INVESTIGATING GEOCHEMISTRY OF THE WHEELER FORMATION OF UTAH, USA: INSIGHTS INTO THE REDOX ENVIRONMENT OF BURGESS SHALE-TYPE FOSSIL PRESERVATION

Burgess Shale-type preservation has given us an unprecedented window into the early evolution of animals. However there remains much speculation about the chemical environment that contributed to this type of fossil preservation. Here we will present a variety of geochemical measures — iron speciation, redox-sensitive trace metals, organic carbon contents, and the sulfur isotope composition of pyrite and sulfates — to explore the sedimentary redox setting of a Burgess Shale-type deposit: the Wheeler Formation of Utah, USA. The Wheeler Formation preserves a full spectrum of shallow to deep water sedimentary facies permitting us to integrate our geochemical data with the sedimentary and stratigraphic architecture and reconstruct the redox settings in this Cambrian sedimentary basin. Geochemical data from the shallow water facies of the Wheeler indicate deposition under an oxic water column and is consistent with the abundant evidence of a rich benthic biota. Pyrite iron to highly reactive iron ratios (Fe_py/Fe_HR) and degree of pyritization (DOP) values from deeper water facies hosting Burgess-type preservation reveal that these facies were not deposited under sulfide-rich, i.e. euxinic condition, but under anoxic and iron replete, or ferruginous conditions. The persistence of ferruginous marine waters — a state thought to be restricted to Archean and Proterozoic oceans — into the earliest Paleozoic may be one of the key environmental conditions that permitted Burgess Shale-type fossil preservation. If correct, then the closure of the Burgess preservation window later in the Ordovician may be linked to the demise of ferruginous marine waters.