Stable carbon and oxygen isotope geochemistry of the upper Visingso Group (early Neoproterozoic), southern Sweden

A newly measured and sampled section in the upper part of the early Neoprot erozoic Visingso Group, south central Sweden, has been investigated for its stable carbon and oxygen isotope geochemistry. The 30 m-thick succession c rops out on the southeastern shore of Lake Vattern, close to the village of Boeryd, and consists mainly of black mudstones with carbonate lenses, some of which have been dolomitized. High H/C ratios of the kerogen preserved i n the investigated succession indicate that organic carbon generally is wel l preserved. The observed delta(13)C(org) values are comparable to previous ly observed Neoproterozoic organic carbon values, with the exception of a f ew exceptionally C-13-depleted kerogens, with delta(13)C(org) greater than or equal to-41.1%. The latter are interpreted to be the result of fermentat ive bacterial reworking of sedimentary organic matter. The Boeryd carbonate s display an unusual array of heavy delta(18)O values (with delta(18)O less than or equal to+ 3.3%) that are interpreted as reflecting deposition in a n arid climate. Geochemical data indicate that the carbonates have been dia genetically altered, but only to a limited extent, and a range of delta(13) C(carb) values from +2.4 to +4.7% appears to be representative of the prima ry seawater composition for the time of deposition of the upper Visingso un it. Positive carbonate isotope values are typical for lower Neoproterozoic, pre-Varangerian successions elsewhere. The C isotope values obtained from the Boeryd section, although not conclusive by themselves, are consistent w ith the formerly biostratigraphically and radiometrically inferred early Ne oproterozoic age for the upper Visingso Group.