EVIDENCE FOR THE DEVELOPMENT OF ANOXIA DURING THE CAMBRIAN SPICE EVENT FROM THE NOLICHUCKY AND EAU CLAIRE FORMATIONS OF KENTUCKY AND OHIO, USA

The Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) is recognized as a major oceanographic event encompassing a large perturbation of the global carbon cycle coupled with a paired shift in the sulfur cycle evidenced by increasing isotopic values of sedimentary pyrite and sulfate over its duration. One proposed mechanism for these observed isotopic trends is a transient increase in the areal extent of anoxic conditions within the oceans. In this scenario the enhanced burial of ¹²C enriched organic carbon and ³²S enriched pyrite under anoxic conditions drove the ocean reservoirs of these elements towards more positive isotopic values. Here we explore the relationship between anoxia and the SPICE using core material from the mixed carbonate-siliciclastic facies of the Nolichucky and Eau Claire Formations (Conasauga Gp) from three locations in Ohio and Kentucky. This material represents sediments deposited in and around the Rome Trough, a rift graben on the Laurentian margin. The three locations chosen represent a proximal-distal transect enabling investigation of differences in geochemical trends related to position within the basin.

Our data display the initiation of the SPICE, recorded in d¹³C_carb and d¹³C_org, at all three study locations. The δ³⁴S of sedimentary pyrite also shows an increase at all locations concurrent with the SPICE and the overall magnitude of this shift increases toward the basin center (from ~+20‰ to ~+50‰). Iron speciation data show a transition from oxic to anoxic, ferruginous conditions at the two more proximal locations synchronous with the onset of the SPICE. However the distal location, in the Rome Trough, where the thickness of the Nolichucky Fm is three times greater, displays a consistent oxic signal before and during the SPICE. This signal may be primary, recording an environment with low productivity and weak water-column oxygen demand at the basin center, and high productivity and bottom water anoxia along the basin margins. Alternatively, anoxia may have developed throughout the basin but is obscured in the iron data by the relatively high rates of sedimentation at the basin center. In either scenario, our data suggest anoxic bottom waters developed in this basin during the SPICE, which is consistent with the proposed driving mechanism for the isotopic trends.