Seasonal changes in biosphereatmosphere carbon exchange influence atmospheric CO2 concentration

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Seasonal changes in biosphere-atmosphere carbon
exchange influence atmospheric CO2 concentration
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Marine primary production and the global carbon
cycle
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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)

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Ocean currents create radically different
environments Centers of gyres have little mixing
Off-shore currents cause upwelling Warm ocean
s have high vertical stability
(not much vertical mixing, thus low nutrients!)
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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 re
duce nutrient limitation
Nutrient concentrations of euphotic zone are
highest in upwelling currents Always depleted
at surface by algal uptake
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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)

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Major upwelling zones off Peru, Africa Outer
Banks, North Pacific California, North Africa
Wind-mixing off Antarctica
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Marine Primary Production Production is highest
in continental margins and shallow seas,
because Upwelling transports nutrients to the s
urface Nutrient runoff from land
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Production is low in blue water, the open
ocean. Nutrient availability is low in some are
as 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 worlds oceans, including the Southern
Ocean, Equatorial and subarctic North Pacific
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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 t
race Fe had been verydifficult, but new, more
precise methods by the 80smade it possible to
make these measurementsaccurately
Martin measured Fe in the HNLC zones, and fou
nd it to be exceedingly low, or non-existent
(below detection limits)
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In Antarctica, Martins 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.
(Graph courtesy U.S. Joint Global Ocean Flux
Study, based on data from K. Johnson and K.
Coale.)
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Give me half a tanker filled with iron, and Ill
give you another ice age John Martin (1989)
Claimed that iron levels could in part be respon
sible 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?
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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 d
epletion of atmospheric CO2
dust deposition is also correlated with the
accumulation of organic carbon in ocean sediments
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Large-scale, open-ocean experiments the true
test of the iron hypothesis
During the 1993 Iron Enrichment Experiment
(IRONEX), researchers dumped iron into a
64-square-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.
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Monitoring CO2 levels in the water showed
increasedphotosynthetic activity where the iron
had been released
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But the results were truly dramatic, as reported
in Science News, 148220 (1995), Nothing ha
d prepared them for the color of the water. The
oceanographers watched in awe as the R. V.
Melville pliedPacific waves dyed a soupy green
by a bumper crop of tinyocean plants.
The tint was abnormal. Only a day before, this p
atch 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.
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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
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Fe
NPP
Ocean carbon storage
? AtmosphericCO2
???
We have demonstrated that we have the key now
for turningthis system on and off. I think some
will be encouraged bythese findings. Therein
lies the dilemma. Kenneth Coale, lead scienti
st in the IRONEX experiments
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Another possible, geoengineering fix deep
ocean CO2 injection Recall that, of the global
reservoirs of C, the deep ocean is the second
largestCO2 in the deep ocean has a very slow
turnover time, many thousands of years (longer
than wood or soil) But delivery to the deep oce
an by physical dissolution and oceantransport is
quite slow So, why not speed up this natural pr
ocess, by directly injecting pure liquid CO2
into deep ocean waters???
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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 ch
anges locally (near sites of injection)
could occur on a worldwide scale
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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) estima
te 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 carbo
nic 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,, 20
01 Potential impacts of CO2 injection on
deep-sea biota. Science 294, 319-320.  
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Local changes in pH will be even larger
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Carbon Sequestration Capacity
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What do you think? Should we fertilize with ir
on? Why or why not? Should we inject CO2
into the oceans, into oil fields, saline
beds??? Why or why not?
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