Authigenesis, biomineralization, and carbon-sulfur cycling in the Ediacaran ocean
Kaufman, Alan J.
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Fossil record of the Ediacaran Period (635-541 Ma) reveals unprecedented rise of early animal life (metazoan) in Earth history. Coupled with this evolutionary milestone, the Earth’s atmosphere and hydrosphere experienced dramatic redox fluctuations. In order to better constrain the redox architecture of the Ediacaran ocean margin, an integrated chemostratigraphic correlation of the Doushantuo Formation in basin scale was conducted (see Chapter 2). The revised redox model suggests that euxinic conditions on the platforms were mainly restricted in lagoonal settings, which helps us to better understand Ediacaran fossil distributions and fluctuated δ13C records in the Ediacaran strata in South China. One of the most distinct features of the Ediacaran chemostratigraphy is the δ13C negative excursion (i.e. Shuram Excursion, or SE) reported globally, which is the largest known C cycle anomaly in Earth history. In order to understand the biogeochemical processes that gave rise to the SE expressed in the upper Doushantuo Formation, systematic petrographic and geochemical investigations were conducted for the outer shelf sections in the Yangtze block (see Chapter 3). Methane-derived authigenic calcite cements and nodules with extreme 13C-depletion were discovered and interpreted as the first empirical evidence of authigenic mineralization associated with the SE. In light of these novel observations, it is proposed that the globally distributed SE may be formed by widespread syndeposition of authigenic carbonates in a sulfate-methane transitional zone positioned at the sediment-water interface in response to a global seawater sulfate increase. Finally, to provide environmental context for the terminal Ediacaran biomineralization of animals, we conducted a high-resolution elemental and isotopic study of the richly fossiliferous Gaojiashan Member (see Chapter 4). Coincident with the first appearance of Cloudina are significant C-S-Ca-Sr cycle anomalies. It is proposed that the onset of calcarious biomineralization of animals may have coincided with an increase in terrestrial weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin. Enhanced concentration of Ca ion in seawater may have promoted the calcarious biomineralization of the early animals. Integrated chemo-, bio- and litho-stratigraphy of the Doushantuo and Dengying formations presented in this dissertation emphasized intimate co-evolution of Earth-life system during the Ediacaran Period.