Black smokers
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On the cruise this summer we are returning to the site of the Black Smoker field we discovered last summer to map the field in more detail and take more samples.
After finding the 11m tower, which was named Loki's Castle after a Norwegian mythical figure renowned for mischievousness, we found a group of at least five smaller chimneys before the cruise time ran out. There may be more to find based on the size of the plume and the apparent size of the mineral mound, which suggest that this field is of significant size. We are anxious to learn more.
What are hydrothermal vents?
Hydrothermal vents have been features of the seafloor since the oceans first formed. Today's vents are sites of lush, thriving faunal communities that have developed around chemical energy sources in the absence of light.
First discovered in 1977 at a spreading ridge in the pacific Ocean near the Galapagos Islands, hydrothermal venting has now been detected along all open-ocean spreading centres. Learning more about hydrothermal circulation is important because it affects
- the composition of the ocean crust
- ocean chemistry
- the formation of sizeable mineral deposits
- the generation of chemical energy sources for biological communities in the deep sea
The composition of the vent fluids depends on a number of factors:
- heat source (magma, chemical reactions)
- types of substrates
- depths / temperatures involved
"Black smokers" tend to be warmer than "white smokers". They can be over 400°C. Black smokers contain particles with high levels of sulphides (sulphur-bearing minerals). White smokers contain more lighter-hued mineral particles (often containing barium, calcium, and silicon).
Initially it was thought that slow and ultra-slow ocean spreading centres were too removed from magma sources to have enough heat for extensive venting activity. Recent discoveries are showing that this is not the case, and in fact, venting at the slower centres may result in potentially larger deposits of seafloor minerals.
Read more in a special issue feature in Oceanography March 2007, written by Senior Scientist Margaret Kingson Tivey at Woods Hole Oceanographic Institution.
tivey-vents.pdf
Richard Herrington, of the department of mineralogy at The Natural History Museum, London writes a paper entitled: Hydrothermal Vents: Deep and Dark Oases Isolated from the Earth's Changing Surface
21feat_hydrothermal_vents-3008.pdf
picture of the basic vent mechanism from the University of Illinois at Chicago, summer school 2002
Vent minerals
The mineral contents of the vent fluids precipitate out as the hotter vent fluids interact with the cold seawater. Deposition occurs both below and above the seafloor. It depends on
- the composition, temperature and density of the vent fluid
- the structure of the oceanic crust
- the style of mixing of the vent fluid and the seawater
Vent waters contain concentrations minerals such as sulphur, copper, zinc, gold, and iron. It is believed that the richest ore deposits now found on land, were once deep sea vents far back in time.
Margaret Tivey from Woods Hole tells us "How to Build a Black Smoker Chimney" - a comprehensive description of how minerals deposit and form a chimney.
Richard Herrington, of the department of mineralogy at The Natural History Museum, London writes a paper entitled: Hydrothermal Vents: Deep and Dark Oases Isolated from the Earth's Changing Surface
21feat_hydrothermal_vents-3008.pdf
What is a plume?
The fluid emerging from hydrothermal vents differs from the surrounding seawater in temperature, composition and density. Because it is warmer it tends to rise. Because it can be full of mineral particles it can look like smoke, hence the name "smoker". Although many of the heavier mineral particles precipitate out quickly and are deposited rapidly either in the chimney itself or forming a mound around the vent, some of the lighter components, especially hydrogen or methane gas can be detected kilometres away.
The plume structure is affected by local current systems, those relating to local topography, large current systems and even tidal movements.
How do we find vents?
Researchers first started to look for vents to explained heat flow measurements relating to plate-tectonics. Thus the first step involves good seafloor maps to identify the locations of geological features such as sea-floor spreading zones, fault lines etc.
As venting occurs diffusely or through small-mouthed chimneys, the next step is to narrow the search down to possible sites a few hundred meters large. To do this scientists look for the plume cloud; anomalies in the water column due to the addition of vent fluids. The tool of choice for this work is a CTD - a rosette of bottles for collecting water samples and a series of instruments that measure the Conductivity, Temperature and Density of the water column. A CTD is dropped over the side of a research vessel and sends a continuous stream of data about the characteristics of the water to scientists on board. When the data show an anomaly - a change in the information - the scientists can send a signal down to the CTD to trigger a sample bottle to close trapping a water sample at that particular location for testing back on board.
Once a plume has been identified and some information about its size and strength collected, scientists can send down an underwater vehicle (manned or unmanned) to visually look for the vent site.
link to CTD pdf
Life near vents
Although researchers had hypothesized the existence of vents before they were actually found, no one expected to find life in such inhospitable locations.
Without light, at great depths with crushing pressure, near sources of extremely hot water containing a toxic brew of chemicals; vents are actually host to a unique ecosystem. That life can exist under these extreme conditions has forced us to revise our definition of the basic requirements for life - and this may provide insights into how life began on earth AND which may provide answers to the question about the possibility of extra-terrestrial life.
The driving energy for the ecosystem comes from a community of chemolithoautotrophic bacteria.
Chemo - getting their energy from chemicals litho - in particular from redox reactions with inorganic materials auto - use inorganic carbon troph - to eat (for energy)
These bacteria interact with other bacteria: each specialised to use a different inorganic energy source or make a different metabolic product. Simple organisms then "eat" the bacteria and their metabolic products. these are, in turn, consumed by larger organisms and an chemsynthetic (based on chemicals not light) ecosystem is formed.
Some vent organisms actually enter into symbiotic partnerships with chemolithoautotrophic bacteria; the organism provides the shelter / substrate and the bacteria provide the energy / food in the form of useable biological compounds.
an article from the Proceedings of the National Academy of Science
life at vents.pdf
Richard Herrington, of the department of mineralogy at The Natural History Museum, London writes a paper entitled: Hydrothermal Vents: Deep and Dark Oases Isolated from the Earth's Changing Surface
21feat_hydrothermal_vents-3008.pdf
Read more about how lifeforms may have arrived at vent locations: Black Smokers, Cold Seeps, Whale Falls
Geobiology - rocks-water-microbe interactions
Researchers are coming to understand that the geosphere and the biosphere are intimately related. Huge global processes and systems, such as element recycling depend on the relationships between rocks, water and microbial organisms. For example, without photosynthetic bacteria, we would not have so much oxygen in our atmosphere. This interaction is actively taking place at hydrothermal vents, with microbes affecting the processes of water circulation, mineral deposition and mineral recycling. Because of the potential global impact of these processes we need to know more about how they function and what organisms are involved.
Commercial interest in vents
Beyond their intrinsic interest as part of global element recycling systems and unique biological systems, vents are attracting commercial interest as sources of minerals and potentially interesting bio-compounds.
key words: hydrothermal mineral deposits, bio-prospecting
Black smokers - links
Wikipedia on hydrothermal vents and black smokers.
This very comprehensive site describes itself as being "A Scrapbook on 'Black Smokers' " and says that it provides "Web resources for creating a Multimedia Scrapbook on Hydrothermal Vents" . It does provide background information sources, photos, maps, diagrams and animations, videos etc. VERY GOOD
Margaret Tivey from Woods Hole tells us "How to Build a Black Smoker Chimney" - a comprehensive description of how minerals deposit and form a chimney.
for schools:
American Museum of Natural History expeditions ...
SEAS: Student Experiments At Sea - Introduction to deep-sea hydrothermal vents