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

Extensive studies of Upper Cambrian strata across several paleocontinents document a global positive shift in the carbon isotope record (δ¹³C) that exceeds +5‰ in an event now known as the Steptoean Positive Carbon Isotope Excursion (SPICE). A handful of studies additionally recognize coeval positive excursions in the organic carbon isotope record and in the carbonate-associated sulfur and pyrite records. Investigations of redox conditions using various other redox-sensitive trace metals reveal notable deviations from baseline values as well. Though the precise driving mechanisms of the SPICE are debated, these signals are commonly interpreted to represent episodes of enhanced organic carbon burial and preservation and changing marine redox conditions (euxinia/anoxia).

The roles of nutrient cycling, local primary production, and changing redox conditions in global carbon cycle perturbations can be investigated using the δ¹⁵N record. Here, we collected high-resolution paired carbonate carbon (δ¹³C_carb) and nitrogen (δ¹⁵N) isotope data of the SPICE from the Rhinehart A-1 drill core from central Iowa. The data reveal a transient negative δ¹⁵N excursion that begins around the onset of the positive δ¹³C excursion, which likely suggests a local increase in nitrogen fixation. An increase in nitrogen fixation by diazotrophs could have supplied the sediment with isotopically low δ¹⁵N values. This bioavailable nitrogen combined with an increase phosphorous via liberation under reducing conditions (as opposed to increased weathering) are consistent with a period of elevated primary production near the onset of the SPICE. The evidence presented here further highlights similarities between the SPICE and other major biogeochemical events and, therefore, plays a pivotal role in both constraining potential driving mechanisms and better understanding how the Earth system operated in the past.