Cyclostratigraphic insights into Silurian climate dynamics - time scale, astronomical forcing, and climatic events during the Silurian

The Silurian Period (443 to 419 million years ago) represents a critical interval in Earth's
history, marked by major climatic shifts, repeated biogeochemical events, and substantial
carbon isotope excursions. Yet, the Silurian remains relatively poorly understood due to a lack
of high-resolution (cyclo)stratigraphic frameworks. The result is some of the largest
chronological uncertainties of the entire Phanerozoic timescale. To address these issues,
stratigraphic correlation, cyclostratigraphy, and radiometric dating were integrated to improve
the spatial and temporal resolution of Silurian successions. The results provide increased
fidelity into the Silurian, allowing us to study the role that astronomical cycles might have
played in pacing Silurian biogeochemical events.
To get better spatial constraints on Silurian successions, we developed a new stratigraphic
correlation framework for Gotland's subsurface based on a set of gamma-ray logs. The
combination of Dynamic Time Warping and Barycenter Averaging techniques enabled a semi-automated correlation of the well-logs. The resulting correlation enabled the identification of
major biogeochemical events in the subsurface and contributed to refining the stratigraphic
architecture of Gotland across a ~60-kilometre transect.
To improve temporal constraints and understand the role of astronomical pacing, the type
Silurian Cellon section in the Carnic Alps of Austria, spanning the Ludlow to Pridoli intervals,
was studied. High-resolution pXRF proxy records and induration patterns from this section
were processed using the newly developed WaverideR R package, allowing us to uncover the
imprint of astronomical cycles, enabling the tracking period (m) of the 405-kyr eccentricity
cycle in continuous wavelet transform scalograms. Monte Carlo simulations incorporating
external age constraints were then used to generate an astrochronologically constrained
numerical age model. The age model yields new numerical ages for Silurian stage boundaries
and conodont zonations. The cyclostratigraphic study also shows that astronomical cycles
could have paced Linde, Klev, and Silurian-Devonian boundary events, since these events all
occur after a 2.4-Myr eccentricity minima. In contrast, the Lau event appears to be decoupled
from this pacing.
To establish more precise numerical age constraints and evaluate the role of astronomical
forcing during the Silurian, we developed a Monte Carlo-based model that integrates
astrochronological constraints, floating astrochronologies, and Bayesian U-Pb age-depth
models. This model was applied to the Telychian-Homerian interval of the Altajme core from
Gotland, Sweden. The new method reduced age uncertainties by an average of 62%
compared to using the Bayesian U-Pb Bchron model alone. The model produced refined ages
and durations for the Ireviken and Mulde biogeochemical events and the Llandovery
Wenlock boundary. The age model also indicates that the maximum rate of change in the
δ¹³Ccarb record reached 0.025‰/kyr and 0.045‰/kyr for the Ireviken and Mulde
biogeochemical events, respectively. The age-calibrated proxy records show the imprint of
astronomical cycles, indicating that the Ireviken and Mulde events could have been paced,
since both events occur after a 2.4-Myr eccentricity minimum.
Together, these case studies demonstrate that the integration of astrochronology, U-Pb
geochronology, and stratigraphic correlation provides a powerful tool for improving Silurian
chronostratigraphy. They further support the hypothesis that long-period astronomical cycles
could have paced Silurian biogeochemical events.