The falling amplitude of carbon isotopic oscillations through the Lower to Middle Cambrian: northern Siberia data

Abstract

Carbon and oxygen isotopes have been studied in Lower to Middle Cambrian fossiliferous carbonates from sections along the Malaya Kuonamka, Olenek, and Muna rivers of the Yudoma-Olenek facies belt, northern Siberian Platform. Marked temporal oscillations in delta 13 C are recorded from shallow-water Nemakit-Daldynian to Atdabanian carbonates, as seen elsewhere on the platform. Rapid sea-level rise during the so-called "Sinsk event" led to starved-basin conditions from the middle Botomian (Bergeroniellus micmacciformis Zone) to late Amgan (Tomagnostus fissus Zone) and to deposition of organic-rich argillaceous carbonates of the Kuonamka Formation. Mayan carbonates were laid down during a phase of renewed and more rapid carbonate sedimentation, and relatively uniform delta 13 C signatures fall from approx. +1.5omicron in the Anomocarioides limbataeformis Zone to near zero throughout the Lejopyge laevigata Zone. These results confirm that a long-term decline took place in maximal delta 13 C values between the late Neoproterozoic and Middle Cambrian, accompanied by a dampening in the amplitude of shorter term oscillations. This was accompanied by a change from rift to drift patterns of crustal subsidence; from low to high sea levels; from an interior basin to outer platform locus for hydrocarbon source rocks; from rare to abundant metazoans in the biosphere; by the first major extinction in the Phanerozoic fossil record; and from "icehouse" to "greenhouse" conditions. We infer that factors leading to changes in the rate of carbon burial were amplified at times of low sea level, high climatic extremes, and a primitive biosphere (late Neoproterozoic), and dampened at times of high sea level, lower climatic variation, and a more advanced biosphere (Middle Cambrian).