Late Devonian Conodont Biostratigraphy And Carbon And Oxygen Isotopic Composition In Western Junggar, NW China

The global Kellwasser Crisis at the Frasnian-Famennian (F/F) transition was one of the major overturns of earth biosphere in the Phanerozoic. The event led to a reduced biodiversity and severely damaged reef ecosystem. Many studies in Europe, Australia and other regions in South China give us a well understanding on the causes and mechanisms of F/F event. In recent years, with continuous reports of Hongguleleng refugia, the public begins to focus on the causes and role of refugia. In order to explain the bio-crisis during the F/F transition and the Hongguleleng refugia, this study provides a high-resolution conodont biostratigraphy, carbon isotope, organic carbon isotope and conodont oxygen isotope accompanied with sedimentary facies/microfacies. Including the Wulankeshun section, many sections of the Hongguleleng Formation were measured. The main conclusions and progressions in research are as following.At the Wulankeshun section,142 conodont samples with a total weight of 0.6 t were collected in 358.3 m strata, which yielded more than 3,000 conodont elements. Fifty-four species or subspecies that belong to Polygnathus, Palmatolepis, Icriodus, Mehlina, Neopolygnathus, Branmehla, Bispathodus, Ancyrognathus and Alveognathus n. gen. were identified. Among them, there were one new genus and sixteen new species or subspecies. However, one new species was not given a name because of the bad preserved condition. Based on rich materials, the first regional conodont zones were established, in ascending order, they were:1). icriodid lineage:Icriodus praealternatus ferus、I. cornutus、I. stenoancylus junggarensis、I. plurinodosus zones; 2.) polygnathid lineage:Neopolygnathus huijunae、Neo. communis communis、Alv. pomeranicus、 Polygnathus cf. szulczewskii、Alveognathus pseudocommunis zones. Moreover, the Famannian was recognized by the first occurrence of Neo. huijunae. They could be correlated with the pelagic "standard zonation".According to the new high-resolution conodont biostratigraphy, the stable isotope samples of Wulankeshun section were measured. The carbonate inorganic carbon isotope showed that there were three obvious excursions in the section. The first excursion (positive) occurred in the F/F transition. The second excursion (negative) occurred at the bottom of Middle Member. The third excursion (positive) occurred at the boundary of Middle and Upper Member. Additionly, the two positive excursions may indicate high primary productivity, while the one negative excursion suggest that volcanism occurred. The organic carbon isotope curve showed frequent fluctuations result from the change of input controlled by sea level change. It suggested the Variscan orogeny was activity in eraly Famennian. Meanwhile, the conodont apatite oxygen isotope samples were analyzed by TC/EA coupled isotope mass spectrometer. The first sea surface temperature change curve of Paleoasian Ocean was established in northwestern Junggar Basin. The result showed that there was a cooling with at least 8 ℃ around F/F boundary. The cooling process was last to crepida zone and subsequently entered a slow cooling stage. The conodont oxygen isotope curve and inorganic carbon isotope curve showed similar pattern and lag behind the latter, which may suggest the pressure of atmospheric CO2 change was the main reason for the globle cooling.The brachiopods, bryozoans, bivalves, ostracods, echinoderms, serpulids and conodonts were found in the Hongguleleng refugia. The fossil assemblages showed a typical characteristics of shallow cold water environment. In Lower Member of Hongguleleng Formation, the freshwater ostracods indicated a brackish water environment. As this reason, it was difficult to reconstruct the absolute palaeotemperature by conodont apatite oxygen isotope. The rapid cooling and the upwelling of anoxia water did not affect the cold water fauna in Hongguleleng refugia. On the one hand, the fauna had a much better tolerance of temperature, on the other, Hongguleleng refugia did not have a very good water exchange condition with outside. Whereas, the land runoff took enough nutrients here. Thus, the so-called F/F Hongguleleng refugia was formed.