Geological and Geochemical Explorations of the Salitre Formation Phosphorite, Eastern Brazil

Citation

Sanders, Cecilia Brooke (2023) Geological and Geochemical Explorations of the Salitre Formation Phosphorite, Eastern Brazil. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rd4m-7x08. https://resolver.caltech.edu/CaltechTHESIS:07202022-060317708

Abstract

This thesis documents my explorations of an ancient seafloor environment through sedimentary geology and stable isotope geochemistry. The geologic record of this seafloor — its life, environmental conditions, lithification, burial and exposure — consists of hundreds of meters of sedimentary rock, outcropping across Bahia and Minas Gerais, Brazil. Though it consists primarily of carbonate grains surrounded by carbonate cements, such as might be found forming in any shallow carbonate platform, this record also contains one of Brazil’s most extensive sedimentary phosphate deposits. In these deposits, phosphate is concentrated as carbonate fluorapatite cements (CFA) in digitate stromatolites, distinctively finger-like, branching accretionary structures likely formed by the accumulation of sediment by microbial mats and biofilms. Chapter 1 introduces the broader motivations of this kind of paleoenvironmental and paleoecological research, for understanding the record of life on Earth and other worlds. Chapter 2 presents new sedimentological and stratigraphic data which interpret the depositional setting of a seafloor 600 million years ago on the rending supercontinent of Gondwana. Chapter 3 presents new carbon and oxygen isotopic measurements and clumped isotope measurements of structural carbonate in phosphatic and non-phosphatic textures of the rock, and uses them to constrain the alteration history of the rock and its effect on the record of primary depositional conditions. Chapter 4 presents new data on the sulfur isotope composition of specific minerals in the rock, combining several disparate analytical methods to draw conclusions about the metabolism of the stromatolites’ microbial architects. Chapter 5 describes the distribution of organic material and style of fossilization, and presents preliminary data which suggest a possible mechanism by which microbial activity may have facilitated phosphate concentration and mineralization. As a whole, this thesis demonstrates the value of multidisciplinary analyses in the reconstruction and understanding of sedimentary phosphorite deposits throughout Earth history, improving our understanding of how and to what extent phosphorites may record the history of life and the environment.

Thesis Files