[1] We report the field observation of hydrate deposits of different crystal structures in the same cores of a mud volcano in the Kukuy Canyon. We link those deposits to chemical fractionation during gas hydrate crystallization. Gas composition and crystallographic analyses of hydrate samples reveal involvement of two distinct gas source types in gas hydrate formation at present or in the past: microbial (methane) and thermogenic (methane and ethane) gas types. The clathrate structure II, observed for the first time in fresh water sediments, is believed to be formed by higher mixing of thermogenic gas. Citation: Kida, M., et al. (2006), Coexistence of structure I and II gas hydrates in Lake Baikal suggesting gas sources from microbial and thermogenic origin,

A unique 800-yr-long record of annual temperatures and precipitation over the south of western Siberia has been reconstructed from the bottom sediments of Teletskoye Lake, Altai Mountains using an X-ray fluorescence scanner (XRF) providing 0.1-mm resolution timeseries of elemental composition and X-ray density (XRD). Br content appears to be broadly correlative with mean annual temperature variations because of changes in catchment vegetation productivity. Sr/Rb ratio reflects the proportion of the unweathered terrestrial fraction. XRD appears to reflect water yield regime and sediment flux. Sedimentation is rather continuous because annual clastic supply and deposited mass are the same. The artificial neural networks method was applied to convert annual sedimentary time-series of XRD, Br content, and Sr/Rb ratio to annual records of temperature and precipitation using a transfer function. Comparison of these reconstructed Siberian records with the annual record of air temperature for the Northern Hemisphere shows similar trends in climatic variability over the past 800 yr. Estimated harmonic oscillations of temperature and precipitation values for both historical and reconstructed periods reveal subdecadal cyclicity.

Lake Baikal is the only freshwater reservoir on Earth with gas-hydrate accumulations in its bottom sediments, partly due to the activity of mud volcanoes. This paper describes a group of mud volcanoes recently discovered on the slope of the Academician Ridge between the northern and central Lake Baikal basins. Our analysis of diatom skeletons in the mud breccia sampled from the study area shows a high abundance of Cyclotella iris et var. These extinct species were also discovered in a core sample from BDP-98 borehole. Based on the biostratigraphic and seismostratigraphic correlations, the age of the mud breccia in the studied mud volcanoes ranges from the Late Miocene to the Early Pliocene (4.6 to 5.6 Ma). The correlations suggest that the material originated from a depth of less than 310 m below the lake bottom.

New high-resolution seismic reflection data from the central part of Lake Baikal provide new insight into the structure and stratigraphy of Academi-

[1] Knowledge of cage occupancies and hydration numbers (n) of naturally occurring gas hydrate in a local environment is important for the improvement in global estimates of hydrate-bound natural gas. We report on local differences in cage occupancies and hydration number of gas hydrates from Lake Baikal. Natural gas hydrates of both structures I and II (sI and sII) and ranging in composition from pure CH 4 to mixed gas hydrate containing up to 15% C 2 H 6 are compared. The average hydration numbers are n = 6.1 for the sI CH 4 hydrates recovered from the Malenky and Bolshoy mud volcanoes, n = 6.2 for the sI hydrates, containing 3-4% C 2 H 6 recovered from the K-2 mud volcano, and n = 6.9 for the sII hydrate containing about 15% C 2 H 6 recovered from the K-2 mud volcano. The differences in hydration number are due to the differences in the small cage occupancy of CH 4 among the samples studied.

Clathrate hydrates are inclusion compounds consisting of a hydrogen-bonded crystal of water molecules with cages that contain a guest molecule. The crystal structures of types I, II, and H are well known, [1] and two other types of clathrate hydrate have recently been reported.[2] Guest-host interactions play a crucial role in the crystal structure, and thus clathrate hydrates are thermodynamically stable only when guest molecules are encaged in the host cages. The relation between structural type of clathrate hydrate and guest molecule is categorized according to the size of the guest molecule.[3] However, the use of these categories is not clear when applied to mixed hydrates; for example, a structural transition in methane (CH 4 ) + ethane (C 2 H 6 ) mixed-gas hydrates from type I to II was observed, even though pure CH 4 and C 2 H 6 each form type I. [4,5] There is not much difference between the molecular size of CO 2 (ca. 5.1 ) and C 2 H 6 (ca. 5.5 ) with respect to the cavities of the hydrate cages, but their effect on hydrate structure may be completely different. Additionally, recent studies have shown that even pure CH 4 and pure CO 2 form type I and type II hydrates in coexistence. It has been shown that the correlation of guest size and formed structure type is not straightforward for pure systems, too.[6]The structural transitions of mixed-gas clathrates are interesting not only from a fundamental point of view but also from the practical viewpoint of natural gas resources and technology.[7] For instance, natural gas clathrate hydrates in oceanic sediments and permafrost, also called natural gas hydrate, are recognized as an alternative natural fuel resource because they contain significant amounts of natural gases. The main gas in the hydrates is CH 4 , but they also contain CO 2 , C 2 H 6 , and small amounts of other gases. In addition, CH 4 + CO 2 mixed-gas hydrates have been studied as a medium to remove CO 2 from air; in particular, natural gas hydrates can be converted to CO 2 hydrate, and thus a fuel is obtained and an unwanted combustion byproduct is removed. [8] We present here our measurements of the lattice expansion of CH 4 + CO 2 and CH 4 + C 2 H 6 hydrates as a function of mixed-gas composition. This is important because lattice expansion plays a crucial role in thermodynamic modeling of hydrate systems. [5, 9] Data for natural gas hydrate from the sediments of Lake Baikal are also reported to examine the effect of small amounts of other gases and the kinetic effect under natural conditions on their crystal structures.X-ray diffraction (XRD) patterns of CH 4 + CO 2 hydrate identified it as type I, which is composed of two 12-hedra and six 14-hedra with space group Pm3n (Figure 1). The crystal structure of CH 4 + C 2 H 6 hydrate changed from type I to type II with increasing C 2 H 6 content in the mixed-gas hydrate, in agreement with previous studies. [4,5] (The type II structure (Figure 1) is composed of sixteen 12-hedra and eight 16-hedra with space group Fd3m.) Since the type I and t...

We report on the first authigenic siderite (FeCO3) concretions recovered from near‐bottom sediments at gas hydrate‐bearing mud volcanoes in fresh water (Lake Baikal, Eastern Siberia). The carbonates appear as firm ‘plate‐type’ formations at the Malenky mud volcano (Southern Baikal Basin) and as soft nodules at the K‐2 mud volcano (Central Baikal Basin). Calcium is the main divalent component which substitutes iron in the carbonate lattice (7 to 20 mol%). The δ13C values of the carbonates (+3.3 to +6.8‰ at Malenky, and +16.5 to +21.9‰ PDB at K‐2) indicate that their formation is due to methanogenesis. The latter was most likely caused by the microbial methyl‐type (acetate) fermentation that is suggested from the isotopic composition of the accompanying methane hydrates and dissolved methane. General depletion of the siderites in 18O (−11.6 to −9.9 ‰ at Malenky, and −13.9 to −12.3‰ PDB at K‐2) is mainly inherited from the isotope composition of pore water (−15.2 to −15.4‰ SMOW) at ambient temperature (3.5°C).

In this paper, we present new seismic and heat‐flow data that show the base of the hydrate stability zone (BHSZ) in Lake Baikal to be locally characterized by abnormal variations in depth, with distinct regions of deeper‐than‐normal and regions of shallower‐than‐normal BHSZ. These variations are related to strong lateral variations in heat flow, and occur in close association with important rift‐basin faults. Areas of shallow BHSZ are also characterized by the presence of several methane seeps and mud volcanoes at the lake floor. We infer that the seeps are the surface expression of escape pathways for overpressured fluids generated by the dissociation of pre‐existing hydrates, in response to a thermal pulse caused by an upward flow of hydrothermal fluids towards the BHSZ. It thus seems that present‐day hydrate dissociation in Lake Baikal is modulated by the tectonic activity in the rift rather than by – climatically controlled – changes in lake level or water temperature.