Study of gas biogeochemical parameters of gaseous fluids from the bottom of the Black Sea

Abstract

Gaseous fluid flows from sediments into bottom water of seas and oceans attract attention of oceanographers, particularly specialists in gas geochemistry [1]. Fluid flows from the Black Sea bottom were the main study object in Cruise 61 of the R/V Professor Vodyanitskii , carried out in the Ukrainian segment of the Black Sea in June‐July, 2004. The expedition was organized and conducted by the Division of Marine Geology and Sedimentary Ore Formation, National Academy of Sciences of Ukraine. Gas biogeochemical parameters were investigated at the Institute of Oceanology (Moscow). These works were aimed at studying spatial variations in gas biochemical parameters in seawater and bottom sediments in the zone of local fluid flows. Bottom water was sampled in fluid discharge areas detected by multibeam sonar as a gaseous plume. Samples were taken closest to the fluid discharge site in the depth interval of 200‐2000 m. Specially designed equipment and methods were used for the study of gas biogeochemical parameters [2]. Water samples were taken with a hermetic bottom bathometer from the bottom layer of 1.5‐2.0 m above the seafloor, with the bathometer cartridge at several meters above the bottom, and by integration of the bathometer from the water column located 100‐150 m above the bottom. Bottom sediments were sampled by a gravity corer with a plastic container that fills with sediments in situ. In the case of spontaneous degassing of gas-saturated sediment after dismantling of the corer on deck, the released gas mixture inflated the container and the gas sample was taken by syringe for chromatographic analysis. Then, sediment samples were taken at different levels of the core. The content of gas components (hydrocarbons, helium, hydrogen, carbon dioxide, and permanent gases) and biochemical parameters, such as the ATP content and alkaline phosphomonoesterase activity (APHA), were simultaneously determined in water and sediment samples. The article presents new factual data on underwater fluid discharge areas. Methane and carbon dioxide appeared to be most informative among gas components. Hydrogen, helium, and homologues of methane were not detected, because their concentration in the analyzed gas mixture was below the sensitivity of the chromatograph. The gas mixture was either sampled immediately from the plastic container of the gravity corer (or hermetic core catcher) or was extracted by phase-equilibrium degassing of seawater and sediment samples. In both cases, air was the main gas carrier. Therefore, oxygen and nitrogen were also measured methodologically. In total, approximately 100 samples were analyzed. The gas mixture contained methane (0.0‐89.8 vol %) and carbon dioxide (0.0‐15.5 vol %). The total content of oxygen and nitrogen varied from 8.65 to 100 vol %. Table 1 presents data on various gas mixtures obtained from the gravity corer container and hermetic core catcher or extracted from water and sediment samples. Biochemical parameters were measured in the same water and sediment samples. In addition, certain intervals of the core and bottom and surface water samples were analyzed to estimate biological activity of seawater medium in study areas. Table 2 presents gasometric and biochemical data. The data from Tables 1 and 2 make it possible to disclose some peculiarities in the distribution of gas biogeochemical parameters. In the study areas, bottom water is saturated with methane and carbon dioxide. The biological activity of water is sufficiently high and