Submarine-hydrothermal exhalative ore layers in black shales from South China and associated fossils - insights into a Lower Cambrian facies and bio-evolution

Early Cambrian (Tommotian) black shales in South China contain submarine-hydrothermal exhalative sulfide ore layers within an are-parallel linear belt extending more than 1600 km. Evidence for an Early Cambrian hydrothermal venting is given by the records of main and trace elements, rare earth elements (REE), sulfur isotopes and the petrological and geological investigations. Mass occurrences of arthropods. sponges and undetermined shelly remains co-occur with hydrothermal vent deposits in the basal Niutitang Formation at the Sansha (Hunan Province) and Zunyi sections (Guizhou Province). This fauna, although not considered as vent fauna sensu stricto. appears linked to the hydrothermal vents via primary and secondary consumption on lithochemotrophic primary producers. Mats of sheathed sulfur bacteria are common in contemporaneous black shales nearby the vent localities. The 'Cambrian explosion', an event of probably rapid biological diversification, is discussed here in context of adaptation to shallow water habitats and major changes in trophic resources. According to our new hypothesis metazoans may have already existed in deep-sea hot vent areas during the Precambrian. This unique ecological niche also supplied favourable habitats for metazoans during the Neoproterozoic supposedly world-wide Varanger glaciation. Cessation of deep ventilation and expansion of dysoxic conditions into shelf areas may have driven the migration of metazoans from hydrothermal vents to shallow water areas in the earliest Cambrian thus enhancing both evolutionary opportunities and the chance of preservation in the geologic record. (C) 2001 Elsevier Science B.V. All rights reserved.