GEOCHEMICAL EVIDENCE FOR SHIFTING REDOX CONDITIONS WITHIN FOSSILIFEROUS STRATA OF THE WHEELER FORMATION, UTAH

Determining the physical and chemical conditions that may result in preservation of Burgess-Shale-type (BST) fossils is an ongoing line of inquiry. Relatively simple models involving sustained anoxia to arrest decay have been replaced by more complex environmental models involving hypersaline pore fluids below an oxygenated water column (Burgess Shale) and fluctuating oxyclines (Wheeler Shale). Paleo-redox conditions at the time of deposition for each of these models is distinct: oxic conditions associated with hypersalinity models and variable to intermediate oxygen levels associated with fluctuating oxycline models. Redox-sensitive trace metals have been studied at fossil localities in the Burgess Shale and Kinzers formations, both indicating oxic conditions in association with BST. Trace metal analysis correlated to secondary physical evidence within the Burgess and Kinzers has determined that Ni/Co and V/Sc provide the clearest signal in these Cambrian shales. Accordingly, these geochemical proxies were applied to samples from the Wheeler Shale of Western Utah, in which fossil evidence has suggested a fluctuating oxycline during deposition.

Forty-five samples from three fossiliferous localities in the Wheeler Shale were analyzed. Each sample contained one of three morphologically simple fossils that have been interpreted as algal in orgin: 1) approximately circular fossils up to 4mm in diameter (Morania fragmenta); 2) non-branching linear fossils up to 1cm long by 0.5mm wide; and 3) filamentous fossils. Three additional non-BST bearing samples from a known oxygenated layer (Asaphiscus zone) were also analyzed. BST-bearing layers yield geochemical proxies that indicate redox conditions ranging from anoxic to oxic. The Ni/Co and V/Sc ranges for layers bearing each of the three fossil types overlap. Asaphiscus Zone samples yielded compositions indicative of oxic to dysoxic conditions. These findings are consistent with the shifting oxycline model for Wheeler Shale, and that preservation of BST occurred in multiple environments. Further, the agreement of geochemical proxies with field evidence at the Burgess and Wheeler sites suggests that this method can be effective in determining paleo-redox conditions in BST fossil beds.