Ninth international conference on - Marum

Abstracts of posters 83
by high content of methane. Deposits of the Black Sea abyssal are characterized by low content of all
homologues of methane. The total content of light homologues of methane averages 0,276x10 -3 cm 3 /kg (a
maximum-1,37x10 -3 cm 3 /kg), heavy homologues - 0,045x10 -3 cm 3 /kg (a maximum - 0,44x10 -3 cm 3 /kg). Among
light homologues ethane prevails.
Attributes of oil and gas presence are expressed in geochemical anomalies of methane of the "mixed" type where
with methane of biogenic origin, migrated to a surface of bottom, thermogenic methane is present, and also by
anomalies of easy and heavy methane homologues. Large breaks are emphasized by a lot of attributes of fluids
migration. Except for geochemical anomalies, anomalies of the increased thermal stream, and also seismic
anomalies of "bright spot" type are marked. The most numerous and contrast seismic anomalies of "bright spot"
type are marked in the West of Shatskiy ridge. Less contrast, but more extensive seismic dynamic anomalies are
in the East-Black Sea basin. We assume a deep gas seep at station 49, dated for a deep break, and on station 84
(CH4=71,2 cm 3 /kg, reef high).
Pore waters. For the first time on Shatskiy ridge pore waters research (182 samples) is carried out, deposits with
mineralized (20÷22 g/l) and strongly freshened (12÷15 g/l) pore water are distinguished. Areas with anomalous
high content of methane homologues are characterized by strong freshening of pore waters of sediments. It is
possible to assume, as the freshened waters together with deep hydrocarbonic fluids rise from depths of
sedimentary thickness. Hence, the chemical content of pore waters is an indirect attribute of migration of fluids
from possible deposits of hydrocarbons.
Thermal stream. Among structures of the Black Sea Shatskiy ridge have a high thermal stream (q), background
value q exceeds on 30 %÷ 40 % the values known for Western and East deep-water basins. On Shatskiy ridge
northwest and southeast areas are detected, the average thermal stream of which is more than 52 mV/m 2 . Local
areas are marked by anomalies exceeding 65÷70 mV/m 2 . Two types of anomalies of thermal stream are notable.
The first type represents the usual localized increase of thermal stream. Anomalies which in a contour of the
increased thermal stream have an area of their lowered values (q=34÷43 mV/m 2 ) concern to the second type. The
most contrast low geothermal parameters are in the East-Black Sea basin. Gas saturated sedimentary thicknesses
and direct gas-bearing traps, being thermal barriers on a way of distribution of a deep thermal stream, cause
presence of negative anomalies of geothermal parameters.
Isotopic composition of dissolved inorganic carbon in the subsurface sediments of the gas
hydrate-bearing mud volcanoes, Lake Baikal.
A. A. Krylov 1,2 , O. M. Khlystov 3 , A. Hachikubo 1 , H. Minami 1 , Y. Nunokawa 1 , H. Tomaru 1 , K. Hyakutake 1 , T. I.
Zemskaya 3 , T. V. Pogodaeva 3 , H. Shoji 1
1 Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan
2 Present address: VNIIOkeangeologia, 1 Angliyskiy pr., St. Petersburg 190121, Russia
3 Limnological Institute SB RAS, 3 Ulan-Batorskaya st., Irkutsk 664033, Russia
Lake Baikal, eastern Siberia, is the largest freshwater basin in the world with a maximal depth of 1642 m (De
Batist et al., 2002) and the only freshwater lake known to contain gas hydrates. First near-bottom methanehydrates
were sampled at the Malenky mud volcanoes in March 2000 (Van Rensbergen et al., 2002; Klerkx et
al., 2003; Matveeva et al., 2003). Investigations of Kida et al. (2006) indicate that the main portion of methane
was produced due to methyl-type fermentation. The authigenic siderites were found in subsurface gas hydratebearing
sediments of the Malenky, K-2 and Irkutsk mud volcanoes. The δ 13 C values of the siderites (+3.3 to
+21.9‰ PDB) indicate that their formation is due to methanogenesis (Krylov et al., 2008).
The isotopic composition of dissolved inorganic carbon (DIC) was measured in the several mud volcanoes
(Malenky, Peschanka, Irkutsk, Goloustnoe). The δ 13 C values of the DIC are varied from the -10‰ PDB in the
uppermost part of sedimentary section up to +20‰ PDB at the layers deeper than 1 m below lake floor.
The oxidation of the rising CH4 in the uppermost sediments is associated with a kinetic isotope effect for carbon
which tends to concentrate the lighter 12 C isotope in the produced CO2. As a result, the 13 C theoretically should
be concentrated in the residual CH4. However, it is not the case for investigated sites where the DIC and CH4
have the general tendency to be depleted in 13 C in the uppermost sedimentary layers. The reason for observed
phenomena is not clear. Since the sulfate-reduction zone in the Lake Baikal freshwater environment is
insignificant, we speculate that the anaerobic methane-oxidation in the near-bottom sediments is, probably,
overwhelmed by the processes of the methane generation.
The strong enrichment of DIC in 13 C at the layers deeper than 1 m below lake floor indicates active
methanogenesis. The methane generation is the exact reason for the enrichments of both siderites and DIC in
13 C.