An episode of remarkable biotic, climatic, sea-level and facies
changes took place during
the late Viruan (Caradoc) epoch in the Baltoscandian area. We studied the
carbon isotopic composition
of carbonate sediments from this period. Data on the stable carbon isotopic
composition
of whole-rock carbonates from three south Estonian core sections, together
with those on ostracode,
distribution are presented. In two core sections, a positive δ13C
shift of 2 ‰ was revealed in the upper
part of the Keila Stage (mid-Caradoc). The comparison of isotope and ostracode
data in the sections
suggested the occurrence of a gap of late Keilan age in the marginal area
of the basin equivalent to the
North Estonian Confacies Belt. The beginning of the excursion approximately
coincided with climatic
and sea-level changes in the shallow shelf area. The peak of the late Keilan
excursion preceded the
biotic crises and maximum black shale accumulation in the Baltoscandian
palaeobasin. An approximately
synchronous carbon isotopic event has been reported from North America,
referring to a possible
global oceanographic event in the Caradoc epoch. Although the positive
carbon isotopic
excursion and related environmental events of the late Keilan age have
some unique features, they
show more similarities to the end-Ordovician and Silurian events, characterized
by oceanic change
from stratified state to thermohaline circulation state, than to warm anoxic
events related to eustatic
sea-level rise.