INCREASED CARBON ISOTOPIC FRACTIONATION DURING THE LATE CAMBRIAN SPICE EVENT AND A PULSE OF ATMOSPHERIC OXYGEN

The Late Cambrian SPICE carbon isotope excursion is documented in carbonate rocks around the world. More recently, study of carbonate associated sulfate reveals a positive shift in sulfur isotopes that closely follows the positive shift in inorganic carbon. Taken together, these parallel isotopic proxy records suggest that organic carbon and pyrite burial increased during the SPICE, and increased the flux of oxygen to the atmospheric reservoir. We report new organic carbon isotope records from carbonate rocks in two cores in Iowa (USA) and Queensland, Australia. The organic carbon isotope curve initially follows the inorganic carbon shift to heavier values. However, during the rising limb of the SPICE excursion, the organic carbon values begin to decrease. The difference between inorganic and organic carbon is below 30 before and after the excursion, but increases to greater than 31 around the peak in inorganic carbon.

If carbon dioxide levels decreased due to organic carbon burial, we would have expected less isotopic discrimination, rather than the increase we observed at the peak of the SPICE. A more likely explanation is that increased oxygen levels drove an increase in isotopic fractionation during photosynthesis. The effect on biological evolution of this burst of oxygen is unknown. However, based on carbon isotope stability in the aftermath of the SPICE, it seems likely that the surface oceans were relatively oxygenated and oligotrophic for tens of millions of years until the middle Ordovician. The Ordovician radiation is known to have occurred during this time interval, and its ultimate causes (intrinsic versus extrinsic) remain an important topic of debate.