The Paleoclimatic and Paleo-oceanic Environment Evolution in Frasnian-Famennian Transition and the Potential Causes of Biotic Crisis

Key words: 
• Frasnian-Famennian transition /
• paleoclimate /
• paleo-oceanic redox condition /
• biotic crisis

Abstract: [Significance] The Late Devonian Frasnian-Famennian (F-F) transition is a critical time interval in geological history during which major marine ecological system changes occurred simultaneously, leading to the mass extinction, known as one of the biggest five in Phanerozoic. This mass extinction (also referred to F-F event or Kellwasser event) was characterized by severe losses of low-latitude shallow water benthic faunas, notably for the reef-dwelling coral and stromatoporoid. High-latitude, deep-sea and terrestrial faunas were affected the least. Until recently, various independent or combined hypotheses, including sea level change, marine anoxia, climate change, volcanic/hydrothermal activities, and bolide impact have been proposed as the causes of this mass extinction event. Among them, climate change and marine anoxia are the most intensively researched and discussed hypotheses in recent years based on the Web of Science data. However, there are still some controversies, and the interactions among these environmental factors are still not clear. [Progress] This study systematically reviews the research papers on the paleoclimatic and paleo-oceanic changes during the F-F transition interval and discusses the anoxic model during this critical period based on the related case studies in South China. Conodont oxygen and strontium isotopes, as well as the carbonate carbon isotope records collectively suggested that the climate became cold during the F-F transition interval, including several rapid warming-cooling fluctuations. Conodont oxygen isotope results suggested that the surface sea-water temperature (SST) had dropped by 5~8 ℃. Strontium isotopes also implied that these warming-cooling fluctuations may have been caused by the short and frequent volcanic activities. Palynology data and carbonate platform exposure/karstification were also the evidence of coeval climate cooling. Additionally, marine anoxia has been extensively hypothesized as a potential killing mechanism for the F–F mass extinction based initially on the occurrence of bituminous limestones (or black shales) named as the Lower and Upper Kellwasser Horizons. Studies on pyrite framboid, biomarker compounds, trace elements, nitrogen isotope, sulfur isotope, Uranium isotope and iron speciation suggested the existence of the Kellwasser anoxic events. However, it generally occurred in geography-specific environments, notably in the pericontinental basins/subbasins proximal to the source hinterlands. Moreover, the extent and degree of anoxia in F-F transition also varied in different study sections all over the world. With regards to anoxic model, researches suggested that the Kellwasser anoxic events should be caused by the increased nutrients input related to the enhanced continental weathering. The marine anoxic studies from three F-F sections belonging to different depositional facies in South China also supported this “top down” anoxic model. [Conclusions and Prospects] Hence, the F-F biotic crisis was not caused by single factor. And, frequent and short-term volcanic activities could have induced the enhanced continental weathering and greenhouse gases emissions, which process, on the one hand, led to the frequent warming-cooling climate fluctuations, and on the other hand promoted the nutrient inputs to the ocean, thereby resulting in eutrophication and anoxia in the shallow-water. Therefore, the mutual interactions of various environmental factors may have exerted great biological pressure in the low-latitude shallow sea, and eventually led to the F-F biotic crisis.