Chapter 7 Benthic deep-sea carbonates: reefs and seeps Carbonates are formed across most latitudes and they are not restricted to shallow water but are also found in all but the deepest abyssal and hadal settings. Deep-water carbonates cover a far larger percentage of the global ocean seafloor than shallow-water tropic carbonates, and they account for nearly half of the deep-water seafloor. Carbonate formation due solely to a high carbonate input occurs in deep water typically by the formation of deep-sea reefs or by association with cold seeps. University 1
1. Carbonate bentho-pelagic coupling Extensive accumulations of deep-water carbonates across the global seafloor are dependent upon pelagic carbonate productivity in the surficial ocean waters. Main contributors to calcareous oozes include planktonic foraminifers, coccolithophores and pteropods. It is estimated that between 20% and 60% of the pelagic marine carbonate production comes from coccoliths with blooms in cocoolithophores. Foraminifers account for a further ~21%. Aragonite and calcite saturation depths are defined by temperature, pressure and partial pressure of CO 2. The average depth limit for the accumulation of calcareous ooze (the calcite saturation depth) in the South Pacific is ~3000 m, rising to less than 1000 m in the North Pacific. It is approximately 4500 m in the Atlantic and average 3500 m in the Indian Ocean. University 2
University 3
2. Calcareous aphotic reefs Higher rates of deep-water carbonate accumulation exist in limited areas due to the processes controlling the formation of benthic reefs and of authigenic carbonate. There are over 5100 coral species, of which more than 65% are aphotic, so-called cold-water (or deep-sea) coral species. The distribution of coral carbonate mounds on continental margins is mainly symptomatic of the ecological tolerances of coral-reef organisms. Two theories for the initiation and development of cold-water coral and coral carbonate mounds: the hydraulic theory and the environmental-control theory. The model for the development of cold-water coral reefs includes four stages: coral colony, thicket, coppice, and eventual to mound. Recently discovered and locally significant accumulation centres for deep-sea carbonates are formed by the deep-sea pycnodontine oyster. University 4
University 5
Figure 6.8 Schematic diagram summarizing the hydraulic and environmental control theories for mound initiation and development. (A) Hydraulic theory. (1) Gas seepage may lead to the development of (2) methanederived authigenic carbonates that may act as a substratum for coral settlement. (3) Gas seepage at the seabed provides a source of food, increasing (4) biomass along the food chain. (5) Cold-water corals are supported by this food chain. (6) Along-slope and hemipelagic sedimentation provides a mineral sediment infill of the reef structure. (B) Environmental-control theory. (7) Erosion of the seafloor by strong currents generates suitable coarse-grained substrata for coral settlement. (8) Surface primary productivity underpins the food chain. (9) This settles through the water column as marine snow and may (10) become concentrated at watermass boundaries and transported to the coral reef, possibly assisted by internal waves. (11) Benthic hydrodynamics helps to enhance the food flux and prevents coral polyp smothering by deposition of fine sediments. (12) Along-slope and hemipelagic sedimentation provides a mineral sediment infill of the reef structure. University 6
(from Christophe Colin) University 7
(from Christophe Colin) Cold-water coral in the northern South China Sea (from cruise photos of manned deep-sea submersible Jiaolong dives in June 2013) University 8
3. Cold seeps and related carbonates Methane and the earth system: continental margins are important areas for the discharge of greenhouse gases and they are sensitive to climate change. Methane escaping from the seafloor plays an important part in the carbon biogeochemical cycle. Submarine cold seepage: often results from the upward migration of fluids from the subsurface through zones of weakness (fault systems, cracks, fissures), affecting the morphology of the seafloor. Mud volcanoes: where substantial, concentrated discharge of subseafloor fluids occur, mud volcanoes are formed and, as a major morphological feature where significant concentration of microbially mediated carbonates are found. Seafloor methane habitats and the marine microbial methane filter: the deep-water methane-seepage environment is commonly nonthermophilic, oxygen-limited, light-free, sometimes brine-impacted, sulphate-containing and sulphide- and methane-rich. University 9
University 10
Figure 6.14 Cold-seep habitats and related chemosynthetic ecosystems at 3-km water depth in the Eastern Mediterranean. (A) Brine-impacted cold-seepage food web. A chemosynthetic crab thrives in environments rich in methane, sulphide and emitting brine solutions. (B) Microbial mat systems in the brine-impacted seepage. (C) Filamentous bacterial mat covering the surface of methaneand brine-saturated mud volcanic deposits. (D) Methane-derived carbonates used as an anchor for sessile seep-associated vestmentifera tube worms. (E) Chemosynthetic sulphide-oxidising filamentous proteobacteria Beggiatoa growing heterotrophically in the presence of oxygen. (F) Methane-saturated brine lake (left) with orange microbial mats covering the brine lake margin (right). Figure 6.16 The diversity of seep carbonates from the Nile deep-sea fan seepage area, Eastern Mediterranean. (A) Carbonate tower outcropping at the seafloor. (B) slate carbonate crust partially outcropping at the seafloor, surrounded by fields of white and brownish-orange dead clams. (C) Laminated carbonate dome. (D) Sampling of one of the chimneys of the carbonate tower (A) with the QUEST- 4000. (E) The chimney, forming a sphere-shaped piece, being hollow inside, with two open conduits. (F) Carbonate sample from a carbonate outcrop in the central part of a pockmark. University 11
Cold-seep carbonate chimney in Taiwan (photos from, July 2013) Mud volcanoes in Taiwan (photos from, July 2013) University 12
University 13