Mineralogical and isotopic analysis of authigenic carbonates from different gas hydrate-bearing seepage structures in the Derugin Basin (Sea of Okhotsk) are presented. The analysis showed the existence of four morphological types of carbonates, with all of them mainly of Mg-calcite.13C values of carbonates generally light owing to the inheritance of carbon from microbial methane. 13C-enriched samples at the VNIIOkeangeologia structure with 13C values of up to +9.3‰ represent carbonate precipitation due to methanogenesis. The calculated equilibrium 18O values of carbonates in general correspond to measured values.

Cold seep-carbonates are the microbially mediated by-products of the anaerobic oxidation of methane (AOM) at seafloor cold seeps, and are widespread about modern continental margins and in the geologic record. Some modern and Miocene examples of cold seep-carbonates from the East Coast Basin, North Island, New Zealand have been analysed in this study, to characterise and determine their carbonate fabrics, elemental and mineralogical composition, and stable δ18O and δ13C isotope signatures, so as to provide insights into the diagenetic changes associated with the lithification and burial of seep-carbonates. The ancient samples were collected from the onshore middle Miocene Tauwhareparae (TWP) seep deposit, while the modern samples were obtained from the National Institute of Water and Atmosphere (NIWA) Cruise TAN0616 (November 2006) from Ritchie Ridge, offshore Hikurangi Margin. A paragenetic sequence of diagenetic events involving early aragonitic phases, followed by late calcitic phases is defined for the seep-carbonates. This sequence likely has relevance for understanding the fluid-cement histories of seep-carbonates more widely. Two main carbonate mineralogies occur in each of the sample groups - modern samples are aragonitic or dolomitic, while the ancient ones consist dominantly of either aragonite or calcite. Thus, aragonite common to both sample groups, and is interpreted to represent the initial primary carbonate precipitate in hydrocarbon seep provinces under specific fluid flux and local pore-water chemistry conditions. Aragonite morphologies range from microcrystalline carbonate ('micarb'), to acicular aragonites that may form botryoids or spherulites. Dolomite occurs in those modern samples which appear to constitute exhumed remnants of a former subsurface 'seep plumbing system', and so are strictly not true seabed 'seep-carbonates', but instead are part of the larger hydrocarbon seep province. Calcite in the ancient samples is either a product of alteration and neomorphic transformation of aragonite, or derives from late stage cementation from burial fluids. As a result of their formation processes, the calcites are generally recrystallised and have equant or 'cellular' textures. Stable δ13C and δ18O isotope cross-plots reveal a large spread of values for the sample groups. Ancient samples range from δ13C -8 to -50 PDB and δ18O -5.5 to +2 PDB. Modern samples have δ13C values from -6 to -41 PDB and δ18O values ranging from +2.6 to +6.7 PDB. The δ13C values suggest the majority of the methane that formed these seep-carbonates is of thermogenic origin, although some mixing from other carbon sources may have occurred. The positive δ18O signatures are suggestive of carbonate formation during dissociation of gas hydrates, while the negative values possibly indicate that some of the formation fluids were warmer than normal in the 17 - 30 C range.

La découverte récente (2013) d'émissions de méthane microbien en rebord du plateau Aquitain (entre 140 et 220 m de profondeur d'eau) sur une bande s'étendant le long de 80 km du nord au sud entre le Cap Ferret et le Capbreton suscite des interrogations sur les modalités de mise en place et d'évolution spatio-temporelle du système fluide associé. Les indices fluides liés à ce système microbien, tels que les carbonates authigènes et les niveaux chargés en gaz, sont localisés dans la zone externe du plateau et s'étendent jusqu'à la partie haute de la pente continentale. Au regard de la géométrie des horizons régionaux, des indices géochimiques et des voies potentielles de migration du gaz, la source de la matière organique pour la génération de ce méthane microbien serait située dans les derniers clinoformes du Pléistocène supérieur, et permettrait la précipitation des carbonates sur une largeur dépendante de l'épaisseur de ces clinoformes. A l'est de ce système, donc plus proche du continent, la présence d'indices potentiels enfouis (carbonates authigènes ou gaz) permet de penser que d'autres systèmes fluide se sont développés au cours de la progradation de la marge Sud Aquitaine, et ce, possiblement depuis le Pliocène. Le développement du système fluide est ainsi étroitement lié à l'évolution de la marge Sud Aquitaine et dépend des flux sédimentaires qui régissent les apports de matière organique dans les unités progradantes. Le caractère unique de ce système microbien dans le Golfe de Gascogne et sa restriction à la marge Sud Aquitaine s'expliquerait essentiellement par le contrôle des apports sédimentaires continentaux. / The recent discovery (2013) of microbial methane emissions along the Aquitaine continental Shelf edge, south west of France (from 140 to 220 m of water depth), extending along 80 km N-S and 8 km E-W, questions about the initiation and temporal evolution of this fluid system. Several fluid evidences related to the microbial system at the edge of the Aquitaine Shelf, such as authigenic carbonates and gas-charged layers extend from the external shelf until the upper part of the continental slope. Based on regional horizon geometry, geochemical evidences and potential migration pathways, the organic source matter for microbial methane is most likely located within the last upper Pleistocene clinoforms of the South Aquitaine margin. Occurrence of potential buried fluid evidence at the east of the fluid system is interpreted as potential evidence for initiation of this fluid system during the Pliocene age. The fluid system initiation and its evolution would be strongly related to the evolution/progradation of the South Aquitaine Margin and thus to the sedimentary regime controlling sediment and organic matter supplies to the shelf and upper continental slope. This relation explains the fact that such fluid system is unique in the Bay of Biscay and is restricted to the South Aquitaine Margin.

Carbonate geochemistry plays an important role in understanding environmental conditions during the time of precipitation. The studies for this dissertation research were focused on carbonate precipitation and crystallization in different chemical and physical environments. The first project aimed to precipitate aragonite at low oxygen levels to identify a correlation between partitioning of trace elements and anoxic and suboxic conditions. The second study focused on the precipitation of amorphous calcium carbonate in varying magnesium concentrations to determine the identity of crystalline material after transformation of ACC. Lastly, the third project was developed to understand transformation of CaCO3 polymorphs. Specifically, whether or not geochemistry is retained from one polymorph to the next. All projects could aid in development of paleoproxies to be used for determining past environmental and climatic conditions in the past.