Hello all,
Thinks look different if one uses push-pump pumps rather than simply
upwelling of nutrients. The upwelled DIC becomes insignificant compared to
the DOC pushed down. Some of you may recall this argument from my GLOBAL
FEVER book from the Univ of Chicago Press, but the following is an excerpt
from my THE GREAT CO2 CLEANUP, chapter six:
Plowing Under a Carbon-fixing Crop
To avoid competing with the world’s food production and supplies of fresh
water, most sequestered carbon must come from new biomass grown in new
places. Here I explore how paired ocean pumps might uplift nutrients and
then sink the new organic carbon back into the ocean depths.
Instead of sinking only the debris that is heavy enough to settle out, as
in iron fertilization, we would be using bulk flow to sink the entire
organic carbon soup of the wind-mixed layer (organisms plus the
hundred-fold larger amounts of dissolved organic carbon) before its carbon
reverts to CO2 and equilibrates with the atmosphere.
The CO2 later produced in the depths by the sunken carbon soup will reach
the surface 400-6,000 years later. Smearing it out over that period greatly
reduces the damaging peaks in ocean acidification and global fever.
...
If we fertilize via pumping up and sink nearby via bulk flow (a push-pull
pump), we are essentially burying a carbon-fixing crop, much as farmers
plow under a nitrogen-fixing cover crop of legumes to fertilize the soil.
Instead of sinking only the debris that is heavy enough, we would be
sinking the entire organic carbon soup of the wind-mixed layer.
Algaculture minimizes respiration CO2 from higher up the food chain and so
allows a preliminary estimate of the size of our undertaking. Suppose that
a midrange 50 g (as dry weight) of algae can be grown each day under a
square meter of sunlit surface, and that half is carbon. Thus it takes
about 10-4 m2 to grow 1 gC each year. To produce our 30 GtC/yr drawdown
would require 30 x 10+11 m2 (0.8% of the ocean surface, about the size of
the Caribbean).
But because we pump the surface waters down, not dried algae, we would also
be sinking the entire organic carbon soup of the wind-mixed surface layer:
the carbon in living cells plus the hundred-fold larger amounts in the
surface DOC. Thus the plankton plantations might require only 30 x 10+9 m2
(closer to the size of Lake Michigan).
The space requirement will be more because downpumps will not capture all
of the new plankton; it might be less because the relevant algaculture
focuses on oil-containing algal species and on harvesting a biofuel crop,
not on plowing under the local species as quickly as possible. The ocean
pipe spacing, and the volume pumped down, will depend on the outflow needed
to optimize the organic carbon production. [The chemostat calculation FYI.]
Only field trials are likely to provide a better estimate for the needed
size of sink-on-the-spot plankton plantations, pump numbers, and project
costs.
Though ocean fertilization is usually proposed for low productivity regions
where iron is the limiting nutrient, another strategy is to boost the
shoulder seasons in regions of seasonally high ocean productivity. For
example, ocean primary productivity northeast of Iceland drops to half by
June as the nutrients upwelled by winter winds are depleted. Continuing
production then depends on recycling nutrients within the wind-mixed layer.
However, to the southwest of Iceland, productivity stays high all summer.
Because not all of the new plankton will be successfully captured and sunk,
fertilization will stimulate the marine food chain locally. Most major
fisheries have declined in recent decades and, even where sustainable
harvesting is practiced, it still results in fish biomass 73% below natural
levels. At least for fish of harvestable size, there is niche space going
unused.
Locating the new plankton plantations over the outer continental shelves is
more likely to supply a complete niche for many fish species, whereas
deep-water plantations will lack variety. (The main commercial catch in
deep water is tuna.) Also, down-pumping near the shelf edge would deposit
the organic carbon in the bottom’s offshore "undertow" stream, carrying it
over the cliff onto the Continental Slope into deeper ocean.
Note that pumps would be tethered to the bottom so that the ocean currents
are always creating a plume downstream: a plume of fertilizer near the
surface and a second plume of carbon soup in the depths. (Pumping up from a
different depth than pumping down will prevent the interaction that
characterizes the oceanographers’ box models.) While the water might come
back around in a thousand years, the plumes for the clean-up will only be
about twenty years long and well diluted by that time.
--
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