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.
Sampling and Pretreatment of Tooth Enamel Carbonate for Stable Carbon and Oxygen Isotope Analysis
