Stratigraphic and structural controls on the location of active methane seeps on Posolsky Bank, Lake Baikal

The distribution and origin of shallow gas seeps occurring at the crest of the Posolsky Bank in Lake Baikal have been studied based on the integration of detailed seismic, multibeam and hydro-acoustic water-column investigations. In total 65 acoustic flares, indicating gas-bubble release at the lake floor (seepage), have been detected within the 630 km² area of the Posolsky Bank. All seeps are located on the Posolsky Fault scarp near the crest of the Posolsky Bank or on similar locations in water depths of -43 m to -332 m. Lake Baikal is the only fresh-water basin in the world where gas hydrates have been inferred from BSRs on seismic data and have been sampled. Our seismic data also portray BSRs occurring up to water depths of -300 m, which is much shallower than the previously reported -500 m water depth. Calculations for hydrate stability, heat flow and topographic effect based on the BSR occurrence and multibeam bathymetry allowed the determination of a methane-ethane gas mixture and heat-flow values wherefore gas hydrates could be stable in the lake sediments at the given ambient conditions. None of the seeps associated with the Posolsky Bank have been detected within this newly established gas-hydrate stability zone. Our observations and data integration suggest that the seeps at the crest of Posolsky Bank occur where gas-bearing strata are cut off by the Posolsky Fault. These gas-bearing layers could be traced down the Posolsky Bank to below the base of the gas-hydrate stability zone (BGHSZ), suggesting that the detected seeps on the crest of the Posolsky Bank are mainly fed by gas coming from below the BGHSZ

Baikal mud volcanoes: thermal features of dynamic gas hydrate systems

In Lake Baikal shallow gas hydrates have already been identified in five mud volcano/seep structures through joint Russian, Japanese and Belgian research. These mud volcano/seep structures are found at different water depths (from 1380 m to as shallow as 440 m) and contain shallow hydrates of both structure I and II. Bottom Seismic Reflections (BSRs), indicative for the presence of deep-seated hydrates, has been observed on nearby seismic profiles. We will report on detailed thermal investigations in association with gravity coring performed over the last three years in the following gas hydrate containing mud volcanoes: “K-2”, “Malenkiy” and “Bolshoy”.The “K-2” mud volcano is located on the flanks of the Kukuy Canyon at a water depth of 900 m water depth. This oval structure of 60 m in height and 800 m in diameter consists of two separate mud volcanoes corresponding to two culminations. Sediment cores have been retrieved in more than 75 sites (15 contained hydrates), with temperature sensors attached to the corer in 22 occasions. Shallow hydrates were only found in two zones of not more 50-100 m diameter: on the top and between the two culminations. These zones also stand out by anomalous low (30-43 mK/m) and high (90-113 mK/m) thermal gradients in comparison to what is measured outside the mud volcano (60-70 mK/m). Cores with hydrates were directly correlated to low thermal gradient and large non-linearity in the temperature-depth profiles. This can be explained in three ways: (1) heat absorption by hydrate dissociation; (2) topographic effect combined with a dynamic hydrate system; and (3) infiltration of cold lake water, possibly induced by local convection and/or water segregation. The localized occurrence of hydrates within the mud volcanoes and a close relation to thermal anomalies was also observed in the mud volcanoes “Malenkiy” and “Bolshoy”, located at a water depth of about 1380m. More than 30 gravity cores in both structures indicate zones with shallow hydrates in local depressions and on culminations. Thermal stations show the presence of anomalous thermal gradients, up to 180 mK/m, at short distances of background values.The mud volcanoes in Lake Baikal do not display a strong activity in terms of acoustic flaring in the water column (almost absent) and large-scale temperature anomalies (< 1 °C). However, they comprise local shallow hydrate systems in close association with anomalous low and high thermal gradients. A dynamic nature of the hydrate system in “K-2” mud volcano has been supported by small shifts of the hydrate occurrence zone within the three year period of investigation

Changing nutrient cycling in Lake Baikal, the world's oldest lake

Lake Baikal, lying in a rift zone in southeastern Siberia, is the world's oldest, deepest, and most voluminous lake that began to form over 30 million years ago. Cited as the "most outstanding example of a freshwater ecosystem" and designated a World Heritage Site in 1996 due to its high level of endemicity, the lake and its ecosystem have become increasingly threatened by both climate change and anthropogenic disturbance. Here, we present a record of nutrient cycling in the lake, derived from the silicon isotope composition of diatoms, which dominate aquatic primary productivity. Using historical records from the region, we assess the extent to which natural and anthropogenic factors have altered biogeochemical cycling in the lake over the last 2,000 y. We show that rates of nutrient supply from deep waters to the photic zone have dramatically increased since the mid-19th century in response to changing wind dynamics, reduced ice cover, and their associated impact on limnological processes in the lake. With stressors linked to untreated sewage and catchment development also now impacting the near-shore region of Lake Baikal, the resilience of the lake's highly endemic ecosystem to ongoing and future disturbance is increasingly uncertain

An inventory of hydrate-related gas seeps in Lake Baikal

Lake Baikal in Siberia, one of the world’s largest rift lakes, is known to be the only fresh-water lake with gas hydrates in the subsurface. Seismic data clearly image the base of the hydrate layer in the Central and Southern Baikal Basins, at both sides of the Selenga delta. Gas seeps and short-lived mud volcanoes were discovered in Lake Baikal’s South Basin at places where the gas hydrate layer shows anomalous thickness variations, which was attributed to localized heat flow anomalies along active fault segments. The gas seeps were interpreted as the result of localized destabilization of gas hydrates by injected thermal water along fault segments; it is probably the inevitable consequence of gas hydrate accumulation in an active rift basin.New data from Lake Baikal’s Central basin, acquired during the summer of 2002, show that the four seeps in the South Basin are not isolated cases. More seeps were discovered, all situated in Baikal’s hydrate accumulation area, all near active fault segments, and all associated with anomalous thickness variations of the underlying hydrate layer. This poster gives an overview of the occurrence of gas seeps in Lake Baikal in relation with thickness anomalies of the gas hydrate layer. The gas seeps in Lake Baikal were found in three areas: 1. Posolsky fault area. Four methane seeps were encountered in the footwall of a small antithetic fault of the Posolsky fault zone. The seeps occur as blow-out craters and conical mud volcanoes. 2. Olkhon fault splay area. Two gas seeps, St.Petersburg and Novosibirsk, were discovered in 2002 at the footwall of a splay of the large Olkhon border fault. The seeps appear to be conical mud volcanoes. 3. The Kukuyu canyon area. The Kukuyu canyon is a large, probably fault-related, canyon, at the northern slope of the Selenga delta. Gas seeps are documented on side scan sonar data and subbottom acoustic profiles.The newly discovered seeps support the interpretation that gas seeps and mud volcanoes in Lake Baikal are caused by the localized dissociation of gas hydrates by thermal input at the base of the hydrate layer. Seepage is probably intense but short-lived, and sometimes accompanied by mud extrusion at the lake floor. They are a rare example where hydrates are the source for intense methane venting, and not vice versa

Mud volcanism and gas seeps in Lake Baikal: causes and consequences

After the discovery in 1999 of a series of mud volcanoes on the deep basin floors of Lake Baikal and of the presence of an anomalous thermally-mixed water layer attributed to methane venting, the lake floor was investigated in more detail in order to identify all possible sources of methane and to understand the processes leading to mud volcanism and methane release at the Baikal lake floor. New data show the presence of at least 4 mud volcano provinces, each consisting of several individual mud volcano structures, and of several areas of gas venting. All mud volcanoes occur in water depths of > 1000 m, within the GHSZ and in areas of abnormally shallow BSR, and are closely associated to large, active faults. They are attributed to hydrate destabilisation at the base of the GHSZ under the influence of a geothermal fluid pulse along the nearby fault. Methane release is not continuous (probably tectonically controlled; most mud volcanous are not active at present) and the source of methane is destabilising gas hydrates at 200-300 m subbottom depth. In addition, a whole series of methane vents (i.e. without distinct morphological expression) occur in shallower-water areas. These venting sites occur mostly in deltaic environments, but some are also associated with faults, and are always outside the GHSZ. Methane release appears to be more continuous (many are now active) and the source of the methane is probably shallow subsurface methane formed by the decomposition of organic matter, although deeper sources can not be excluded.Consequences of methane venting for the waters of Lake Baikal are the presence of a thermally-mixed water layer, wich could (if persisting and increasing in thickness) lead to a permanent stratification of the water column. This could influence the water mixing process and have major influences on the oxygenation of the lake and the benthic biota. In addition, increasing evidence is becoming available that some of these seeps may influence the water column (directly, or via associated temperature-driven circulation effects) up to the surface, causing localised melting or non-freezing of the winter-ice cover and even massive fish deaths. Measurements of surface-water and near-surface air methane concentrations are currently underway. The influence of the methane seeps (up to a few years not even suspected in the largest lake on Earth) on the entire lake system may thus be extremely important

Small-scale turbulence and vertical mixing in Lake Baikal

The water column of Lake Baikal is extremely weakly-but permanently-stratified below 250 m. Despite the thickness of this relatively stagnant water mass of more than 1000 m, the water age (time since last contact with the atmosphere) is only slightly more than a decade, indicating large-scale advective exchange. In the stratified deep water, the fate of water constituents is determined by the combined action of advective processes (deep-water intrusions) and small-scale turbulent vertical diffusion. Here, vertical diffusivity is addressed through the analysis of 25 temperature microstructure profiles collected in the three major basins of Lake Baikal to a depth of 600 m. In addition, in the 1,432-m deep south basin, monthly CTD profiles and a two year record of near-bottom currents were analyzed. Balancing turbulent kinetic energy and small-scale temperature variance leads to the conclusions that (1) vertical diffusivity in the stratified deep water ranges from 10-90 X 10(-4) m(2) s(-1) (between 600 and 250 m), which is three orders of magnitude more than estimated by Killworth et al. (1996), (2) the mixing efficiency is similar to 0.16, comparable to that found in stronger stratification (e.g., the ocean interior), (3) turbulence under ice decays with a time scale of 40 +/- 2 d and (4) the interior of the permanently stratified deep water below 250 m and the bottom boundary layer contribute roughly equally to the TKE production. The latter implies, that mixing in the deep water of Lake Baikal's three sub-basins is dominated by bottom boundary mixing as found in smaller lakes and ocean basins

Increase in the water level of Lake Baikal as a possible cause of changes in methane flux and concentrations in the water column

The construction of a dam for the Irkutsk hydroelectric power plant caused the water level in Lake Baikal to increase by 80 cm between 1958 and 1961. This led to a downward migration of the base of the gas hydrate stability zone (BGHSZ) in the sediments of the lake, which was followed by a long transition process to a new state of equilibrium. In the first stage of the transition, new gas hydrates formed at the BGHSZ, which was accompanied and followed by a decrease in pore pressure, a decrease in methane transfer from the BGHSZ to the lake floor (via fast transfer pathways, such as faults or mud volcanoes), a decrease in the intensity of methane release from the lake floor into the water column, and a decrease in the methane concentrations in the water. In the second stage, which covers the last 10-15 years, methane concentrations in the water column have started to increase again, possibly in response to an uptick in methane flux from the lake floor. In this paper, we look at possible explanations. Mathematical modeling of the migration of the BGHSZ allowed us to estimate how long the transition process takes. The modeled transition times are different for different locations in the lake, depending mainly on the sedimentation rate and the gas hydrate content of the sediments. In the near future, Lake Baikal may reach a quasi-stationary state again similar to that before the construction of the dam. This stationary state likely involves much higher methane concentrations in the water column than what is observed today, as well as adverse effects on biota of pulsed expulsions of methane, sourced from the BGHSZ, into the water column by means of e.g. mud-volcano eruptions. Such effects may include events of mass deaths of the endemic deep-water fish, golomyanka, similar to what was reported to have occurred in the 19th and first half of the twentieth century, prior to the construction of the dam. This study also reemphasizes how variations in the dynamics of a natural gas hydrate system may have a profound impact on the water bodies in which they occur and on the ecosystems within these water bodies. It also highlights which effects can be expected in other hydrate-bearing marine basins where climate-induced sea-level rise will impact the dynamics of the hydrate reservoirs