Paleoclimatic and Paleo-Oceanic Environment Evolution in the Frasnian-Famennian Transition: Potential causes of the biotic crisis.

[Objective] The Late Devonian Frasnian-Famennian (F-F) transition is a critical time interval in geological history. This period saw major simultaneous marine ecological system changes that led to one of the Big Five mass extinctions in the Phanerozoic. The F-F event (also referred to as the Kellwasser event) was characterized by severe losses of low-latitude shallow-water benthic faunas, notably reef-dwelling coral and stromatoporoids. High-latitude, deep-sea and terrestrial faunas were the least affected. Various separate or combined hypotheses have been proposed as the causes of this mass extinction (sea-level change, marine anoxia, climate change, volcanic/hydrothermal activities, and bolide impact) . Of these, climate change and marine anoxia have been the most intensively researched and discussed hypotheses based on the Web of Science in recent years. However, some controversies remain, and the interactions of certain factors are still unclear. [Methods] This study systematically reviews available reports on the paleoclimatic and paleo-oceanic changes during the F-F transition and discusses the anoxic model during this critical period, based on related case studies in southern China. [Results and Conclusions] Conodont oxygen and strontium isotopes, as well as the carbon isotope records for carbonates, collectively suggest a cold climate during the F-F transition interval, with several rapid warming-cooling fluctuations. Conodont oxygen isotopes suggest that the sea-sur face temperature (SST) dropped by 5 ° C-8 ° C. Strontium isotopes also imply that the temperature fluctuations were due to frequent, short-duration volcanism. Palynological data and carbonate platform exposure/karstification are also suggestive of coeval climate cooling. Additionally, marine anoxia has been extensively hypothesized as a possible killing mechanism in the F-F mass extinction, based initially on the presence of bituminous limestones (or black shales), named the lower and upper Kellwasser horizons. Studies of pyrite framboids, biomarker compounds, trace elements, isotopes of nitrogen, sulfur and uranium, and iron speciation have variously suggested the existence of the Kellwasser anoxic events. However, these generally occurred in geographically specific environments notably in the pericontinental basins/sub-basins proximal to source hinterlands. Moreover, the range and degree of anoxia in the F-F transition also reportedly differed between study sections around the world. With regard to the anoxic model, studies have suggested that the Kellwasser anoxic events were caused by the increased nutrient input related to enhanced continental weathering. Marine anoxic studies from three F-F sections comprising different depositional facies in southern China also support this "top down" anoxic model. Clearly, the F-F biotic crisis was not caused by any single factor. Frequent short-term volcanic activity may have enhanced continental weathering and associated greenhouse gas emission, leading to frequent warming-cooling climate fluctuations and also increased oceanic nutrient input. In the latter case, eutrophication and anoxia would have occurred in shallow water, and the mutual interaction of the various environmental factors may have exerted biological pressure in the low-latitude shallow sea, and eventually led to the F-F biotic crisis. [ABSTRACT FROM AUTHOR]