Chapter 2Toward understanding Late Devonian global events: few answers, many questions

• Published in: Developments in Palaeontology and Stratigraphy Vol. 20; pp. 5 - 36
• Main Author: Racki, Grzegorz
• Format: Book Chapter
• Language: English
• Published: Elsevier Science & Technology 2005
• Subjects: anoxia; biodiversity; geochemistry; geochronology; global events; Late Devonian; palaeoclimate; volcanism; geochronology; Late Devonian; biodiversity; geochemistry; anoxia; palaeoclimate; global events; volcanism
• ISBN: 0444521275; 9780444521279
• ISSN: 0920-5446
• DOI: 10.1016/S0920-5446(05)80002-0

The Late Devonian was an epoch of dramatic evolutionary and environmental changes linked primarily with the Frasnian-Famennian (F-F) mass extinction. Current data and ideas support a prolonged, multi-causal nature of the biodiversity crisis, which favor Earth-bound mechanisms rather than a global cosmic catastrophe. The better under-standing of the Late Devonian ocean-climate-biosphere system leads to several questions, and provides an agenda for future research. (1) Magnitude and rank of biotic changes: more detailed biodiversity studies are needed to place the end-Frasnian extinction in its Late Devonian context. In particular, the emerging severity of the end-Givetian and end-Famennian extinctions contrasts with the current overemphasis on the stepwise F-F crisis. (2) Timing of the key boundaries: a lack of radioisotopic dates hampers any estimation of true biodiversity dynamics, and the integrated comparison with reported ages of impact craters and magmatic events. (3) Marine vs. terrestrial events: insight into global ecosystem changes and correlation should be strengthened by chemostratigraphy, exemplified by the carbon isotope link between marine- and land-derived organic materials. (4) High-resolution (bio)geochemical patterns: isotope secular trends are poorly known at the intra-zonal and inter-basin scales, exemplified by prominent carbon isotope shifts across the Lower-Middle Frasnian passage. Further evidence is also awaited to verify cooling (vs. anoxia) pulses as the main stress factor in the F-F and end-Famennian marine settings, as well as climatic feedback with evolving weathering regimes on land and nutrient dynamic in marine realm. (5) Near-equatorial vs. high-latitude domains: refined data from extratropical successions, e.g. from the Kolyma Block, are still awaited. (6) Tectonic and volcanic-activity: an integrated analysis of tectonic and igneous events, possibly triggered by superplume activity. will serve to evaluate any possible link with the Late Devonian biospheric perturbations. (7) Cyclostratigraphical perspective: includes growing research on refined magnetosusceptibility (MS) and various sea-level signatures to test whether they result from variation in Milankovitch frequency orbital variability. In addition, eustatic sea-level trends and their assumed glacioeustatic forcing have only recently been subject to discussion.