A LATE CAMBRIAN OCEAN ANOXIC EVENT

The geologic record contains many large, often globally expressed positive carbon isotope excursions recorded in carbonate rocks. In the Mesozoic these excursions are easily linked to organic-rich deposits formed from enhanced carbon burial under ocean-scale anoxia –i.e., oceanic anoxic events (OAEs). Importantly, voluminous organic carbon and pyrite burial in anoxic settings can be a central player in modulating the amount of oxygen and carbon dioxide in the atmosphere, and many of Earth's major biological extinctions are coeval with ocean-scale anoxia.

In the Paleozoic, physical records of organic carbon burial are scarcer, and we are left with only carbon isotope records, which can be interpreted in multiple ways. These records become less ambiguous when viewed in light of coeval seawater sulfur isotope trends. High-resolution carbonate-C and sulfate-S isotope records of the Late Cambrian SPICE event reveal parallel, positive excursions suggesting enhanced organic C and pyrite S burial. Comparison of the SPICE to isotope data from the Toarcian OAE, including geochemical box modeling, leads us to conclude that the SPICE Event is a prime candidate for an early Paleozoic OAE.

Additional evidence for increased ocean anoxia coincident with the SPICE comes from the Alum Shale of Sweden. Organic carbon isotope data record the SPICE, and iron-sulfur proxies show that the Alum was persistently euxinic before, during and after the event. Consistent with the OAE hypothesis, molybdenum concentrations show muted enrichment during the SPICE, with significant [Mo] occurring at end of the event. These data suggest a depleted seawater Mo inventory associated with a greatly expanded euxinic sink during the SPICE.

An interesting result from geochemical box modeling of the SPICE and Toarcian records is the suggestion of large-scale oxidation of ³⁴S-depleted sulfur at the end of both events. A likely source of this sulfur is the oxidation of destabilized euxinic water masses. The driver of this oxidation was most likely increasing atmospheric pO₂ in conjunction with lowered pCO₂—the result of the enhanced organic carbon and pyrite burial that marked the events. Lowered pCO₂ allowed lower global temperatures, invigorated ocean circulation and thus the delivery of O₂ to the deep ocean leading to the demise of each anoxic event.