FROZEN HEAT | Volume 1

FROZEN HEAT

AOM

Acharax

Calyptogena

Beggiatoa

Gas hydrate

on gas hydrates in the Gulf of Mexico. Studies suggest the

ice worm consumes free-living microbes associated with the

hydrate and that the worm’s activities, which involve forming

depressions and creating small-scale water currents at the

hydrate surface, may promote microbial growth and speed

hydrate decomposition. The association of the ice worm with

gas hydrates occurs both at the sediment-water interface and

at least 10 centimetres below the surface.

Aside from the Gulf of Mexico, there has been limited di-

rect sampling of massive methane hydrates to assess meta-

zoan associations. Exposed methane hydrate at Hydrate

Ridge does not appear to be directly colonized by metazo-

ans (Boetius and Suess 2004), although the presence of gas

hydrates supports dense, colourful bacterial mats that can

lead to high densities of infauna (animals living inside the

sediment) in the near vicinity (Sahling

et al.

2002; Levin

al.

2010; Vanreusel

et al.

2010). The gas hydrates just below

bacterial mats at Hydrate Ridge may actually act as a barrier,

blocking some of the digging clams, tubeworms, and other

species (Sahling

et al.

2002).

2.4.2

Sensitivities of methane-seep

communities to climate change and

geological variations

There are indications in the geological record that warming/

cooling trends and oscillations in eustatic sea level could in-

fluence methane hydrate stability, authigenic carbonate for-

mation, slope stability, and, in turn, the abundance of seep

habitats (Jiang

et al.

2006; Archer 2007; Kiel 2009). Undersea

earthquakes, such as the Grand Banks earthquake, can also

produce methane seeps and chemosynthetic habitats (Mayer

al.

1988). It is, so far, unknown how the gas-hydrate response

to ongoing climate change (Discussed in Volume 1, Chapter

3) will affect chemosynthetic communities. Dissociation could

create completely new habitats by increasing methane seepage,

or rapid gas hydrate dissociation and disappearance might de-

crease the horizontal extent of existing seep habitats.

fields formed by the engineering/foundation species and the

microbially-precipitated carbonates, creates a heterogeneous,

highly patchy habitat structure that contributes significantly

to the overall biodiversity of seep ecosystems and continental

margins (Cordes

et al.

2010; Vanreusel

et al.

2010).

The animals present at cold seeps are rarely in direct contact

with gas hydrates. Only a single large taxon, the ice worm

Hesiocaeca methanicola

(See Chapter 1, Fig. 1.2) (Desbruyeres

and Toulmond 1998; Fisher

et al.

2000), has been document-

ed to live directly in or on methane hydrates. This species

attains relatively large size (2–4 centimetres) and occurs at

high densities (2 500 to 3 000 individuals per square metre)

Figure 2.7:

Chemosynthetic habitats. Chemosynthetic habitats

generated by different fluid flow rates, including transport of

methane, as well as the sulphide resulting from anaerobic oxidation

of methane (AOM), are colonized by different fauna. Left: free-living,

sulphur-oxidizing bacteria mats (e.g.,

Beggiatoa

spp.) in sediments

with highest fluxes. Centre: vesicomyid clams (e.g.,

Calyptogena

spp.) in sediments with high-to-moderate fluxes. Right: solemyid

clams (e.g.,

Acharax

spp.) in sediments with low flux.