Methane-derived authigenic carbonates from the northern Gulf of Mexico — MD02 Cruise

Authigenic carbonates were sampled in piston cores collected from both the Tunica Mound and the Mississippi Canyon area on the continental slope of the northern Gulf of Mexico during a Marion Dufresne cruise in July 2002. The carbonates are present as hardgrounds, porous crusts, concretions or nodules and shell fragments with or without carbonate cements. Carbonates occurred at gas venting sites which are likely to overlie gas hydrates bearing sediments. Electron microprobe, X-ray diffraction (XRD) and thinsection investigations show that these carbonates are high-Mg calcite (6–21 mol% MgCO₃), with significant presence of framboidal pyrite. All carbonates are depleted in ¹³C (δ¹³C = − 61.9 to − 31.5‰ PDB) indicating that the carbon is derived mainly from anaerobic methane oxidation (AMO). Age estimates based on ¹⁴C dating of shell fragments and on regional sedimentation rates indicate that these authigenic carbonates formed within the last 1000 yr in the Mississippi Canyon and within 5500 yr at the Tunica Mound. The oxygen isotopic composition of carbonates ranges from + 3.4 to + 5.9‰ PDB. Oxygen isotopic compositions and Mg²⁺ contents of carbonates, and present in-situ temperatures of bottom seawater/sediments, show that some of these carbonates, especially from a core associated with underlying massive gas hydrates precipitated in or near equilibrium with bottom-water. On the other hand, those carbonates more enriched in ¹⁸O are interpreted to have precipitated from ¹⁸O-rich fluids which are thought to have been derived from the dissociation of gas hydrates. The dissociation of gas hydrates in the northern Gulf of Mexico within the last 5500 yr may be caused by nearby salt movement and related brines.     

Halogen systematics in the Mallik 5L-38 gas hydrate production research well,Northwest Territories,Canada: Implications for the origin of gas hydrates under terrestrial permafrost conditions

The authors report here halogen concentrations in pore waters and sediments collected from the Mallik 5L-38 gas hydrate production research well, a permafrost location in the Mackenzie Delta, Northwest Territories, Canada. Iodine and Br are commonly enriched in waters associated with CH₄, reflecting the close association between these halogens and source organic materials. Pore waters collected from the Mallik well show I enrichment, by one order of magnitude above that of seawater, particularly in sandy layers below the gas hydrate stability zone (GHSZ). Although Cl and Br concentrations increase with depth similar to the I profile, they remain below seawater values. The increase in I concentrations observed below the GHSZ suggests that I-rich fluids responsible for the accumulation of CH₄ in gas hydrates are preferentially transported through the sandy permeable layers below the GHSZ. The Br and I concentrations and I/Br ratios in Mallik are considerably lower than those in marine gas hydrate locations, demonstrating a terrestrial nature for the organic materials responsible for the CH₄ at the Mallik site. Halogen systematics in Mallik suggest that they are the result of mixing between seawater, freshwater and an I-rich source fluid. The comparison between I/Br ratios in pore waters and sediments speaks against the origin of the source fluids within the host formations of gas hydrates, a finding compatible with the results from a limited set of ¹²⁹I/I ratios determined in pore waters, which gives a minimum age of 29 Ma for the source material, i.e. at the lower end of the age range of the host formations. The likely scenario for the gas hydrate formation in Mallik is the derivation of CH₄ together with I from the terrestrial source materials in formations other than the host layers through sandy permeable layers into the present gas hydrate zones.