cold seep

cold seep. A cold seep is a location on the ocean floor where hydrogen sulfide, methane, and other hydrocarbon-rich fluid seepage occurs, often forming a brine pool. Unlike hydrothermal vents, the fluids are near ambient temperature. These seeps support unique chemosynthetic communities and are significant for carbon cycle and climate studies. They were first discovered in 1983 in the Gulf of Mexico by researchers including Charles D. Hollister.

Definition and discovery

Cold seeps are defined by their emission of cold, methane-rich fluids and brines through the seafloor. The seminal discovery occurred in 1983 at the Florida Escarpment in the Gulf of Mexico by a team from the Woods Hole Oceanographic Institution using the submersible DSV *Alvin*. This finding, led by Charles D. Hollister, revealed dense biological communities independent of photosynthesis. Subsequent major discoveries include seeps at the Barbados Accretionary Wedge and within the Mediterranean Sea, such as the Nile Deep Sea Fan. Key research vessels like RV Knorr and institutions like IFREMER have been instrumental in global mapping efforts.

Formation and geology

Cold seeps form primarily in geologically active regions where fluid migration is driven by tectonics and sediment compaction. Common settings include convergent margins like the Cascadia Subduction Zone, passive continental margins such as the Blake Ridge, and within sedimentary basins like the Eel River Basin. The seepage often originates from the dissociation of methane hydrates or the thermal breakdown of organic matter. Expulsion features include pockmarks, mud volcanoes as seen in the Black Sea, and extensive carbonate deposits formed by microbial activity, such as those at the Hydrate Ridge.

Biology and ecosystems

Cold seep ecosystems are oases of life dominated by chemosynthetic symbiosis. Foundation species include bathymodiolin mussels like *Bathymodiolus childressi*, vestimentiferan tube worms such as *Lamellibrachia luymesi*, and diverse clams like *Calyptogena magnifica*. These organisms host symbiotic bacteria that oxidize methane or sulfide. Associated fauna include galatheid crabs, polychaete worms, and specialized snails like *Provanna*. Notable communities exist at the MBARI-studied Monterey Canyon, the Gulf of Cadiz, and the Japan Trench. These ecosystems show parallels with those at hydrothermal vents and whale falls.

Chemistry and biogeochemistry

The geochemistry of cold seeps is characterized by anaerobic oxidation of methane, often mediated by consortia of Archaea and sulfate-reducing bacteria in the sulfate-methane transition zone. This process produces hydrogen sulfide and leads to the precipitation of authigenic carbonates, forming structures like chimneys and pavements. Key chemical species include dissolved methane, higher hydrocarbons, and brines rich in chloride. These seeps are significant sources of methane to the ocean, though much is consumed microbially; they influence global carbon cycle budgets and are studied by projects like the International Ocean Discovery Program. The stability of subsurface gas hydrate reservoirs is a critical control on fluid flux.

Global distribution and significance

Cold seeps are found along most continental margins, from the Arctic Ocean's Haakon Mosby Mud Volcano to the Antarctic's Ross Sea. Major concentrations occur in the Atlantic Ocean along the West African margin, the Pacific Ocean near Costa Rica and New Zealand, and in enclosed basins like the Sea of Okhotsk. Their global significance is multifaceted: they are potential sources of fossil fuels and gas hydrates, act as natural laboratories for studying extremophiles, and play a role in long-term climate regulation by sequestering carbon. They also represent analogs for potential extraterrestrial chemosynthetic environments. Ongoing research is coordinated by entities like the Census of Marine Life and the InterRidge program.

Category:Oceanography Category:Marine geology Category:Extreme environments