12 Paleontology/Sedimentology/Stratigraphy

Thursday, 20 March 2008
8:00 AM - 12:00 PM

GEOCHEMICAL EVIDENCE OF ENHANCED OCEAN ANOXIA DURING THE LATE CAMBRIAN SPICE EVENT AND IMPLICATIONS FOR TRILOBITE BIOMERE EXTINCTIONS

Paper number: 12-3
Program: Technical Sessions
Time: 8:40 AM – 9:00 AM

Authors

Abstract

The Upper Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) was first described from carbonate sections in the Great Basin. Subsequent investigation of the SPICE has shown that this event records a perturbation to the global carbon cycle. We present carbonate-C, sulfate-S and pyrite-S records from sections from the Great Basin and other globally distributed sections that show parallel, positive isotope shifts suggesting enhanced organic C and pyrite S burial. Differences in the magnitude of sulfur isotope excursion between basins suggest a low and isotopically heterogeneous marine sulfate reservoir. This idea is supported by the Δ34S records, which show zero and negative values at the peak of the sulfur isotope excursion, which indicate a marine sulfate reservoir of less than <1mM.

Geochemical box modeling confirms that the SPICE excursions were caused by organic carbon and pyrite burial. An interesting result from geochemical box modeling of the SPICE record is the suggestion of large-scale oxidation of 34S-depleted sulfur at the end of the event. Possible sources of this sulfur are the oxidation of destabilized euxinic water masses or the weathering of shelf pyrites during a sea level drop coincident with the peak of the isotope excursions.

Comparison of the SPICE to similar isotope data from the Toarcian Ocean Anoxic Event (OAE), along with the results from geochemical box modeling of both events, lead us to conclude that the SPICE event is a prime candidate for an early Paleozoic OAE. Other evidence for increased anoxic deposition coincident with the SPICE comes from the Alum Shale of Sweden. Molybdenum concentrations show muted enrichment during the extent of the SPICE, despite data that show the basin was persistently euxinic before, during and after the event. Significant increases in molybdenum concentration occur only immediately after the event, suggesting a depleted seawater Mo inventory associated with a greatly expanded global anoxic Mo sink during the SPICE.