Silicon and oxygen stable isotopes in chert as indicators of diagenesis and ocean paleo-environmental conditions

• Main Author: Tatzel, Michael
• Other Authors: m, Prof. Dr. Friedhelm von Blanckenburg, Prof. Louis A. Derry, PhD
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2016
• Subjects: stable isotopes; diagenesis; chert; silica; silicon; oxygen; Precambrian; ddc:550; Southern Ocean
• Online Access: https://refubium.fu-berlin.de/handle/fub188/3709; https://doi.org/10.17169/refubium-7909; https://nbn-resolving.org/urn:nbn:de:kobv:188-fudissthesis000000102205-6

This thesis investigates the influence of early marine diagenesis of siliceous sediment (comprised of opal-A) on the silicon stable isotope composition, the significance of Si stable isotopes in chert from the Precambrian-Cambrian boundary (PcC-boundary), as well as a re-evaluation of oxygen isotopes in chert. Chert forms during burial diagenesis from opal on a time scale of tens of million years. Amorphous opal (opal-A) commonly first transforms to opal-CT (composed of cristobalite and tridymite), which constitutes the sedimentary rock porcellanite. Whether the primary Si isotope composition of the sediment is preserved and what the silicon isotope composition of chert signifies is unknown. To study the impact of early diagenesis on the silicon (Si) isotope record of chert, I have determined the δ30Si (the normalized 30Si/28Si isotope ratio) of Pleistocene and Pliocene siliceous sediment and intercalated early diagenetic porcellanite layers from the Southern Ocean using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). This study implies that the Si isotope composition of siliceous sediment deposited around the Pc-C boundary is not significantly shifted during early diagenesis. At this time siliceous sponges appear in the rock record and were recently hypothesized to have fundamentally contributed to the Late Neoproterozoic Oxygenation Event by way of filtering organic carbon from seawater (for feeding). Oxygenation is thought to have caused the Cambrian “explosion”, i.e. the rapid proliferation of higher life on Earth. The effect of sponges on seawater oxygenation would depend on their abundance, a size that cannot be obtained from the fossil record due to incomplete spicule preservation. Fractionated Si from siliceous sponge spicules, however, is preserved in chert and allows estimating the relative abundance of sponges, and hence for the first time to test this hypothesis. To do so, Si isotopes and redox-sensitive elements were determined on chert and siliceous shale from the deep ...