| The Frasnian-Famennian (F-F) transition was a period of dramatic evolutionary and environmental changes characterized by the Kellwasser or F-F mass extinction, which represents one of the big five Phanerozoic mass extinction events. This crisis not only caused a severely decline in biodiversity, but also inflicted the ecosystem of the Late Devonian. There was a good research background of stratigraphy (including biostratigraphy, chemostratigraphy and cyclostratigraphy etc.) in the Upper Devonian from South China, which could provide important theoretical bases for exploring the process and ultimate cause of the F-F event. This study focuses on high-resolution conodont biostratigraphy and reconstruction of paleotemperature based on conodont oxygen isotope accompanied with carbon isotope, microspherules, and microfacies during the F-F transition with the goal of understanding the relationships between organisms and environments. So, in order to explain the process and environmental background of this bio-crisis during the F-F transition, two important sections were chosen from Guangxi, South China, including the Yangdi section whose sediments represent platform slope facies and the Nandong section whose sediments represent basinal facies. And following results and conclusions have been gained in this paper.At the Yangdi section, Ninety-one conodont samples with a total weight of 344.0 kg were collected by centimeter-scales in 53.3 m strata which from the Xiangtian Formation to the Lower part of the Wuzhishan Formation, including twenty-one samples were collected in the F-F boundary interval, the thickness of which is about 1.9 m. A total of 18,135 conodont species (P1) were obtained by this work, more than 11,828 were well-preserved. Fourty-three species and thirteen subspcies that belong to Palmatolepis, Polygnathus, Icriodus, Ancyrodella, Ancyrognathus and Pelekysgnathus are identified. Based on rich conodont materials, five conodont zones are revised and one new conodont zone is recognized, in ascending order, they are:Upper Pa. rhenana Zone, Pa. linguiformis Zone, Lower Pa. triangularis Zone, Middle Pa. triangularis Zone, Upper Pa. triangularis Zone and Lower Pa. crepida Zone. Based on the distribution of conodonts from this study, the lower limit of previous conodont zones that have been used for more than twenty years were revised from several tens centimeters to decade meters. The lower limit of the Upper Pa. rhenana Zone was placed at the bottom of the Xiangtian Formation, which is about 1430 cm lower than previously reported. The lower limit of the Pa. linguiformis Zone was about 580 cm lower than previous research, i.e., it should be placed at the bottom of the bed overlying the brecciated limestones. The lower limit of the Lower Pa. triangularis Zone (F-F boundary) was placed at the uppermost of the Xiangtian Formation, the level of which is about 88 cm lower than previously reported. The lower limit of the Middle Pa. triangularis Zone was placed at the base of the Wuzhishan Formation, the level of which is about 110 cm lower than previous research. The lower limit of the Upper Pa. triangularis Zone was placed at the 431 cm above of the Wuzhishan Formation, the position of which is the base of Bed 49. The lower limit of the Lower Pa. crepida Zone was defined at the base of Bed 63, which is positioned at the 2,127 cm above the Wuzhishan Formation. However, we cannot constrain its top because Pa. termini was not found in any samples from current collection.At the Nandong section, seventy-four conodont samples with a total weight of 290.4 kg were collected by centimeter-scales in 35.46 m strata, which from the upper part of the Liujiang Formation to the Lower part of the Wuzhishan Formation. A total of 3,549 conodont species (P1) were obtained by this work, more than 2,205 were well-preserved. Thirty-two species and nine subspcies that belong to Palmatolepis, Polygnathus, Icriodus, Ancyrodella, Ancyrognathus and Pelekysgnathus are identified. Based on new conodont data, five conodont zones are revised, in ascending order, they are:Lower Pa. rhenana Zone, Upper Pa. rhenana Zone, Pa. linguiformis Zone, Lower Pa. triangularis Zone, Middle Pa. triangularis Zone. Based on the distribution of conodonts from this study, the lower limit of previous conodont zones were revised by several meters. The lower limit of the Lower Pa. rhenana Zone was placed at the level that is about 4.9 m lower than previous research. The lower limit of the Upper Pa. rhenana Zone was revised down by about 5.0 m. The lower limit of the Pa. linguiformis Zone was revised up by about 5.3 m. The lower limit of the Lower Pa. triangularis Zone should be placed at the level that is about 5.7 m higher than previous research. The lower limit of the Lower Pa. triangularis Zone was revised up by about 1.0 m.Based on the changes in abundance and diversity of conodonts, ostracods and tentaculitids, combining the correlation with the Upper Kellwasser Horizon (UKH) from foreign sections in centimeter-scale during the F-F boundary interval at the Yangdi section, the UKH in South China was precisely dated for the first time. And two step-wise pelagic faunas extinctions were recognized during the F-F boundary interval at the Yangdi section. The first step occurred at the transition between the lower and upper Pa. linguiformis Zone. A massive decrease in the total number of conodont elements (P1) occurred in this change, and is marked by the disappearance of Pa. linguiformis, accompanied by the successive disappearances of Pa. eureka, Pa. rhenana, Pa. nasuta, Pa. gigas gigas, Pa. g. paragigas, Pa. g. extensa, Ancyrodella ioides and Ad. nodosa. Meanwhile, Icriodids show its first outburst by multiplying their frequency within the fauna. The second step occurred well below the brownish marlstone horizon, which is equivalent to the uppermost UKH, and is characterized by the elimination of the Frasnian conodont elements, such as, Polygnathus webbi, Pol. decorosus, Pol. lodinensis, Pol. macilentus, Ad. curvata, Pa. rotunda. Furthermore, the decimation of ostracods and tentaculitids also occurred at this step. Integrating the previous research results of benthic faunas including rugose corals, brachiopods and ostracods with the extinction pattern of pelagic faunas from this study, we summarized three steps of the Upper Kellwasser bio-crisis in South China. The first step decimated diverse brachiopods (including most atrypids), and occurred at the base of the Upper Kellwasser Horizon, the level of which corresponds to the Event 6 summarized by Sandberg et al., i.e., representing the start of anoxia. The second step is characterized by massive disappearance of conodonts (mostly are Palamtolepids), and occurred at the transition between the lower and upper Pa. linguiformis Zone, which may correspond to the Event 8 summarized by Sandberg et al., i.e., representing the start of rapid eustatic fall. The third step may represent the mass extinction, which exterminated both benthic (rugose corals, brachiopods and ostracods) and pelagic faunas (conodonts, pelagic ostracods and tentaculitids), and it is not only equivalent to the Event 9 summarized by Sandberg et al., but also equivalent to the Event C proposed by Schindler. Moreover, according to the previous results from cyclostratigraphy, the third episode of the bio-crisis and the whole Upper Kellwasser crisis are estimated to have lasted 16-18 kyr and～200 kyr, respectively.The new high-resolution conodont biostratigraphy not only provides with high precise time framework, but also changes the views to explain the biological-environmental events. The carbonate inorganic carbon isotope (813Ccarb) results from the Yangdi and Nandong section show that there are two positive excursions during the F-F transition. The first positive excursions in the Yangdi and Nandong section occurred in the Upper Kellwasser Horizons with amplitudes about 2.2%o and 3.1%o, respectively. The second positive excursions appeared at the lower part of the Middle Pa. triangularis Zone with amplitudes about 2.4%o and 3.1‰, respectively. Therefore, the trend of δ13Ccarb perturbation is consistent with previous studies. However, it is show that the time of δ13Ccart, perturbation in this study is earlier than previous results based on new conodont biostratigraphy. At the Nandong section, based on new conodont biostratigraphy, the δ13Ccarb positive excursion not only overturns the traditional view of δ13Ccarb negative excursion during the Upper Kellwasser Horizon, but also indicates that the δ13Ccarb positive excursions show global characteristics during the F-F transition. Two positive excursions may indicate anoxic circumstance or high primary productivity, while the negative excursion and concurrent hot climate may suggest that volcanism occurred in the Late Devonian strata, which is positioned at the underlying layer of the Upper Kellwasser Horizon. The change of vertical gradients in δ13Ccarb indicated that the water column of the Late Devonian Ocean oscillated between mixing and stratification, which was coupling with climate. The stratification of sea water column occurred in warm climate and the mixing of sea water column appeared in cool climate. The negative value of the vertical gradients in δ13Ccarb during the Upper Kellwasser Horizon indicated intense mixing of sea water column.Abundant perfectly-preserved phosphatic microspherules have been discovered across the F-F transition from the Yangdi section by acid cracking method. They are mostly spherical or elliptical in shape and about 150μm in diameter with smooth exterior surfaces. Each microspherule consistently possesses a small dimple on the surface. The internal texture of microspherules consists of concentric light-colored apatite and dark-colored organic matter bands alternating around a central core. Conodonts have also been found preserved together with phosphatic microspherules in the same horizon, and the abundance of the former is obviously higher than that of the latter. Laser Raman spectral studies show close similarities in spectral patterns between the outer shells of phosphatic microspherules and the blade of the conodont genus Palmatolepis sp., as well as between the microspherule nucleus and the platform of the same conodont genus. Furthermore, the statistical results and geochemical data demonstrate that the elevated abundance of phosphatic microspherules roughly coincides with the blooms of bacteria and algae, but is later than the sharp increase of oceanic nutrients. The phosphatic microspherules are interpreted here to be the otoliths secreted by an uncertain type of fishes based on the compositional similarities and their interrelated abundances, as well as on morphological and textural similarities between modern fish otoliths and phosphatic microspherules. As a result, the formation of phosphatic microspherules is probably related to seawater eutrophication.At the Yangdi section,106 conodont apatite oxygen isotope samples from the F-F transition have been analyzed by TC-EA coupled isotope mass spectrometer. Based on the high-resolution conodont biostratigraphy, the high-resolution paleotemperature curve was established for the first time in South China. The result shows that the paleotemperature of South China ranges from 27℃ to 36℃ with four alternate changes between hot and cool. At the upper part of the Pa. linguiformis Zone, there were two rapid (lasted about 200 kyr) and dramatic (amplitude of 7～8℃) abnormal perturbations of climate that were concurrent with two major episodes of biotic crisis. The first rapid (lasted about 200 kyr) and dramatic (amplitude of 7℃) warming event occurred in the underlying layer of the Upper Kellwasser Horizon, the level of which corresponds to the first episode of biotic crisis that is characterized by demise of shallow-water communities. The second rapid (lasted about 200 kyr) and dramatic (amplitude of 8℃) cooling event took place in the Upper Kellwasser Horizon, which is concurrent with the main pulse of extinction that is characterized by decimation of shallow-water and deep-water communities. The paleoclimate not only affected the biodiversity, but also were closely related with the carbon cycle. From the Pa. linguiformis Zone to the Pa. crepida Zone, there are consistent perturbations between carbonate inorganic carbon isotope and conodont oxygen isotope, and the beginning perturbation time of the former is earlier than that of the latter, which may indicate that the partial pressure of atmospheric CO2 controlled the evolution of paleoclimate during the F-F transition.Integrating all obtained data above, the relationship between the organism and environment could be roughly summarized as follows. Volcanism brought about the rapid and dramatic warming climate that induced anoxia and rise of sea level, which resulted in the decimation of shallow-water communities. Subsequent mountain building event not only enhanced the terrestrial input of nutrients, but also drived the climate from warming to cooling. The cooling climate and intense disturbance of water current enhanced the circulation veloicity between deep and shallow water, which further increased the nutrient and decreased the oxygen content in shallow water, and then brought about eutrophication. The eutrophication induced deteriorative ecosystem that was characterized by anoxia, lightless and toxin, which caused the disappearance of shallow-water and deep-water communities. |
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