A. Yu. Leina, O. Yu. Bogdanovaa, Yu. A. Bogdanova, and L. I. Magazinab
a Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia
b Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow,
Russia
e-mail: lein@geo.sio.rssi.ru
Received March 15, 2006; in final form, April 19, 2006
Abstract—Two genetically different types of authigenic carbonate mounds are studied: those within an active
hydrothermal field related to serpentinite protrusions in the zone of intersection of a transform fracture zone
and the Mid-Atlantic Ridge, and those in an active field of methane seepings in the Dnieper canyon of the Black
sea. The general geochemical conditions under which authigenic carbonate formation occurs in the two fields
considered were found. They include the presence of reduced H2S, H2, and CH4 gases at the absence of free
oxygen; the high alkalinity of the waters participating in the carbonate formation; the similarity of the textural
and structural features of authigenic aragonite, which represents the initial mineral of the carbonate matter of
the mounds; the paragenesis of aragonite with sulfide minerals; and the close relation of carbonate mounds with
communities of sulfate-reducing and methane-oxidizing microorganisms. A new mechanism of formation of
hydrothermal authigenic carbonates is suggested; it implies their microbial sulfate reduction over the hydrogen
of the fluid in the subsurface zone (biosphere) of mixing between the hydrothermal solution and the adjacent
seawater.
DOI: 10.1134/S000143700704011X
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