadest possible scale.

SPICE is near enough in time to comparable Neoproterozoic carbon isotope excursions to serve as a model for global carbon cycling in a high-CO₂, unvegetated world. All evidence points to enhanced burial of marine organic matter and reduced sulfur as the driving mechanism. Systematic changes in Δ¹³C throughout the event at widely separated sites imply CO₂ drawdown, global cooling, and a substantial increase in the amount of oxygen in the atmosphere. Similar fluctuations may have occurred on a global scale throughout the later Proterozoic and early Phanerozoic. The challenge now is to use proxy records and numerical models to forecast the onset and development of such events in realistic ways.