Cold-seep carbonates
Cold seeps are regions of the seafloor where cold methane-rich water escapes. They are the cold-water version of hydrothermal vents, where hot water escapes from the seafloor. Like hot-water vents, they can be associated with a vibrant community of organisms as well as build-ups of carbonate deposits.
Cold seeps explained
Dr Ashley Rowden, a principal scientist with the NIWA Marine Benthic Ecology group based in Wellington, explains what cold seeps are, where they are located in New Zealand and why rich chemosynthetic communities of specialised organisms develop around them.
Discovered in the 1980s, cold seeps are found at depths ranging from a few hundred metres to thousands of metres. They are most often present along continental margins where there is tectonic plate activity. For example, along the modern Hikurangi Margin within New Zealand’s offshore East Coast Basin lie numerous cold-seep sites. It is in this region that the Pacific tectonic plate is subducted under the Indo-Australian plate. Tectonic stresses squeeze the seafloor sediments, pushing fluids rich in hydrocarbons through cracks and fissures to the surface.
Hikurangi Margin
Map showing the location of the Hikurangi Margin. Ancient cold-seep sites onshore are shown along with the location of offshore cold-seep carbonate deposits.
Cold seeps generally indicate large amounts of hydrocarbons below the seabed, as the water that escapes is rich in dissolved hydrocarbons like methane as well as some hydrogen sulfide. The origin of these compounds is from ancient marine life (organic matter) that has been degraded either by high temperatures or microorganisms.
Cold-seep communities
On the seafloor around the seep site, large populations of microorganisms (Archaea) are found that can use the methane and hydrogen sulfide as an energy source. These microorganisms form the basis for a community of living things that are often associated with these sites. The conditions at the seep site are extremely inhospitable, with the absence of light, high pressures, cold temperatures and high levels of toxic chemicals. Each member species of the group of organisms that forms these communities has evolved adaptations to deal with these extreme conditions.
Cold seeps and living things
NIWA's Dr Ashley Rowden describes how a community of organisms develops around a cold-seep site. Forming the base of such a community are specialised bacteria that use methane gas present in the seep water as their energy source. Other organisms such as clams, mussels and tubeworms use these chemosynthetic bacteria by developing symbiotic relationships with them.
Cold seeps and cold-seep carbonates
Dr Steve Hood explains that a cold seep is the expulsion of hydrocarbon-rich fluids at the seafloor. Over time. a specialist type of limestone can form. These are quite unlike shelf limestones found on land. Ancient cold-seep limestone deposits have been found on land in the East Coast region of the North Island.
Points of interest
Look for the following micrographs:
Micrograph a: Thin section image as seen under cathodoluminescent light of cold-seep limestone. The worm tube cross-section is infilled with radiating needles of aragonite crystals that have a blue colour.
Micrograph b: Thin section image as seen under cathodoluminescent light of cold-seep limestone. Needles of aragonite crystals growing off a bivalve shell fragment.
Cold-seep carbonate deposition
The microorganisms found in and around cold-seep sites are chemosynthetic. They use methane and hydrogen sulfide as their energy source. One of the byproducts produced by these organisms as they use up the methane is bicarbonate. In the slightly alkaline conditions present in the surrounding waters, this bicarbonate is mostly precipitated out as calcium carbonate in the aragonite crystal form.
HCO3-
+
OH-
+
Ca2+
→
CaCO3(s)
bicarbonate from bacterial action
alkaline seawater
calcium ions present in seawater
calcium carbonate deposited
Over long time periods, these deposits of calcium carbonate build up and can form chimneys, solid pipes, cemented slabs or pavements, and irregular mounds. These deposits are referred to as methane-derived authigenic carbonate (MDAC). Authigenic means ‘made in place’ or ‘on site’. If seafloor surveys indicate the presence of MDAC, it can serve as a key indicator to the presence of hydrocarbons deep within the subsurface.
Shelf and cold-seep limestones compared
Surrounded by an array of samples of limestone, Dr Steve Hood compares a typical New Zealand shelf limestone with an ancient cold-seep carbonate from the North Island east coast region. Cold-seep carbonates have unique fossil faunas never seen in shelf limestones as well as the aragonite mineral form of calcium carbonate as opposed to calcite
Points of interest
Look out for the following micrographs in this clip:
Micrograph a: Thin section image as seen under plane polarised light of a temperate limestone dominated by bryozoans and foraminifera. Skeletons have pressure-dissolved into one another.
Micrograph b: Thin section image as seen under plane polarised light of a temperate limestone dominated by bryozoans. A tight, well cemented structure.
Micrograph c: Thin section image as seen under plane polarised light of cold-seep limestone from the east coast of the North Island. The white veins are mineralised fractures that run through the limestone.
Micrograph d: Thin section image from c above, as seen under cathodoluminescent light. The red regions are due to the presence of calcite crystals.
Micrograph e: Thin section image as seen under plane polarised light of cold-seep carbonate. The brown outer ring is fossilised microbial mat, and the worm tube cross-section is infilled with needle-shaped aragonite crystals.
Micrograph f: Thin section from e above, as seen under cathodoluminescent light.
Ancient cold seeps
Recent New Zealand research conducted through Earth Sciences Departments at Auckland and Waikato Universities has focused on rocky limestone outcrops located in the east coast region of the North Island. Careful examination has revealed that the origin of these limestones was from ancient cold seeps.
The presence of solid pipes along with the fossilised remains of symbiotic molluscs, clams and tubeworms provide strong evidence to suggest that these limestone deposits were formed at cold-seep sites millions of years ago during the Miocene epoch
Ancient cold-seep carbonate section
Thin section of ancient cold-seep carbonate viewed with a light microscope. The paler areas are filled with calcium carbonate in the aragonite crystal form, the darker areas are hydrocarbon residues.
To date, 16 sites have been identified, and further investigations are continuing.
When samples of these ancient cold seep carbonates are analysed in the lab a number of interesting features are revealed:
When broken up, a strong hydrocarbon smell is experienced.
When a thin section is viewed under a light microscope, paler regions full of aragonite crystals are seen along with darker areas of hydrocarbon material.
When analysed for the presence of the isotope 13C, a much lower value is obtained compared with temperate shelf limestones.
All of these features link these rocks with a hydrocarbon origin even though they were formed up to 20 million years ago.
Cold seeps and methane hydrate
Often associated with cold seeps and deep within the marine sediment are deposits of methane hydrate. This gas hydrate is formed under very specific temperature and pressure conditions when molecules of methane become trapped within a cage of water ice molecules. The gas hydrate has the appearance of a white ice-like substance. When brought up to the surface, the reduction in pressure and the increase in temperature cause the hydrate to break down, releasing methane gas.
Methane hydrate
Methane hydrate consists of methane gas trapped in the crystal structure of water. When brought to the surface, methane hydrate can be easily ignited. As the ice crystal structure of the methane hydrate collapses, the trapped methane gas is released. The gas can be easily ignited, producing the ‘burning ice’ effect.
Vast deposits of methane hydrate exist in the marine sediments of the Hikurangi margin, and these could well be mined in the future to provide an additional source of natural gas to service the energy needs of the country.
Nature of science
Cold seeps were not discovered until the 1980s. Scientific advancement is often dependent upon the technologies available at the time. It was the development of better remote-sensing devices like unmanned submersibles known as ROVs (remotely operated vehicles) that allowed this discovery to be made.
Related content
Find out more about Exploring for cold-seep sites and Cold-seep communities.
