A GLACIAL DIVIDE BETWEEN EDIACARAN EXTINCTION AND THE CAMBRIAN EXPLOSION OF LIFE

Complex Ediacara organisms lived in a short interval between two profound biogeochemical anomalies. During these events, at the Ediacaran-Cambrian boundary and the Shuram excursion, δ¹³C values plunge to -10‰ or more. While the Shuram perturbation is related to significant sea level fall, the Ediacaran-Cambrian boundary excursion has not been related to glaciation. In arctic Siberia along the Khorbusuonka River and its tributaries, a thick diamictite of glacial origin is sandwiched between Turkut Formation carbonates, which contain the boundary excursion, and the Kessyusa Formation, which has abundant Early Cambrian trace and small shelly fossils. The freshly-exposed glacial diamictite has a green-grey sandy to clayey calcareous matrix with abundant ice-rafted debris in a weakly stratified pile. In weathered outcrop the matrix becomes orange in color. The randomly-oriented clasts range from granules to boulders mostly derived from the Turkut and underlying Khatyspyt formations, but they also consist of shale, occasional green igneous rocks, and metamorphic rocks of exotic origins. Sub-rounded clasts are often faceted although no striations were observed. To expose Khatyspyt lithologies base level must have fallen at least 100 meters during the ice age. The diamictite is abruptly truncated by ferruginous shale with sub-rounded cobbles likely deposited during post-glacial transgression. This unit is overlain by a fluvial pebbly conglomerate and then siltstones bearing abundant animal traces. The glacial interpretation provides an explanation for the boundary anomaly and the demise of the Ediacara biota through drawdown of sea level and enhanced upwelling of H₂S and HCO₃^- charged waters produced through sulfate reduction. The ecospace vacated by the Ediacara biota was re-populated with Cambrian faunas in the glacial aftermath, in likely response to rising oxygen.