IODINE TO CALCIUM RATIOS (I/CA) SUGGEST INCREASED SHALLOW WATER DEOXYGENATION DURING THE LATER CAMBRIAN SPICE EVENT

The SPICE event (Steptoean Positive Isotopic Carbon Excursion) was a +3-6 ‰ excursion in δ¹³C found in later Cambrian successions (~497 Ma). It is globally detected in both marine carbonates and organic matter, suggesting that it represents a perturbation in global carbon cycling. One hypothesis posits that this event represents widespread marine deoxygenation that led to the enhanced burial of organic carbon, resulting in the positive carbon isotope excursion. A trilobite extinction event that occurs near the beginning of the SPICE hints at links between these events and their underlying driver(s). Although global proxies have produced evidence of redox changes surrounding the SPICE, assessment of local redox variations has been difficult since most studied sections recording the SPICE are carbonate and therefore not suitable for the application of most traditional geochemical local redox proxies. The current study uses the novel iodine-to-calcium ratios (I/Ca) paleoredox proxy to ascertain local redox conditions in carbonate dominated sections, bridging the gap in knowledge between local conditions and previously published global trends.

Here we present I/Ca data from six globally distributed SPICE sections: Scotland, western Utah, Newfoundland, Australia, North China, and Maryland. Together these successions represent a diverse set of carbonate environments deposited in a variety of water depths. While diagenesis can remove iodine from rocks, creating lower values, all the data from all six of our studied sections show I/Ca values below the modern oxic threshold. Our collective data suggest that later Cambrian oceans had generally lower oxygen content than modern oceans due to relatively low I/Ca values. Importantly, at least three sections from three separate continental margins show clear decreasing trends in I/Ca during the rising limb and peak of the SPICE. These findings suggest that the SPICE was coupled with shallow water deoxygenation which affected a variety of environments on a global scale.