Lecture 5 Lecture 5 碳的前世今生 碳的前世今生 – Understanding the global carbon cycleUnderstanding the global carbon cycle

What is Biogeochemistry? Biogeochemistry and Carbon Cycle The Breathing of Gaia Carbon Cycling Carbon Cycling

BioGeoChemistry

• 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.

BioGeoChemistry• 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.

BioGeoChemistry• 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.

BioGeoChemistry

• 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

Biogeochemistry 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.

A sketch of carbon cycle illustrating fluxes and reservoirs (From: SeaWIFS project)

Biogeochemistry 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.

Reservoirs 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.

Why 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 Earth’s 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.

Seafloor 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.

Initial Forcings

Initial Forcings

Negative Feedback Loop

Negative Feedback Loop

Chemical WeatheringChemical Weathering

HCO3-: Bicarbonate

Negative Feedback From Chemical WeatheringNegative 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 Earth’s Climate: Past and Future)

BLAG Carbon Cycle

On Land: CaSiO3 + CO2 -> CaCO3 + SiO2

Subduction: CaCO3 + SiO2 -> 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.

Uplift (Weathering) HypothesisMaureen 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 =>

以上兩個假說看待 chemical weathering 有些許差異 :

BLAG假說把 chemical weathering 視為是為了調整海底版塊擴張、火山活動注入大氣層的 CO2 增加後 , 而被動回應的負回饋作用 ◦ 它的調整速度有地域性 ; 在暖溼地帶 , 速度加快 ◦

uplifting假說中則是把 chemical weathering 本身視為是氣候變遷的驅動力 , 而非負回饋作用 ; 此驅動力直接作用在因年輕而通常地形較破碎的舉升區 ◦

The weathering on land (CaSiO3 + CO2 -> CaCO3 + SiO2) was first proposed by Harold Urey in 1950s to understand the fundamental process of removing CO2 from atmosphere.

According to Urey’s model, the amount of atmospheric CO2 is regulated by the presence hydrologic cycle.

Is this really valid? Is this really valid? 1.Atmospheric CO2 also comes out of volcanoes. 2.The rate at which this happens is presumably independent from the surface reactions described in Urey’s proposal. 3.After entering the atmosphere, some of CO2 is concentrated in the soil by the action of plants (and bacteria, fungi).

Is this really valid? Is this really valid? 4.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. 5.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).

The above thought analysis of Urey’s 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 Urey’s model.

Therefore, in foreseeing what will happen in human’s timescale, the changes in our ecosystem talk.

=>=> The breathing of Gaia

Lessons we learn are: Lessons we learn are:

Important indication of Keeling curveImportant 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.

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.

The breathing of GaiaThe breathing of Gaia

The Carbon cyclingThe Carbon cycling

The exchange of carbon between the atmosphere and the ocean/land takes place in several ways:

The Carbon cyclingThe Carbon cycling

1. The physical carbon pump2. The biological carbon pump3. The marine carbon cycle4. The terrestrial carbon cycle

1. The most important mechanism is through physical mixing of the ocean (i.e. vertical deep mixing). When seawater is cooler it takes up more.

2. 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.

3. Vertical circulation acts as an enormous carbon pump, giving the ocean more carbon than if equilibrium with the surface ocean.

The physical carbon pumpThe physical carbon pump

Sketch illustrating the concept of vertical deep mixingSketch illustrating the concept of vertical deep mixing

What will happen if the ocean become warmer (or cooler)?

Warming Warming the oceans: A Thought Experimentthe oceans: A Thought Experiment

1.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.

Warming Warming the oceans: A Thought Experiment the oceans: A Thought Experiment (cont.)(cont.)

2. 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.

Warming Warming the oceans: A Thought Experiment the oceans: A Thought Experiment (cont.)(cont.)

3. 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.

CoolingCooling the oceans: Another Thought the oceans: Another Thought ExperimentExperiment

1. Cooling also takes place from the top by removing heat because of evaporation, freezing, and infrared radiated to the sky.

2. 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.

CoolingCooling the oceans: Another Thought the oceans: Another Thought Experiment (cont.)Experiment (cont.)

3. 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.

CoolingCooling the oceans: Another Thought Experiment the oceans: Another Thought Experiment (a corollary)(a corollary)

a. 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.

b. 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).

CoolingCooling the oceans: Another Thought Experiment the oceans: Another Thought Experiment (a corollary)(a corollary)

c. With intensified mixing, the thermocline initially could release additional CO2 to the atmosphere, counteracting the positive feedback from cooling.

d. 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.

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.

Lessen learnedLessen learned

The biological carbon pumpThe biological carbon pumpOcean gets a disproportionate share of the CO2 available to the ocean-atmosphere system (about 50 times larger).

The biological carbon pumpThe biological carbon pump1. The main reason: CO2 readily reacts with water

(H2O) to make soluble species of ions, the bicarbonate (HCO� 3

-).

2. 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.

3. The last reason for the ocean’s 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.

The biological pump: A Thought ExperimentThe biological pump: A Thought Experiment1. We start with a well-mixed ocean, dark and quite cold

throughout. 2. We then turn on the Sun and heat the ocean from

above. 3. A warm-water layer develops on top of the ocean, and

since it is euphotic, green algae will now grow in this layer => CO2 is being fixed into carbon compounds (photosynthesis, you know).

4. Some of these particles of the algae (dead organic stuff) sink out of the euphotic zone into the deeper cold waters.

5. Others could be re-mineralized: decay by the action of bacteria, releasing CO2 back to the water.

But how long can this process of carbon fixation (item 3), carbon settling (item 4), and carbon recycling (item 5) continue in our experiment?

The biological pump: A Thought Experiment The biological pump: A Thought Experiment (cont.)(cont.)

Answer: It can continue until all the nutrients that are necessary for photosynthesis have been used up.

Used up all the nutrients?

The sketch of oxygen profile with an oxygen minimum zone (OMZ) at mid-depth (typically 1-km below sea surface)

What 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.

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.

The biological pump: A Thought Experiment The biological pump: A Thought Experiment (cont.)(cont.)

Oceanic biological pumpOceanic biological pump

1) 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.

The Redfield RatioThe Redfield Ratio

1. Removing the nutrients from the surface layer, carbon also is being removed. The content of total dissolved carbon in the surface layer decreases.

2. At the same depth as the nutrient maximum there is a maximum in total dissolved carbon as well.

3. 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.

The Redfield Ratio The Redfield Ratio (cont)(cont)

4. Typical numbers describing the composition of phytoplankton are C:N:P = 106:16:1C:N:P = 106:16:1. 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).

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.

The biological pumpThe biological pump

The biological pumpThe 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.

The Marine Carbon Cycle (MCC)The 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.

The Marine Carbon Cycle (MCC)The 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

The Marine Carbon Cycle (MCC)The 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.

The Marine Carbon Cycle (MCC)The Marine Carbon Cycle (MCC)

Typical marine phytoplanktons: diatoms (left) and Coccolithophores (right)

Good guy Bad guy

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 (� 球石藻 ) 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.

The Marine Carbon Cycle (MCC)The 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.

The Marine Carbon Cycle (MCC)The 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.

The Marine Carbon Cycle (MCC)The Marine Carbon Cycle (MCC)

The terrestrial carbon cyclingThe terrestrial carbon cycling

The terrestrial vs. oceanic biosphereThe terrestrial vs. oceanic biosphereCarbon 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.

The terrestrial vs. oceanic biosphereThe 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?

Changing COChanging CO22 and terrestrial response 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.

Changing COChanging CO22 and terrestrial response and terrestrial responseQ1: 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?

Annual growth and decayAnnual 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 ?

Annual growth and decayAnnual growth and decayThe 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.

Figure shows the increased rate of green vegetation during the growing season (May – September) between 1982 and 1990 (from Myneni et al. Nature, 1997)

Increased plant growth in the northern forestsIncreased plant growth in the northern forests

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.

Annual growth and decayAnnual growth and decay

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.

Plant Growth Factors and GreeningPlant 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.

Plant Growth Factors and GreeningPlant Growth Factors and Greening

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.

The Soil CycleThe Soil Cycle

End of Lecture 4