Oceans cover seventy percent of the earth's surface and ninety percent of the ocean surface is a desert. The "blue"-ness of the Earth, the blue water seen in satellite images, is a desert.
Productive waters are green. The reason for the almost complete absence of green in the world's oceans is a lack of nutrient in the surface waters where life flourishes.
The green phytoplankton, "grass of the sea" at the bottom of ocean food chains, quickly eat the little nutrient available to them. The global marine ecosystem, including fish, is limited by nutrient.
Half of the nutrient enters the oceans from rivers and winds blowing dust. Upwellings carry the other half, but only happen naturally in 0.1% of the ocean.
Oceans are generally stratified, unlike the atmosphere which is upwelling everywhere as clouds. This stratification is the ultimate cause of low overall ocean productivity:
The phytoplankton also need light, so bloom only in surface layers which then become nutrient poor in a general absence of upwellings.
So the ocean surface becomes life rich relative to middle ocean strata, which remain nutrient rich by being life poor, while being continuously nutrified.
Hence a need to mix middle and surface layers of stratified ocean, produce artificial upwellings using our Nutrient Megapump.
Phytoplankton requires nitrate, phosphate, iron and silicate in order to thrive and these are absent from much of the ocean's surface.
Unlike deserts on land where the missing ingredient is usually water which may lie hundreds of kilometres away, the nutrients needed by the marine ecosystem are only hundreds of metres distant; vertically downwards at depths where the sun's rays hardly penetrate.
All that is needed for the desert ocean to bloom is to mix these nutrient-rich deeper waters into the sunlit surface layer so that photosynthesis can take place. Mixing nutrients into the desert-like parts of the ocean in this way will have two desirable effects. It will produce a:
• New marine ecosystem comprising phytoplankton, zooplankton, small fish, big fish eating smaller fish. Big commercial fish, and a
• Net loss of carbon dioxide from the atmosphere.
However it requires energy to mix the ocean. Below about 50m depth, seawater is highly stratified and colder, saltier and denser.
So energy is required to lift it up to the surface against the action of gravity. Where is this energy to be found? Wind and wave energy are candidates. Ocean thermal energy conversion (OTEC) is also a candidate energy source.
The mixing needs to be done on a very large scale. Flows comparable with major rivers need to be generated. It is doubtful if the just-mentioned energy sources would be sufficiently powerful for the task.
There is another energy source in the ocean: Running along the middles of the oceans are mid-oceanic ridges (MORs), ~60,000 km of them. These are fissured mountain ranges raised by the upwelling of hot subterranean magma.
The magma comes to within a few kilometers of the seafloor through the crust along MORs where it is porous due to subsidence, seafloor falls away from high MOR elevations. Very hot water (200-400 deg C) then wells up through the seafloor as hydrothermal vents (HTVs).
HTVs were first seen in 1977 as "black smokers". The dark colour of their chimneys is due to metal sulphides dissolved out of hot rock and precipitated where vent water mixes with cold seawater. There are grey and white smokers also.
As possible energy sources these are very good candidates indeed. Most vents put out ~10 MW (Megawatts) of power (in the form of heat) and there are many of them. Only ~10% of MORs have been explored. Wherever MORs have been explored, hydrothermal vents have been found.
The thermal power of the flow from a single vent in the TAG field on the Mid-Atlantic Ridge is 1.7 GW. The total power from all HTV fields is ~10 terawatts, about the same as humankind's total energy usage, 2010.
Unfortunately this HTV energy only comes in the form of heat. It will require mechanical energy to mix the ocean. It is easy to convert mechanical energy to heat (via friction) but it is much harder to convert heat to mechanical energy because of the Second Law of Thermodynamics.
Nevertheless it can be done. Machines which do this are called heat engines. The steam turbine which generates electricity is an example of a heat engine. However it gets better than that.
The hot water from the HTVs is not just any old hot water, it is superheated hot water. Its temperature is ~360 degC, well above the boiling point at the surface of the ocean and comparable with the temperatures at which steam turbines run.
The only reason this water does not boil at the HTVs is because, at that depth, typically ~2500m, the water is under huge pressure, ~250 atmospheres. The higher the pressure the greater the temperature needed to make water boil.
So the vent water does not normally boil. It rises up a couple of hundred metres then spreads out horizontally like smoke from a chimney on a frosty night.
Now suppose we gather the vent water up with a funnel and bring it closer to the surface in an insulated pipe. As it gets closer to the surface and the pressure decreases it will start to boil.
There is a change of state from water to steam. All that is needed to make a heat engine work, beyond invention, design and so on, is such a change of state.
This change of state generates three things: energy, momentum and buoyancy. We have found that using the buoyancy is the best way to bring about ocean mixing.
We use the buoyancy of the newly generated steam in the form of huge slugs of steam injected into a surrounding, concentric "fat pipe" to produce a very large bubble pump. Hitherto bubble pumps have only been made on a very small scale, in gas driven refrigerators and so on.
It turns out that bubble pumps can be made to work and can be used to mix the surface layers of the ocean very economically at "biggest machine in the world" scales.
Our modelling suggests that a huge Nutrient Megapump utilising an ~2 GW HTV would cover its ~$1 billion costs from the value of its fisheries alone, while its similarly valued Carbon Credits would be a further income.
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