CARBON ISOTOPIC FRACTIONATION ACROSS A LATE CAMBRIAN CARBONATE PLATFORM: A REGIONAL RESPONSE TO THE SPICE EVENT AS RECORDED IN THE GREAT BASIN, WESTERN UNITED STATES

Geochemical models have proposed that the late Cambrian was characterized by a greenhouse climate with high pCO_². Furthermore, stable-isotope analyses within the Great Basin have documented a large carbon isotope (δ¹³C_carb) excursion, known as the Steptoean Positive Carbon Isotope Excursion (SPICE). This event has been documented globally, and is interpreted as having resulted from enhanced organic carbon burial. Unless the size of carbon reservoirs in the Cambrian ocean was significantly different from those of the Cenozoic, this forcing should have resulted in a comparable excursion in organic carbon (δ¹³C_org). It is also predicted that increased organic carbon burial would lower atmospheric CO₂. Organic carbon isotope data are presented here from Shingle Pass, Nevada and House Range, Utah. At Shingle Pass, δ¹³C_org values record a positive excursion that roughly mirrors δ¹³C_carb values at a similar magnitude, suggesting an oceanographic control on the carbon isotope trend. In the House Range section, although δ¹³C_org values show a rough positive shift associated with δ¹³C_carb, the magnitude is smaller and values show minor shifts across the excursion. We interpret this to reflect a larger chemoautotrophic biomass contribution in the relatively deep water, semi-restricted basinal setting. The difference between organic and inorganic carbon isotope values (Δ¹³C) averages 27-28 per mil across both sections, but increases to 30 per mil at the peak of the excursion and falls to as low as 25 per mil immediately after the Sauk II/III sequence boundary. We interpret the parallel but reduced excursion in δ¹³C_org values did reflect the production changes temporally across the SPICE event. During greenhouse conditions, Δ¹³C is less sensitive to changes in atmospheric CO₂. Thus the increased organic fractionation during the SPICE event may have rather been due to diminished growth rates concomitant with sea level fall and a potential drop in atmospheric carbon dioxide.