Microbial Activity In Sediments At An Active Margin; Control By Fluid Flux
This simulation is a depiction of a convergent margin showing the subduction of one tectonic plate beneath another, the associated scraping of the accretionary prism, the forearc basin, and distant volcanic activity.
Closer view of prism showing movement of subducting plate, resistance of overlying plate, discontinuous nature of scraped sediments on prism (folding, buckling), and relatively laminar sediments of forearc basin.
Microbes are left in place in sediments through sedimentation of particles as "marine snow" or are mobilized in the sediments by fluid movement controlled by any of the processes that are depicted in this simulation. Microbial activity in the sediments is controlled by the flux of chemical energy proximal to the cells. Cells require some minimal inputs of energy (e.g., acetate, H2) and oxidants (e.g., CO2, sulfate) as well as removal of wastes (e.g., methane, sulfide). This mechanical flux on an active margin may be driven by regional- or global-scale processes such as:
· Compaction and fluid expulsion
· Hydrate instability
· Seafloor slumping
· Tidal forces
· Tectonism
COMPACTION
The mass of overlying sediments gradually packs sediment particles thereby squeezing water and gases out during the compaction process.
Cells attached to the sediment particles benefit from fluid flux past them. Coarse grained strata are likely to be especially prone to high flux rates and may be habitats where high microbial biomass and activities would be expected.
HYDRATE INSTABILITY
Changes in the gas hydrate stability zone (GHSZ) may occur as a result of temperature or pressure changes in the sediments. The example shows a gradual warming of bottom waters near the seafloor which would cause hydrates in the sediments to decompose. When hydrate forms or decomposes there are associated changes in the gas phase, that is, methane may change from existing as a free gas, as a dissolved gas, or as a hydrated (solid) gas. Changes in hydrate stability are reversible.
Cells that remain attached to the sediments may be alternately exposed to dissolved, free or hydrated methane, depending on the location of GHSZ. The formation of each phase is likely to cause local, micro-scale flux of porewater. These phase changes are likely to be gradual, tempered by sediment thickness and the rapidity of pressure and temperature changes which lead to change in gas hydrate stability.
SEAFLOOR SLUMPING
Hydrates that form on a slope act to hold the sediments together. However, following events that might destabilize the hydrates (e.g., bottom-water warming or seismic events) the hydrate dissociates, gas is released, and a seafloor slump may occur.
Initially, cells are surrounded by the hydrate and are relatively quiescent. The destabilization of the hydrate causes both the aforementioned changes in gas phase and a massive disruption of the sediments as they move down slope. Sediments and the attached microbes are dramatically resorted and mixed. Cells are exposed to the rapidly mixed porewater and porewater constituents.
TIDAL FORCES
The diurnal, monthly, and seasonal effects of tides have a measurable impact on hydrostatic pressure in sediments. The resulting cyclic change in hydrostatic pressure influences the rate of flux of fluids out of sediments and, by inference, the movement of these fluids within the sediments. Open sediment systems (i.e., subject to the hydrostatic pumping of tides) are exposed to relatively low hydrostatic pressure during low tides and release more pore fluids from the sediments compared to when they are exposed to high hydrostatic pressure during high tides.
Cells that are attached to sediments in open systems experience higher flux rates of fluids during low tides than at high tides. Such cells may be metabolically adapted to tidal cycling.
TECTONIC EVENTS
Tectonic events along plate boundaries increase or change fluid movement in the overlying sediments. Porewaters and their constituents are relatively static under pre-event conditions in shallow sediments. The seismic event mediates shock waves that create large as well as localized (pore-scale) mixing and potentially significant fluid flow through fissures and fractures. Preferred flowpaths for fluids may be newly opened as a result of such an event.
Between events, microbial cells may be relatively inactive due to depletion of electron donors and electron acceptors in their immediate surroundings. The pressure wave exerted by the seismic event may redistribute these microbial energy sources and sinks in a manner that exceeds normal diffusion in the sediments.
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