CARBON ISOTOPE EVIDENCE FOR ENHANCED SURFACE OXIDATION FOLLOWING A POTENTIAL NEOPROTEROZOIC ICE AGE IN DEATH VALLEY, CALIFORNIA

Four carbon isotopic biogeochemical events are recorded in the Neoproterozoic sedimentary succession of Death Valley, California, two of which are associated with cap carbonate lithofacies above glacial diamictite.  Two others (at the base of the Beck Spring Dolomite and within the Rainstorm member, Johnnie Formation) have textural similarities with cap carbonates worldwide, but no glacial phenomena are observed. The Rainstorm event begins in the Johnnie oolite, a dolomite marker bed deposited on a sequence boundary in shallow, agitated open marine waters, and continues through a sequence of green shale and fine-grained micrite with needle-like seafloor cements.  In three separate sections, d¹³C values trend from –3 to –7‰ across the Johnnie oolite, then plummet to -11‰ in the overlying seafloor precipitates. The top of the unit is truncated by a major erosional surface.  Not including the oolite, this class of cap carbonate lithofacies is recognized in a number of examples worldwide.  Considering the carbon isotopic composition of riverine inputs, sustained d¹³C values of <-5‰ (a typical value for Neoproterozoic cap carbonates) are unlikely without an additional flux of ¹³C-depleted carbon to the depositional environment. A singular methane hydrate release as the source of the additional light carbon is possible, but seems unlikely given the long stratigraphic duration of the negative anomaly.  On the other hand, if the event were synchronous with an atmospheric buildup of oxygen, it is possible that exposure and oxidation of fossil organic matter in platform sediments could provide a prolonged flux of light carbon to shallow marine environments. A similar carbon isotope trend is noted in the broadly correlative Wonoka Formation of Australia (in possibly restricted conditions) and across the Krol B/C transition in the Lesser Himalayas of India (in demonstrably open marine conditions).  This hypothesis is consistent with model results suggesting a rapid buildup of oxygen during the terminal Neoproterozoic associated with high rates of sedimentation and organic carbon burial associated with Pan African orogenesis.