IDENTIFYING PRECESSION AND OBLIQUITY CYCLES IN THE ORDOVICIAN?

The theory of astronomical climate forcing has become an essential part of our understanding of Earth’s climatic past. Deep-time studies of astronomical climate forcing are complicated by issues related to the preservation of the strata, the quality and availability of independent stratigraphic constraints, and uncertainties in astronomical solutions and parameters. Whereas the 405-kyr eccentricity cycle is believed to be a relatively stable astronomical metronome back in deep-time, larger uncertainties exist on the precession, obliquity and other eccentricity components as a result of tidal dissipation (Earth-Moon system) and chaotic transitions in the Solar System. As most of the insolation power is actually situated in the obliquity and precession bands – and not in the eccentricity band – the identification of individual precession and obliquity cycles does not only allow for more detailed timescale reconstructions but equally gives more insight in astronomically forced climate dynamics.

The first crucial step to reconstruct deep-time precession and obliquity cycles is a reliable identification. Such reconstructions are only possible in the presence of clearly astronomically forced signals in a well-constrained independent temporal framework. Here, we present two different Ordovician settings where we identify individual precession and obliquity cycles. The first example comes from the Middle Ordovician in Baltoscandia. Recent mm-resolution XRF elemental core scanning data serve as high-resolution cyclostratigraphic proxies in a detailed litho-; bio- and chemostratigraphic framework. This record suggests the presence of eccentricity amplitude modulated precession cycles in dissolution features in the so called “orthoceratite limestone”. The second example comes from the well-studied Upper Ordovician Anticosti sections in Laurentia. Regular multi-meter alternations between clay and shale rich intervals show an imprint of precession, obliquity and eccentricity. These studies pave the way for further reconstruction of Ordovician precession and obliquity cycle parameters. Such reconstructions are an example of how we can use the geological record to help to constrain the history of the Earth-Moon and Solar Systems.