TERMINAL NEOPROTEROZOIC OCEAN OXYGENATION: EVIDYENCE FROM THE HUQF SUPERGROUP, SULTANATE OF OMAN

Carbon and sulfur chemostratigraphy of the Huqf Supergroup [~635 – 540 Ma] indicate progressive oxygenation of the ocean during the terminal Neoproterozoic.  Following the Fiq (Marinoan-equivalent) glaciation, fractionation between sulfate and sulfide d³⁴S_{sulfate-sulfide} increases from ~ <10‰ to ~28‰, indicating a post-Marinoan increase in seawater sulfate concentrations beyond the threshold for significant microbial fractionation.  Across the same stratigraphic interval, bitumen d¹³C exhibits a ~2‰ depletion relative to kerogen; however, bitumen is equivalent or enriched in ¹³C relative to kerogen in older Huqf strata.  Isotopic depletion of bitumen d¹³C, found in younger Huqf strata and throughout the Phanerozoic, signals a fundamental change in carbon cycling.

 

The overlying Shuram Formation contains a negative excursion >13‰ in carbonate d¹³C, which is correlated with a similar excursion in other globally-distributed localities.  Sulfate d³⁴S remains relatively invariant throughout deposition of the Shuram, indicating that significant sulfide oxidation did not accompany the d¹³C excursion.  Coupled organic carbon and carbonate d¹³C data reveal non-steady state carbon cycling in which organic d¹³C is buffered from changes in coexisting carbonate d¹³C.  Thus, the Shuram d¹³C excursion may be explained by oxidation of a vast reservoir of dissolved organic carbon in the deep ocean.  Following the Shuram excursion, coeval organic and carbonate d¹³C exhibit covariance, including across a 7‰ negative excursion in d¹³C at the Precambrian/Cambrian (PC-C) boundary.

 

In strata younger than the Shuram d¹³C excursion, sulfate d³⁴S increases dramatically from ~23‰ to a maximum of ~42‰ at the PC-C boundary.  An increase in pyrite d³⁴S (from ~-5‰ to ~15‰) parallels that found in sulfate d³⁴S.  Consistency in d³⁴S_{sulfate-sulfide} indicates that this increase in d³⁴S results from an increase in pyrite burial. Buildup of oxidants derived from an increase in burial of pyrite may have been sufficient to oxygenate the deep ocean, as is consistent with a decline in sulfate d³⁴S following the PC-C boundary.  Together these data indicate a stepwise reorganization of the global carbon and sulfur biogeochemical cycles in the terminal Neoproterozoic resulting in the ventilation of the deep ocean.