Lecture 4 碳的前世今生 – Understanding the global carbon cycle

1. Lecture 4 碳的前世今生 – Understanding the global carbon cycle What is Biogeochemistry? Biogeochemistry and Carbon Cycle The Breathing of Gaia Carbon Cycling

2. 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.

3. 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.

4. 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.

5. 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

6. 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.

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

8. 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.

9. 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.

10. 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.

11. Seafloor Spreading Rate Hypothesis is also known as BLAG Hypothesis to denote its initial authors, the geochemists Robert Berner, Antonio Lasaga, And RoberGarrels. 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.

12. Initial Forcings Negative Feedback Loop Initial Forcings Negative Feedback Loop

13. Chemical Weathering HCO3-: Bicarbonate

14. Negative 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)

15. 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.

16. Uplift (Weathering) Hypothesis Maureen Raymo and her colleagues (1986) proposed a second 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 =>

17. 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 ◦

18. 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.

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 Urey’s proposal. • After entering the atmosphere, some of CO2 is concentrated in the soil by the action of plants (and bacteria, fungi). • 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).

20. 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

21. Important 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.

22. The 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 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.

23. The Carbon cycling

24. The 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

25. The 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.

26. Sketch illustrating the concept of vertical deep mixing What will happen if the ocean become warmer (or cooler)?

27. Warming the oceans: A Thought Experiment Warming of ocean waters takes place at 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. The ocean will yield some of its own CO2 and slow its uptake of CO2 from the atmosphere. The deep cold water also 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 oxygen is entirely used up, nitrate (NO3) is used by the bacteria as an oxygen source instead. In this process, nitrous oxide (N20) and molecular nitrogen (N2) are made while nitrate is being destroyed.

28. Warming the oceans: A Thought Experiment 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.

29. Coolingthe oceans: Another Thought Experiment Cooling also takes place at 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. 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.

30. Coolingthe oceans: Another Thought Experiment A corollary to (1)-(3) is that the water column, after cooling, is quite well mixed, which was not necessarily the case 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). 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.

31. Lessen 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.

32. The biological carbon pump Ocean gets a disproportionate share of the CO2 available to the ocean-atmosphere system (about 50 times larger). The main reason is that CO2 readily reacts with water to make soluble species of ions, the bicarbonate� (HCO3-). Another reason is 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 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 distributing it to the deeper water layers.

33. The 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 => 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.

34. The biological pump: A Thought Experiment 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?

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

36. 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.

37. The biological pump: A Thought Experiment • 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.

38. Oceanic 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.

39. The 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. Typical numbers describing the composition of phytoplankton are C: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 Alfred Redfield.

40. The 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.

41. The 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.

42. The Marine Carbon Cycle • 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.

43. 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

44. 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.

45. The Marine Carbon Cycle (MCC) Typical marine phytoplanktons: diatoms (left) and Coccolithophores (right)

46. The 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 sillicate. 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.

47. 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. • Blooms of carbonate-fixing plankton, like coccolithophores and coral, would have the effect of bringing CO2 from surface waters to the air. What precisely causes the blooms of coccolithophores and whether their intensity is increasing or decreasing as the planet warms are not known at present.

48. The terrestrial carbon cycling

49. The terrestrial vs. oceanic biosphere • Carbon on land is locked up in soils (soil carbon) and in trees (biosphere reservoir). • Mass of oceanic biosphere is small compared with that of carbon in wood. • Plants on land appear, for some reason, to be about twice as efficient in fixing carbon during photosynthesis than organisms in the ocean.

50. The 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?