THE SULFUR ISOTOPIC EVOLUTION OF NEOPROTEROZOIC SEAWATER SULFATE

We have analyzed the sulfur isotopic composition of trace sulfate extracted from Neoproterozoic carbonates collected in Namibia, South Australia and Death Valley, CA, in order to construct the isotopic evolution of Neoproterozoic seawater sulfate.  Biogeochemical processes impose significant and predictable isotopic fractionations on sulfur species as they are cycled between oxidized and reduced forms. Consequently, the isotopic composition of sedimentary sulfates are sensitive indicators of rates of these processes at a global scale.  Previous secular d³⁴S_sulfate trends for the Neoproterozoic have been developed using rarely occurring evaporitic gypsum and barite deposits that provide only a low-resolution glimpse of sulfate evolution.  Carbonates are significantly more abundant in the Neoproterozoic than sedimentary sulfate deposits and therefore can provide a relatively continuous marine sulfate sulfur isotopic signal that may be utilized to assess environmental change.  The nearly pure carbonates contain 0 to 300 ppm sulfate.

d³⁴S_sulfate and d¹³C_carbonate values of Neoproterozoic carbonates are relatively stable until about 850 Ma, with d³⁴S values ranging from 10-25‰ and d¹³C values near 0‰.  By contrast, the mid to late Neoproterozoic (850–550 Ma) is characterized by rising d³⁴S values reaching 50‰ with high-amplitude short-term variation between 10‰ and 40‰.  Additionally, d¹³C values rise to 8‰ with sharp minima.  We suggest that weathering inputs of sulfate and inorganic carbon were high during the late Mesoproterozoic and early Neoproterozoic, with an anoxic-sulfidic deep ocean supporting a moderate pyrite burial rate.  By contrast, most of the late Neoproterozoic was characterized by low weathering rates and high burial ratios of pyrite/total sulfur and organic carbon/total carbon, but low burial rates.

There are no definitive trends in d³⁴S_{sulfate-pyrite} through this period, but a minimum of 0‰ as late as 580 Ma may indicate low sulfate concentrations and near total reduction of oceanic sulfate to sulfide.  The available data cannot support or refute the proposed development of a non-photosynthetic sulfide-oxidizing biota at 800 Ma.