Jin, Xin (2019) The Carnian (Late Triassic) Extreme Climate Event: Comparison of the Italian Tethys and South China Geological Records. [Ph.D. thesis]
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Abstract (italian or english)
As one of the most complex climate events in Triassic, the Carnian extreme climate event or Carnian Pluvial Episode (CPE) has been widely documented in the Tethyan regions. It occurred at the boundary of the two substages of the Carnian: Julian 1 and Julian 2. The CPE was mainly characterized by Tethys-wide humid climate associated with abundant terrigenous sediment and freshwater input to the shallow marine carbonate platforms. A contemporary crisis of carbonate production was recognized in the marginal sea in western Tethys. The CCD in Lagonegro Basin (southern Apennines, southern Italy) arose temporarily. A remarkable temperature was inferred from oxygen isotopes of conodont apatite during this interval. The onset of the CPE was found coincident with a major negative δ13C perturbation recorded by bulk organic matter and n-alkanes. This was interpreted as a massive injection of CO2 into the atmosphere-ocean system, possibly linked to the eruption of the large igneous province of Wrangellia. The CPE has been studied for decades in western Tethys, but much less data are available from other parts of Paleo-Tethys. In this thesis, I focused on the yet poorly investigated Sichuan Basin, which was located at the margin of South China Block in the eastern Tethys. We selected four stratigraphic sections encompassing the Carnian: Ma’antang (MAT), Qingyan Gou (HWQ), Guanyin Ya (HWG) and Jushui (JS) with the goal of determining their biostratigraphy and retrieving carbon and oxygen stable isotope record. Quantitative modal analysis was also carried out for comparing the carbonate facies of the Sichuan Basin with those from the Dolomites, northern Italy, that in the Carnian occupied a paleogeorgraphic position in the western Tethys. New diagnostic fossil findings were presented together with carbonate δ13C and δ18O records and organic carbon isotopes from bulk organic matters and woods from the MAT, HWQ, HWG and JS sections in northwestern Sichuan Basin. The late Carnian conodonts associations indicated that the demise of the hexactinellid sponge reef in Hanwang and Jushui areas was late Carnian in age. A late Carnian ammonoid association with tropitids was found at the boundary between carbonates and siliciclastics, overlain by late Carnian ammonoid associations. Along with ammonoids, the late Carnian bivalve Halobia cf. H. septentrionalis was found. These new biostratigraphic data suggested a late Carnian to early Norian age for the base of the siliciclastic succession, which was confirmed by the occurrence of the DKC and DLC palynological associations of Li and Wang (2016) in the HWQ and HWG sections. In light of this comprehensive biochronologic evidence, we concluded that major facies changes of the Upper Triassic of the Sichuan Basin should not be ascribed to the CPE. Moreover, the Carnian/Norian boundary interval was pinpointed in a short stratigraphic interval around 10 meters thick of the HWQ section in the Sichuan Basin. The carbon isotopic excursions recorded by bulk carbonate rock and microbial grains of the HWQ section were interpreted and discussed in the light of a petrographic evaluation of the diagenetic overprint. Smooth positive trends of carbon stable isotopes recorded by wood and bulk organic matter of the HWQ, HWG and JS sections implied that no isotopic excursion was present across the Carnian/Norian transition. The wood samples showed dispersed values, in agreement with the normal variability of isotopic composition of wood tissue. A strong negative isotopic perturbation of organic carbon isotopes at the MAT was dated by early Carnian ammonoid and conodont associations, and was comparable with other CPE records of South China. A quantitative petrological study was performed on carbonate microfacies of the MAT, HWQ and HWG sections from Sichuan Basin and sections of Southern Apls. The results revealed that the demise of microbial factories within CPE was not a local phenomenon, but interested the entire Tethy. The microbial carbonate platforms totally recovered between the latest Carnian to early Norian, both in western and eastern Tethys.
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