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Biogeochemical connections between the atmosphere and the ocean
Biogeochemical Connections Between the Atmosphere and the Ocean Peter S. Liss School of Environmental Sciences, University of East Anglia, Norwich, UK
In comparison with other planets in the solar system, the chemical composition of the earth's atmosphere clearly shows the influence of biological processes. The level of carbon dioxide is much lower and of oxygen much higher due to the photosynthesis of land and marine plants, and many reduced gases exist at concentrations far in excess of what purely inorganic processes would allow. In this chapter the emphasis is on the role of biological activity in the oceans in affecting the chemical (and physical) properties of the atmosphere. Air-sea exchanges of ozone, carbon dioxide, dimethyl sulphide, dimethyl selenide and ammonia are described with regard to their importance for processes in the atmosphere, together with the role of atmospheric dust inputs on ocean biology with its potential for concomitant change in trace gas fluxes to the atmosphere.
A. INTRODUCTION
chemical composition with that of other planets in the solar system strongly shows the influence of biological processes. Many reduced gases exist, at concentrations far in excess of what purely inorganic processes would allow. In the oceans the biota play a strong role in determining the amounts of gases important in controlling the earth's radiation balance, such as carbon dioxide and dimethyl sulphide (DMS), as well as the chemistry of the atmosphere and geochemical cycling of several key elements including nitrogen, selenium and the halogens. It has recently been shown how delicately poised the oceanic biota are in terms of availability of the element iron, and hence in their ability to control levels of several gases of importance in the atmosphere. This chapter begins with a brief overview of the effect of the biosphere on the overall composition of the earth's atmosphere. There follows a section on the role of the marine biota in producing or consuming various trace gases which are important for the chemistry (and physics) of the atmosphere. After this there is a complementary section on the role that inputs from the atmosphere can play in affecting the level of biological activity in the oceans, which activity can in turn affect the gaseous emissions discussed in the previous section. This cyclic interaction between the atmospheric and oceanic reservoirs leads to a short concluding section on the global nature of biogeochemical inter-reservoir transfers; a concept encouraged by the holistic view of the earth obtained by observing it from the moon.
Just over 30 years ago mankind had the opportunity to view planet earth from the moon. It can be argued that this represented a major turning point in how we have regarded our home planet ever since. Seen from the moon, the earth looks very isolated in space. From that viewpoint its unity is obvious, which reinforced the idea that our planet must be seen and studied as a whole, rather than split into its component reservoirs, as is so often done in academic research. The obvious dominance of the oceans in terms of coverage relative to land led to the idea that the earth should really have been called "ocean". Finally, the blue oceans, green/brown land and white clouds all looked very different to the colouring of the other planets we can see using telescopes from earth. It cannot be said that any of this understanding was new, since intellectually all of it had been known for many years. What was new was the perspective that viewing our planet from an outside observation post provided. Thus the isolation, beauty and very special properties of earth were made visually apparent. The extraordinary properties made manifest in the blues, greens, browns and whites of the earth as seen from space are, of course, due in large measure to the presence of living organisms on the planet. In this chapter some of the ways in which the functioning of the earth's (particularly marine) biota control important properties of the atmosphere, oceans and land will be discussed. For the atmosphere, comparison of its
Meteorology at the Millennium ISBN 0-12-548035-0
249
Copyright © 2002 by Academic Press All rights of reproduction in any form reserved.
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BIOGEOCHEMICALCONNECTIONS BETWEEN THE ATMOSPHERE AND THE OCEAN
B. T H E C H E M I C A L THE EARTH'S
COMPOSITION ATMOSPHERE
OF
The gaseous composition of the earth's atmosphere clearly shows the influence of biological processes on land and in the sea. This is well illustrated in Fig. 1, which shows a comparison of the present composition of the atmosphere (i.e. with life present), with that predicted if no life existed on the earth. The prediction of the lifeless composition is made by assuming the earth and its atmosphere to be at thermodynamic equilibrium, with the presence of life disturbing that equilibrium. It should be noted that in Fig. 1 the percentage contribution of each gas to the total composition is shown as a histogram on a logarithmic scale for both the life present and the life absent cases. A clear example of the effect of life on atmospheric composition is the 3-4 orders elevation of 02 and concomitant decrease in CO2 arising from the uptake of the latter and release of the former as a result of photosynthesis by land plants and marine phytoplankton. Furthermore, several trace gases, including the important greenhouse gases CH4, N20, occur 2-3 orders of magnitude higher in concentration in our present atmosphere than would be predicted in the absence of life. Other gases including NH3, HCI and DMS (not shown) are present in low but meas-
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urable amounts in the current atmosphere, whereas they would be at significantly lower levels or totally absent in the air of a lifeless earth. Some important gases, e.g. water vapour and ozone, are omitted from the figure because, although the biosphere and its products are important for their cycling, it is difficult to estimate concentration levels for them in the pre-life atmosphere. C. AIR-SEA EXCHANGE OF GASES OF IMPORTANCE IN THE ATMOSPHERE Many gases cross the atmosphere-ocean interface, and in Table 1 a summary is given of the gases involved, as well as an indication of their importance in the atmosphere, both troposphere and stratosphere. For most of the gases, the net flux direction is from sea to air, i.e. the oceans are a source of the gases to the atmosphere. However, there are some exceptions, two of which are now discussed. 1. O z o n e Ozone (03) is one exception, since for this gas the oceans are an absolute sink (i.e., there is no re-emission, due to the high reactivity of ozone with iodide ions and organic material at the sea surface, leading to its destruction in the water (Garland e t a l . , 1980). Although the rate of uptake of 03 by the oceans is small compared with deposition to land surfaces, their large area means that they may still be a significant sink in the global budget of the gas (Ganzeveld and Lelieveld, 1995). Another possible exception is ammonia where the net flux direction is somewhat uncertain and is probably highly variable, a situation which will be discussed further later. 2. M a n m a d e C a r b o n D i o x i d e
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Another exception is manmade C O 2 where the oceans take up about 40% of the total amount of the gas emitted to the atmosphere from the burning of fossil fuels (IPCC, 1996). This contrasts with the situation for natural CO2 where, although in some locations and for some seasons the net fluxes are both in and out of the oceans, averaged for the whole of the oceans and over a yearly cycle there is near balance between the gross up and down flows. The observed seasonal and spatial variability is due to both temperature affecting the solubility of CO2 in seawater, and biological uptake and release of the gas during photosynthesis and respiration, respectively. However, with the increase in atmospheric CO2 resulting from anthropogenic activities the natural balanced two-way fluxes have a net downwards component superimposed on them. The importance of biological activity is not only for
251
C. AIR---SEA EXCHANGE OF GASES OF IMPORTANCE IN THE ATMOSPHERE TABLE 1
Air-Sea Exchange of Gases of Importance in the Atmosphere
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Gas
Importance in atmosphere
1"
Dimethyl sulphide (DMS)
Acidity Cloud condensation nuclei Budgeting S cycle Particles Particles Alkalinity Aerosol composition Oxidation capacity Budgeting I (and Br?) cycle(s) Oxidation capacity Oxidation capacity Oxidation capacity Oxidation capacity Radiation balance Radiation balance Radiation balance Radiation balance Budgeting Se cycle Budgeting Hg cycle
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Carbon dioxide ( C O 2 ) (manmade) Nitrous oxide (N20) Methane (CH4) Dimethyl selenide (DMSe) Elemental mercury (Hg)
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the small-scale and short-term changes in C O 2 uptake indicated above. Changes in ocean productivity also have the potential to strongly influence levels of CO2 in the atmosphere on much longer time-scales. This is illustrated in Fig. 2. The figure shows the results of a study by Watson and Liss (1998) in which a simple model (Sarmiento and Toggweiler, 1984) was used to examine how far changes in marine biological activity can affect atmospheric CO2 partial pressures. In each run the model was initialized at pre-industrial steady state (approximately 260 ppm). At t = 100 years a perturbation was applied. For the "Strangelove ocean" all
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marine biological activity was stopped instantaneously. "Cold surface water productivity = 0" and "cold surface water productivity = max" show the possible extremes to which atmospheric CO2 can be driven by modulating the efficiency of the cold-water marine biota. After 1000 years the difference between the atmospheric CO2 level produced by the lifeless and maximum productivity oceans is about 250 ppm (180-430 ppm). Although this is clearly an extreme range, it does illustrate that modulation of biological activity in the oceans has the potential to have a very substantial effect on atmospheric CO2 concentrations. Turning now to gases in Table 1 whose direction of net flux is from the oceans to the atmosphere, several examples are considered.
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F I G U R E 2 Effects of marine biota on atmospheric CO2 from a simple model (Sarmiento and Toggweiler, 1984); the various scenarios are explained in the text (Watson and Liss, 1998).
Dimethyl sulphide (DMS) is a gas produced in ocean surface waters by the activities of phytoplankton. The DMS itself is formed by the enzymatic cleavage of a precursor molecule (dimethyl sulphoniopropionate, DMSP), which the organisms make as an osmolyte or as a cryoprotectant. The commonest way for the DMSP to DMS transition to occur is on death of the plankton by zooplankton grazing or attack by viruses. Once released to the seawater there is a range of bacterial and photochemical transformations which the DMS can undergo (see Liss et al., 1997, for a review of production and destruction processes of DMS in seawater). However, there is almost always a measurable residual
252
BIOGEOCHEMICALCONNECTIONS BETWEEN THE ATMOSPHERE AND THE OCEAN
concentration of DMS in the water and it is this which drives the net flux of the gas to the atmosphere. Because of its rapid dispersion and reaction in the atmosphere, air concentrations are generally only a few percent of the equivalent water values and are often ignored in air-sea flux calculations with little loss of accuracy. Once in the atmosphere, DMS is subject to oxidation by free radicals such as hydroxyl and nitrate to form a variety of products (Ravishankara et al., 1997), the most important of which appear to be methane sulphonic acid (MSA) and sulphur dioxide, which can be further oxidized to sulphate either in water droplets or in aerosols, including sea-salt particles (see e.g., Sievering et al., 1992). All of these products are acidic and give the natural acidity to aerosol particles, rain and other forms of precipitation in the marine atmosphere (Charlson and Rodhe, 1982). Of course, over industrialized/urbanized parts of the globe, particularly in the Northern Hemisphere, human activities have strongly perturbed the natural sulphur cycle, with concomitant increase in both sulphate aerosol numbers and decrease in pH of precipitation and particulates. In areas remote from heavily populated land, particularly in the Southern Hemisphere, the natural cycle is still dominant. Chin and Jacob (1996) have calculated that in terms of the total atmospheric burden of sulphur the percentage contributions of the three main sources are currently: biogenic (mainly from oxidation of DMS) 42%, anthropogenic 37% and volcanic 18%. The transfer of sulphur in the form of DMS from the sea to the marine atmosphere, with some of the oxidation products ultimately deposited on land, is a vital component of the budget of the element in the earth system, since material balance cannot be achieved without it (Brimblecombe et al., 1989). A further important property of DMS and specifically submicron particles formed via its oxidation is in affecting climate. Such particles can interact both directly with sunlight (Shaw, 1987) and indirectly by acting as nuclei on which cloud droplets form (termed cloud condensation nuclei, CCN). The number density of CCN is a major determinant in cloud formation and thus of the albedo of the planet (Charlson et al., 1987). Over land there is generally an abundance of CCN resulting from soil dust blown into the air and from pollution sources. However, over the oceans, far from land (i.e. the majority of the globe), the main source of CCN appears to be from oxidation of marine-produced DMS. This is supported by various lines of evidence, including the chemical composition of the CCN being largely sulphate (neutralized to some extent by uptake of ammonia gas from the atmosphere, see later), and the coherence in the seasonality of both CCN number
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density and cloudiness (as measured by optical depth), which in turn correspond to the times when marine plankton are actively producing DMS in seawater (Boers et al., 1994; Liss and Galloway, 1993). Twomey (1991) has attempted to quantify the magnitude of the effect of CCN number density on cloud albedo (and hence reflectance), as illustrated in Fig. 3. This shows that the CCN-albedo relationship is highly nonlinear. Thus, the effect of change in CCN number on the incremental increase in albedo for each particle added (called susceptibility in Fig. 3) is much more pronounced at low compared with high particle numbers. For example, once the air contains several hundred particles or droplets per cubic centimetre, addition of further particles has little effect on cloud reflectance. Over and close to land, CCN numbers generally exceed this level. Thus, we must look to oceanic areas, and particularly the Southern Hemisphere, with its large ratio of sea to non-ice-covered land, for the greatest potential climatic impact of changing CCN numbers, whether the particles arise, for example, from alteration in oceanic DMS emissions or from inputs of SO2 from (increased) fossil fuel burning. To graphically illustrate the point. Twomey cites a calculation which indicates that equal quantities of sulphur entering the atmosphere in the two hemispheres will have 25 times the effect on cloud albedo in the Southern compared with the Northern Hemisphere. 4. D i m e t h y l Selenide Selenium is the element immediately below sulphur in Group VI of the Periodic Table. For this reason sim-
D. IMPACT OF ATMOSPHERIC DUST ON OCEAN BIOGEOCHEMISTRY
ilarities in behaviour between the two elements might be expected, in this case for their methylated forms DMS and DMSe. For almost a quarter of a century it has been known that, as in the case for sulphur already discussed, there is a discrepancy in the global budget of selenium which can only be rectified by a flux of some, probably volatile, form of the element from the ocean to the atmosphere (Mosher and Duce, 1987). However, until recently, difficulties with the sensitivity of the analytical techniques available for measuring the very low levels at which selenium exists in the environment have meant that it has not been possible to confirm this and establish the size and form of the sea to air flux by direct measurement. Notwithstanding these difficulties, a recent measurement campaign initiated by the author and Drs David Amouroux and Olivier Donard of the University of Pau has thrown significant light on this topic (Amouroux et al., 2001). Volatile forms of selenium were measured at picomolar concentration levels in water samples collected on a research cruise in the north Atlantic in the summer of 1998. The results show that the main volatile forms of selenium are DMSe and the mixed selenium-sulphur compound DMSeS. In addition, simultaneous measurements of DMS and biological parameters clearly show a correlation between the concentrations of DMSe and DMS and strongly indicate a biogenic source for the volatile selenium gases measured. Calculation of the size of the sea-to-air flux of volatile selenium and extrapolation to the oceans as a whole indicates that it is likely to be sufficient to balance the global budget for the element. This implies that the sea is a source of selenium to the land, a result with important implications for human health, as recently reviewed by Rayman (2000).
5. Ammonia Ammonia (NH3) is another example of a gas found in the atmosphere which would not occur there but for biological activity. In Fig. 1 the lifeless earth's atmosphere shows essentially no NH 3, whereas with life present there is a small but significant amount. The ammonia comes from a variety of sources on land as well as from the oceans; here we will consider only the latter source because it can be treated as a companion air-sea flux to that of DMS discussed previously. As in the case of sulphur, "the nitrogen cycle has also been substantially amended through a variety of human activities. In order to examine the role and behaviour of NH 3 in its natural setting, we now have to go to parts of the globe furthest from man's activities. Since such areas are also far from land, this enables the marine part of the cycle to be examined without major influence from
253
terrestrial source regions (this is possible because the atmospheric lifetime of NH 3 after emission from land (or sea) is only a matter of days). Figure 4 shows measurements made by Savoie et al. (1993) on atmospheric aerosol particles collected over a 5-year period at a coastal sampling site in Antarctica. A clear seasonal cycle is evident for ammonium (the form of ammonia in the aerosol), with highest values in the spring and summer seasons and minimum levels in autumn and winter. Also remarkable in Fig. 4 is the similarity of the ammonium to the MSA and non-sea-salt sulphate (both oxidation products of DMS) seasonal cycles. These similarities strongly suggest a marine biological origin for the NH3, as well as for the DMS products. Support for this idea comes from a more detailed analysis of the data in Fig. 4, from which it can be calculated that the molar ratio of ammonium to non-sea-salt sulphate in the aerosols is close to 1:1, implying a chemical composition of ammonium bisulphate (NH4HSO4). The final piece of the jigsaw comes from measurements made in the North and South Pacific Ocean by Quinn et al. (1990), from which they were able to calculate the fluxes of both NH3 and DMS being emitted from the ocean to the atmosphere. Although both fluxes show quite a lot of spatial variability, the mean values are again reasonable close to a 1:1 ratio, supporting the idea that the aerosol composition in terms of both nitrogen and sulphur (the main constituents of the total mass) can be explained by marine biogenic emissions.
D. IMPACT OF ATMOSPHERIC D U S T ON OCEAN BIOGEOCHEMISTRY As discussed in the previous section, trace gases produced or consumed by biological activity in the oceans clearly affect the chemical and physical properties of the atmosphere. However, the atmosphere also provides input of nongaseous material to the oceans in the form of land-derived mineral dust and particulates formed during burning and the use of fertilizers in agriculture. These solids can have a potentially large impact on marine productivity, particularly in certain parts of the oceans. As with land plants, marine plankton need nutrient elements (e.g. nitrogen, phosphorus, iron, etc.) for optimal growth. It is generally agreed that of these nutrients it is deficits of nitrogen and iron which present the greatest potential for limitation and for them atmospheric inputs are also likely to be important. In the case of nitrogen there is growing concern about the impact of the increasing input of human-derived nitrogen compounds to regions of the open oceans where nitrogen is the nutrient-limiting biological activity, such
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Graphs on the left show the actual time series of weekly-average concentrations of methane sulphonic acid (MSA), non-sea-salt sulphate (NSS SO42-) and ammonium NH4 ~) at Mawson, Antarctica. Graphs on the right illustrate the composted seasonal cycles, showing the monthly means and their 95% confidence intervals (from Savoie et al., 1993).
as the subtropical gyres of the North and South Pacific. While estimates suggest that, at present, atmospheric nitrogen accounts for only a few percent of the annual new nitrogen delivered to surface waters in these regions, the atmospheric input to the ocean is highly episodic, often coming in large pulses extending over only a few days when it can play a much more important role. Further, such inputs are increasing as the developing nations of the world increase their production and use of fixed nitrogen. For example, a modelling study by Galloway et al. (1994) developed maps of the "recent" (1980) and expected (2020) annual deposition of oxidized forms of nitrogen, which includes nitrate, nitrite and nitric acid derived from combustion processes, from the atmosphere to land and ocean surfaces. In Fig. 5 the projected ratios of the estimated deposition of oxidized nitrogen in 2020 to the values in 1980 are shown. From 1.5 to 3 times, and in some limited areas up to 4 times, the recent rate of deposition will occur by 2020 over large areas of coastal and open oceans, and these estimates do not include possible changes (almost certainly increases) in reduced and organic nitrogen inputs. This leads to the possibility of regional biogeochemical impacts in both coastal and open ocean areas. Turning now to the case of iron, it has been known
for almost a hundred years that marine biological activity, as judged by underutilization of "traditional" plant nutrients in the water such as phosphate and nitrate, appears limited in areas of the ocean far from land. Examples of these high-nutrient low-chlorophyll (HNLC) areas are the southern oceans, and the equatorial and sub-Arctic Pacific. It has been speculated for a long time that the nutrient limiting biological activity in these areas is iron. The fact that the underutilization of nitrate and phosphate occurs most clearly in waters remote from land is in accord with the idea that an important source of the iron is from deposition of soil dust carried from the continents via the atmosphere. Figure 6 shows the distribution of the global flux of iron to the oceans. The distribution is clearly very nonlinear (the scale on the figure is logarithmic), with highest deposition close to deserted continental areas and the remote H N L C regions listed above being amongst the regions receiving least. However, because of the difficulties of measuring iron at the very low concentrations at which it occurs in seawater, it has until recently been difficult to test whether it is indeed an important limiting nutrient for plankton growth. The first experiments were done onboard ship using flasks of seawater containing plankton and the
255
D. IMPACTOF ATMOSPHERIC DUST ON OCEAN BIOGEOCHEMISTRY
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256
BIOGEOCHEMICAL CONNECTIONS BETWEEN THE ATMOSPHERE AND THE OCEAN
results seemed to confirm that adding iron did indeed enhance plant growth (e.g., Martin and Fitzwater, 1988). However, questions remained about how well the flasks mirrored conditions in the oceans and there were also problems of contamination during collection of the seawater (Fitzwater et al., 1996). These difficulties led to the idea of conducting such experiments by enriching a small patch of ocean water with iron in situ (Watson et al., 1991). Several such experiments have been carried out to date, two in the equatorial Pacific (Martin et al., 1994; Coale et al., 1996) and most recently in the southern ocean (Boyd et al., 2000). In all cases there was clear evidence in support of the iron limitation hypothesis. From the viewpoint of ocean biology affecting the properties of the atmosphere, these in situ studies showed that the enhanced biological activity arising from iron amendment leads to enhanced drawdown of carbon dioxide (Cooper et al., 1996; Watson et al., 2000) and increased production of DMS (Turner et al., 1996).
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We conclude this chapter with two examples which attempt to place the air-sea exchange of DMS and iron, discussed earlier, into a broader whole earth context. 1. D M S a n d t h e C L A W H y p o t h e s i s The case that in the unpolluted atmosphere DMS is the major source of acidity and aerosol particles (including CCN) was discussed earlier. However, some major workers in this field (Charlson, Lovelock, Andreae and Warren, 1987; the acronym CLAW derives from the initial letters of their surnames) have gone further and proposed that there is a feedback mechanism through which production of DMS by plankton can act as a climate regulating mechanism. This is illustrated diagramatically in Fig. 7. On the lefthand side of the figure the production of DMS in the sea, its emission to the atmosphere and conversion to a variety of products is shown. Following round the cycle, this leads to the role of sulphate CCN in cloud formation with its obvious link to climate. The novel proposal made by Charlson et al. (1987) is shown on the righthand side of the diagram. In this it is suggested that a change in atmospheric temperature, arising from, for example, altered solar input or concentration of greenhouse gases, would lead to a change in DMS production in the sea, the sign of which acts to back-off the initial atmospheric perturbation. Thus, an increase in air temperature would give rise to the plankton making more
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DMS, which would lead to more CCN and hence enhanced cloudiness, which in turn would have a negative feedback on the initial temperature change. When proposed this was a very original and provocative concept, which has led to a large volume of research aimed at testing it. Although the many parts of the complex chain of processes from DMS to climate have been studied individually, it is still not clear whether the overall CLAW hypothesis is an important one, or even whether the climate-DMS feedback has the necessary negative sign. Some evidence against comes from the past record of atmospheric sulphur retained in an ice core from Antarctica and shown in Fig. 8. What this seems to indicate is that during the last ice age, atmospheric levels of both non-sea-salt sulphate and MSA were higher than now, presumably due to greater oceanic biological activity then, which is contrary to what would be predicted by the CLAW hypothesis. However, other ice core results seem to tell a somewhat different story, for example Hansson and Saltzman (1993) present data from a Greenland core which show an apparent opposite trend for MSA but a similar one for non-sea-salt sulphate. Although the ice core records potentially allow a more global view of the validity of the CLAW to be taken, at present there is insufficient data to draw definite conclusions.
REFERENCES
2. Iron Although the results discussed earlier clearly indicate that iron can play an important role in controlling biological activity in the oceans, extrapolation of the findings to larger areas let alone globally (typical in situ iron-enriched patch sizes were approximately 100 km 2) and to longer timescales (enrichment experiments lasted for less than 20 days) is clearly a major problem. However, as with the CLAW hypothesis discussed above, data from ice cores and other proxy records of past environmental conditions can prove useful in this context. Figure 8 shows a compilation of ice core and deep-sea sediment records spanning from the last ice age to the present. Figure 8a shows ~lSO as a marker of temperature as well as iron and MSA (a proxy for DMS, since we know of no other way in which it can be formed in the environment); both show higher levels in the last ice age. Figure 8b shows atmospheric CO2 as recorded in air bubbles trapped in the ice, as well as several markers of ocean productivity found in deep-sea sediments (alkenones, dinosterol, TOC) all of which indicate greater production than at present. The data are concor-
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year x 10 a, b.p. F I G U R E 8 Ice core and sediment data for the Holocene and end of the last ice age. (a) Methane sulphonic acid (MSA) and 8180 in an ice core from Dome C, east Antarctica; estimated Fe concentration in an ice core from Vostok, Antarctica• (b) CO2 concentration in an ice core from Vostok, Antarctica; total organic carbon (TOC), alkenones and dinosterol in a sediment core from the eastern tropical Pacific (from Turner et al., 1996).
257
dant with an hypothesis that says that iron deposition from the atmosphere was greater during the last glaciation and that this led to higher productivity in the oceans, which resulted in lower atmospheric CO2 and higher DMS (due to greater drawdown of CO2 and enhanced production of DMS). Although such correlations are fascinating and certainly necessary in order to establish the role of iron in global biogeochemistry, the case is far from proved since we lack confirmatory data from a range of locations and, as usual, correlation cannot of itself prove causation.
References Amouroux, D., P. S. Liss, E. Tessier, M. Hamren-Larsson and O. F. X. Donard, 2001: Role of the oceans in the global selenium cycle. Earth Planet. Sci. Lett., in press. Andreae, M. O., 1990: Ocean-atmosphere interactions in the global biogeochemical sulfur cycle. Marine Chemistry, 30, 1-29. Boers, R., G. P. Ayers and J. L. Gras, 1994: Coherence between seasonal variation in satellite-derived cloud optical depth and boundary-layer CCN concentrations at a midlatitude Southern Hemisphere station. Tellus B, 46, 123-131. Boyd, P. W., A. J. Watson, C. S. Law and 32 coauthors, 2000: A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature, 407, 695-702. Brimblecombe, P., C. Hammer, H. Rodhe, A. Ryaboshapko and C. F. Boutron, 1989: Human influence on the sulphur cycle. In Evolution of the Global Biogeochemical Sulphur Cycle (P. Brimblecombe and A. Yu Lein, Eds), John Wiley, pp. 77-121• Charlson, R. J. and H. Rodhe, 1982: Factors controlling the acidity of natural rainwater. Nature, 295, 683-685. Charlson, R. J., J. E. Lovelock, M. O. Andreae and S. G. Warren, 1987: Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326, 655-661. Chin, M. and D. J. Jacob, 1996: Anthropogenic and natural contributions to tropospheric sulfate: a global model analysis. J. Geophys. Res., 101, 18691-18699. Coale, K. H. et al. 1996: A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature, 383, 495-501. Cooper, D. J., A. J. Watson and P. D. Nightingale, 1996: Large decrease in ocean-surface CO2 fugacity in response to in situ iron fertilization. Nature, 383, 511-513. Donaghay, P. L., P. S. Liss, R. A. Duce, D. R. Kester, A. K. Hanson, T. Villareal, N. W. Tindale and D. J. Gifford, 1991: The role of episodic atmospheric nutrient inputs in the chemical and biological dynamics of oceanic ecosystems. Oceanography, 4, 62-70. Duce, R. A., P. S. Liss, J. T. Merrill and 19 coauthors, 1991: The atmospheric input of trace species to the world ocean. Glob. Biogeochem. Cycles, 5, 193-259. Fitzwater, S. E., K H. Coale, R. M. Gordon, K. S. Johnson and M. E. Ondrusek, 1996: Iron deficiency and phytoplankton growth in the equatorial Pacific. Deep-Sea Res. Pt II, 43, 995-1015. Galloway, J. N., H. Levy II, and P. S. Kasibhatla, 1994: Consequences of population growth and development on deposition of oxidized nitrogen. Ambio, 23, 120-123• Ganzeveld, L. and J. Lelieveld, 1995: Dry deposition parameterization in a chemistry general circulation model and its influence on the distribution of reactive trace gases. J. Geophys. Res., 100, 20999-21012. Garland, J. A., A. W. Elzerman and S. A. Penkett, 1980: The mech-
258
BIOGEOCHEMICALCONNECTIONS BETWEENTHE ATMOSPHERE AND THE OCEAN
anism for dry deposition of ozone to seawater surfaces. J. Geophys. Res., 85, 7488-7492. Hansson, M. E. and E. S. Saltzman, 1993: The first Greenland ice core record of methanesulfonate and sulfate over a full glacial cycle. Geophys. Res. Lett., 20, 1163-1166. IGBP, 1992: Global Change: Reducing Uncertainties. International Geosphere-Biosphere Programme. IPCC, 1996: Climate Change 1995. The Science of Climate Change, Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change (J. T. Houghton, L. G. Meiro Filho, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell Eds). Liss, P. S. and J. N. Galloway, 1993: Air-sea exchange of sulphur and nitrogen and their interaction in the marine atmosphere. In Interactions of C, N, P and S Biogeochemical Cycles and Global Change (R. Wollast, F. T. Mackenzie and L. Chou, Eds), Springer, pp. 259-281. Liss, P. S., A. D. Hatton, G. Malin, P. D. Nightingale and S. M. Turner, 1997: Marine sulphur emissions. PhiL Trans. R. Soc. Lond. B, 352, 159-169. Margulis, L. and J. E. Lovelock, 1974: Biological modulation of the Earth's atmosphere. Icarus, 21, 471-489. Martin, J. H. and S. E. Fitzwater, 1988: Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature, 331, 341-343. Martin, J. H., K. H. Coale, K. S. Johnson and 41 coauthors, 1994: Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean. Nature, 371, 123-129. Mosher, B. W. and R. A. Duce, 1987: A global atmospheric selenium budget. J. Geophys. Res., 92, 13289-13298. Quinn, P. K., T. S. Bates, J. E. Johnson, D. S. Covert and R. J. Charlson, 1990. Interactions between the sulfur and reduced nitrogen cycles over the Central Pacific Ocean. J. Geophys. Res., 95, 16405-16416. Ravishankara, A. R., Y. Rudich, R. Talukdar and S. B. Barone, 1997:
Oxidation of atmospheric reduced sulphur compounds: perspective from laboratory studies. Phil. Trans. R. Soc. Lond. B, 352, 171-182. Rayman, M. P. 2000: The importance of selenium to human health. Lancet, 356, 233-241. Sarmiento, J. L. and J. R. Toggweiler, 1984: A new model for the role of the oceans in determining atmospheric CO2. Nature, 308, 621-624. Savoie, D. L., J. M. Prospero, R. J. Larsen, F. Huang, M. A. Izaguirre, T. Huang, T. H. Snowdon, L. Custals and C. G. Sanderson, 1993: Nitrogen and sulfur species in Antarctic aerosols at Mawson, Palmer Station and Marsh (King George Island). J. Atmos. Chem., 17, 95-122. Shaw, G. E., 1987. Aerosols as climate regulatorsma climate biosphere linkage. Atmos. Environ., 21, 985-986. Sievering, H., J. Boatman, E. Gorman, Y. Kim, L. Anderson, G. Ennis, M. Luria and S. Pandis, 1992: Removal of sulphur from the marine boundary layer by ozone oxidation in sea-salt aerosols. Nature, 360, 571-573. Taylor, S. R. and S. M. McLennon, 1985: The Continental Crust: Its Composition and Evolution. Blackwell Scientific, Oxford. Turner, S. M., P. D. Nightingale, L. J. Spokes, M. I. Liddicoat and P. S. Liss, 1996. Increased dimethyl sulphide concentrations in sea water from in situ iron enrichment. Nature, 383, 513-516. Twomey, S., 1991. Aerosols, clouds and radiation. Atmospheric Environment, 25A, 2435-2442. Watson, A. J. and P. S Liss, 1998: Marine biological controls on climate via the carbon and sulphur geochemical cycles. PhiL Trans. R. Soc. Lond. B., 353, 41-51. Watson, A. J., P. S. Liss and R. A Duce, 1991: Design of a small scale iron enrichment experiment. Limnol. Oceanogr., 36, 1960-1965. Watson, A. J., D. C. E. Bakker, A. J. Ridgwell, P. W. Boyd and C. S. Law, 2000: Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2. Nature, 407, 730-733.
In comparison with other planets in the solar system, the chemical composition of the earth's atmosphere clearly shows the influence of biological processes. The level of carbon dioxide is much lower and of oxygen much higher due to the photosynthesis of land and marine plants, and many reduced gases exist at concentrations far in excess of what purely inorganic processes would allow. In this chapter the emphasis is on the role of biological activity in the oceans in affecting the chemical (and physical) properties of the atmosphere. Air-sea exchanges of ozone, carbon dioxide, dimethyl sulphide, dimethyl selenide and ammonia are described with regard to their importance for processes in the atmosphere, together with the role of atmospheric dust inputs on ocean biology with its potential for concomitant change in trace gas fluxes to the atmosphere.
A. INTRODUCTION
chemical composition with that of other planets in the solar system strongly shows the influence of biological processes. Many reduced gases exist, at concentrations far in excess of what purely inorganic processes would allow. In the oceans the biota play a strong role in determining the amounts of gases important in controlling the earth's radiation balance, such as carbon dioxide and dimethyl sulphide (DMS), as well as the chemistry of the atmosphere and geochemical cycling of several key elements including nitrogen, selenium and the halogens. It has recently been shown how delicately poised the oceanic biota are in terms of availability of the element iron, and hence in their ability to control levels of several gases of importance in the atmosphere. This chapter begins with a brief overview of the effect of the biosphere on the overall composition of the earth's atmosphere. There follows a section on the role of the marine biota in producing or consuming various trace gases which are important for the chemistry (and physics) of the atmosphere. After this there is a complementary section on the role that inputs from the atmosphere can play in affecting the level of biological activity in the oceans, which activity can in turn affect the gaseous emissions discussed in the previous section. This cyclic interaction between the atmospheric and oceanic reservoirs leads to a short concluding section on the global nature of biogeochemical inter-reservoir transfers; a concept encouraged by the holistic view of the earth obtained by observing it from the moon.
Just over 30 years ago mankind had the opportunity to view planet earth from the moon. It can be argued that this represented a major turning point in how we have regarded our home planet ever since. Seen from the moon, the earth looks very isolated in space. From that viewpoint its unity is obvious, which reinforced the idea that our planet must be seen and studied as a whole, rather than split into its component reservoirs, as is so often done in academic research. The obvious dominance of the oceans in terms of coverage relative to land led to the idea that the earth should really have been called "ocean". Finally, the blue oceans, green/brown land and white clouds all looked very different to the colouring of the other planets we can see using telescopes from earth. It cannot be said that any of this understanding was new, since intellectually all of it had been known for many years. What was new was the perspective that viewing our planet from an outside observation post provided. Thus the isolation, beauty and very special properties of earth were made visually apparent. The extraordinary properties made manifest in the blues, greens, browns and whites of the earth as seen from space are, of course, due in large measure to the presence of living organisms on the planet. In this chapter some of the ways in which the functioning of the earth's (particularly marine) biota control important properties of the atmosphere, oceans and land will be discussed. For the atmosphere, comparison of its
Meteorology at the Millennium ISBN 0-12-548035-0
249
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250
BIOGEOCHEMICALCONNECTIONS BETWEEN THE ATMOSPHERE AND THE OCEAN
B. T H E C H E M I C A L THE EARTH'S
COMPOSITION ATMOSPHERE
OF
The gaseous composition of the earth's atmosphere clearly shows the influence of biological processes on land and in the sea. This is well illustrated in Fig. 1, which shows a comparison of the present composition of the atmosphere (i.e. with life present), with that predicted if no life existed on the earth. The prediction of the lifeless composition is made by assuming the earth and its atmosphere to be at thermodynamic equilibrium, with the presence of life disturbing that equilibrium. It should be noted that in Fig. 1 the percentage contribution of each gas to the total composition is shown as a histogram on a logarithmic scale for both the life present and the life absent cases. A clear example of the effect of life on atmospheric composition is the 3-4 orders elevation of 02 and concomitant decrease in CO2 arising from the uptake of the latter and release of the former as a result of photosynthesis by land plants and marine phytoplankton. Furthermore, several trace gases, including the important greenhouse gases CH4, N20, occur 2-3 orders of magnitude higher in concentration in our present atmosphere than would be predicted in the absence of life. Other gases including NH3, HCI and DMS (not shown) are present in low but meas-
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urable amounts in the current atmosphere, whereas they would be at significantly lower levels or totally absent in the air of a lifeless earth. Some important gases, e.g. water vapour and ozone, are omitted from the figure because, although the biosphere and its products are important for their cycling, it is difficult to estimate concentration levels for them in the pre-life atmosphere. C. AIR-SEA EXCHANGE OF GASES OF IMPORTANCE IN THE ATMOSPHERE Many gases cross the atmosphere-ocean interface, and in Table 1 a summary is given of the gases involved, as well as an indication of their importance in the atmosphere, both troposphere and stratosphere. For most of the gases, the net flux direction is from sea to air, i.e. the oceans are a source of the gases to the atmosphere. However, there are some exceptions, two of which are now discussed. 1. O z o n e Ozone (03) is one exception, since for this gas the oceans are an absolute sink (i.e., there is no re-emission, due to the high reactivity of ozone with iodide ions and organic material at the sea surface, leading to its destruction in the water (Garland e t a l . , 1980). Although the rate of uptake of 03 by the oceans is small compared with deposition to land surfaces, their large area means that they may still be a significant sink in the global budget of the gas (Ganzeveld and Lelieveld, 1995). Another possible exception is ammonia where the net flux direction is somewhat uncertain and is probably highly variable, a situation which will be discussed further later. 2. M a n m a d e C a r b o n D i o x i d e
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Another exception is manmade C O 2 where the oceans take up about 40% of the total amount of the gas emitted to the atmosphere from the burning of fossil fuels (IPCC, 1996). This contrasts with the situation for natural CO2 where, although in some locations and for some seasons the net fluxes are both in and out of the oceans, averaged for the whole of the oceans and over a yearly cycle there is near balance between the gross up and down flows. The observed seasonal and spatial variability is due to both temperature affecting the solubility of CO2 in seawater, and biological uptake and release of the gas during photosynthesis and respiration, respectively. However, with the increase in atmospheric CO2 resulting from anthropogenic activities the natural balanced two-way fluxes have a net downwards component superimposed on them. The importance of biological activity is not only for
251
C. AIR---SEA EXCHANGE OF GASES OF IMPORTANCE IN THE ATMOSPHERE TABLE 1
Air-Sea Exchange of Gases of Importance in the Atmosphere
Net flux direction
Gas
Importance in atmosphere
1"
Dimethyl sulphide (DMS)
Acidity Cloud condensation nuclei Budgeting S cycle Particles Particles Alkalinity Aerosol composition Oxidation capacity Budgeting I (and Br?) cycle(s) Oxidation capacity Oxidation capacity Oxidation capacity Oxidation capacity Radiation balance Radiation balance Radiation balance Radiation balance Budgeting Se cycle Budgeting Hg cycle
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the small-scale and short-term changes in C O 2 uptake indicated above. Changes in ocean productivity also have the potential to strongly influence levels of CO2 in the atmosphere on much longer time-scales. This is illustrated in Fig. 2. The figure shows the results of a study by Watson and Liss (1998) in which a simple model (Sarmiento and Toggweiler, 1984) was used to examine how far changes in marine biological activity can affect atmospheric CO2 partial pressures. In each run the model was initialized at pre-industrial steady state (approximately 260 ppm). At t = 100 years a perturbation was applied. For the "Strangelove ocean" all
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marine biological activity was stopped instantaneously. "Cold surface water productivity = 0" and "cold surface water productivity = max" show the possible extremes to which atmospheric CO2 can be driven by modulating the efficiency of the cold-water marine biota. After 1000 years the difference between the atmospheric CO2 level produced by the lifeless and maximum productivity oceans is about 250 ppm (180-430 ppm). Although this is clearly an extreme range, it does illustrate that modulation of biological activity in the oceans has the potential to have a very substantial effect on atmospheric CO2 concentrations. Turning now to gases in Table 1 whose direction of net flux is from the oceans to the atmosphere, several examples are considered.
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Dimethyl sulphide (DMS) is a gas produced in ocean surface waters by the activities of phytoplankton. The DMS itself is formed by the enzymatic cleavage of a precursor molecule (dimethyl sulphoniopropionate, DMSP), which the organisms make as an osmolyte or as a cryoprotectant. The commonest way for the DMSP to DMS transition to occur is on death of the plankton by zooplankton grazing or attack by viruses. Once released to the seawater there is a range of bacterial and photochemical transformations which the DMS can undergo (see Liss et al., 1997, for a review of production and destruction processes of DMS in seawater). However, there is almost always a measurable residual
252
BIOGEOCHEMICALCONNECTIONS BETWEEN THE ATMOSPHERE AND THE OCEAN
concentration of DMS in the water and it is this which drives the net flux of the gas to the atmosphere. Because of its rapid dispersion and reaction in the atmosphere, air concentrations are generally only a few percent of the equivalent water values and are often ignored in air-sea flux calculations with little loss of accuracy. Once in the atmosphere, DMS is subject to oxidation by free radicals such as hydroxyl and nitrate to form a variety of products (Ravishankara et al., 1997), the most important of which appear to be methane sulphonic acid (MSA) and sulphur dioxide, which can be further oxidized to sulphate either in water droplets or in aerosols, including sea-salt particles (see e.g., Sievering et al., 1992). All of these products are acidic and give the natural acidity to aerosol particles, rain and other forms of precipitation in the marine atmosphere (Charlson and Rodhe, 1982). Of course, over industrialized/urbanized parts of the globe, particularly in the Northern Hemisphere, human activities have strongly perturbed the natural sulphur cycle, with concomitant increase in both sulphate aerosol numbers and decrease in pH of precipitation and particulates. In areas remote from heavily populated land, particularly in the Southern Hemisphere, the natural cycle is still dominant. Chin and Jacob (1996) have calculated that in terms of the total atmospheric burden of sulphur the percentage contributions of the three main sources are currently: biogenic (mainly from oxidation of DMS) 42%, anthropogenic 37% and volcanic 18%. The transfer of sulphur in the form of DMS from the sea to the marine atmosphere, with some of the oxidation products ultimately deposited on land, is a vital component of the budget of the element in the earth system, since material balance cannot be achieved without it (Brimblecombe et al., 1989). A further important property of DMS and specifically submicron particles formed via its oxidation is in affecting climate. Such particles can interact both directly with sunlight (Shaw, 1987) and indirectly by acting as nuclei on which cloud droplets form (termed cloud condensation nuclei, CCN). The number density of CCN is a major determinant in cloud formation and thus of the albedo of the planet (Charlson et al., 1987). Over land there is generally an abundance of CCN resulting from soil dust blown into the air and from pollution sources. However, over the oceans, far from land (i.e. the majority of the globe), the main source of CCN appears to be from oxidation of marine-produced DMS. This is supported by various lines of evidence, including the chemical composition of the CCN being largely sulphate (neutralized to some extent by uptake of ammonia gas from the atmosphere, see later), and the coherence in the seasonality of both CCN number
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density and cloudiness (as measured by optical depth), which in turn correspond to the times when marine plankton are actively producing DMS in seawater (Boers et al., 1994; Liss and Galloway, 1993). Twomey (1991) has attempted to quantify the magnitude of the effect of CCN number density on cloud albedo (and hence reflectance), as illustrated in Fig. 3. This shows that the CCN-albedo relationship is highly nonlinear. Thus, the effect of change in CCN number on the incremental increase in albedo for each particle added (called susceptibility in Fig. 3) is much more pronounced at low compared with high particle numbers. For example, once the air contains several hundred particles or droplets per cubic centimetre, addition of further particles has little effect on cloud reflectance. Over and close to land, CCN numbers generally exceed this level. Thus, we must look to oceanic areas, and particularly the Southern Hemisphere, with its large ratio of sea to non-ice-covered land, for the greatest potential climatic impact of changing CCN numbers, whether the particles arise, for example, from alteration in oceanic DMS emissions or from inputs of SO2 from (increased) fossil fuel burning. To graphically illustrate the point. Twomey cites a calculation which indicates that equal quantities of sulphur entering the atmosphere in the two hemispheres will have 25 times the effect on cloud albedo in the Southern compared with the Northern Hemisphere. 4. D i m e t h y l Selenide Selenium is the element immediately below sulphur in Group VI of the Periodic Table. For this reason sim-
D. IMPACT OF ATMOSPHERIC DUST ON OCEAN BIOGEOCHEMISTRY
ilarities in behaviour between the two elements might be expected, in this case for their methylated forms DMS and DMSe. For almost a quarter of a century it has been known that, as in the case for sulphur already discussed, there is a discrepancy in the global budget of selenium which can only be rectified by a flux of some, probably volatile, form of the element from the ocean to the atmosphere (Mosher and Duce, 1987). However, until recently, difficulties with the sensitivity of the analytical techniques available for measuring the very low levels at which selenium exists in the environment have meant that it has not been possible to confirm this and establish the size and form of the sea to air flux by direct measurement. Notwithstanding these difficulties, a recent measurement campaign initiated by the author and Drs David Amouroux and Olivier Donard of the University of Pau has thrown significant light on this topic (Amouroux et al., 2001). Volatile forms of selenium were measured at picomolar concentration levels in water samples collected on a research cruise in the north Atlantic in the summer of 1998. The results show that the main volatile forms of selenium are DMSe and the mixed selenium-sulphur compound DMSeS. In addition, simultaneous measurements of DMS and biological parameters clearly show a correlation between the concentrations of DMSe and DMS and strongly indicate a biogenic source for the volatile selenium gases measured. Calculation of the size of the sea-to-air flux of volatile selenium and extrapolation to the oceans as a whole indicates that it is likely to be sufficient to balance the global budget for the element. This implies that the sea is a source of selenium to the land, a result with important implications for human health, as recently reviewed by Rayman (2000).
5. Ammonia Ammonia (NH3) is another example of a gas found in the atmosphere which would not occur there but for biological activity. In Fig. 1 the lifeless earth's atmosphere shows essentially no NH 3, whereas with life present there is a small but significant amount. The ammonia comes from a variety of sources on land as well as from the oceans; here we will consider only the latter source because it can be treated as a companion air-sea flux to that of DMS discussed previously. As in the case of sulphur, "the nitrogen cycle has also been substantially amended through a variety of human activities. In order to examine the role and behaviour of NH 3 in its natural setting, we now have to go to parts of the globe furthest from man's activities. Since such areas are also far from land, this enables the marine part of the cycle to be examined without major influence from
253
terrestrial source regions (this is possible because the atmospheric lifetime of NH 3 after emission from land (or sea) is only a matter of days). Figure 4 shows measurements made by Savoie et al. (1993) on atmospheric aerosol particles collected over a 5-year period at a coastal sampling site in Antarctica. A clear seasonal cycle is evident for ammonium (the form of ammonia in the aerosol), with highest values in the spring and summer seasons and minimum levels in autumn and winter. Also remarkable in Fig. 4 is the similarity of the ammonium to the MSA and non-sea-salt sulphate (both oxidation products of DMS) seasonal cycles. These similarities strongly suggest a marine biological origin for the NH3, as well as for the DMS products. Support for this idea comes from a more detailed analysis of the data in Fig. 4, from which it can be calculated that the molar ratio of ammonium to non-sea-salt sulphate in the aerosols is close to 1:1, implying a chemical composition of ammonium bisulphate (NH4HSO4). The final piece of the jigsaw comes from measurements made in the North and South Pacific Ocean by Quinn et al. (1990), from which they were able to calculate the fluxes of both NH3 and DMS being emitted from the ocean to the atmosphere. Although both fluxes show quite a lot of spatial variability, the mean values are again reasonable close to a 1:1 ratio, supporting the idea that the aerosol composition in terms of both nitrogen and sulphur (the main constituents of the total mass) can be explained by marine biogenic emissions.
D. IMPACT OF ATMOSPHERIC D U S T ON OCEAN BIOGEOCHEMISTRY As discussed in the previous section, trace gases produced or consumed by biological activity in the oceans clearly affect the chemical and physical properties of the atmosphere. However, the atmosphere also provides input of nongaseous material to the oceans in the form of land-derived mineral dust and particulates formed during burning and the use of fertilizers in agriculture. These solids can have a potentially large impact on marine productivity, particularly in certain parts of the oceans. As with land plants, marine plankton need nutrient elements (e.g. nitrogen, phosphorus, iron, etc.) for optimal growth. It is generally agreed that of these nutrients it is deficits of nitrogen and iron which present the greatest potential for limitation and for them atmospheric inputs are also likely to be important. In the case of nitrogen there is growing concern about the impact of the increasing input of human-derived nitrogen compounds to regions of the open oceans where nitrogen is the nutrient-limiting biological activity, such
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FIGURE 4
Graphs on the left show the actual time series of weekly-average concentrations of methane sulphonic acid (MSA), non-sea-salt sulphate (NSS SO42-) and ammonium NH4 ~) at Mawson, Antarctica. Graphs on the right illustrate the composted seasonal cycles, showing the monthly means and their 95% confidence intervals (from Savoie et al., 1993).
as the subtropical gyres of the North and South Pacific. While estimates suggest that, at present, atmospheric nitrogen accounts for only a few percent of the annual new nitrogen delivered to surface waters in these regions, the atmospheric input to the ocean is highly episodic, often coming in large pulses extending over only a few days when it can play a much more important role. Further, such inputs are increasing as the developing nations of the world increase their production and use of fixed nitrogen. For example, a modelling study by Galloway et al. (1994) developed maps of the "recent" (1980) and expected (2020) annual deposition of oxidized forms of nitrogen, which includes nitrate, nitrite and nitric acid derived from combustion processes, from the atmosphere to land and ocean surfaces. In Fig. 5 the projected ratios of the estimated deposition of oxidized nitrogen in 2020 to the values in 1980 are shown. From 1.5 to 3 times, and in some limited areas up to 4 times, the recent rate of deposition will occur by 2020 over large areas of coastal and open oceans, and these estimates do not include possible changes (almost certainly increases) in reduced and organic nitrogen inputs. This leads to the possibility of regional biogeochemical impacts in both coastal and open ocean areas. Turning now to the case of iron, it has been known
for almost a hundred years that marine biological activity, as judged by underutilization of "traditional" plant nutrients in the water such as phosphate and nitrate, appears limited in areas of the ocean far from land. Examples of these high-nutrient low-chlorophyll (HNLC) areas are the southern oceans, and the equatorial and sub-Arctic Pacific. It has been speculated for a long time that the nutrient limiting biological activity in these areas is iron. The fact that the underutilization of nitrate and phosphate occurs most clearly in waters remote from land is in accord with the idea that an important source of the iron is from deposition of soil dust carried from the continents via the atmosphere. Figure 6 shows the distribution of the global flux of iron to the oceans. The distribution is clearly very nonlinear (the scale on the figure is logarithmic), with highest deposition close to deserted continental areas and the remote H N L C regions listed above being amongst the regions receiving least. However, because of the difficulties of measuring iron at the very low concentrations at which it occurs in seawater, it has until recently been difficult to test whether it is indeed an important limiting nutrient for plankton growth. The first experiments were done onboard ship using flasks of seawater containing plankton and the
255
D. IMPACTOF ATMOSPHERIC DUST ON OCEAN BIOGEOCHEMISTRY
1~:1
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F I G U R E 5 Ratio of the estimated deposition of oxidized forms of nitrogen to ocean and land surfaces in 2020 relative to 1980 (adapted from Galloway et al., 1994).
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256
BIOGEOCHEMICAL CONNECTIONS BETWEEN THE ATMOSPHERE AND THE OCEAN
results seemed to confirm that adding iron did indeed enhance plant growth (e.g., Martin and Fitzwater, 1988). However, questions remained about how well the flasks mirrored conditions in the oceans and there were also problems of contamination during collection of the seawater (Fitzwater et al., 1996). These difficulties led to the idea of conducting such experiments by enriching a small patch of ocean water with iron in situ (Watson et al., 1991). Several such experiments have been carried out to date, two in the equatorial Pacific (Martin et al., 1994; Coale et al., 1996) and most recently in the southern ocean (Boyd et al., 2000). In all cases there was clear evidence in support of the iron limitation hypothesis. From the viewpoint of ocean biology affecting the properties of the atmosphere, these in situ studies showed that the enhanced biological activity arising from iron amendment leads to enhanced drawdown of carbon dioxide (Cooper et al., 1996; Watson et al., 2000) and increased production of DMS (Turner et al., 1996).
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We conclude this chapter with two examples which attempt to place the air-sea exchange of DMS and iron, discussed earlier, into a broader whole earth context. 1. D M S a n d t h e C L A W H y p o t h e s i s The case that in the unpolluted atmosphere DMS is the major source of acidity and aerosol particles (including CCN) was discussed earlier. However, some major workers in this field (Charlson, Lovelock, Andreae and Warren, 1987; the acronym CLAW derives from the initial letters of their surnames) have gone further and proposed that there is a feedback mechanism through which production of DMS by plankton can act as a climate regulating mechanism. This is illustrated diagramatically in Fig. 7. On the lefthand side of the figure the production of DMS in the sea, its emission to the atmosphere and conversion to a variety of products is shown. Following round the cycle, this leads to the role of sulphate CCN in cloud formation with its obvious link to climate. The novel proposal made by Charlson et al. (1987) is shown on the righthand side of the diagram. In this it is suggested that a change in atmospheric temperature, arising from, for example, altered solar input or concentration of greenhouse gases, would lead to a change in DMS production in the sea, the sign of which acts to back-off the initial atmospheric perturbation. Thus, an increase in air temperature would give rise to the plankton making more
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FIGURE 7 Proposed feedback cycle between climate and marine DMS production. Signs indicate whether an increase in the value of the preceding parameter in the cycle is expected to lead to an increase (+) or a decrease ( - ) in the value of the subsequent parameter (from Andreae, 1990).
DMS, which would lead to more CCN and hence enhanced cloudiness, which in turn would have a negative feedback on the initial temperature change. When proposed this was a very original and provocative concept, which has led to a large volume of research aimed at testing it. Although the many parts of the complex chain of processes from DMS to climate have been studied individually, it is still not clear whether the overall CLAW hypothesis is an important one, or even whether the climate-DMS feedback has the necessary negative sign. Some evidence against comes from the past record of atmospheric sulphur retained in an ice core from Antarctica and shown in Fig. 8. What this seems to indicate is that during the last ice age, atmospheric levels of both non-sea-salt sulphate and MSA were higher than now, presumably due to greater oceanic biological activity then, which is contrary to what would be predicted by the CLAW hypothesis. However, other ice core results seem to tell a somewhat different story, for example Hansson and Saltzman (1993) present data from a Greenland core which show an apparent opposite trend for MSA but a similar one for non-sea-salt sulphate. Although the ice core records potentially allow a more global view of the validity of the CLAW to be taken, at present there is insufficient data to draw definite conclusions.
REFERENCES
2. Iron Although the results discussed earlier clearly indicate that iron can play an important role in controlling biological activity in the oceans, extrapolation of the findings to larger areas let alone globally (typical in situ iron-enriched patch sizes were approximately 100 km 2) and to longer timescales (enrichment experiments lasted for less than 20 days) is clearly a major problem. However, as with the CLAW hypothesis discussed above, data from ice cores and other proxy records of past environmental conditions can prove useful in this context. Figure 8 shows a compilation of ice core and deep-sea sediment records spanning from the last ice age to the present. Figure 8a shows ~lSO as a marker of temperature as well as iron and MSA (a proxy for DMS, since we know of no other way in which it can be formed in the environment); both show higher levels in the last ice age. Figure 8b shows atmospheric CO2 as recorded in air bubbles trapped in the ice, as well as several markers of ocean productivity found in deep-sea sediments (alkenones, dinosterol, TOC) all of which indicate greater production than at present. The data are concor-
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257
dant with an hypothesis that says that iron deposition from the atmosphere was greater during the last glaciation and that this led to higher productivity in the oceans, which resulted in lower atmospheric CO2 and higher DMS (due to greater drawdown of CO2 and enhanced production of DMS). Although such correlations are fascinating and certainly necessary in order to establish the role of iron in global biogeochemistry, the case is far from proved since we lack confirmatory data from a range of locations and, as usual, correlation cannot of itself prove causation.
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BIOGEOCHEMICALCONNECTIONS BETWEENTHE ATMOSPHERE AND THE OCEAN
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