REDOX CONDITIONS OF TERMINAL EDIACARAN AND EARLY CAMBRIAN LAGERSTÄTTEN IN RUSSIA

Redox geochemical measurements were performed on shale samples from two localities in the Russian Federation, the Cambrian Stage 2 Chuskuna Formation in the Olenek Uplift southwest of the Lena Delta in arctic Siberia, and the Ediacaran Zimnie Gory and Erga formations in the White Sea region. The Chuskuna Formation, which is 20 million years older than the Burgess Shale, contains a remarkable lagerstätte (deposit of extraordinary fossils with exceptional preservation) of fossilized organic cuticles representing as many as 15 separate animal clades, while the White Sea shale contains exceptionally preserved Ediacaran fossils of the globally-distributed White Sea assemblage. To understand the redox conditions that allowed for exceptional preservation of fossils, sample powders were acidified to determine the % carbonate and sequentially leached to determine iron abundance of carbonate, oxide, magnetite, and poorly reactive silicate fractions. Yields of sulfur in the form of Ag₂S from CRS (chromium reducible sulfur) extraction provided the iron abundance of the sulfide fraction of samples. Iron speciation data serves to estimate local redox conditions of the sediments and water column at the time of shale formation. For the Chuskuna Formation, thallium (Tl) isotope ratio measurements were collected to provide information about global redox conditions at the time of formation. In both shale localities, iron speciation reveals low levels of iron in authigenic carbonate, oxide, and magnetite phases, initially suggesting oxic depositional conditions. The measured iron in the poorly reactive silicate phases is high, but not enough to account for the majority of iron in the sediments, suggesting that most of the iron is detrital in origin. Observations of glauconite in thin sections of the Chuskuna supports anoxic or dysoxic conditions, and near-crustal Tl isotope compositions suggest global seawater anoxia or euxinia. The lack of discernible redox-sensitive metal enrichment in this Cambrian Stage 2 shale is either reflective of: 1) local oxic conditions during deposition, which is inconsistent with the expectation that the sediments would likely need to be anoxic to preserve the fossil cuticles, or 2) the long-term depletion of metals from the oceanic inventory due to prolonged global euxinia through the terminal Ediacaran and basal Cambrian intervals.