A. N. Derkachev

Massive barite deposits and carbonate mineralization in the Derugin Basin, Sea of Okhotsk: precipitation processes at cold seep sites

An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite buildups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples showa Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals ( V1%) in the barite fabric imply the presence of H2S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative N 13 C values of these carbonates (s343.5x PDB) indicate methane as major carbon source; N 18 O values between 4.04 and 5.88x PDB correspond to formation temperatures, which are certainly below 5‡C. One core also contained shells of Calyptogena sp. at different core depths with 14 C-ages ranging from 20 680 to s 49 080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO 2�

Authigenic carbonate and barite mineralization in sediments of the Deryugin Basin (Sea of Okhostsk)

Results of the analysis of mineralogy, geochemistry, and isotope parameters (δ13C, δ34S, δ18O, and 87Sr/86Sr) of carbonates and barites from sediments of the Deryugin Basin in the Sea of Okhotsk are presented. The diagenetic nature of carbonates and barites, which are formed as a result of the prolonged activity of cold seeps acting along a fracture zone and supplying methane–and barium-saturated fluids, is determined. Any signs for hydrothermal activity were not observed.

Chapter 17 Multivariate Analysis of Heavy Mineral Assemblages of Sediments from the Marginal Seas of the Western Pacific

Abstract Heavy mineral data of sediments from the marginal seas of the Western Pacific (Bering, Okhotsk, Japan, East China, Philippine, Banda, etc.), from the Tonga and Vanuatu Trenches and from adjacent areas have been evaluated using multivariate statistics and correlation, factor, cluster and discriminant analyses. The overall characteristics of heavy mineral assemblages and the regional variability of mineral associations are described and interpreted. Despite the large latitudinal and environmental range of the study basins, the complexity of source rocks and volcanic signatures are clearly reflected in the distribution of heavy mineral assemblages. Results allowed identification of several mineralogical provinces, linked to distinct hinterland complexes and associated tectonic controls. Q-mode factor analysis differentiated four major groups of assemblages of distinctive provenance. R-mode factors helped to define representative mineral associations, which best characterise the mineral composition of the sediments of the individual marginal seas, and led to the delineation of eight provinces. Lithogeodynamic discriminant diagrams, based on average values of heavy mineral compositions, identify assemblages which are most typical of differing types of magmatic and metamorphic rocks, and discriminate between them and those associated with other crustal lithologies. Changing compositions of the heavy mineral assemblages indicate a clear trend from simple geodynamic environments to composite ones, including oceanic uplifts, young island arcs, evolved and mature island arcs, marginal-continental volcanic ranges, fold belts, ancient cratons and platforms.

Possible variation in methane flux caused by gas hydrate formation on the northeastern continental slope off Sakhalin Island, Russia

The Sakhalin Slope Gas Hydrate Project (SSGH) is an international collaborative effort by scientists from Japan, Korea, and Russia to investigate natural gas hydrates (GHs) that have accumulated on the continental slope off Sakhalin Island, Okhotsk Sea. From 2009 to 2011, field operations of the SSGH-09, -10, and -11 projects were conducted. GH-bearing and -free sediment cores were retrieved using steel hydro- and gravity corers. The concentrations of sulfate ions in sediment pore waters were measured to investigate sulfate concentration–depth profiles. Seventeen cores showed linear depth profiles of sulfate concentrations. In contrast, eight cores and two cores showed concave-up and -down profiles plausibly explained by sudden increase and decrease in methane flux from below, respectively, presumably caused by the formation of gas hydrate adjacent to the core sampling sites.

Fe-Mn crusts from underwater rises of the Kashevarov Trough (Sea of Okhotsk)

Dredging on nonvolcanic underwater rises in the Kashevarov Trough area (Sea of Okhotsk) carried out during Cruise 178 of R/V “Sonne” yielded sedimentary (conglomerates), metamorphic (biotite hornfels, tectonic breccia with inclusions of biotite hornfels), and igneous (granodiorite) rocks as well as Fe-Mn crusts up to 15 cm thick. The study of the structural-textural properties and the thorough analysis of the mineral and chemical compositions of these crusts, including the REE and Y distribution, show that the ore sediments are hydrogenic in nature.

Manifestation of carbonate-barite mineralization around methane seeps in the Sea of Okhotsk (the western slope of the Kuril Basin)

First data are reported on a new manifestation of carbonate-barite mineralization found at a site of methane emanations on the western slope of the Kuril Basin, Sea of Okhotsk. Morphological types of barite, aragonite, and low-magnesian calcite are considered in detail; the results of carbon and oxygen isotope study of carbonate concretions and crusts are presented. It is shown that the barite was formed in sediments owing to the diffusion infiltration of the barium-rich fluids through sedimentary succession. The component and isotope compositions of gases are determined and the relatively elevated content of heavy hydrocarbons is revealed. It was assumed that the relatively heavy isotope composition of carbonates is caused by the influence of fluid released from deep sedimentary horizons owing to the dehydration of clay minerals during post- sedimentation transformations. Obtained data show that the origin of carbonate-barite mineralization is related to the migration of hydrocarbons (mainly methane) and barium-bearing cold gas-fluid flows, which were derived not only from near-surface reservoirs but also from deeper-seated sources.

Sediments of buried paleovalleys on the shelf of East Korean Bay as indicators of holocene paleogeographic settings

The lithology and stratigraphy of Holocene fine-grained sediments were studied in buried paleovalleys on the coastal shelf of the East Korean Bay. The typical natural setting of recent nepheloid near-continental sedimentation in the near-mouth area is defined. It is established that the silting apron and transit-linear settings are untypical of recent sedimentation. Their formation was determined by paleoclimatic changes during the Holocene.

Phosphate inclusions in tuffites from the Krishtofovich Rise (Sea of Japan)

Tuffites of the core taken from the underwater Krishtofovich Rise in the southeastern Sea of Japan contain phosphate inclusions that were studied by scanning electron microscopy and chemical methods. The tuffites relatively enriched in phosphorus represent products of Miocene alkaline volcanism and are overlain by a thin layer of sediments partly transformed into smectites and containing microfossils presumably of the Late Miocene age. The phosphate inclusions variable in shapes and consistency are largely confined to the lower sedimentary layer, although occurring as single fragments of compact rock and nonlithified small inclusions in voids of altered tuffs as well. By their structure, microtextures, and main chemical composition, the phosphate inclusions are similar to the phosphorites from other areas of the ocean, which indicates their similar biogenic-diagenetic origin. Judging from the REE composition in the most phosphatized matter, they originate largely from seawater and, to a lesser extent, from tuffites. The redistribution of mobile phosphorus enclosed in the tuffites results in a small-scale phosphatization insufficient for ore formation.

Mineral composition of sediments from the Eastern Laptev Sea shelf and East Siberian Sea

The mineral composition of the heavy fraction in surface sediments of the Laptev and East Siberian seas has been analyzed. The multivariate statistics methods were used to define the main heavy mineral assemblages with outlining regional differences in their compositions and principal factors responsible for the formation of sediment mineral affinity in the study areas. It is shown that the distribution of the determined heavy mineral assemblages is controlled by the geostructural positions of sedimentation basins.

Characteristic features of the occurrence of gas-fluid emanations on the northeastern slope of Sakhalin Island, Sea of Okhotsk

This paper summarizes the results of combined studies in the areas of the occurrence of gas-fluid emanations in the water column, on the seafloor, and in the upper part of the sedimentary section on the northeastern slope of Sakhalin Island (Sea of Okhotsk). The phenomena typical of methane seeps were characterized in detail: the presence of acoustic anomalies, high concentrations of methane in the water and sediments, and the specific features of the sedimentary section (the textural and structural features, the character of the mineral inclusions and gas hydrates, and the chemical composition). The component composition of the gases (including the accompanying gas hydrates), which is almost free of heavy hydrocarbon homologues of methane; the light carbon isotopic compositions of the methane and carbonate concretions; and the low temperatures of the near-bottom water and sediments suggest that the occurrence of numerous methane emanations is related to gas generation in the upper part of the sedimentary sequence. No evidence was found for the existence of a deep center of gas formation within the seafloor area considered. Typical submarine mud volcanic edifices were also not found.