Astronomical Pacing of Early to Middle Devonian Anoxic Events at Multi-myr Timescales (oued Ferkla, Se Morocco)

The Devonian was marked by numerous ocean anoxic events, many of which are associated with sea-level fluctuations, carbon-cycle perturbations, and faunal turnovers. Yet the mechanisms driving these events and the factors controlling their recurrence remain poorly understood and strongly debated. Growing evidence suggests that astronomical forcing influenced the timing and pacing of Late Devonian anoxic events, most notably through the ~100‑kyr eccentricity (e.g., Kellwasser Crisis) and ~2.4‑Myr grand‑eccentricity (e.g., Annulata, Dasberg, Hangenberg events) Milankovitch cycles. Earlier events, however, remain largely understudied despite their importance in contributing to protracted environmental stress and their resemblance to later, more severe extinction events. We present evidence from the Early to Middle Devonian Oued Ferkla section, which spans the global Daleje, Choteč, Kačák, and pumilio events. New high‑resolution geochemical analyses (XRF) show strong variability in total detrital input and redox-sensitive elements, suggesting distinct shifts in the hydrological cycle and depositional environment. Cyclostratigraphic analysis of the detrital signal reveals strong astronomical control by precession, obliquity, and eccentricity, confirming visual identification of lithological patterns identified on the field. An ~18-Myr floating astrochronology was constructed by tuning to the 405-kyr long eccentricity metronome and placed into global context using δ13Ccarb chemostratigraphy. A conspicuous ~6-Myr cycle was identified that appears to exert primary control on the timing of the anoxic events investigated here. While the Daleje, Choteč, and Kačák event intervals are characterized by stark increases in total detrital input, paced by local ~6‑Myr maxima, the pumilio events occur around a minimum of this long‑period cycle and show only minimal, coarse detrital input. These observations suggest that the recurrence of Early to Middle Devonian anoxic events was paced by a previously unrecognized ~6-Myr astronomical cycle, hinting at the possible role of long-period Milankovitch cycles in shaping Paleozoic climate variability.