CARBON AND SULFUR ISOTOPIC EVIDENCE FOR THE TERMINAL NEOPROTEROZOIC DEEP OCEAN OXYGENATION, HUQF SUPERGROUP, SULTANATE OF OMAN

High resolution carbon and sulfur chemostratigraphy of the Huqf Supergroup [~635 – 540 Ma] indicate the progressive oxygenation of the ocean during the terminal Neoproterozoic (~580 – 542 Ma).   Detailed analysis of carbonate d¹³C reveals multiple positive and negative excursions including a globally-extensive negative excursion of >13‰ recorded within the Shuram Formation.  This “Shuram” excursion extends through >500m of stratigraphic section in Oman.  Sulfate d³⁴S remained relatively invariant throughout deposition of the Shuram, indicating that the oxidation of depleted carbon responsible for the Shuram d¹³C excursion was not accompanied by significant oxidation of sulfide.  Analysis of coupled organic carbon and carbonate d¹³C reveals non-steady state carbon cycling in which organic d¹³C is buffered from changes in coexisting carbonate d¹³C.  In this case, the Shuram excursion may be explained by the oxidation of a vast reservoir of dissolved organic carbon in the deep ocean.  Following the Shuram excursion, paired organic and carbonate d¹³C indicate that the carbon cycle evolved to a quasi-steady-state.  At the time of this transition, sulfate d³⁴S increased dramatically from ~23‰ to a maximum of ~42‰ at the Precambrian/Cambrian boundary.  Pyrite d³⁴S suggests the increase in sulfate d³⁴S results from an increase in the fractionation between sulfate and sulfide, possibly coupled with increased pyrite burial.  The earliest Cambrian strata in Oman record a decrease in sulfate d³⁴S to ~39‰.  This decline, which follows the Precambrian/Cambrian boundary, reflects a progressive increase in the concentration of seawater sulfate as oceanic oxygen levels increased.  Together these data indicate a stepwise reorganization of the global carbon and sulfur biogeochemical cycles in the terminal Neoproterozoic and the ventilation of the deep ocean.