THE REDOX FRAMEWORK OF PRECAMBRIAN-CAMBRIAN TRANSITION IN THE ARCTIC SIBERIA

That there are frequent oscillations of marine redox signature across the Ediacaran–Cambrian (E-C) boundary interval has long been appreciated, but the controlling mechanisms and the casual relationships with the coeval ecological and evolutionary changes remain obscure. We studied iron speciation and paleoenvironmental proxies for the E–C transition represented by the calcareous sandstones and sandy calcimudstones interbedded with thin shales and siltstones of the Syhargalakh Formation (SF) and the siltstones-shale couplets interbedded with fine-grained sandstones of the Mattaia Formation (MF) of the Olenek Uplift, NE Siberia. The iron speciation analysis through the boundary interval shows a distinct trend from a ferruginous water column in distal facies (SM) towards a dysoxic/oxic one in proximal facies (MF). The uppermost SF is also marked by an increase in P/Al, Ba/Al, and Cd/Al suggesting higher bioproductivity, as well as an increase in U/Al and U/Th indicating the expansion of anoxic conditions within the sediment. Furthermore, the same interval shows higher Ni/Al, Cu/Al, and Zn/Al values associated with high micronutrient concentrations. The paleoenvironmental proxies distribution is here interpreted as the evidence for an increase in nutrient input boosting bioproductivity and a rise in oxygen demand leading to anoxia in the sediment and/or bottom waters, which is in agreement with the iron speciation data for the uppermost SF. In contrast to the SF, the overlying lowermost MF is characterized by lower P/Al, Ba/Al, Cd/Al, U/Al, and U/Th values suggesting the apparent lack of anoxia, again in agreement with the iron speciation. Coincidently, the SF/MF boundary is marked by the first appearance of Treptichnus pedum in the section, the index-ichnotaxon for the Cambrian GSSP. Whether or not T. pedum can be reliably used for correlation of the E-C boundary, the first appearance of this trace fossil in NE Siberia is correlated with the marine redox facies change towards the oxic conditions. A reliable redox framework is needed for the E–C boundary interval to reconstruct the redox landscape of Ediacaran and Cambrian evolutionary events.

The research was funded by RFBR # 20-35-70016, and government contract 0331-2019-0002 (Ministry of Education and Science of the Russian Federation).