Authigenic Carbonate Rock from Geologic Settings with and without Gas Hydrate from the Gulf of Mexico

Jung, Woodong¹, Roger Sassen² (1) Texas A&M University, College Station, TX (2) Texas A & M University, College Station, TX

Authigenic carbonate rocks associated with gas hydrate show a considerable difference in the mineralogy between geochemical environments that favor gas-hydrate crystallization and environments that retard gas-hydrate crystallization. This may be a useful tool for gas-hydrate exploration. Authigenic carbonate samples from the seafloor are recovered from the Atwater Valley 425, Mississippi Canyon 118, and the The Elbow 711 in the Gulf of Mexico. We document the carbonate isotopes (δ¹³C and δ¹⁸O) in the geochemical and microbiologic context in which it is found. The δ¹³C of authigenic carbonate rock rarely corresponds to the δC of source carbon such as CH₄. We use petrographic thin-sections to photograph and describe the texture, identify carbonate species and other minerals such as barite, strontium, gypsum, and pyrite. We discover that the δ¹³C of the carbonate is usually heavier than the source carbon (CH₄). This is because carbonate sometimes encloses substantial normal-marine skeletal carbonate. For example, we have studied nodules formed of foraminifera (normal marine carbonate) that are cemented with carbonate strongly depleted in ¹³C. The mineralogy and trace-metal composition provides vital insight to the origin of the fluids from which the carbonate forms. We further characterize the unusual mineralogical components using an electron microprobe to detail map the distribution of unusual elements in the thin-sections.