Diagenetic history of the cenozoic carbonate sedimentary rocks of the Northwestern Yucatan Peninsula, Mexico

Includes bibliographical references (pages [289]-304).The goal of this study is to find how the large Terminal Cretaceous Chicxulub Impact Crater controlled the geologic evolution of the northwestern Yucatan Peninsula, Mexico, during the Cenozoic Era. The Crater is roughly occupied by the Chicxulub Sedimentary Basin, which is circumscribed by the Cenote Ring (a ring of karstic sinkholes). Lithologic, petrographic, and geochemical data of 11 cores from inside the Cenote Ring (depths range: 1,100 to 21 mbls) were compared to those obtained on 8 cores from outside the Ring (depths range: 671 to 98 mbls). They reveal that the Chicxulub Sedimentary Basin concentrated the deposition of pelagic and outer-platform sediments during the Paleocene and Eocene, and it filled during the Middle Miocene. Shallow-water deposition predominated outside the Chicxulub Basin during the Cenozoic. δ¹⁸O and δ¹³C in whole-rock, non-vug-filling calcite from cores ranges from −7.14‰ to +0.85‰ PDB, and −4.62‰ to +3.38‰ PDB, respectively. The abundance of the possible features of subaerial exposure (subaerial crusts, vugs, karst, dedolomite) increases upward and outward from the center of the Basin. δ18O and δ13C in whole-rock, non-vug-filling dolomite from cores ranges from −4.62‰ to +3.38‰ PDB, and −4.62‰ to +3.38‰ PDB, respectively. Most samples have negative δ¹⁸O and positive δ¹³C. XRD-determined mole % CaCO₃ varies between 47.28 and 57.24. Polyhedral dolomite, montmorillonite, mixed-layer clay, zeolite, quartz, celestite, and potassium feldspar are present in UNAM 5 core between 270 and 198 mbls. The relative abundance of dolomite is greater outside the Ring. Replacive dolomite is larger, more euhedral, and less stoichiometric inside the Ring than outside. Replacive dolomite and vug-filling dolomite cement formed under shallow-burial conditions in the presence of a fluid dominated by freshwater and/or an anomalously high geothermal gradient. Dedolomitization features (hollow crystals or dolomite rhombs partially replaced by calcite) are more common outside the Ring. Petrographic, microprobe, XRD, and stable isotope data indicate that formation of diagenesic low-magnesium calcite and dolomite was more frequent outside the Chicxulub Basin than inside. Sediment deposition and diagenesis also were controlled by the structural features of the crater (peaks, rings).Ph.D. (Doctor of Philosophy

Sulfur Isotope Fractionation as an Indicator of Biogeochemical Processes in an AMD Passive Bioremediation System

Sulfate, the main dissolved contaminant in acid mine drainage (AMD), is ubiquitous in watersheds affected by coal and metal mining operations worldwide. Engineered passive bioremediation systems (PBS) are low-cost technologies that remediate sulfate contamination by promoting (1) precipitation of sulfate-bearing compounds, such as schwertmannite and gypsum; and (2) microbially-mediated sulfate reduction (BSR) to sulfide with subsequent precipitation of sulfide minerals. In this study, chemical and sulfur isotopic data are used to infer multiple pathways for sulfate sequestration in the Tab-Simco PBS. By simultaneously monitoring sulfate concentrations and δ34SSO4 values at four sampling points across the PBS, we (1) identified that the organic layer within the bioreactor was the primary site of BSR processes contributing to sulfate sequestration; (2) observed seasonal variations of BSR processes; (3) estimated that initially the BSR processes contributed up to 30% to sulfate sequestration in the Tab-Simco bioreactor; and (4) determined that BSR contribution to sulfate sequestration continuously declined over the PBS operational lifetime. Together, our results highlight the utility of combining geochemical and microbial fingerprinting techniques to decipher complementary processes involved in sulfur cycling in a PBS as well as the value of adding the sulfur isotope approach as an essential tool to help understand, predict, prevent and mitigate sulfate contamination in AMD-impacted systems