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.

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.

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-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.

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

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.

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.