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Astronomical signatures in Late Devonian black shales of the Rhenish Massif.

Subject Area Palaeontology
Term from 2020 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 451461400
 
Final Report Year 2024

Final Report Abstract

The Devonian period (419-359 million years ago) saw significant disruptions to the global carbon cycle, marked by widespread deposition of organic-rich black shales and elevated extinction rates. These events, which played a crucial role in shaping the evolution of life on Earth, were likely triggered by a combination of climatic, environmental, and possibly astronomical factors. This study investigated whether astronomical cycles, particularly the ~2.4-million-year grand eccentricity cycle, influenced climate changes and anoxic (oxygenpoor) conditions that led to mass extinctions in the Late Devonian. We focused on four major Late Devonian events: the Kellwasser Crisis (~372 Ma), the Annulata Crisis (~365 Ma), the Dasberg Crisis (~363 Ma), and the Hangenberg Crisis (~359 Ma). The Kellwasser and Hangenberg events are among the most severe mass extinctions in Earth's history. Fieldwork was conducted at three locations in Germany’s Rhenish Massif (Winsenberg, Effenberg, and Oberrödinghausen), where rock layers were sampled at centimeter-scale resolution. Using portable X-ray fluorescence (pXRF) analysis, we collected elemental data to track changes in weathering and runoff, providing insights into past climate shifts. At all three sites, time-series analysis of these pXRF records revealed cycles corresponding to changes in Earth’s orbit. Our findings suggest that the ~2.4-million-year eccentricity cycle likely played a crucial role in determining the timing of these crises. Before each event, we observed a 350-to-700-kyr long period of increased weathering, associated with thickening regolith (the loose layer of soil and rock) on the continents. These phases each coincided with a long-term eccentricity minimum, which may have allowed for climate stability. As eccentricity increased again after such a long-term minimum, climate variability intensified, leading to regolith erosion and a greater flow of nutrients into the oceans. This surge in nutrients likely caused eutrophication (excessive nutrients leading to oxygen depletion), which in turn triggered marine anoxia and extinctions. After each crisis, weathering intensified again, likely due to fresh, weatherable material made available after widespread regolith erosion during the events. The key takeaway is that these ancient climate crises were not random but likely paced by astronomical forces, with Earth's changing orbit influencing weathering on land. These changes then had consequences for oceanic conditions, and, ultimately, life on Earth. For the public, these findings emphasize the role that natural cycles play in shaping Earth's environment, with insights into how ancient continental environments and climate processes interacted to produce dramatic shifts in marine ecosystems. Understanding these past events can help us appreciate the complexity of Earth's climate system and its potential vulnerabilities in the face of modern climate change and man-made eutrophication events.

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