The carbon and oxygen isotopic compositions of some Early Neoproterozoic (?early Cryogenian) successions on the Kola Peninsula, northwest Russia {specifically the Sredni and the Rybachi Peninsulas and Kildin Island, Murmansk Coast, and the Chapoma River, Tiersky Coast) have been studied. Organic carbon isotope data were obtained from all investigated rocks, whereas carbonate carbon data, with one exception, were acquired solely from the Kildin Island and the Tiersky Coast. Critical evaluation of the organic carbon preservation showed that approximately two-thirds of the samples contained isotopically altered organic matter. The ?¹³C_org values regarded as refiecting primary depositional values range between -33.4 and -28‰. Various approaches aimed at assessing the signal quality in terms of retention of the primary signal of 13 investigated carbonates indicate that only three carbonates provided a close-to-primary signal. From the Kildinskaya Group on Kildin Island, the least ¹³C-depleted sample yielded ?¹³C_carb = +0.7‰, a value interpreted to reflect the primary depositional value of sea water at the time of deposition of the group.

Joakim Samuelsson, Institute of Earth Sciences, Micropalaeontology, Norbyvägen 22, S-752 36 Uppsala, Sweden.
Present address: Department of Geology and Pedology, Kriegslaan 28158, B-9000 Gent, Belgium