Paleo-Ocean environmental fluctuations and their constraints on biomes during mid Early Silurian in upper Yangtze region, southwest China

The Upper Yangtze Ocean (region) in southwest China was a stagnant and hypoxic environment during early Early Silurian when the Lungmachi Formation deposited and turned to an oxidized environment during the mid Early Silurian... [ view full abstract ]

The Upper Yangtze Ocean (region) in southwest China was a stagnant and hypoxic environment during early Early Silurian when the Lungmachi Formation deposited and turned to an oxidized environment during the mid Early Silurian when the Shihniulan Formation was formed. This paper systematically studied the paleo-ocean environmental fluctuations and their constraints on biomes in Shihniulanthe Ocean (now southeastern Chongqing City and northern Guizhou Province, southwest China) during the Early Silurian. Results show that, in a context of falling sea-level, the Yangtze Ocean had experienced 4 times of sea level rising and falling during mid Early Silurian. The sea level changes caused continuous fluctuations of redox conditions with 4 phases of reduction and 3 phases of oxygen increase. Deposited material in the region was apparently affected by supplies from the Xuefeng Mountain in the east and the ancient land in southern Guizhou Province. Continuous terrigenous clastics supply from the two sources resulted in the formation of shallow mixed sedimentary environment, which restricted the development of biomes in it. The environmental fluctuations in the Upper Yangtze Ocean during the mid Early Silurian can be divided into 4 stages. The first stage was a transitional period with its inherited stagnation anoxic marine environment from the previous period despite the fact that the oxygen content of the water was increased for the first time. The second stage experienced again a growth of oxygen content and a burgeoning biomes in the ocean. In the third stage, the development of biomes was restricted one more time as the sea level rose again, which induced a short period of hypoxic environment, and terrigenous clastics poured in. The fourth stage experienced another oxygen content rise, which was followed by spreading organic reef and biological boom. However, rising sea level by the end of this stage soured the environment and caused a massive biological die-off, leaving behind reef and paleontological biomes in situ. It is therefore suggested that the fluctuations of the sea level, terrigenous clastics supply and redox conditions, all restricted the evolution of biological communities of Upper Yangtze Ocean during the mid Early Silurian.