Upper Ordovician Astronomical Climate Forcing from the Vauréal Formation, Anticosti Island, East Canada

The early Paleozoic paleotropical carbonate succession exposed on Anticosti Island in the Gulf of the St. Lawrence,
Canada, is one of the most complete, thickest, most richly fossiliferous and diagenetically unaltered carbonate sections
in the world across the Ordovician/Silurian boundary. Our cyclostratigraphic study focused on the Katian
Vauréal Formation using data from continuously exposed coastal sections and complete stratigraphic drill cores.
The high-resolution (dm-scale) proxy data include natural gamma ray profiles, pXRF measurements and bulk
carbonate stable isotope values of oxygen (18O) and carbon (13C). Time-series analysis of the proxy series
demonstrates meter-scale periodicities in lithological alternations (carbonate versus clay content), which are hypothesized
to result from sea-level variations. Similar periodicity patterns can be seen in the stable isotope data.
According to the available age constraints, the observed periodicities could be related to astronomical periods. The
working hypothesis is that astronomical changes in insolation were driving sea-level variations by the waxing and
waning of the Late Ordovician ice sheets. This hypothesis is also supported by a similar signal in the 18O record.
Our results demonstrate the potential for constructing a high-resolution (10^4 yr) age model for the Vauréal Formation
as well as for the younger units exposed on Anticosti Island. Such an astronomically based age model and
corresponding climatic interpretations should shed more light on the dynamics of the Late Ordovician glaciations
and the mass extinction event.