Authigenic carbonate rock (ACR) is derived from microbial oxidation of methane, biodegradation of crude oil, and oxidation of sedimentary organic matter. The precipitation of ACR was characterized petrographically, mineralogically, and geochemically. ACR collected from the seafloor in the Gulf of Mexico (GOM) and ACR recovered from drilled cores in the Krishna-Godawari (KG) basin offshore India were used. All study sites are associated with hydrocarbon gas venting, seepage, free gas, or gas hydrate. ACR from the GOM is densely cemented and extremely irregular in shape, whereas ACR from offshore India is generally an oval-shaped smooth nodule and also densely cemented. The dominant mineral in ACR is authigenic calcite. ACR contains carbon derived from sedimentary organic carbon oxidation that geologically sequesters much fossil carbon. Bulk carbon and oxygen isotopes of ACR were measured. ACR from the GOM is strongly depleted in 13C with ?13C of ?42.5? and enriched in 18O with ?18O of 4.67?. The ?13C of hydrocarbon is typically more depleted in 13C than in the associated ACR. The reason is that authigenic carbonate cements from hydrocarbon oxidation generally enclose skeletal material characterized by normal marine carbonate. Three groups that represent different hydrocarbon sources to ACR were classified in this study: primary carbon sources to ACR from (1) methane plus biodegraded oil, (2) methane, or (3) biodegraded oil. Wide ranges in ?13C (?49.12 to 14.06?) and ?18O ( 1.27 to 14.06?) were observed in ACR from offshore India. In sediments, the ?13C may be affected by differences in the rate of organic carbon oxidation, which generate varying ?13C with depth during methanogenesis. Based on the wide range in ?13C, ACR from offshore India was classified: (1) ?13C may reflect high rates of organic carbon oxidation, (2) ACR may be derived primarily from methane oxidation, and (3) ?13C may reflect low rates of organic carbon oxidation. ?18O values are heavier than those of normal marine carbonates. The ?18O may be caused by reaction with deep-sourced water that was isotopically heavier than ambient seawater. Some samples may reflect heavy ?18O from gas hydrate decomposition, but it would not cause significant heavy oxygen isotopes.

It has been hypothesized that authigenic carbonate minerals, formed within the pore spaces of marine siliciclastic formations during early diagenesis, may have had a substantial influence on the global carbon cycle, particularly in times of low oxygen in Earth history. According to this idea, alkalinity is generated via anaerobic organic matter degradation, resulting in carbonate oversaturation and the precipitation of low δ13C carbonate cements. If a substantial amount of 13C-depleted carbonate was sequestered in this authigenic sink, the δ13C of dissolved inorganic carbon (DIC) in the global ocean would be driven to more positive values without significant organic carbon burial - a signal which would be recorded in marine carbonates. Research presented herein tests this hypothesis from newly acquired lithostratigraphic and coupled stable carbon and oxygen isotope data of Upper Devonian limestone and black shale formations preserved within the Western Canada Sedimentary Basin. The Late Devonian includes a mass-extinction event, and is characterized by pervasive ocean anoxia and a dramatic reduction in platformal carbonate sediment deposition. As such, it has been hypothesized to represent an ideal time for the emergence of an active authigenic carbonate sink. Results show that both basinal shale (Besa River and Exshaw formations) and platform carbonates (Wabamun Group and its equivalents), record a δ13C signal that is within the expected range of Devonian seawater (3‰ to -2‰), signifying that precipitated authigenic carbonate had no influence on the isotopic composition of DIC. It was observed, however, that evaporitic depositional settings can accumulate carbonate sediment with low δ13C values (down to -8.4‰), potentially caused by local water column organic matter respiration during prolonged water-mass residence in a restricted marginal marine setting. If such depositional environments were globally pervasive, such as during during global sea-level lows, it is plausible that the carbon isotope mass balance would be affected. / Graduate / 2021-06-18

Methane derived authigenic carbonate (MDAC) precipitation occurs within marine sediments as a byproduct of the microbial anaerobic oxidation of methane (AOM). While these carbonates form in chemical and isotopic equilibrium with the fluids from which they precipitate, burial diagenesis and recrystallization can overprint these signals. Plane polarized light (PPL) and cathodoluminescent (CL) petrography have allowed for detailed characterization of carbonate phases and their subsequent alteration. Modern MDACs sampled offshore in northern Cascadia (n =33) are compared with paleoseep carbonates (n =13) uplifted on the Olympic Peninsula in order to elucidate primary vs. secondary signals, with relevance to interpretations of the carbonate record. The modern offshore environment (S. Hydrate Ridge and Barkley Canyon) is dominated by metastable acicular and microcrystalline aragonite and hi-Mg calcite (HMC) that with time will recrystallize to low-Mg calcite (LMC). The diagenetic progression is accompanied by a decrease in Mg/Ca and Sr/Ca ratios while variation in Ba/Ca depends upon the Ba-concentration of fluids that spur recrystallization. CL images discern primary carbonates with high Mn/Ca from secondary phases that reflect the Mn- enrichment that characterizes deep sourced fluids venting at Barkley Canyon. Methane along the Cascadia continental margin is mainly of biogenic origin, where reported strontium isotopic values reflect a mixture of seawater with fluids modified by reactions with the incoming Juan de Fuca plate. In contrast, the Sr-isotopic composition of carbonates and fluids from Integrated Ocean Drilling Program (IODP) Site U1329 and nearby Barkley Canyon point to a distinct endmember (lowest ⁸⁷Sr/⁸⁶Sr = 0.70539). These carbonates also show elevated Mn/Ca and δ¹⁸O values as low as -12‰, consistent with a deep-source of fluids feeding thermogenic hydrocarbons to the Barkley Canyon seeps. Two paleoseep carbonates sampled from the uplifted Pysht/Sooke Fm. have ⁸⁷Sr/⁸⁶Sr values similar to those of the anomalous Site U1329 and Barkley Canyon carbonates (⁸⁷Sr/⁸⁶Sr = 0.70494 and 0.70511). We postulate that the ⁸⁷Sr-depleted carbonates and pore fluids found at Barkley Canyon represent migration by the same type of deep, exotic fluid as is found in high permeability conglomerate layers down to 190 mbsf at Site U1329, and which fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52-57 Ma igneous Crescent Terrane, which is located down-dip from both Barkley Canyon and Site U1329. This previously unidentified endmember fluid in northern Cascadia may have sourced cold seeps in this margin since at least the late Oligocene. / Graduation date: 2012