Astronomically-paced climate and carbon-cycle feedbacks in the lead-up to the Late Devonian Kellwasser Crisis

Abstract. Repeated carbon isotope excursions and widespread organic-rich shale deposition mark the Middle and Late Devonian series. Various explanations such as extensive volcanism and land plant evolution have been given for these perturbations and the general sensitivity of the Devonian to oceanic anoxia, but their repeated nature suggests that astronomical forcing may have controlled their timing. Here, a cyclostratigraphic study of the Kellwasser Crisis at the Frasnian-Famennian stage boundary (ca. 372 Ma) is carried out. The Kellwasser Crisis was one of the most ecologically impactful of the Devonian perturbations and is ranked among the ‘Big Five’ Phanerozoic mass extinctions. The studied site is the Winsenberg Road Cut section in the Rhenish Massif, Germany, which represents a quiet tropical shelf basin setting. Centimetre-scale elemental records, generated by portable X-Ray scanning, allow for testing of the hypothesis that a 2.4 Myr eccentricity node preceded the Upper Kellwasser event. The study’s results are supportive of this hypothesis. We find enhanced chemical weathering (K₂O/Al₂O₃) during the period leading up to the Upper Kellwasser, and a peak in distal detrital input (SiO₂/CaO) and riverine runoff (TiO₂/Al₂O₃) just prior to the start of the Upper Kellwasser. We interpret this pattern as the long-term eccentricity minimum facilitating excessive regolith build-up in the absence of strong seasonal contrasts. The Earth’s system coming out of this node would have rapidly intensified the hydrological cycle, causing these nutrient-rich regoliths to be eroded and washed away to the oceans where they resulted in eutrophication and anoxia. An astronomical control on regional climate is observed beyond this single crisis. Wet-dry cycles were paced by 405-kyr eccentricity, with both the Lower and Upper Kellwasser events taking place during comparatively drier times. A precessional-forced monsoonal climate system prevailed on shorter timescales. Intensification of this monsoonal system following the node may have caused the widespread regolith erosion. We estimate the total duration of the Kellwasser Crisis at ca. 900 kyr, with the individual events lasting for ca. 250 and 100 kyr, respectively. If astronomical control indeed operated via regolith build up in monsoonal climates, land plants may have played an important role. Not by certain evolutionary steps triggering specific perturbations, but by permanently strengthening the climatic response to orbital forcing via soil development – creating soils thick enough to meaningfully respond to orbital forcing – and intensifying the hydrological cycle.