Chemostratigraphic Correlation of Terminal Neoproterozoic Units from Caborca, Mexico

At least two prominent negative carbon isotope excursions have been recognized globally in Ediacaran successions between ~580 Ma and the Precambrian-Cambrian boundary. Here, we present geochemical data from Neoproterozoic units near Caborca, Sonora, Mexico, to compare to the global record. Two negative carbon isotopic excursions occur in the Caborca succession; the lower excursion occurs in the Clemente Formation and reaches an isotopic nadir of –10‰ VPDB, whereas the upper excursion occurs in the La Cienega Formation and begins within strata containing Cloudina. The overlying members of the Puerto Blanco Formation contain the Precambrian-Cambrian boundary-marking trace fossil Treptichnus pedum. δ³⁴S of carbonate-associated sulfate (CAS) from the lower excursion displays a negative shift from 20 to 12‰ VCDT and an increase in CAS concentration, concomitant with the carbon isotopic excursion. δ³⁴S_CAS values rise to over 30‰ in the strata between the two carbon isotopic excursions.

The lithologic, isotopic and CAS concentration trends are quite similar to the Death Valley Succession, located ~700 kilometers to the northwest, although the absolute value of δ³⁴S_CAS between Caborca and Death Valley is slightly different. Sulfur isotope data from Oman and China do not match the trends recognized in Death Valley and Caborca, nor do they agree with one another. The differences in δ³⁴S_CAS among these sites suggests that such proxies can be influenced by regional and/or diagenetic processes and do not necessarily reflect a global oceanic sulfate isotopic signal.

The cause(s) of these sulfur and carbon perturbations is unknown. One possibility is that an increase in terminal Neoproterozoic oceanic O₂ (or alternate oxidizing compound) caused the oxygenation of isotopically-light, reduced phases of sulfur and carbon. An increase in oceanic oxygen was likely a necessary threshold to overcome in order to allow the proliferation of more complex life forms during the Cambrian Explosion.