The paper presents the results of U–Pb SHRIMP-II dating and isotope-geochemical data on zircon xenocrysts extracted from igneous rocks of the Shaka Ridge, South Atlantic. We compared the obtained data and the results of studying young “oceanic” zircons from rocks of the Atlantis Bank, Central Atlantic. The age of xenogenic zircon grains, recorded in the U–Pb isotope system, changes from Proterozoic (ca. 1.1 Ga) to Mesozoic (ca. 180 Ma). Zircon possesses geochemical characteristics of igneous origin: The Th/U ratio is from 0.01 to 19.4 and REE distribution spectra differentiated from light to heavy lanthanides with pronounced positive Ce and negative Eu anomalies. In the discrimination diagrams showing ratios between Yb, U, and Y, the studied zircon falls within the field of continental crustal rocks, including the field of continental granitoids. The crustal source of zircon xenocrysts is also suggested by the higher Li content in zircon. The data on oxygen isotope composition in zircon indicate either the presence of crustal signatures or probable hydrothermal effect. Most likely, the results indicate the multiplicity of sources from which zircon xenocrysts were transported.

Представлены результаты геохимических исследований (главные, редкие и редкоземельные элементы) для зерен клинопироксена, выделенных из габброидов хребта Шака, расположенного в Южной Атлантике. Исследуемый клинопироксен принадлежит к Ca-Mg-Fe-типу и характеризуется плавным изменением химического состава от центра к краю, выражающимся в снижении значения магнезиальности Mg#, возрастании суммарного содержания REE и более отчетливом проявлении отрицательной Eu-аномалии. На основании геохимических и морфологических особенностей изученных зерен сделан вывод о существенном влиянии фракционной кристаллизации на состав клинопироксена в ходе его образования. Оценка P-T-параметров по нескольким методикам позволила обозначить узкий диапазон значений температур (1225–970 °С) и давлений (3–1 кбар) при которых кристаллизовался клинопироксен.

Ferromanganese (Fe-Mn) crusts collected from two seamounts of the Vietnam continental margin, central South China Sea were analyzed for mineralogical and chemical compositions, and dated using Th-230(excees) technique and cobalt chronometry. The studies of dissolved oxygen content in water column at the area by CTD-profiling display Oxygen Minimum Zone locates at the depth 420 to 1100 m with O2 concentration of 1.6-1.8 ml/l. The composition and morphology of studied crusts are different from Prime Crust Zone crusts in Pacific Ocean and partially similar to ferromanganese crusts and nodules from northern South China Sea due to unique environment of basin. The Fe-Mn crusts from Vietnam margin show high ratios of Fe relative to Mn and Ni relative to Co, high Th and Pb concentrations and comparable to PCZ deposits low contents of REE. Few main mechanisms were dominated in crust formation at Vietnam margin including a permanent input of detrital material sourced by river discharge, shelf sediments and dust plume while Fe and Mn (oyxhydr)oxides slow precipitate from seawater as initially colloidal particles within oxygen-minimum zone predominantly at the surface of biogenic limestone substrate. Upwelling and high coastal primary productivity are important factors in Fe, Ni, Th and Pb transportation and accumulation. The main factor of high Fe/Mn ratio is the significant flux of Fe released by redox cycling in the sediment. SCS crusts are enriched in Ni relative to Co due to influx of Ni from coastal laterite weathering and optimum conditions for its transportation to crust formation area. Thorium and lead related to significant continental detrital input and eolian supply, especially Asian dust. Due to diversity of crust thicknesses local hydrographic current regime is also suggested as crucial factor in crust formation. Calculated growth rate using Th-230(excees) technique is 11 +/- 1 mm/Myr and 4.4 +/- 0.5 mm/Myr for two measured crusts. The crust growth in South China Sea might be initiated around 1.5 Myr ago, the period which coincide with the Southern Ocean glacial-induced restructuring dated 1.6 Myr.

Permafrost thawing leads to mobilization of the vast carbon pool into modern biogeochemical cycling through the enhanced release of dissolved organic matter (DOM) and production of greenhouse gases (CO2 and CH4). In this work, we focus on the study of methane and DOM distribution and genesis in the ground ice samples of thermodenudational exposure in the Central Yamal (Russian Arctic). We propose that the liberation of the ice-trapped CH4 and generation of CO2 by DOM mineralization are the earliest factors of atmospheric greenhouse gases emission as a result of permafrost thawing. The observed enormously “light ” isotope signatures of methane (δ13C < −80‰, δD < −390‰) found in the tabular ground ice units significantly divergent in morphology and localization within the exposuremay be related to subzero (cryogenic) carbonate reduction a as significant factor of the local methane enrichment. DOM is mainly formed (>88%) by biochemically refractory humic acids. Distribution of the labile protein-like DOM reflects the specific features of carbon and nitrogen cycles in the tabular ground ice and ice wedge samples. Tabular ground ice units are shown to be a significant source of methane and high quality organic matter as well as dissolved inorganic nitrogen (DIN). Ice wedges express a high variation in DOM composition and lability.

Driven by rising bottom water temperatures, the thawing of subsea permafrost leads to an increase in fluid flow intensity in shallow marine sediments and results in the emission of methane into the water column. Limiting the release of permafrost-related gas hydrates and permafrost-sequestered methane into the global carbon cycle are of primary importance to the prevention of future Arctic Ocean acidification. Previous studies in the South Kara Sea showed that abundant hydro-acoustic anomalies (gas flares) induced by seafloor gas discharge into the water column occur in water whose depth is >= 20 m. This distribution of gas flares could indicate the outer extent to which continuous permafrost restricts upward fluid flow. This paper reports on a geochemical analysis of a 1.1m long sediment core located in an area of shallow fluid flow off of the Yamal Peninsula coast (South Kara Sea) using high-resolution seismic data. Our results reveal a thin zone of Anaerobic Oxidation of Methane (AOM), a sharp shallow sulfate-methane transition (SMT) located at a sub-bottom depth of 0.3 m, and significant temporal variation in methane discharge confirmed by the pyrite (FeS2) distribution in the core sample. A concave up pore water chloride profile depicts upward fresh/brakish water advection in subsurface sediments. The terrestrial/fresh water genesis of methane from the sampled core is deduced from the stable isotopic signatures (delta C-13 and delta D). We propose two mechanisms for the observed fluid flow: i) convection of thaw water from subsea permafrost; and/or ii) lateral sub-permafrost ground water discharge marking the outer extent of continuous permafrost off of the central Yamal Peninsula coast at (similar to)45m water depth.

Since the Last Glacial Maximum (~19 ka), coastal inundation from sea‐level rise has been thawing thick subsea permafrost across the Arctic. Although subsea permafrost has been mapped on several Arctic continental shelves, permafrost distribution in the South Kara Sea and the extent to which it is acting as an impermeable seal to seabed methane escape remains poorly understood. Here we use >1300 km of high‐resolution seismic data to map hydroacoustic anomalies, interpreted to record seabed gas release, on the West Yamal shelf. Gas flares are widespread over an area of at least 7500 km 2 in water depths >20 m. We propose that continuous subsea permafrost extends to water depths of ~20 m offshore and creates a seal through which gas cannot migrate. This Arctic shelf region where seafloor gas release is widespread suggests that permafrost has degraded more significantly than previously thought.