Canadas Blasco, Fuencisla; (2021) Biogeochemistry of late Ediacaran organic-rich shales in South China. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Late Ediacaran Member IV shales, deposited in the Nanhua Basin (South China), record the recovery from the most negative carbon isotopic excursion in Earth history, known as the Shuram excursion. This member is organic-rich and contains distinctive mineralogical and sedimentological features, described for the first time within the stratigraphic context of all relevant paleoenvironments. The study of this member allows a better understanding of the oceanic paleoenvironmental conditions that governed during the deposition of these organic-rich deposits. In addition, and more interestingly, this study describes the principal biogeochemical processes in the carbon and nitrogen cycles that controlled the production and preservation of organic matter and influenced the recovery from the Shuram excursion. A multidisciplinary methodology comprising nitrogen and carbon geochemistry, Raman spectroscopy and organic facies distribution modelling suggest different but complementary interpretations. Evidence from geochemical analyses suggests a dynamic late Ediacaran scenario where a new equilibrium in the C and N cycles was established whereby photosynthetic primary production overcame secondary production as the main source of organic matter in sediments. New Raman and SEM results support this scenario describing different biomass sources and mineralogical-sedimentological signals of organic matter remineralization and primary production. Consistently, quantitative analyses of paleoproductivity point to high levels of organic matter production in shallow waters during the late Ediacaran in South China. These new observations point to the variable dominance of distinct microbial communities in the late Ediacaran ecosystems, and that extensive photosynthetic primary production and organic matter burial modulated the recovery from the most negative δ13Ccarb excursion in Earth history. Thus, this study fills in a critical knowledge gap by providing a new answer to the recovery of the Shuram excursion. Moreover, this study highlights that Member IV, which is generally overshadowed by the Precambrian-Cambrian boundary research, offers an exceptional opportunity to understand the profound environmental changes during the late Ediacaran Period, at the dawn of the Cambrian explosion of animals.

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