Introduction: Geochemical Cycles and Global Changes

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On behalf of the guest editors, Drs. S. Montani, H. Obata, T. Ono, Y. W. Watanabe and M. Yamamoto, I (S. Tsunogai)would like to describe the aim of this special issue.

Chemistry is an extremely useful tool when we study global change issues, especially global warming. This isbecause chemistry can provide a tremendous amount on quantitative data of many system components, includinginorganic and organic compounds, stable isotopic ratios and radioisotopes contained in natural and artificial matteron Earth. More accurate or more sensitive data usually give more meaningful information on the Earth’s changingproblems. Chemists, therefore, are apt to compete to obtain such unexplored data by inventing a new method andapplying it as soon as possible to samples collected from the Earth’s environment.

This effort is important and valuable for resolving our environmental problems, but the chemists’ intention oftenchanges gradually from earth science to pure and applied analytical chemistry. This may be due to the complex natureof the global change issues, and it is difficult for us to achieve a high level goal, especially using chemistry as a singletool. We have discussed this point at two symposia: Sapporo symposium in celebration of my retirement from HokkaidoUniversity on 22–23 March 2002 and a JOS symposium “Geochemical cycles and global change” on 31 March 2002.This special issue comprises mostly the papers presented at the symposia.

The structure of our global change research can be likened to a cloth, composed of the warp of observed chemi-cal data in this case and the woof of phenomena occurring in nature. Our research subjects are found at some intersec-tion points of the warp and the woof. We chemists, of course, must acquire chemistry, including technical skills,generating the warps before starting research on the global change issues. It is, however, next to impossible for us toclarify any phenomenon arising from the complicated Earth system using some data on only one component. We mustthus take up a more important woof, a phenomenon to be solved first, and next select a most appropriate componentor a set of components based on some concepts of pure chemistry. The selection and application of these components,however, are not easy, because they are controlled by many factors, namely many woofs. To minimize the difficulty,we must utilize the accumulated knowledge on the geochemical cycles of chemical components on the whole Earthsystem, not merely the oceanic part.

On the other hand, some chemical oceanographers are not necessarily well versed in pure chemistry. For exam-ple, they are not well aware of the definition of adsorption, scavenging, suspension, to dissolve or solution, etc. Thisitself, however, is not serious, because we can learn what is necessary by reading a textbook. The important task israther to combine fundamental chemistry and the phenomena occurring in the ocean. As an example of this, I wouldmention the oceanic carbonate system (which is one of my major subjects), using results obtained by myself but notusing chemical symbols and formulae. I believe that the younger generation in this field will rewrite such sentencesusing their own results.

1. Atmospheric carbon dioxide dissolves into seawater forming carbonic acid (dissolved but not dissociatedcarbon dioxide), which partly dissociates into bicarbonate ion and carbonate ion. The proportions of carbonic acid,carbonate ion and bicarbonate ion are, respectively, usually less than 1%, around 10% and nearly 90% of total car-bonate (Tsunogai, 1989a, b).

2. The carbonic acid concentration in equilibrium with atmospheric carbon dioxide increases by about 4% per1°C depression in water temperature (Tsunogai, 1989a, b).

3. The total carbonate content (around 2000 µM: M = mol/l, although oceanographers use a unit of mol/kg ofseawater) also increases by about 0.4% per 1°C with decreasing water temperature (Tsunogai, 1989a, b; Tsunogai etal., 1993).

4. For a certain increase in the concentration of atmospheric carbon dioxide, as a result, the colder the water is,the smaller the increment in the equilibrium concentration of total carbonate. For instance, if the atmospheric carbondioxide increases by 80 ppm from 280 ppm, the total carbonate in seawater will increase by 60 µM at 30°C, but onlyby 40 µM at 0°C (Tsunogai, 1989a, b; Tsunogai et al., 1993).

5. Formation of solid calcium carbonate in the surface water enhances the escape of carbon dioxide into theatmosphere, and its dissolution in the deep ocean makes the absorption of atmospheric carbon dioxide easy afterrising to the surface (Tsunogai, 1991a, 2000).

6. If about 100 µM of solid carbonate is dissolved in seawater like the Pacific Deep Water, the water canabsorb nearly the same amount of atmospheric carbon dioxide. This is one of the reasons why the surface water in thenorthern North Pacific absorbs so much atmospheric carbon dioxide (Tsunogai et al., 1993; Tsunogai, 1997, 2000).

648 Introduction

7. Formation of organic matter in the surface water means that the atmospheric carbon dioxide can be ab-sorbed into the water, and its decomposition in the deep water allows the carbon dioxide in the water to escape to theatmosphere (Tsunogai, 1989a, 1997, 2000, 2002).

8. If about 200 µM of carbon as well as nutrients regenerates in the abyss, like the Pacific Deep Water, nearlythe same amount of carbon dioxide can escape from the upwelled deep water, and the escape is accelerated when thewater is warmed. The equatorial East Pacific is an example of this water (Tsunogai et al., 1993; Tsunogai, 1997, 2000,2002).

9. When the upwelled water has lost all the nutrients due to photosynthesis and has cooled to the deep watertemperature, the carbon dioxide lost in the upwelling region returns to the ocean. Examples of this condition are thenorthwestern North Pacific and the northern North Atlantic (Tsunogai, 1997, 2000, 2002).

10. In the actual ocean, the amount of carbon dioxide returning to the upwelled water described above is greaterthan that escaping from the upwelling region (Tsunogai, 1997, 2000, 2002). This is due to three causes, describedbelow.

11. The first cause is that the amount of ‘preformed carbonate’ is less than those of the preformed nutrientsequivalent to carbonate when the Pacific Deep Water is formed. This is due to some carbon dioxide loss but noappreciable depletion in nutrients in the Antarctic Ocean in winter. This also means that the region was a source ofatmospheric carbon dioxide in the preindustrial era. The second cause is the increased alkalinity as described in theitem 6 and the third one is the increased concentration of atmospheric carbon dioxide after the Pacific Deep Water hasbeen formed (Tsunogai, 1989a, 1989b, 1997, 2000, 2002).

12. This means that the more the amount of upwelling Pacific Deep Water, the more the amount of carbondioxide absorbed into the ocean, and vice versa (Tsunogai, 1997, 2000, 2002).

13. It takes a considerable time to attain a gas-exchange equilibrium between the atmosphere and the oceans.There are two major barriers to the exchange: the air-sea interface and the vertical mixing or stratification of seawater(Tsunogai, 1989a, 1989b, 2002).

14. In the well-mixed surface 100 m water, the time constant (mean time) for gas exchange is roughly 20–30days for nitrogen and oxygen, but it takes 10 times longer for carbonate at the molecular level and 100 times longerfor carbon isotopes at the atomic level (Nakayama et al., 2000; Tsunogai, 2002).

15. Of course, the time constant (gas transfer velocity), which is important for the air-sea exchange, dependslargely on the region and season: the velocity is extremely high under heavy storms (Tsunogai and Tanaka, 1980;Tsunogai, 2002; Kawabata et al., 2003).

16. Air bubbles may play major role in the gas exchange, making the transfer velocity of carbon dioxide consid-erably larger than that of a sparingly soluble gas (Nakayama et al., 2000, 2002; Tsunogai, 2002).

17. The time constant for vertical mixing is about 1000 years for the entire ocean water, but it is about 100 yearsor less for the intermediate water (Tsunogai, 1981).

18. This means that, if the intermediate water occupies an oceanic volume that is not less than one tenth of thedeep water’s volume, it plays a more important role than the deep water in the absorption of anthropogenic carbondioxide emitted during the last several decades (Tsunogai, 1987, 1995; Tsunogai et al., 1995).

19. The Continental shelf pump has been proposed as a mechanism transferring carbon dioxide from the atmos-phere to the ocean, besides the biological and solubility pumps. Denser water containing more dissolved carbon isformed on the continental shelf bottom due to cooling, sea ice formation and evaporation, and transports carbon intothe subsurface layer of the open ocean by isopycnal mixing. The reverse flow of carbon is prevented by the welldeveloped pycnocline during the warming season (Tsunogai et al., 1997, 1999, 2003; Tsunogai, 2002).

20. Iron is essential for plankton growth, but its residence time in the ocean is short. This means that the olddeep water contains substantially no iron but much regenerated nutrient (Tsunogai, 1985, 2001).

21. Due to the lack of iron, phytoplankton cannot actively propagate in the upwelling deep water even thoughnutrients are present. The propagation should commence if iron is added to the water (Tsunogai, 1985, 2001).

22. Only a little iron is necessary for the propagation and is supplied gradually from land via the atmosphereand rivers. If the upwelled deep water stays at the surface, all the nutrients in the surface water are ultimately trans-formed into organic matter, implying only a delay in the assimilation (Tsunogai et al., 1985; Tsunogai, 1991b, 2001).

23. The effectiveness of iron fertilization encounters at a problem if the nutrients in the upwelled water return tothe abyss before being taken up by phytoplankton (Tsunogai, 2001).

24. There are two mechanisms for transporting the surface water to the abyss: the formation of deep and inter-mediate waters (advection), and the vertical mixing (diffusion). These physical processes control the effectiveness ofiron fertilization (Tsunogai, 1972a, 1987, 1995).

Introduction 649

25. One more problem is the production of solid carbonate, operating as a source of atmospheric carbon diox-ide. The ratio of carbonate carbon to organic carbon in settling particles regionally and vertically varies. At present,the ratio at 1 km depth is about 1/2 as a median, meaning that the biological activity certainly works as a sink foratmospheric carbon dioxide (Tsunogai and Noriki, 1987, 1991).

26. The ratio of carbonate carbon to organic carbon in settling particles is extremely low in the Antarctic Oceanand the northwestern North Pacific, where the dissolved silicate concentration is high. The ratio increases with depthdue to the more rapid decomposition rate of organic debris, and it is greater than unity for particles settling at 1 kmdepth in the eastern North Pacific (Tsunogai et al., 1979, 1986, 1990; Tsunogai, 1987, 2002; Tsunogai and Noriki,1991).

27. In the eastern North Pacific, therefore, even if the iron fertilization increases biological production with thepresent carbonate carbon to organic carbon ratio, it is of almost no value for the absorption of atmospheric carbondioxide, while it is effective in the northwestern North Pacific (Tsunogai and Noriki, 1991; Tsunogai, 2001, 2002).

28. Since silicate is gradually supplied to the ocean surface similarly to iron, the rapid propagation ofphytoplankton achieved thanks to the iron fertilization may increase the carbonate carbon to organic carbon ratio,which reduces the effectiveness of the biological pump (Tsunogai, 1979, 1991b; Tsunogai and Watanabe, 1983; Tsunogaiet al., 1985).

29. I conclude that the iron fertilization in the northern North Pacific may only accelerate the nutrient consump-tion to be used after flowing out in a southwesterly direction nearer the coast, and it may make a different ecosystemrich in calcareous organisms, which is worse for the absorption of atmospheric carbon dioxide (Tsunogai, 2001,2002).

30. Of course, since the synthesized organic carbon is mostly regenerated at shallower depths, even if the ironfertilization is effective, it must be maintained. Otherwise, the regenerated carbon will return to the atmosphere whenthe fertilization is interrupted (Tsunogai, 1972b, 2002; Tsunogai and Noriki, 1987; Tsunogai et al., 1990).

Finally I would like to add a spiritual aspect of this topic. A Japanese word corresponding to ‘Cycle’ is ‘Junkan’,but it has one more meaning, ‘Circulation’. Therefore, Japanese physical oceanographers also use the word, ‘Junkan’,for the motion of ocean water. In order to distinguish the dual meanings in ‘Junkan’, I have proposed to use a term‘Rinne’ for ‘Cycle’. The word, ‘Rinne’, comes from Buddhism, meaning the never-ending cycle of reincarnation,which has a more specific sense than ‘Cycle’ in English for chemical substances in the four-dimensional world. Ibelieve ‘Rinne’ is more appropriate than ‘Junkan’ in Japanese and ‘Cycle’ in English for the geochemical cycling inthe present global environmental issues. Furthermore, I would like to propose one more word ‘Kikou Henka’ for theman-made climate changes, because the word ‘Henka’ means ‘to change irreversibly’, and ‘Kikou Hendou’ for thenatural climate change, because the most popular meaning of ‘Hendou’ is ‘fluctuation’, like a sine curve returning tothe original value. An English word ‘Climate Change’ can not distinguish the present and future man-made climatechanges from the past natural climate changes. The climate change issue is a serious problem for future human kind,and thus my definition is better and we should use ‘Kikou Henka’ for the present great ordeal.

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J. Oceanogr., 59, this issue, 685–693.

Guest Editor-in-ChiefShizuo Tsunogai

Professor emeritus of Hokkaido University,Shirako 2-4-19, Wako 351-0101, Japan

Guest EditorsShigeru Montani, Graduate School of Fisheries Sciences, Hokkaido UniversityHajime Obata, Ocean Research Institute, University of TokyoTsuneo Ono, Hokkaido National Fisheries Research InstituteYutaka W. Watanabe, Graduate School of Environmental Earth Science, Hokkaido UniversityMasanobu Yamamoto, Graduate School of Environmental Earth Science, Hokkaido University