A paired carbonate–organic δ13C approach to understanding the Cambrian Drumian carbon isotope excursion (DICE)

Abstract

The middle Cambrian (Cambrian Series 3) Drumian carbon isotope excursion (DICE) represents a pronounced negative carbon isotopic excursion, and has been proposed as a key chemostratigraphic marker for identifying the base of Cambrian Drumian Stage and global correlation. However, the precise interbasinal correlation of the DICE to the South China remains unsolved and the driving mechanism of the DICE is to be further understood. In this study, we report new paired δ13Ccarb and δ13Corg data from the Wangcun section in South China. Both δ13Ccarb and δ13Corg profiles capture a large negative excursion (DICE) near the FAD of P. atavus. We suggest that the parallel δ13Ccarb and δ13Corg negative excursions during the DICE may have resulted from the shoaling of anoxic deep waters associated with the transgressive event. Shoreward and upward incursion of deep anoxic waters into oxygenated shallower water column during transgression could lead to the enhanced oxidation of dissolved organic carbon (DOC) and introduce additional 13C-depleted carbon to the surface water, resulting in the parallel δ13Ccarb and δ13Corg negative excursions. More importantly, we use a box model to quantify the flux of DOC oxidation needed to produce a carbon isotope excursion of the magnitude and duration of the DICE we observed. The model results indicate that oxidation of DOC during the DICE may require massive amounts of oxidants (i.e., sulfate and oxygen), probably causing further expansion of anoxia in the shallow marine environment. We suggest that shoaling of anoxic water during the DICE could have delayed the recovery of metazoan reef systems from the early–middle Cambrian mass extinctions.