Evidence for anoxia at the Ediacaran–Cambrian boundary: the record of redox-sensitive trace elements and rare earth elements in Oman

The Ediacaran–Cambrian boundary in Oman is characterized by a globally correlative negative Ccarb excursion in platform carbonate rocks, and deposition of basinal organic-rich rocks. These observations may relate to the development of local, and possibly global, anoxia. We have studied redoxsensitive trace elements (TE) and REE from basinal and platform successions. Detrital input and authigenic element enrichment were the main factors controlling the record of TE and REE. Trace elements in the basin are enriched relative to crustal concentrations. In siliceous rocks, TE enrichment was decoupled from detrital input, suggesting authigenic concentration of TE under anoxic (possibly sulphidic) water column conditions. Trace elements and REE indicate higher detrital input in shales. Element enrichment was minimally influenced by detrital input in the platform carbonate rocks; it is strongest in the basal platform section, coincident with the carbon isotope excursion. The results suggest development of anoxia in a stagnant basinal water mass. Overturn or upward expansion of the deep water charged with TE and isotopically light C produced anoxia on the platform, and was probably related to development of the isotope excursion. These results are consistent with anoxia in boundary strata of Iran, and with the hypothesis of global anoxia at this time. The Ediacaran–Cambrian transition is a key period for the understanding of the evolution of life and its relationship to biogeochemical changes. A large-magnitude negative carbon isotope excursion has long been recognized in close proximity to the Ediacaran–Cambrian boundary (e.g. Grotzinger et al. 1995; Bartley et al. 1998; Walter et al. 2000; Amthor et al. 2003). The excursion was preceded by a prolonged period with high burial rates of organic material (Grotzinger et al. 1995; Saylor et al. 1998). Burial led to C enrichment in surface waters, as recorded in carbonate rocks, and caused accumulation of isotopically light CO2 in basins (e.g. Knoll et al. 1996; Holser 1997a; Kimura et al. 1997). At the same time, heavy S in sulphate rocks indicates S enrichment in surface waters and burial of reduced sulphur in organic-rich basin sediments (e.g. Walter et al. 2000; Schröder et al. 2004). These observations led previous workers to suggest anoxia for several Ediacaran– Cambrian boundary sections (e.g. Walter et al. 2000; Kimura & Watanabe 2001), but direct studies of redox conditions in this time interval are sparse. Overturn or simple expansion of an anoxic water mass could have delivered isotopically light CO2 to shallow platform areas, thus affecting the shallow-water carbon isotope record (Knoll et al. 1996; Kimura et al. 1997; Bartley et al. 1998). However, the global or local nature of such conditions is still poorly constrained. Firm radiometric ages and suitable facies development are major requirements for this kind of study. In the Ara Group of Oman, a negative carbon isotope excursion occurs in uraniumenriched platform carbonate rocks, which correlate with organicrich shales and siliceous rocks (Mattes & Conway Morris 1990; Amthor et al. 2003). Organic biomarkers for chemoautotrophic bacteria, water stratification, and heavy S in platform evaporites suggest anoxic conditions in the basin (Grantham et al. 1987; Schröder et al. 2004; Amthor et al. 2005). The good age constraints for this succession (Amthor et al. 2003) and the availability of excellent sample material from hydrocarbon exploration wells allow a detailed study of redox conditions, environmental differences in platform and basin strata, and of how detrital input may have diluted the authigenic record of redox conditions (see Piper 1994). Samples along a platform-tobasin transect were selected for the analysis of redox-sensitive trace elements (TE) and REE. The results from this study are discussed, followed by a reconstruction of redox conditions at the time of deposition.