CHROMIUM ISOTOPE AND MICROSTRATIGRAPHIC LASER ABLATION ANALYSIS OF THE BURGESS SHALE FORMATION: NEW IMPLICATIONS FOR THE PALEOREDOX CONDITIONS OF THE CAMBRIAN LAGERSTÄTTE

The middle Cambrian Burgess Shale Formation, British Columbia, has contributed substantially to our understanding of the evolutionary paleobiology and paleoecology associated with the Cambrian explosion and its aftermath. The paleoredox conditions of the Burgess Shale environment are therefore of significant importance to our understanding of both the habitat and subsequent geological preservation of early Paleozoic communities, however debate about the impact of post-lithification metamorphism on the reliability of sedimentary geochemical signals from the Burgess Shale continues. Here we present bulk geochemical and laser-ablation ICP-MS analyses of Burgess Shale rocks from the Walcott Quarry, Raymond Quarry and Campsite Cliff members, all of which bear exceptional carbonaceous compression fossils. Traditional redox-sensitive trace metals (U, Mo, V) do not show significant elemental enrichments relative to average shale values. Microscale laser-ablation profiles of redox-sensitive trace metal concentrations illustrate that concentrations of U, Mo and V are consistently below average shale values across millimetre-scale bedding planes. Rare concentration peaks do not correlate among traditionally redox sensitive elements, suggesting that enrichment-based trace metal proxies are in themselves inadequate paleoredox indicators for the Burgess Shale. However, chromium isotopes exhibit significant fractionations from crustal values and significant within-member variation, with similar 𝛿⁵³Cr ranges in each member studied, suggesting authigenic chromium enrichment throughout the Burgess Shale Formation. A large range of 𝛿⁵³Cr values suggests significant authigenic chromium enrichments and partial reduction of chromium during aqueous phase diagenesis. These features combined are most consistent with predominantly ‘suboxic’ redox conditions, allowing for chromium enrichments but preventing significant enrichments in other redox sensitive metals that become scavenged in more reducing conditions.