A CHROMIUM ISOTOPE PERSPECTIVE ON BURGESS-SHALE-TYPE PRESERVATION

Burgess-Shale-type preservation, exceptional preservation of soft-bodied organisms as carbonaceous compressions, is most widely observed in the Late Ediacaran to Middle Cambrian, largely disappearing thereafter. Conflicting geochemical evidence has argued for both oxic and anoxic depositional conditions ­– there are no enrichments in redox sensitive elements Mo or U and iron speciation is frequently ambiguous. There may be Cr and V enrichments in these samples, but potential for variability in detrital inputs of these elements makes tracking these enrichments based on concentration data alone ambiguous. Therefore, the nature of environmental conditions surrounding these enigmatic assemblages has remained the subject of debate. Herein, we present Cr isotope data from a suite of Burgess-shale-type deposits including the Burgess Shale, the Nuititang Formation, the Miaohe Konservat-Lagerstätte, and the Kaili Formation. We observe a highly variable and fractionated Cr isotope signal, indicative of the preservation of an authigenic enrichment resulting from a weakly reducing depositional environment. This enhanced understanding of the redox environment has significant implications from both a taphonomic perspective, as well as for the relationship between the paleoenvironment and the lifestyles of the observed biota.