Intrinsic land-surface response times delayed Late Devonian oceanic anoxia

Devonian strata are repeatedly interrupted by black shales, indicative of widespread ocean anoxia. Astronomical forcing, particularly the 2.4-Myr eccentricity cycle, has been proposed as a pacemaker for these recurring carbon cycle disruptions. Yet, a mechanistic explanation remains lacking. Here, we present K/Al, P/TOC and Cu/Al records, derived primarily from X-ray fluorescence (XRF), from three stratigraphic sections in the Rhenish Massif (Germany) spanning four major Late Devonian anoxic events: the Kellwasser, Annulata , Dasberg, and Hangenberg crises. A consistent trend in regional weathering intensity and nutrient input prior to black shale deposition emerges, with intensified weathering values preceding anoxia by 700–800 kyr. Both the nature and timing of these trends are explained by 2.4-Myr eccentricity forcing, yet the characteristic lead time of the pre-anoxic weathering pulse (700–800 kyr) does not correspond to any primary Milanković period, suggesting a nonlinear response to orbital forcing. In the proposed model, a 2.4-Myr eccentricity minimum allows for the persistence of perennially wet, stable conditions for ∼800 kyr rather than a single 405 kyr cycle. This enables vegetation to expand into formerly arid areas, developing into forests underlain by nutrient-rich soils. During this interval, one or more 405 kyr-scale climate extremes are effectively skipped, permitting soils to develop and become progressively depleted over timescales comparable to their intrinsic response time. Subsequent erosion of this extensive soil cover once climatic variability increases drives widespread eutrophication. These results support a regionally driven land-surface-driven ('top-down') control on Late Devonian anoxia, implicating orbitally-paced climate change and vegetation thresholds as key drivers of nutrient fluxes.