A HIGH-RESOLUTION INVESTIGATION OF THE δ13CCARB AND δ15N RECORDS OF THE LATE CAMBRIAN SPICE EVENT

The Cambrian Period (~539 – 487 Ma) marks the transition from the highly unstable, volatile δ¹³C record of the Neoproterozoic to the increasingly more stable Paleozoic. The final major positive excursion of this period is the globally documented Steptoean Positive Carbon Isotope Excursion (SPICE), which is characterized by a +3-5‰ shift in the carbon isotope record and thus represents an interval of disequilibrium in the cycling of carbon between the linked atmospheric, oceanic, and lithospheric reservoirs. This biogeochemical event additionally produced shifts in sulfur isotopes and various redox-sensitive trace metals, which can be associated with changing marine redox conditions (euxinia/anoxia) and episodes of enhanced organic carbon burial.

Here, we conducted a high-resolution study of the δ¹³C_carb and δ¹⁵N records of the SPICE using a core from central Iowa (Rhinehart A-1 Core). Examining δ¹⁵N, though only a more localized signal, provides novel insight into the role of nutrient cycling, primary production, and redox conditions leading up to and through the SPICE and reveals a transient negative excursion in δ¹⁵N that closely aligns with the onset of the positive δ¹³C excursion. The negative shift observed here likely denotes a local increase in nitrogen fixation, in turn providing copious amounts of diazotrophic nitrogen that is preserved in sediments with low δ¹⁵N values. An abundance of phosphorous, likely due to remobilization in anoxic and/or euxinic settings as opposed to increased weathering, coupled with bioavailable nitrogen are findings consistent with an interval of enhanced local primary productivity at the initiation of the SPICE. Examining the nitrogen cycle of the SPICE, especially in light of the parallels to the Ireviken (Silurian) and Hangenberg (Devonian-Carboniferous) events, underscores the potential for similarities in the driving mechanisms behind these large positive carbon isotope excursions.