Locations: 1. Trenches in subduction
zones e.g. Japan, Chile, Aleutians
2. Accretionary prisms e.g. Barbados, Mediterranean, Makran,
3. Base of Continental Slopes e.g. Florida
4. Sediment slumps or slides e.g. Laurentian Fan
5. Sedimentary basins e.g. Gulf of Mexico, North Sea, Skagerrak,
Guayamas Basin
6. Faults over aquifers e.g. Baltic
Causes: 1. Sediment compression
leading to de-watering (1-3)
2. Upward migration of hydrocarbons or other fluids of low density
(e.g. fresh water) (5,6)
3. Exposure of deeper sediments (4.)
Reducing chemicals present:
Methane, higher hydrocarbons, hydrogen sulphide, ammonia
Origin of :
Methane
- methanogens, thermogenic, primordial
Higher hydrocarbons - thermogenic
Hydrogen sulphide - sulphate reducing bacteria, sometimes
in association with methanogens?
Ammonia - anaerobic breakdown of organic matter
Geochemical relationships: Anaerobic breakdown of organic matter to methane occurs via methanogens in the sediment and to methane and higher hydrocarbons by high temperature processes deeper into the crust. Methane may be oxidised anaerobically as well as aerobically in the micro-oxic zone. Anaerobic oxidation is believed to occur by a reverse methanogenic reaction followed by scavenging the hydrogen formed by sulphate reducers to form sulphide.
Properties of methane:
Like all gases it has a higher solubility under pressures
Rising bubble streams carry water with them
Under suitable temperature-pressure conditions methane combines
with water to form a gas hydrate (clathrate) with 6-8 moles of
water per mole of methane.
The base of clathrate layers is shown by the bottom simulating
reflector (BSR), the position of which is controlled by the geochemical
gradient.
Clathrate stability increases if higher hydrocarbons are present
Clathrate melting causes a 200-fold increase in volume
Clathrates are found on the seafloor in areas of deeper water
(normally >400m) with low bottom temperatures
World-wide methane hydrate reserves are estimated to be between
2,000 and 4,000,000 Giga tonnes (Gt) of C, compared with 3.6 Gt
C in the atmosphere, 3 Gt in the marine biota and 5,000 Gt in
all other fossil fuel reserves.
Salinities of seeps
These vary from fresh water at freshwater aquifer outlets and
low salinities associated with methane hydrate dissolution to
very high salinities where seeping water collects dissolved salts
from evaporite deposits.
Physical characteristics
Pockmarks (depressions in seabed caused by fluid expulsion)
Carbonate deposits and cemented sediment
Mud volcanoes
Detection of seeps
Most seep communities investigated to date have been found either
as a result of chance during submersible dives examining geological
features, through investigations of topographic anomalies on the
seabed, e.g. pockmarks, or through looking for the sources of
hydrocarbon traces seen on the surface. Only relatively recently
have seeps been detected by looking for the source of methane
plumes in the water column. Cold seeps in the Aleutian subduction
zone were found after detecting a methane anomaly only 2 x the
background in the bottom water.
Communities
Like hydrothermal vents, cold seeps frequently support a large
biomass based on chemosynthetic production, e.g. off Sagami Bay,
Japan at 900-1200 m depth at the foot of the escarpment the biomass
of Calyptogena and vestimentifera reaches > 10 kg m-2 while
in the Nankai Trough the biomass surrounding the seeps reaches
20 kg m-2.. This is 1000-fold higher than non-seep benthic communities
at the same depth.
Aleutian subduction zone
Seeps in this are caused by the pressure of the Pacific Plate
subducting under the N. American Plate, causing a fluid flow out
of the sediment of 3.4 m y-1. This flow causes entrainment of
interstitial fluid and draw-down of bottom water so that sulphate
in the bottom water is mixed with methane-rich fluids from deeper
in the sediment. This sulphate is reduced to sulphide and the
methane is oxidised to carbon dioxide leading to calcium carbonate
precipitation and the formation of sediment concretion. The seep
fluid contains 4.5 mM hydrogen sulphide, 3 mM ammonia, and 0.3
mM methane. Flows of fluid around the Calyptogena colonies were
estimated at 5.5 litres m-2 d-1.
Gulf of Mexico
Numerous hydrocarbon seeps are present at water depths of 400
- 2,200 m venting, in different proportions, fluids containing
methane, hydrogen sulphide and higher hydrocarbons. Some of the
seeps overly shallow salt diapirs which may be eroded to form
brine pools on the bottom. Such pools are anoxic and contain high
concentrations of dissolved methane.
In places methane hydrates are exposed on the seabed, often surrounded
by Bathymodiolus spp.. Recently a new species of polychaete has
been found to live in holes in the edge of the exposed hydrate.
Monterey Bay, California
Seeps occur towards the base of the slope. Up to 5 spp. of Calyptogena
are found. The two most abundant spp. are C. pacifica which
is abundant in sediments with low sulphide concentrations (<1
mM) and C. kilmeri, which is abundant at high sulphide
concentrations (> 5 mM). C. pacifica has a low sulphide-binding
affinity (accumulating up to 2 mM sulphide at 0.4 mM ambient concentration)
but a fast growth rate, up to 100 mm in 10 years. In contrast,
C. kilmeri, has a high sulphide binding affinity (accumulating
up to 4 mM sulphide at 0.1 mM ambient concentration) but is less
tolerant of high concentrations and grows slowly, 60 mm in 100
years.
COLD SEEP REFERENCES
Cary, S. C., Fisher, C. R. & Felbeck, H., 1988. Mussel growth supported by methane as sole carbon and energy source. Science, 240, 78-80.
Colbourne, E. B. & Hay, A. E. An acoustic remote sensing and submersible study of an Arctic submarine spring plume. Journal of Geophysical Research. C. Oceans. 95, 13219-13234 (1990).
Dando, P. R., Bussmann, I., Niven, S. J., O'Hara, S. C. M., Schmaljohann, R. & Taylor, L. J., 1994. A methane seep area in the Skagerrak, the habitat of the Pogonophore, Siboglinum poseidoni, and the bivalve molluscThyasira sarsi. Marine Ecololgy Progress Series, 107, 157-167.
Dando, P. R. & Hovland, M., 1992. Environmental effects of submarine seeping natural gas. Continental Shelf Research, 12, 1197-1207.
Juniper, S. K. & Sibuet, M., 1987. Cold seep benthic communities in Japan subduction zones: spatial organization, trophic strategies and evidence for temporal evolution. Marine Ecology Progress Series, 40, 115-126.
MacDonald, I. R., Guinasso, N. L., Reilly, J. F., Brooks, J. M., Callender, W. R. & Gabrielle, S. G., 1990. Gulf of Mexico hydrocarbon seep communities: VI. patterns in community structure and habitat. Geo-Marine Letters, 10, 244-252.
Paull, C. K., Hecker, B., Commeau, R., Freeman-Lynde, R. P., Neumann, C., Corso, W. P., Golubic, S., Hook, J. E., Sikes, E. & Curray, J., 1984. Biological communities at the Florida Escarpment resemble hydrothermal vent taxa. Science, 226, 965-967.
Vacelet, J., Boury-Esnault, N., Fiala-Medioni, A. & Fisher, C. R. A methanotrophic carnivorous sponge. Nature, London 377, 296 (1995).