Alla Yu Lein

Methane seep community of the Håkon Mosby mud volcano (the Norwegian Sea): composition and trophic aspects

The Håkon Mosby mud volcano (the Norwegian Sea, depth c. 1250u2005m) was studied in July 1998 by a joint Russian–German–USA–Norwegian expedition on the 40th cruise of the RV Akademik Mstislav Keldysh using the two Mir submersibles. The benthic community is dominated by two species of symbiotrophic pogonophores, Sclerolinum contortum (more abundant) and Oligobrachia haakonmosbiensis. The biomass of S. contortum reaches at least 435u2005gu2005m ; for O. haakonmosbiensis the value is 350u2005gu2005m . Different benthic organisms form associations with each species of pogonophore. The total list of known benthic fauna includes 46 species. A zoarcid fish, Lycodes squamiventer, is a common member of the benthic community. Bacterial mats are found over an extensive part of the crater. The background benthic community is much poorer and is dominated by ophiuroids (Ophiocten gracilis, Ophiopleura borealis). Pycnogonids (Collossendeis proboscidea), buccinid gastropods and asteroids are also present. Stable carbon isotope data showed significant depletion of C in both species of pogonophores: in S. contortum δ C was up to −48.3‰, in O. haakonmosbiensis the value varied from −51.1 to −56.1‰. It can be suggested that the methane carbon contributes to the nutrition of the pogonophoran O. haakonmosbisensis. Carbon isotopes also revealed incorporation of non‐photosynthetic carbon into local trophic webs: δ C in Metacaprella horrida (amphipod) showed −44.9‰, in the tube‐dwelling polychaete (Amphinomidae) −40.6‰. In the bacterial mat δ C varied from −17.6 to −53.0.

FRACTIONATION OF STABLE ISOTOPES OF CARBON AND SULFUR DURING BIOLOGICAL PROCESSES IN THE BLACK SEA

The paper presents literature and authors’ own data on the isotopic composition of sulfur and carbon compounds in the water column and bottom sediments of the Black Sea. The fractionation factors of stable isotopes have been compared with the rates of sulfate reduction, photoand chemosynthesis, methanogenesis, and anaerobic oxidation of methane. In the water column and bottom sediments, the inverse relationship between 32S and 34S fractionation and sulfate reduction rate (measured in situ with the use of NaSO4) was observed. The isotopic composition of hydrogen sulfide in the water column (δ34S = -40.0‰) differs greatly from δ34S of the reduced sulfur compounds of bottom sediments; this confirms the hypothesis that H2S forms in the water column itself. Seasonal dynamics of δ13C of phytoplankton-produced organic carbon was revealed; it was demonstrated that some changes in the isotopic composition of POC occur in the chemocline as a result of the photoand chemosynthetic activity of microorganisms. The data on the isotopic composition of the three main sources of the Black Sea methane are presented. δ13C of the biogenic methane produced in bottom sediments reaches -67.6‰; δ13C of methane from cold methane seeps reaches -65.8‰; δ13C of methane from mud volcanoes ranges from -30.0‰ to -75.0‰. The large-scale process of microbial oxidation of methane results in the production of methane-derived carbonates (δ13C values range from -27.2‰ to -45.6‰). Using the data on the rates of methanogenesis and anaerobic oxidation of methane as well as the data on the isotopic composition of methane, the balance between the methane flux into the water column and its oxidation has been calculated. It was found that the annual methane production and oxidation in the anoxic zone of the Black Sea are 62.9 and 77.7·1010 mol m−2, respectively. About 80% of methane production is concentrated in the water column and 20% of methane is produced in mud volcanoes and cold seep areas (10% each).

Processes of Early Diagenesis in the Arctic Seas (on the Example of the White Sea)

During an early diagenetic stage of sedimentation, a continuous exchange between bottom water and pore solution happens. Throughout the diagenetic stage, an oxygen, SO42−, Ca2+, and Mg2+ diffuse into pore solution from near-bottom water, but the gases generated within sediments (CO2, NH3, H2S, CH4, etc.), and other pore waters’ components (Fe2+, Mn2+) are transferring from surface sediments into near-bottom water. At the same time, as a result of some ions’ oversaturation, a precipitation from pore water of a number of chemical compounds takes place followed by formation of authigenic minerals. Their composition depends on oxidation-reduction conditions within sediments. In oxidizing conditions (in upper sediment layers), ferromanganese mineral associations, as well as glauconite, phosphates, etc., are formed. As the environment becomes reducible after loss of free oxygen due to diagenetic processes, mainly, metal sulfides and carbonate minerals are formed. Such is an idealized scheme of transformation of seawater into pore waters in a humid zone.