Title: Lecture 5 ??????
1Lecture 5 ?????? Understanding the global
carbon cycle
- What is Biogeochemistry?
- Biogeochemistry and Carbon Cycle
- The Breathing of Gaia
- Carbon Cycling
2BioGeoChemistry
- life processes on earth are, in essence, carbon
chemistry. - The carbon cycle, movement of carbon atoms
through various places of storage on earth
(reservoirs), is tied to life processes. - In studying the carbon cycle, biology and
geochemistry merge to form a new scientific
discipline biogeochemistry.
3BioGeoChemistry
- The all-important role of life processes in
maintaining Earth's environments was stressed by
the Russian mineralogist, Vladimir Vernadsky
(1863-1945), the father of biogeochemistry. - The American geochemist G. Evelyn Hutchinson
(1903-1991) first outlined the principles. - The basic elements of biogeochemistry have been
popularized by the James Lovelock (1919 -), under
the label of Gaia Hypothesis.? - Gaia Hypothesis a concept that life processes
regulate the radiation balance of Earth to keep
it habitable.
4BioGeoChemistry
- Biogeochemists study the carbon cycle and its
interactions with the cycles of other elements
involved in life processes nitrogen, oxygen,
phosphorus, sulfur and iron, etc. - It is the hydrological cycle that helps drive the
carbon cycle, and this is where the climate and
carbon cycle are most intimately connected. - Biogeochemistry studies the history of the great
carbon reservoirs in the crust of Earth (e.g.
limestone rocks coal deposits) and distribution
of nitrate and phosphate in oceans.
5BioGeoChemistry
- Biogeochemistry seeks to explain the composition
of the atmosphere as a result of bacterial action
and photosynthesis. - It records the exchange of matter at the
interfaces - (1) decay of organic matter in soils and
resulting gases released into the air - (2) the uptake of oxygen by oceans and its
utilization at depth - (3) leaching of nutrients from soil and their
transport into ocean
6Biogeochemistry and carbon cycle
- Carbon cycle is the core of biogeochemistry. It
describes the movement of carbon atoms through
the life-support systems on the surface of the
planet. - Models of the carbon cycle consist of
"reservoirs" of carbon and the "fluxes" between
these reservoirs. - Reservoirs include ocean, atmosphere, biosphere,
soil carbon, carbonate sediments, and organic
carbon sediments. - Fluxes describe the rate at which atoms move from
one reservoir into another. E.g., flux could be
the rate of movement of carbon between organic
matter produced in ocean surface and the
sediments in the ocean floor.
7A sketch of carbon cycle illustrating fluxes and
reservoirs (From SeaWIFS project)
8Biogeochemistry and carbon cycle
- The crucial questions concern the mechanisms that
control the fluxes, and how these controls change
as the planet is warming. - What controls the productivity of the ocean, and
what controls the proportion of the matter
produced that reaches the ocean sediment? How
does the amount of plankton change with a warming
ocean, and how does the flux of organic matter to
the seafloor change as a result? - As for future projection, we first must
understand what has happened in the past and what
has happened so far.
9Reservoirs of carbon (in GtC) and fluxes between
reservoirs (arrows)
Reservoirs differ greatly in size and in their
ability to respond to changes, a property called
reactivity.? Large reservoirs with small fluxes
in and out are not very reactive. Small
reservoirs with relatively large fluxes in and
out are very reactive - as far as carbon is
concerned, the atmosphere is such a Reservoir.
Fortunately, the atmosphere is closely coupled to
the ocean, a large Reservoir that can offset this
problem and stabilize the atmosphere.
Unfortunately, the atmosphere's dependency on the
ocean has a drawback if the ocean reacts to
climate change by giving off a small proportion
of its CO2, the atmosphere, with its low
concentrations of CO2, greatly amplifies the
effect. In other words, what seems a small
adjustment for the ocean results in a big change
in the atmosphere.
10Why So Little Carbon in our Atmosphere?
- Plants, algae and shell-making organisms are
responsible for the large-scale solidification of
CO2 within carbonate minerals (in limestone) and
organic materials. Making coal and other organic
matter has also led to splitting the carbon from
the oxygen, with much of the oxygen staying in
the air. This has produced an atmosphere
fundamentally different from those of Venus and
Mars. - Earth would be chemically out of balance and
therefore "unsustainable" were it not for Earths
ongoing life processes. - The low CO2 in atmosphere are a result of the
biologically-mediated movement of CO2 from
reactive reservoirs (the atmosphere and ocean) to
much less reactive reservoirs (limestones and
organic matter). - Although these long-term reservoirs can be heated
(through subduction by plate tectonics),
rereleasing the CO2 into atmosphere, weathering
and life processes then cycle them back into the
long-term storage, continuously keeping the
values low.
11Seafloor Spreading Rate Hypothesis is also known
as BLAG Hypothesis to denote its initial authors,
the geochemists Robert Berner, Antonio Lasaga,
And Rober Garrels. It proposes that the
tectonic-scale climate changes are driven by
variations in the global average rate of seafloor
spreading that leads to the variations of
volcanic and in turn could alter the amount of
CO2 emitted into the atmosphere.
12Initial Forcings
Negative Feedback Loop
Initial Forcings
Negative Feedback Loop
13Chemical Weathering
HCO3- Bicarbonate
14Negative Feedback From Chemical Weathering
- The chemical weathering works as a negative
feedback that moderates long-term climate change. - This negative feedback mechanism links CO2 level
in the atmosphere to the temperature and
precipitation of the atmosphere. - A warm and moist climate produces stronger
chemical weathering to remove CO2 out of the
atmosphere ? smaller greenhouse effect and colder
climate.
(from Earths Climate Past and Future)
15BLAG Carbon Cycle
- On Land CaSiO3 CO2 -gt CaCO3 SiO2
- Subduction CaCO3 SiO2 -gt CaSiO3 CO2
On tectonic timescale
BLAG hypothesis provides a long-term regulatory
mechanism to the climate system by moving a
roughly constant amount of total carbon back and
forth between the rocks and the atmosphere.
16Uplift (Weathering) Hypothesis
Maureen Raymo and her colleagues (1986) proposed
a secondary hypothesis to explain how the plate
tectonic activity might moderate the amount of
atmospheric CO2 level. The uplifting of mountains
and plateaus (mainly caused by the collision of
continents) inevitably results in several
processes favoring/accelerating the chemical
weathering to remove atmospheric CO2 level gt
17????????chemical weathering ????? BLAG???chemical
weathering ????????????????????????CO2???,???????
???? ? ?????????? ?????,???? ? uplifting??????che
mical weathering ?????????????,??????????????????
????????????? ?
18- The weathering on land (CaSiO3 CO2 -gt CaCO3
SiO2) was first proposed by Harold Urey in 1950s
to understand the fundamental process of removing
CO2 from atmosphere. - According to Ureys model, the amount of
atmospheric CO2 is regulated by the presence
hydrologic cycle.
19- Is this really valid?
- Atmospheric CO2 also comes out of volcanoes.
- The rate at which this happens is presumably
independent from the surface reactions described
in Ureys proposal. - After entering the atmosphere, some of CO2 is
concentrated in the soil by the action of plants
(and bacteria, fungi).
20- Is this really valid?
- The reactions of CO2 with silicate minerals
within the soil, therefore, do not proceed
according to the concentration of atmospheric
CO2. In addition, the rate of dissolution of
rocks is contingent not only on the presence of
water, but also the presence of microscopic
organisms on the surface of the rocks. - Moreover, the precipitation of the carbonate and
silica is made possible not only by inorganic
processes but also by organisms (algae, corals,
and foraminiferans produce carbonate and diatoms
and sponges make silica).
21Lessons we learn are
- The above thought analysis of Ureys approach
point to the very importance of life in
influencing the atmospheric CO2 levels. - The reactions that govern the long-term storage
of carbon are rate-dependent and these rates are
determined not only by the plate tectonics BUT
ALSO by the life processes, factors not included
in Ureys model. - Therefore, in foreseeing what will happen in
humans timescale, the changes in our ecosystem
talk.
gt The breathing of Gaia
22Important indication of Keeling curve
CO2 changes seasonally over quite a large range.
In addition, continuing the measurements showed
that the values drift upward from one year to the
next. After these discoveries, the science of the
carbon cycle had changed forever. Since then, the
"Keeling curve" has become the symbol of the
ever-changing chemistry of the atmosphere and the
associated warming of our planet.
23The breathing of Gaia
- Is it the ocean with its large reservoir, warming
and cooling? Or is it processes on land, having
to do with plant growth indicated in Keeling
curve? - The answer is actually land plants. Since most of
the land is located in NH, the fluctuations are
greatest here. (If the ocean were to blame, we
should see a larger effect in SH.) - Gaia breathes?on an annual cycle.
- Expect an equally vigorous exchange within the
ocean? Yes, such an exchange does exist and it
results in a rather short residence time of the
carbon in the atmosphere, less than 10 years.
24The Carbon cycling
25The Carbon cycling
The exchange of carbon between the atmosphere and
the ocean/land takes place in several ways
- The physical carbon pump
- The biological carbon pump
- The marine carbon cycle
- The terrestrial carbon cycle
26The physical carbon pump
- The most important mechanism is through physical
mixing of the ocean (i.e. vertical deep mixing).
When seawater is cooler it takes up more. - Vertical circulation makes sure that CO2 is
constantly being exchanged between ocean and
atmosphere and is ultimately responsible for the
fact that cold water fills the depths of the
ocean. - Vertical circulation acts as an enormous carbon
pump, giving the ocean more carbon than if
equilibrium with the surface ocean.
27Sketch illustrating the concept of vertical deep
mixing
What will happen if the ocean become warmer (or
cooler)?
28Warming the oceans A Thought Experiment
- Warming of ocean waters takes place from the top,
so at first a little more CO2 is released into
the air from below. The warm current is not as
cool it used to be when it reaches high
latitudes. It then takes up less CO2 than it
would otherwise and, in addition, it does not
sink as deeply.
29Warming the oceans A Thought Experiment (cont.)
- The ocean also yields some of its own CO2 and
slows its uptake of CO2 from the atmosphere. The
deep cold water no longer participates very
actively in the vertical circulation and tends to
stagnate. Oxygen (O2) is used up while CO2 is
being produced from organic matter on the sea
floor and from organic matter still falling down
from above. In places where O2 is entirely used
up, nitrate (NO3) is used by the bacteria as an
oxygen source instead. In this process, nitrous
oxide (N20 a greenhouse gas) and molecular
nitrogen (N2) are made while nitrate is being
destroyed.
30Warming the oceans A Thought Experiment (cont.)
- By warming the oceans and weakening the physical
pump, we have created a deep ocean reservoir rich
in CO2 and poor in nutrients. When this cold
water returns to the surface, it will now bring
CO2 back to the atmosphere, without the means to
recapture it by photosynthesis (for which
nutrients are needed). Such a process could have
contributed to the pulsed nature of CO2 rise
during deglaciation, as revealed by the ice cores.
31Cooling the oceans Another Thought Experiment
- Cooling also takes place from the top by removing
heat because of evaporation, freezing, and
infrared radiated to the sky. - As it cools, the water will uptake more CO2 and
readily mix vertically (cold water is heavier
than warm water), sinking to the depth level
appropriate for the density of the sinking water.
32Cooling the oceans Another Thought Experiment
(cont.)
- On the whole, the atmospheric CO2 is drawn down
and the cooling process initiates further cooling
due to the loss of greenhouse gas, a case of
positive feedback. This might trigger the
reglaciation.
33Cooling the oceans Another Thought Experiment (a
corollary)
- A corollary to (1)-(3) is that the water column,
after cooling, is quite well mixed, which was not
necessarily the case (previous warm stage)
before. - If the mixing was slower before (during the
previous warm stage), CO2 could have accumulated
in intermediate waters within the subsurface
layer of water (called the thermocline).
34Cooling the oceans Another Thought Experiment (a
corollary)
- With intensified mixing, the thermocline
initially could release additional CO2 to the
atmosphere, counteracting the positive feedback
from cooling. - This might help explain why during the initial
phase of reglaciation, the atmospheric CO2 tend
to stay high upon cooling as evidenced in ice
cores.
35Lessen learned
- The above thought experiments illustrate how
complicated things can get when considering the
exchange of CO2 between ocean and atmosphere upon
changing the climate. - Whether the scenarios outlined in the thought
experiments have much resemblance to the reality
is another matter (perhaps they do. Maybe they
don't). - But it is this kind of thinking that needs to be
exercised before going into the mathematical
models to make them responsive to simulate
climate change.
36The biological carbon pump
Ocean gets a disproportionate share of the CO2
available to the ocean-atmosphere system (about
50 times larger).
37The biological carbon pump
- The main reason CO2 readily reacts with water
(H2O) to make soluble species of ions, the
bicarbonate? (HCO3-). - Another reason the physical pump described
previously cold water holds more CO2 in solution
than warm water. This cold, CO2-rich water is
then pumped down by vertical mixing to depths. - The last reason for the oceans big share of
carbon is its?biological pump removing CO2 from
the surface water of the ocean, changing it into
living matter and transporting it to the deeper
water layers.
38The biological pump A Thought Experiment
- We start with a well-mixed ocean, dark and quite
cold throughout. - We then turn on the Sun and heat the ocean from
above. - A warm-water layer develops on top of the ocean,
and since it is euphotic, green algae will now
grow in this layer gt CO2 is being fixed into
carbon compounds (photosynthesis, you know). - Some of these particles of the algae (dead
organic stuff) sink out of the euphotic zone into
the deeper cold waters. - Others could be re-mineralized decay by the
action of bacteria, releasing CO2 back to the
water.
39The biological pump A Thought Experiment (cont.)
But how long can this process of carbon fixation
(item 3), carbon settling (item 4), and carbon
recycling (item 5) continue in our experiment?
Answer It can continue until all the nutrients
that are necessary for photosynthesis have been
used up.
Used up all the nutrients?
40The sketch of oxygen profile with an oxygen
minimum zone (OMZ) at mid-depth (typically 1-km
below sea surface)
41What about the recycling of nutrients
(phosphorous, sulfur, and nitrogen) through decay
of organic matter?
- Yes, the decay of the organic particles not only
recycles carbon, but also the nutrients. - However, the amount that is being recycled is
diminished as the export of particles to deeper
layers (and ocean bottom) continues. - At some point, the recycling (item 5)becomes
negligible because all the nutrients have been
exported to the cold layers below and nothing can
grow anymore.
42The biological pump A Thought Experiment (cont.)
- Vertical profile of nutrients concentrations
shows practically nothing in the warm layer, a
maximum below the warm layer where bacteria have
remineralized many of the particles received from
above, and an exponential decay with depth, as
there is less and less left for the bacteria to
remineralize. - At the point of the nutrient maximum, right below
the upper warm layer, there would also be an
oxygen minimum zone (OMZ). - If we now add a slow upward movement of the water
to simulate the process of deep circulation, we
have a first-order model of the oxygen minimum in
the oceans.
43Oceanic biological pump
- CO2 is fixed by photosynthesis,
- 2) this organic matter sinks into deeper waters,
- 3) bacterial decay releases CO2 and other
nutrients, making them available to be used again
by phytoplankton, until - 4) ultimately deposition locks away the carbon
in sediments.
44The Redfield Ratio
- Removing the nutrients from the surface layer,
carbon also is being removed. The content of
total dissolved carbon in the surface layer
decreases. - At the same depth as the nutrient maximum there
is a maximum in total dissolved carbon as well. - How much carbon is exported from the surface
layer in the process of losing all the nutrients?
To estimate this amount, one must know the ratio
of nutrient atoms to carbon atoms within the
organic matter settling out of the euphotic zone.
45The Redfield Ratio (cont)
- Typical numbers describing the composition of
phytoplankton are CNP 106161. Whenever 106
carbon atoms are fixed into organic matter (by
photosynthesis), 16 nitrogen atoms are fixed
(taken from nitrate, NO3- , and ammonia, NH3), as
well as one phosphorus atom. This sequence of
numbers is called the "Redfield Ratio" after
American oceanographer Alfred Redfield (1934).
46The biological pump
- Oceanic upwelling attempts to bring both carbon
and nutrients back to the surface. - However, the biologic activity in the surface
layer (aided by sunlight) keeps removing the
nutrients and causing them to settle back down,
together with the appropriate amount of carbon
(determined by the Redfield Ratio). - This is a way of pumping nutrients and carbon
down, against the upward movement of upwelling,
and hence the term "biological pump. It aids to
hide some of the carbon into sediment reservoir.
47The biological pump
- If the biological pump were turned off,
atmospheric CO2 would rise to about 550 ppm
(compared to the current 375 ppm). - If the pump were operating at maximum capacity
(that is, if all the oceanic nutrients were used
up) atmospheric CO2 would drop to 140 ppm. - Thus, if we change the overall concentration of
nutrients in the ocean there is a net effect on
carbon cycle.
48The Marine Carbon Cycle (MCC)
- The "physical carbon pump" and the "biological
carbon pump" illustrate the mixing of the ocean
and the biological processes in the sunlit zone
of the ocean. - They are of prime importance in controlling the
carbon budget of the sea and the exchange with
the atmosphere. - Also, we have mentioned the ways in which carbon
is stored in sediments and recycled. - Together, these concepts define the marine carbon
cycle.
49The Marine Carbon Cycle (MCC)
- MCC involves the production and recycling of two
types of carbon-rich materials organic matter
and carbonate (CaCO3). The latter processes about
four times more carbon atoms than the former. - The production of solid CaCO3 (so called
carbonate precipitation?) occurs in the surface
waters, both - organically - by organisms that build their
shells from CaCO3, AND - inorganically according to the chemical
equilibrium in the oceans - Ca 2 2HCO3- ? CaCO3
CO2 H2O
50The Marine Carbon Cycle (MCC)
- Surprisingly, the deposition of large quantities
of calcium carbonate actually tends to raise the
atmospheric CO2. - However, carbonate precipitation is closely
coupled to the "real" organic biological pump
(discussed earlier). - The net effect the carbonate cycle (NOT carbon
cycle) acts as a dragging force on the biological
pump. - The amount of drag can be modified by changing
the ratio of the number of carbon atoms that are
involved in the carbonate cycle to those
partaking in the organic biological cycle.
51The Marine Carbon Cycle (MCC)
Bad guy
Good guy
Typical marine phytoplanktons diatoms (left) and
Coccolithophores (right)
52The Marine Carbon Cycle (MCC)
- In ocean, this is done mainly by changing the
amount of silicate (SiO4). - Marine organisms called diatoms grow rapidly in
the presence of silicate. They fix carbon into
organic matter and take much of it down to deep
waters (at the end of their life cycle). - If silicate is little, organisms called
coccolithophores?(???) grow more readily than
diatoms. They precipitate lots of carbon into
carbonate. But they remove calcium carbonate from
surface waters by precipitation, which makes
these waters reject CO2 and thus tend to raise
the atmospheric CO2.
53The Marine Carbon Cycle (MCC)
- Therefore, any process favoring the growth of
organisms made from silicate (e.g. diatoms), over
organisms made from carbonate (e.g.
coccolithophorids) will tend to lower the
atmospheric CO2, and vice versa. - Factors controlling the diatoms vs.
coccolithophorids species include temperature,
nutrient levels, and light. More subtle indirect
factors, however, are not yet understood.
54The Marine Carbon Cycle (MCC)
- Blooms of carbonate-fixing plankton, like
coccolithophores and coral, would have the net
effect of bringing CO2 from surface waters to the
atmosphere. - What precisely causes the blooms of
coccolithophores and whether their population is
increasing or decreasing as the planet warms
remain unclear at present.
55The terrestrial carbon cycling
56The terrestrial vs. oceanic biosphere
- Carbon on land is locked up in (1) soils (soil
carbon) and (2) in trees (biosphere reservoir). - Mass of oceanic biosphere is small compared with
that of carbon in wood. - Plants on land appear, for some reasons, to be
about twice as efficient in fixing carbon during
photosynthesis than organisms in the ocean.
57The terrestrial vs. oceanic biosphere
- It is not easy to make a direct comparison
between ocean and land carbon reservoirs. On land
(carbon mainly moves through wood), we can
measure "productivity" fairly simply the mass of
carbon in trees divided by their average age. - In contrast, measurements of oceanic productivity
are much more difficult. One reason is because
many of the carbon-fixing organisms are extremely
short-lived. - So, is there even a purpose in comparing the
fixation of carbon by photosynthesizing bacteria
and other phytoplankton in the ocean with the
fixation of carbon in wood on land? What do you
think?
58Changing CO2 and terrestrial response
- There are two carbon cycles of interest on land
- (a) The cycle involving annual growth and
decay, and (b) the cycle involving long-term
storage of carbon in wood, remains in soil, and
near-surface organic deposits. - Both cycles have the atmosphere as intermediary.
59Changing CO2 and terrestrial response
Q1 How will the terrestrial biosphere and soil
carbon (has plenty of bacteria) respond to
global warming? Q2 How will its feed back
into the climate?
60Annual growth and decay
- Decays return CO2 to the air, a reservoir from
which CO2 can be extracted for renewed growth. - The sensitivity of atmosphere to land plant
growth and decay is evident from the Keeling
curves Upon close inspection of the annual
cycles, the amplitude of the annual cycles is
found to increase with time.
What is your Interpretation here ?
61Annual growth and decay
- The favored interpretation terrestrial biosphere
is growing and decaying at an increasing rate
(particularly true in NH). - It is difficult to see how tropical forests could
be expanding because they are burned and
disappearing and because they are in the tropics
lacking seasonal variation. - It is thus concluded that most of the observed
biosphere expansion comes from temperate and
northern forests.
62Increased plant growth in the northern forests
Figure shows the increased rate of green
vegetation during the growing season (May
September) between 1982 and 1990 (from Myneni et
al. Nature, 1997)
63Annual growth and decay
- A puzzle Under human interference, why does the
figure above show that the terrestrial biomass is
expanding? - Something is disguising the observed trend of
deforestation, or - there is some compensating process making it
appear as if the biosphere is getting bigger, or - Maybe it is more vigorous growth (and decay) of
annuals, deciduous trees, and bushes that are
responsible for the increase in amplitude of the
Keeling curve.
64Plant Growth Factors and Greening
- High CO2 indeed stimulate plant growth.
- Plant has to balance its need for letting CO2
into its photosynthetic factories without letting
water inside the plant escape, a result of the
plant opening its pores (called stomata) during
photosynthesis. - If more CO2, the pores on leaves do not need to
open as much to get the same amount and water can
be retained better within the plant. - Plant thus grows more vigorously in places where
water is a limiting factor (e.g. blooming in arid
area). - Increased precipitation due to warming further
favors an overall increase in annual growth and
decay.
65Plant Growth Factors and Greening
- However, the above story is not complete and
there is a downside. - At higher latitudes as the seasons change in
response to warming, the programming of the
various trees (time to shed leaves when the days
are short) will be out of synch. - Opportunistic shrubs (not so programmed) that
tend to hang on until it gets too cold will take
an advantage. - Thus plants adapted to warmer climates very well
but trees do not spread across landmasses very
fast. - Thus, quite likely, we will see an increased
turmoil in the plant world weakened tree stands
increasingly susceptible to infestation of fire.
66The Soil Cycle
- Plant debris are deposited and buried in the
soil. - Global warming is expected to increase the rate
at which bacteria and fungi digest the deposited
organic material. - This is true for the portion of soil carbon that
have always been frozen or close to freezing,
like the vast areas of tundra and peat deposits
of high northern latitudes. - Scientists thus worry that the response of soil
carbon will be a positive feedback, making our
climate even warmer.
67End of Lecture 4
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