Orbital control on cyclical primary productivity and benthic anoxia: Astronomical tuning of the Telychian Stage (Early Silurian)

The identification of Milankovitch cycles and the establishment of a high-resolution astronomical time scale for the Palaeozoic represent a research frontier in cyclostratigraphy and astrochronology. A continuous core drilled through the Telychian Stage (Llandovery, Silurian) of the Pasłek Formation in the Baltic Basin (Poland) is characterized by repetitive centimetre-scale alternations of homogeneous greenish-grey mudstones and faintly laminated dark-grey-to-black mudstones. Such lithological rhythmicity reflects a cyclical variation in redox conditions at the sediment-water interface, and therefore was used to construct a floating astronomical time scale (ATS) for the Telychian Stage. The cyclostratigraphic analysis was performed on a 5 mm resolution greyscale scan based upon photographs of the slabbed core, with lower values associated with greenish-grey mudstones and higher values related to dark-grey-to-black mudstone facies. Spectral density estimation by means of the multitaper method (MTM) reveals significant peaks rising above the 95% red noise confidence level that we interpret as the 405-kyr long-eccentricity, short-eccentricity, obliquity and precession components. The MTM evolutive power spectral analysis (EPSA) shows a gradual increase in the velocity of sedimentation from 5.03 m/Myr in the lowermost interval to 5.48 m/Myr towards the uppermost part of the studied sequence. We postulate that the observed cycles reflect orbitally-driven climatic variations from stable wet conditions to monsoon-like high seasonal contrasts that affected weathering intensity, runoff and nutrient supply. These cyclical variations led to rhythmic variations in organic matter fluxes and benthic anoxic conditions. In the analysed record, orbital precession influenced the deposition of the greenish-grey mudstone and dark-grey-to-black mudstone couplets, occurring on average every 10 cm. Furthermore, long-eccentricity modulation of precession controlled the relative depositional predominance of one facies over the other. Observed periodicities resemble those of Cenozoic and Mesozoic orbitally controlled records, thus suggesting that during the Silurian the orbital eccentricity forcing on the carbon flux acted in the same way as in the Cenozoic and Mesozoic. The orbital tuning of the studied interval, calibrated by means of accurate acritarch biostratigraphy, allowed for the estimation of an astronomically calibrated duration for the Telychian Stage of about 5.46 Myr.