BENTHIC FORAMINIFERA AS INDICATORS OF "DEGASSING PIPES" RELEASING DEEP HYDROCARBON FLUIDS ON THE BLACK SEA BOTTOM

The Black Sea basin is the largest hydrocarbon reservoir on Earth. According to the theory of global fluidogenesis, deep fluids enriched mainly with methane move from the mantle upward through “degassing pipes” to the seafloor. These pipes are formed by tectonic ruptures that create weakened zones in the sedimentary cover allowing filtration streams of fluids to rise to the seafloor surface. Currently, about 80 billion m³ of methane are dissolved in Black Sea sediments and water. Methane saturation has an impact on biota, particularly meiobenthos. This presentation describes how benthic foraminifera reveal the presence of "degassing pipe" outlets through their taxonomic composition, quantitative distribution, as well as morphology, size, geochemistry, and the isotopic signatures of their tests.

The study areas are located on the Black Sea northwestern shelf and continental slope and were investigated within the frameworks of the HERMES (water depth 71-905 m, salinity of bottom water 18.6-22.0 psu) and ODESSA (water depth 16.8-34.8 m, salinity 8.7-17.2 psu) projects. 45 and 46 stations, respectively, have been sampled and studied by a variety of methods (e.g., Kadurin et al., this volume).

Micropaleontological analysis was performed in the Micropaleontological Laboratory of Odessa I.I. Mechnikov National University. Isotopic study of foraminiferal tests was performed at the Mass Spectrometric Center for Solid-Phase, Gas, Isotope and Trace Element Analysis at the Institute of Geochemistry, Mineralogy and Ore Formation named after M.P. Semenenko of the National Academy of Sciences of Ukraine.

29 and 10 species, respectively, of benthic foraminifera were identified in bottom sediments of each study area. Locations of "degassing pipe" outlets on the seafloor were characterized by a decrease in the quantitative and taxonomic characteristics of foraminifera along with an increase in the relative abundance of lagenids and of pyritized and deformed tests, and a decrease in ¹³C and size of foraminiferal tests. Moreover, all these parameters permitted the division of "degassing pipe" outlets into central, marginal, and outer parts—similar to the divisions based on hydrocarbon gas geochemistry as shown in Kadurin et al. (this volume).