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Seasonal changes in biosphere-atmosphere carbon
exchange influence atmospheric CO2 concentration
Marine primary production
and the global carbon cycle
What limits marine production?
• Water? (no, except intertidal)
– Strong contrast with terrestrial systems, where
water is the dominant limiting factor
• CO2? (no, except sometimes intertidal)
– CO2-bicarbonate-carbonate equilibrium supplies
CO2
• Light? (always at depth)
• Nutrients? (usually)
Ocean currents create radically different environments
Centers of gyres have little mixing
Off-shore currents cause upwelling
Warm oceans have high vertical stability
(not much vertical mixing, thus low nutrients!)
Euphotic zone of oceans are frequently nutrient poor
– spatial separation of light and nutrients
– Terrestrial plants overcome this via vascular transport
Some phytoplankton swim
or alter buoyancy
to reduce nutrient limitation
Nutrient concentrations of
euphotic zone are highest
in upwelling currents
Always depleted at surface
by algal uptake
Latitudinal gradients in productivity
• Polar oceans are most productive
– More effective mixing of nutrients from depth
because of lower surface T, and weaker vertical T
gradient
• Polar lands are least productive
– Less rapid nutrient release from SOM
• Consequences
– Bipolar bird, fish and mammal migrations to
capitalize on spring blooms of phytoplankton
– Polar distribution of anadromous fish (eat marine,
breed fresh)
Major upwelling zones off
Peru, Africa
Outer Banks, North Pacific
California, North Africa
Wind-mixing off Antarctica
Marine Primary Production
• Production is highest in continental margins and shallow seas, because
– Upwelling transports nutrients to the surface
– Nutrient runoff from land
• Production is low in ‘blue water’, the open ocean.
– Nutrient availability is low in some areas of the open ocean
– But in others, there are vast expanses of areas with high nutrients (N and P)
but low chlorophyll (i.e., low NPP)
– These are called HNLC (“High Nutrient, Low Chlorophyll”) zones
– They occur in about 1/5th of the world’s oceans, including the Southern Ocean,
Equatorial and subarctic North Pacific
Oceanographers hypothesized that zooplankton grazers were so
active in these areas, that they kept populations of phytoplankton low
But there was no evidence for this idea.
In 1981, John Martin began to tackle this “mystery of the desolate
zones”
He speculated that iron could be responsible
Up until then, measuring trace [Fe] had been very
difficult, but new, more precise methods by the 80s
made it possible to make these measurements
accurately
Martin measured [Fe] in the HNLC zones, and
found it to be exceedingly low, or non-existent
(below detection limits)
In Antarctica, Martin’s team collected clean water and added iron to some
samples and left others untreated.
The samples were placed in baths on the deck of the ship.
The phytoplankton in the iron-dosed jar flourished after a few days.
(Graph courtesy U.S. Joint Global Ocean Flux Study, based on data from K. Johnson and K. Coale.)
“Give me half a tanker filled with iron, and I’ll give you another ice
age” John Martin (1989)
Claimed that iron levels could in part be responsible for past ice
ages
During an ice age much of the fresh water on the continents is
locked up in the ice caps, and the exposed landmasses become drier
than they are today.
If large amounts of iron were swept off these arid landmasses by wind
and dumped into the ocean's “desolate zones,” the resulting growth of
phytoplankton would effectively pump vast amounts of carbon dioxide
from the atmosphere deep into the seas.
What does the ice core record show?
From the ice core record, dust inputs are correlated with oceanic
productivity over the past several hundred thousand years:
dust deposition (Fe inputs) is correlated with depletion of atmospheric CO2
dust deposition is also correlated with the accumulation of organic carbon
in ocean sediments
Dust
Atm
CO2
Large-scale, open-ocean experiments: the true test
of the iron hypothesis
During the 1993 Iron
Enrichment Experiment
(IRONEX), researchers
dumped iron into a 64square-kilometer area and
measured the response of
phytoplankton.
The photograph above
shows researchers at the
Naval Postgraduate School
preparing iron to be
dumped in the sea.
Monitoring CO2 levels in the water showed increased
photosynthetic activity where the iron had been released
But the results were truly dramatic, as reported in
Science News, 148:220 (1995),
“Nothing had prepared them for the color of the water. The
oceanographers watched in awe as the R. V. Melville plied
Pacific waves dyed a soupy green by a bumper crop of tiny
ocean plants.
The tint was abnormal. Only a day before, this patch of water
near the Galapagos Islands had sparkled with electric blue clarity,
a quality owing to the general absence or phytoplankton.
They had transformed this marine desert into a garden simply by
sprinkling a dilute solution of iron into the water.”
The results of the
Southern Ocean Iron
Enrichment
Experiment (SOIREE)
experiment in 1999
were captured by the
Sea-viewing Wide
Field-of-view Sensor
(SeaWiFS).
The bright comma in
the image indicates
phytoplankton growth
stimulated by iron
added during the
course of the
experiment.
(Image courtesy Jim Acker,
Goddard Distributed Active
Archive Center, the SeaWiFS
Project, NASA/Goddard
Space Flight Center, and
ORBIMAGE
+ Fe
+ NPP
+ Ocean carbon
storage
 Atmospheric
[CO2]
???
“We have demonstrated that we have the key now for turning
this system on and off. I think some will be encouraged by
these findings. Therein lies the dilemma.”
Kenneth Coale, lead scientist in the IRONEX experiments
Another possible, ‘geoengineering’ fix… deep ocean CO2 injection
Recall that, of the global reservoirs of C, the deep ocean is the
second largest
CO2 in the deep ocean has a very slow turnover time, many
thousands of years (longer than wood or soil)
But delivery to the deep ocean by physical dissolution and ocean
transport is quite slow
So, why not speed up this natural process, by directly injecting
pure liquid CO2 into deep ocean waters???
Would it stay there?
very likely, yes
Is it economically feasible?
under investigation!
What would the environmental impacts be?
pH changes in the ocean
very large changes locally (near sites of injection)
could occur on a worldwide scale
Potential Impacts of Deep Ocean CO2 Injection:
ecosystems at such depths are very to changes in biogeochemistry
particularly to changes in pH that surely would result from such large infusions
of CO2
Seibel and Walsh (2001) estimate that sequestration of enough atmospheric CO2
to stablilize atmospheric concentrations at 550 ppm (twice the pre-industrial
level) would decrease ocean pH globally by about 0.1 by 2100.
The pH goes down due to the formation of carbonic acid:
H2O + CO2  H2CO3 (carbonic acid)
Because of the high sensitivity of most deep-sea organisms to rapid changes in
pH, such massive CO2 disposal likely would have significant adverse consequences
on deep-sea ecosystems.
Reference
Seibel, B. A., and P. J. Walsh,, 2001: Potential impacts of CO2 injection on deepsea biota. Science 294, 319-320.
Local changes in pH will be even larger…
Simulated time evolution of
pH contours near the CO2
injection point
- background current velocity: 5
cm/s
- liquid CO2 droplet initial
radius (as injected): 7 mm
- liquid CO2 injection mass flow
rate: 1 kg/s
The pH scale ranges from 5.9
to 7.9, increasing in 0.2
increments. The background
pH (shown in red) was taken as
8.0.
After approximately 45
minutes, the plume reaches a
steady state within 100 m of
the CO2 release
point.
Carbon Sequestration Capacity
What do you think?
Should we fertilize with iron?
Why or why not?
Should we inject CO2 into the oceans, into oil fields,
saline beds…???
Why or why not?