BG3.2

Methane in aquatic sediments: sources, chemical interactions, fluxes, and biotopes
Convener: A. Stadnitskaia  | Co-Conveners: H. Niemann , T Feseker 
Oral Programme
 / Tue, 21 Apr, 08:30–11:15  / Room 21
Poster Programme
 / Attendance Tue, 21 Apr, 15:30–17:00  / Poster Area BG
Atmospheric methane plays an essential role in climate change, but many questions regarding its formation, flux and degradation are poorly constrained. In aquatic sediments, methane is an important hydrocarbon of which large quantities can escape to the overlying hydrosphere. Methane is either formed by microbial decomposition of organic matter under anoxic conditions, or by thermal cracking of organic matter at high pressure in the deep subsurface. Methane is buoyant and thus inclines to migrate upward making gas seepage into the hydrosphere a natural and dynamic process. Migration is strongly associated with geological and tectonic factors which determine its mechanism and magnitude. These factors may prohibit fluid passage, resulting in free gas or gas hydrate accumulations in subsurface sediments, or induce fluid and gas transport which can lead to seepage. Methane seepage is an important geological phenomenon and associated to a remarkable heterogeneity in physical properties of bottom sediments. Commonly, seepage is expressed in the surface relief e.g. mud volcanoes and pock marks. Acoustic anomalies in sub-surface sediments caused by fluid migration and hydrocarbon accumulations are also often encountered.

Production, accumulation, and consumption rates of methane in oceanic and lacustrine environments are globally significant. However, specifically the contribution of the oceans to the atmospheric methane budget appears to be small. A significant fraction of the methane flux is either consumed microbially in marine sediments and the water column or temporarily stored as gas hydrates in the subsurface. In lacustrine environments, comparably little methane is retained in sediments and the contribution of these systems to the global methane budget could be higher than previously estimated.

Methane efflux is efficiently controlled by methane-oxidizing aerobic and anaerobic microorganisms (some of which were only discovered recently) that thrive along redox gradients in sediments or the water column. Their biomass and metabolic products are an important basis for a diverse but mostly unknown food-web structure consisting of chemosynthetic organisms, such as bivalves and tube worms as well as scavengers and detritus feeders. The biomass of this community can be orders of magnitude higher in comparison to adjacent habitats. Furthermore, methane-rich environments often bear authigenic mineral fabrics. The precipitation mechanisms are not fully understood yet but the activity of methanotrophs appears to play an essential role.

Unfortunately, the diversity of the methanotrophic community, their biogeochemical functioning as well as the magnitude of methane production and consumption are still poorly constrained. Similarly, many questions as to the geological driving forces governing methane migration, the magnitude of methane venting as well as its (abiotic) fate in sediments and the hydrosphere are still unanswered. This session will provide a platform to discuss recent approaches in identifying present and past methane seepage environments, quantifying methane fluxes, production, transport, consumption, associated processes and organisms as well as geological settings supporting high methane fluxes in marine and lacustrine environments.