An imprint of astronomical climate forcing in the Baltoscandian MiddleOrdovician ‘orthoceratite limestone’?

The Middle Ordovician (∼460-470 Ma) in Baltoscandia is mainly represented by the so called ‘orthoceratite lime-stone’. This is a geographically wide-spread condensed sedimentary succession of a few tens of meters thicknessconsisting of fine-grained limestones deposited at middle paleolatitudes. These successions have a long history ofdetailed litho- and biostratigraphical research, which in recent years is complemented with chemostratigraphicalinvestigations (mainly stable carbon isotopes). New mm-resolution XRF elemental core scanning data from theTingskullen-1 and Kårehamn P4 cores from the island of Öland, Sweden, reveal the potential to further test thehypothesis of the presence of Milankovitch scale cyclicity in these records. This new high-resolution data alsoresolves the regular occurrence of hardgrounds in this ‘orthoceratite limestone’, as for example expressed in min-ima in carbonate content and peaks in detrital elements. These dissolution surfaces have been suggested to berelated to carbonate production-dissolution alternations. It is interesting that many of the relative changes in theelemental profiles seem to be related to these dissolution horizons and show a certain regularity in occurrence ondifferent spatial levels. The ratio between the different periods, in combination with specific patterns of amplitudemodulation - both visually and statistically inspected - suggests the presence of a precessional scale variability,bundled in eccentricity cycles. Besides the existing integrated stratigraphical framework, these interpretations canbe tested by ongoing geochronological work (CA-TIMS U-Pb dating of zircons from volcanic ash layers present inthese successions). The prospective of building an astronomical timescale for the Middle Ordovician can shed newlight on its climate dynamics and associated tempo and duration of important events such as the Great OrdovicianBiodiversification Event (GOBE) and the middle Darriwilian carbon isotope excursion (MDICE).