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

Abstract

A newly measured and sampled section in the upper part of the early Neoproterozoic Visingsö Group, south central Sweden, has been investigated for its stable carbon and oxygen isotope geochemistry. The 30 m-thick succession crops out on the southeastern shore of Lake Vättern, 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 in the investigated succession indicate that organic carbon generally is well preserved. The observed δ13Corg values are comparable to previously observed Neoproterozoic organic carbon values, with the exception of a few exceptionally 13C-depleted kerogens, with δ13Corg[ges ]−41.1 ‰. The latter are interpreted to be the result of fermentative bacterial reworking of sedimentary organic matter. The Boeryd carbonates display an unusual array of heavy δ18O values (with δ18O[les ]+3.3 ‰) that are interpreted as reflecting deposition in an arid climate. Geochemical data indicate that the carbonates have been diagenetically altered, but only to a limited extent, and a range of δ13Ccarb values from +2.4 to +4.7 ‰ appears to be representative of the primary seawater composition for the time of deposition of the upper Visingsö unit. 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 with the formerly biostratigraphically and radiometrically inferred early Neoproterozoic age for the upper Visingsö Group.