Cold seep ecosystems

Background

At a continental margin, methane is produced as a final step of the chemical transformation of the organic matter in sediments. A variable part of the methane may locally rise to the seafloor along high permeability conduits such as faults, mud volcanoes or sand layers. Methane seeps are defined as seafloor sites where methane-rich fluids escape the seafloor. At methane seeps, the methane flux sustains exceptionally rich ecosystems. These are characterised by a high biomass and a unique faunal composition. Species are often endemic. Despite their fragmented distribution, methane seeps may greatly influence the deep-sea biota and may play a major role in the carbon and sulphur turnovers at the margin.

Left: Methane bubbling from seafloor at the Hakon Mosby Mud Volcano, Norwegian margin. Image taken from the ROV Victor during the HERMES Vicking expedition aboard the RV Pourquoi Pas? to the Norwegian margin in summer 2006. Image courtesyAWI/Ifremer.

Methane seeps have been discovered only relatively recently on European margins, and particularly on the Norwegian slope, in the Gulf of Cadiz and in the eastern Mediterranean. The presence of polychaetes, tube worms and bivalves is one of the most striking observations at methane seeps. In turn, the presence of these fauna is one of the best 'indicators' of methane-rich fluid emissions at the seafloor. Biological communities live in the soft surficial sediments or fixed on hard authigenic carbonate crusts. Their composition and productivity have been observed to strongly depend on gas fluxes and local environmental factors. At methane seeps, biological communities include large invertebrates living in symbiosis with chemotrophic bacteria using methane and/or hydrogen sulfide as energy source. In areas of high methane flux, the benthic biomass produced through chemosynthetic processes can be 1,000 to 50,000 times greater than the biomass resulting from photosynthetic production at a similar depth.

The global inventory of methane seeps at continental margins is growing rapidly, but the geological, chemical and biological processes operating at methane seeps remain little known. Among key issues, the diversity of species, from microbes to metazoans, their geographical distribution and the degree to which fragmented habitats on the ocean margin are inter-connected, the various factors that control the dynamics of seeps and their ecosystems, need further investigations. How methane seep ecosystems respond to global warming is an important issue.

Right: Carbonate crusts and bacterial mats at the Napoli mud vocano, eastern Mediterranean.
Image courtesy Ifremer/MEDECO 2007

Key results

The HERMES project established a field programme of detailed investigations of cold seeps in four main target areas along Europe’s ocean margin: the Norwegian margin, the Gulf of Cadiz, the eastern Mediterranean and the Black Sea. This multidisciplinary field programme has made full use of advanced technology mapping and in-situ measurement tools to achieve bathymetric, visual and chemical analyses of the deep seafloor. Results indicate a vast diversity of hydrocarbon-fuelled chemosynthetic ecosystems in the Europe’s seas, including pockmarks, gas chimneys, mud volcanoes and brine ponds, and new seepage sites continue to be discovered. These cold seeps are often associated with ‘hot spots’ of increased biological activity. A better knowledge of their distribution and nature has been achieved through the HERMES project (Foucher et al. 2009; Van Reusel et al. 2009).

The fluid flow regimes at cold-seep systems vary on spatial scales of metres to hundreds of metres and host different types of communities. At several seepage sites selected for focused studies within the HERMES project, such as the Hakon Mosby Mud Volcano (HMMV) on the Norwegian margin, the Carlos Ribeiro and Arutyunov mud volcanoes in the Gulf of Cadiz, and the Amon mud volcano in the eastern Mediterranean sea, discharge fluid rates have been estimated between several cm and up to 10 m per year.

Left: sidescan data collected on the TTR16 cruise superimposed onto seafloor bathymetry for the giant Yuma and Ginsberg mud volcanoes in the Gulf of Cadiz (image courtesy TTR16).

High-resolution mapping and in-situ observations have emphasized the ecosystem zonation at individual seeps as a function of the intensity of the methane flux and active biogeochemical processes in response to this flux. Cold seep ecosystems provide niches for chemosynthetic megafauna species by emitting reduced compounds such as methane and sulphide, which supply energy to CO₂-fixing symbiotic bacteria. These chemosynthetic symbioses between invertebrates and thiotrophic and/or methanotrophic symbionts are only found in highly reduced environments, and are an obvious example of how cold seep ecosystems add biodiversity to the marine deep-water life.

During our research in the HERMES project, we were able to identify the dominant chemosynthetic symbioses on Europe’s continental margins, but most likely much more remain to be discovered. In many seep ecosystems, especially those that remain active for tens of thousands of years, hardgrounds form due to the extensive carbonate precipitation that is a by-product of microbial hydrocarbon oxidation. High fluid flow rates of >5 m per year are often associated with gas ebullition, mud displacement and disturbed seafloor surfaces. Due to the absence of electron acceptors in subsurface fluids, such high fluid flow velocities can restrict microbial turnover of methane and sulfide to a few mm below the seafloor. Medium flow rates are often associated with diverse types of microbial mats, some of which may cover hundreds of meters of seafloor. These mats typically consist of giant vacuolated sulfur-oxidizing bacteria, which exploit the high AOM-derived sulfide fluxes to the seafloor. Such bacteria can use internally stored nitrate to oxidize sulfur and fix CO₂ for growth, thus coupling the carbon, nitrogen and sulfur cycles in the seep sediments. Chemosynthetic bivalves and tubeworms dominate low fluid flow systems, and show special adaptations on order to exploit subsurface sulfide and hydrocarbon pockets. New biological species have been reported, and the identification of the sampled biological material wsill continue for some time.

Right, top: fauna living on carbonate crust surrounding an active seep site at HMMV. Bottom: specialist sampling equipment at work at the HMMV site. Images courtesy AWI/Ifremer from the Vicking Expedition, summer 2006.

A challenging issue remains the temporal variability of the seepage activity and hence the energy source to chemosynthetic communities. In the absence of free gas, gas hydrate in sediment beneath cold seeps may regulate the methane flux and sustain communities of chemosynthetic biota for periods of several thousand years at sites where hydrate was formed during past events of large gas release. On average, the pore space of the seabed within the gas hydrate stability zone contains 1-10% gas hydrate, resulting in a global gas hydrate reservoir of 1,000-22,000 Gt C. Gas hydrates form when gas saturates the pore water at water depths >400m and ambient temperatures of <4°. Gas hydrate associated chemosynthetic ecosystems have been detected in the Nordic margin, the Gulf of Cadiz and the eastern Mediterranean as well as in the Black Sea. However, little is known about the dynamics of hydrate distribution and the susceptibility of hydrate-associated chemosynthetic ecosystems to global warming reaching deep waters.

Links to recent cold seep expeditions:

RV Pourquoi pas? cruise to the central and eastern Mediterranean cold seeps, autumn 2007
RRS James Cook cruise (Leg 1) to the mud volcanoes in the Gulf of Cadiz, summer 2007
RV Meteor cruise, central Mediterranean cold seeps, autumn 2006
HERMES Vicking cruise, Norwegian margin cold seeps, summer 2006
Ionian Sea mud volcanoes - RV OGS Explora, summer 2005

**NEW** Image library of cold seeps and mud volcanoes