Sources and sinks of methane in Lake Baikal: A synthesis of measurements and modeling

Abstract

We studied the methane (CH 4 ) budget of Lake Baikal, the most voluminous lake in the world and the only freshwater body with known occurrences of methane hydrates in the sediments. CH 4 concentrations were measured in water samples taken during six expeditions between October 2002 and June 2004; these expeditions covered the entire lake volume. A one-dimensional model was applied to (1) estimate the large-scale vertical CH 4 fluxes within the South Basin of Lake Baikal, (2) determine the exchange with the atmosphere, and (3) constrain the CH 4 inputs from seeps and mud volcanoes to the deep water. Fluxes were generally several orders of magnitude below previous estimates. The annual internal source of CH 4 to the pelagic surface mixed layer was roughly estimated to be 40 Mg CH 4 . A large part of this input diffuses downwards and is consumed in the water column, with a CH 4 residence time of about 4 yr. The input of CH 4 from deep gas seeps and mud volcanoes is less than a few 10 Mg CH 4 yr 21 , most of which is oxidized before reaching the surface. The net CH 4 flux between the atmosphere and the main waterbody distant from shallow areas is negligible and not significantly different from zero. However, occasional high CH 4 concentrations, both in the surface water and in the atmosphere, indicate that the region near the Selenga delta is a local CH 4 source to the atmosphere. CH 4 fluxes in the Central Basin are very similar to those in the South Basin, whereas in the North Basin, the shallow CH 4 sources are weaker.1 Corresponding author (martin.schmid@eawag.ch). AcknowledgmentsWe thank the captains and crews of RV Vereshchagin and RV Titov for their hard work during so many surveys; A. Ageev, V. Blinov, V. Druzhinin, R. Gnatovsky, A. Salyuk, M. Schurter, and O. Vereshchagina for their invaluable support in the field work; and I. Tyryshkin and Y. Ponomarev for their assistance in the design of the methane gas analyzer. Finally, the constructive input from two reviewers helped to improve the clarity of the manuscript.This project was supported by the INTAS grant 2001-2309 (multidisciplinary study of natural gas seeps in Lake Baikal-MULTISGAS). The water temperature measurements were supported by the EU-project CONTINENT (High-resolution CONTINENTal paleoclimate record in Lake Baikal: A key-site for Eurasian teleconnections to the North Atlantic Ocean and monsoonal system; EVK2-CT-2000-00057).

Modeling lakes and reservoirs in the climate system

“…Process-based models have been developed for the fate of organic carbon in lakes (Hanson et al 2004), the release of methane bubbles from lake sediments (Makhov and Bazhin 1999), the dissolution of methane during the rise of the bubbles through the water column (McGinnis et al 2006), and gas exchange between the lake surface and the atmosphere (Guérin et al 2007). A 1-D reaction-transport model has been applied to the Lake Baikal methane balance (Schmid et al 2007). Whole lake and reservoir GHG flux modeling approaches have included the development of regression models (Bastviken et al 2004) and methane budgets (Bastviken et al 2008), as well as dynamical numeric approaches (Abril et al 2005;Thérien and Morrison 2005).…”

Cyclonic circulation and upwelling in Lake Baikal

“…As a result of increased stability of the metalimnion below the surface mixed layer (Schmid et al 2007), vertical mixing decreases during the summer period. Major factors influencing vertical exchange processes during summer stratification can include vertical movements of water masses within cyclonic currents driven by the winds (Shimaraev and Ladeyshchikov 1986) as well as horizontal heterogeneity of water temperature and density.…”

Oxic water column methanogenesis as a major component of aquatic CH4 fluxes

“…This assumption certainly holds for small, shallow ecosystems where the surface layers are in relatively close contact with sediments. Yet CH 4 supersaturation is also prevalent in large, deep lakes 3,5,[7][8][9][10][11][12] and oceanic surface waters [13][14][15][16][17] , where deeper water columns result in significantly reduced surface water exchange with anoxic sediments. Studies of CH 4 dynamics in surface waters of oceans and large lakes have indeed concluded that pelagic CH 4 supersaturation cannot be sustained either by lateral inputs from the littoral or from benthic inputs alone 9,10,[13][14][15][17][18][19] .…”