Study On The Biota And Its Co-evolution To The Paleoenvironment In The Early Cambrian Of The Eastern Yangtze Gorges And Western Zhejiang,China

The period surrounding the Precambrian-Cambrian boundary (～542Ma) is a key interval in the history of Earth and Life in several different and complementary ways. From the Late Ediacaran to the Early Cambrian, the sea water geochemistry and the biotic communities witnessed drastic and significant changes, accompanied by a global climate change,the strong reorganization of plates, mass extinctions and the development of metazoans.During this transitional interval, the most striking phenomenon is the disappearance of the soft bodied Ediacaran fauna followed by the explosive radiation of skeletonized animals in the Early Cambrian. These events are widely known from black rock series in the Lower Cambrian of the Yangtze Block, China On the one hand, the radiation of metazoans includes the explosive growth of diversity and abundance, increasingly complicated food chain and lifestyle. On the other hand, their living environment changed from shallow sea under normal marine conditions to a protected basin and slope area underanoxic to euxinic waters.How did the new oxidation process begin and evolve? Did the ocean circulation re-start? What were the hypoxia degrees of bathyal and deep sea areas? How did the levels of primary productivity evolve? Was it sufficient to support a large outbreak of organisms? So far, the available data are not enough to fully answer these questions. In particular, the data on the southeast edge of the sea, slope and basin are still very rare.In this dissertation, I report abundant andwell-preserved metazoan fossils and small shelly fossils (SFFs) from the black rocks of the Shuijingtuo Formation in Zigui. I also document the occurrence of some SSFs and acritarchs from the black shale and cherts of the Hetang Fm. in west Zhejiang area. Here I focused my study on the sedimentary environment, paleoxygen, the features and level of the primary productivity during this period. Several lines of conclusion can be drawn:1. Paleontonological features. To investigate the potentiality of the presence of source rocks in the Early Cambrian of the Three Gorges area, Ifound abundant and well preserved fossils from several sections close to Zigui. Combined with previous findings, I propose the definitionof the "Shujingtuo Biota". The so-called Shuijingtuo biota includes more than ten orders of organisms, such as Sponges, Chancelloriids, Archaeocyatha, Brachiopoda, Hyolitha, Gastropoda, Monoplacophora, Lobopodia, Arthropoda and some undetermined fossils and micro-fossils. Trilobites (e.g., Zsunyidiscus), Sponges, Bradoriida, Phyllocarida and few radiolarians were found at the lower part of the Shuijingtuo Fm., sharing similar features with Songlin fauna from the base of thef Niutitang Fm. in Guizhou province. Chancelloriids, Archaeocyatha, Brachiopoda and Monoplacophora fossils found from the limestone concretion at the upper part of the Fm. are similar with those of the "Xihaoping fauna" in the north Hubei. The sponges, Bradoriida, Cnidaria such as?Byronia and Sphenothallus are similar with Zunyi biota from the upper part of Niutitang Fm. in Guizhou, and are the atypical type of Lagerstatten type fossils (sponges are excluded because of the lack of adequate mining).Although further work is needed, the discovery of the Shuijingtuo biota has completed the biota series from the earliest to the middle Cambrian in the Three Gorges to west Hubei area, including the Yanjiahe biota, Shuijingtuo biota, Shipai fauna and Lagerstatten type fossils preserved in Jingshan, west Hubei.I also report a few SSFs from the black shale from the Hetang Fm. These SSFsgenerally occur in the limestone interbeds or nodules in the black cherts. These foundings suggest that (/) SSFs could also be found in deep water facies;(ii) the discovery of SSFs in the silicates is limited by the traditional pre-treatment approach;(iii) the composition and characteristics of these SSFs from the limestone concretion in the upper part of the Shuijingtuo Fm. are consistent with Rhombocorniculum cancellatum zone of Adabatian in the North Hubei and South Shaanxi area It shows they could well correlate with each other in terms of age. Those SSFs such as Amphigeisina which are widely distributed in South Chinacould be erected as a new SSFs zone fossil replacing the "Rhombocorniculum cancellatum zone" occurring only in shallow areas.2. Paleoxygen evolution and its characteristics. I carried out a comprehensive research on geochemistry, framboidal pyrites and TOC on both the Shuijingtuo Fm. in Zigui and the Hetang Fm. in west Zhejiang. It indicates that these two rock series bothdeposited under hypoxic sulfide environment, but they also display important differences. In the protected basin of Zigui area, the evolution of the paleoxygen levels can roughly be divided into four stages.Compared with the relatively steady Stage1, the lower part of Stage1corresponds to hypoxic conditions ameliorating upwards. Oxygen rich surface waters are occasionally brought into nearby water and sediment interface in stages1and2, which could have supported the life of benthic animals.In Stage3, the fossils are absent, with strong enrichment of TOC and redox sensitive elements. The redox indexes thus reach their peak, indicating the enlargement of anoxia or euxinia.At the bottom of Stage4, abundant fossils are found from the rhythmic alternance of black limestone and black mud-shale. The diversity of metazoans reaches its maximum. Among them, an important proportion of benthic organisms are found in situ while the others are infaunal. Both the geochemical and biological evidences fluctuate and indicate that the area frequently displayed temporary aerobic conditions, enough to allow and sustain the radiation of life.The black shale-carbonate laminations in upper part of the Shuijingtuo Fm. in Zigui, also yielded the contradiction between the section observations and the geochemical indicatorssuch as V/(V+Ni), U/Th, V/Cr and Ni/Co, recording a rapid reduction. On the other hand, substantial increases are not only observed in the abundance of organisms but also the degree of differentiation. By contrast, the median and mean values of the framboidal pyrites show some decline, indicating that the bottom water is in severe anoxia or even euxinia.These apparent oxygenation fluctuations were detected by the high-resolution ICP-MS. Such fluctuations between aerobic and anaerobic values are the signs of (ⅰ) the beginning of the normalization of the bottom water, that is to say, the size and depth of the oxidation of seawater is expanding;(ⅱ) no permanent oxygenated water developed on this region. In other words, different types or sources of water masses have not been fully flow and mixing yet.(ⅲ) This repeated wrestling between oxygenic and anoxic bottom waters seems to be tightly related to the radiation of metazoan organisms, rendering the conditions alternatively proper and improper for their radiation and preservation.The general evolution of the paleoxygenation on Yangtze paltform is characterized by the periodical weakening of the anoxia in relation with the sea level changes, upwelling and activities of surface water. This progressing of oxygenation can also generally be correlated with other areas in South China such as the Yuanshan Fm. in Yunnan province, Niutitang Fm. in Guizhou, and Yangjiaping Fm. in Hunan. Th/U and FeHR/FeT ratios reveal that bottom waters on the southwestern Yangtze Platform were commonly anoxic and ferruginous in the Cambrian, but became oxic in the late Adabatian, which elsewhere saw the development of the Chengjiang Biota, Shujingtuo Biota and Zunyi Biota.Based on the biological or geochemical features, there is a clear turning point after the anoxic event. In the heavily anaerobic situtations prevailing before this point, fossils are rare. The crossing of this turning point marks the significant increase of the abundances and diversity of organisms together with the decrease of the degree of anoxia. A clear order and directions of the oxygenation progress can be distinguished thanks tobiostratigraphic correlations:the massive oxygenation progress occurred earlier in the shallower Kunming area and later expanded to the the protected basin where the Shujingtuo Fm. deposited.However, chemical evidences provide the opposite conclusion for the Hetang Fm. from the South Anhui to West Zhejiang area. The bottom water is under anoxic to euxinic condition, some argue these conditions are not permanent. The paleoxygenation condition of the bottom water didn’t change until the top of the Hetang Fm. The activities of seawater from the upper part of the Fm. have failed to substantially bring oxgen down to the bottom of the water column. Additionally, the radiation of metazoans didn’t reach the west Zhejiang area possibly because of the low oxygen content of bottom water. So the fossils in this area are relatively rare. Those distinct differences document the recovery status and progress of the marine environment after the lower Cambrian anoxic events, as well as the scope and extent of the oxygenated water masses.3. Primary Productivity. There are three major producers in the Precambrian-Cambrian ocean, including macro-and micro-plants (including acritarchs) and microbialites. Their bloom has a close relationship with the increase of the oxygen content in both the atmosphere and sea, food supply of consumers, black sediments and accompanying mineral, oil and gas resources. The valuable geochemistry productivity proxies and TOC data obtained from the Shuijingtuo and Hetang Fms indicate that the ocean experienced a long lasting relatively high primary productivity during the early Cambrian. In the certain circumstances, the primary productivity was stimulated by the terrestrial input. The primary productivity in this period was steadily developing according to the stable carbon isotopic characteristics of the Three Gorges area. However, several indicators of primary productivity are highly correlated with the bottom water condition. It implies that the preservation of the organic carbon and primary productivity sensitive metal are mainly controlled by paleoxygen condition of bottom water.The statistical work on the inertinite and ovitrinite shows that they don’t display an obvious relationship with other productivity proxies. The organisms in Shuijingtuo, Hetang and Niutitang Fms were mainly interpreted as remnants of in situ bacterially reworked organic matter of cyanobacteria/algal type. This interpretation is totally distinct from the organisms in black rocks in Kuanchuanpu Fm. of earliest Cambrian which were mainly from benthic macro algae.