Tian, Zheyu; (2022) Reconstructing Biogeochemical Cycles During and After the Late Ediacaran DOUNCE/Shuram Excursion. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, I made carbon, oxygen, and sulfur chemostratigraphic study of three representative sections in South China (Lianghong, Sishang and Qinglinkou sections). These data confirm that the DOUNCE/Shuram (EN4) was a basin-wide event; Data from the Qinglinkou section also confirm the other negative excursions (EN1=CANCE, EN2=WANCE, EN3=BAINCE). Field observations and C isotope data from the Qinglinkou section suggest that the ‘middle carbonate’ unit, which occurs between two black shale layers of the upper Doushantuo Formation, corresponds to a slumped unit that likely derives from the overlying Hamajing Member (Lower Dengying Formation). I designed and compared three Carbonate-associated sulfate (CAS) extraction methods to minimize the influence caused by contaminants. My results suggest that a sequential NaOCl, H2O2, and NaCl pre-leaching method is most effective for eliminating contaminants. For purer carbonate samples, more traditional sequential leaching of NaCl and DI water should be sufficient. My CAS data illustrates that sulfate δ34S (~21‰), Δδ34Ssulfate-sulfide (~30‰) and estimated seawater sulfate concentrations (~29mM) show similar values to modern levels during the DOUNCE/Shuram. However, after the DOUNCE/Shuram, the sulfate reservoir shrank to pre-excursion levels (as low as 0.6 mM), influencing the S cycling pathways and resulting in a decoupled C-S isotope trend. The seawater 87Sr/86Sr during the DOUNCE/Shuram suggests high continental weathering during that interval, which would have contributed to elevated nutrient, Fe, and sulfate fluxes into the ocean. In summary, my work shows how global tectonic forces might have driven the DOUNCE/Shuram. I also analyzed the representative samples for U isotope and redox-sensitive trace elements (RSTE), e.g., U, Mo, V, concentrations. Overall, the data reveal a pulsed oxygenation history in late Ediacaran and indicate that the DOUNCE/Shuram interval is not a simple oxygenation event. Moreover, a final oxygenation event of the late Ediacaran was recognized that might directly link with the ‘Cambrian Explosion'.

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