Carbon isotopic fractionation across a late Cambrian carbonate platform: A regional response to the spice event as recorded in the Great Basin, United States.

摘要

Geochemical models have suggested that the late Cambrian was characterized by a greenhouse climate with high pCO2. Furthermore, stable-isotope analyses within the Great Basin have documented a large carbonate isotope (delta13Ccarb) 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 carbon-isotope excursion in organic matter (delta13 Corg). It is also predicted that increased organic carbon burial would lower atmospheric CO2, which may cause global cooling and a reduction in carbonate-organic carbon isotope fractionation. To test these predictions, paired carbonate and organic carbon isotope data are reported here from carbonate stratigraphic sections at Shingle Pass, Nevada and in House Range, Utah. At Shingle Pass, delta 13Corg values record a positive excursion that roughly mirrors delta 13 Ccarb values at a similar magnitude, suggesting an oceanographic control on the carbon isotope trend. In the House Range section, although delta 13Corg values show a rough positive shift associated with positive delta13Ccarb, the magnitude is smaller and values show minor shifts across the excursion. However, constructing a time-equivalent overlay of data from both sections using key stratigraphic boundaries resolved apparent discrepancies, suggesting a regional control on carbon isotopic fractionation. The difference between carbonate and organic carbon isotope values (Delta13C = delta13C carb -- delta13Corg) averages 27‰ to 28‰ in both sections, but increases to 30‰ at the peak of the excursion and falls to as low as 25‰ immediately after the Sauk II/III sequence boundary. Temporal variations in Delta13C do not follow the predicted atmospheric CO2 changes before the delta 13Ccarb peak of the SPICE, as might have been derived from the increased organic carbon burial model for the origin of the SPICE event, and indicates that the carbon isotope fractionation was less sensitive to atmospheric CO2 changes when ambient CO2 was high. The abrupt drop in Delta13C after the delta 13Ccarb peak of the SPICE is consistent with low atmospheric CO2 and the potential evolution of photosynthetic organisms in adapting to CO2-limited environments with stronger bicarbonate uptake during carbon fixation.