Be

be concentrated in low and mid-latitudes, especially downwind of sub-tropical deserts such as the Sahara, Arabia, and the Gobi (Fig. 4.7). Desertification is likely to increase such fluxes, and so potentially the biological production stimulated by aeolian iron.

4.2 Climatically active products of marine biological processes

Carbon dioxide is a major participant in marine biological processes, as well as being an important greenhouse gas. It has been discussed in several places already, and will be further considered in the next section. There are a number of other by-products of marine biological activity that are active in the climate: these include several carbon-, nitrogen-, and sulphur-based gases, methyl chloride and alkyl iodides. Estimates of the global fluxes of some of these, and other, gases across the air-sea interface are given in Table 3.2.

4.2.1 Carbon compounds other than CO2

Methane, CH4, and carbon monoxide, CO, are the principal two climatically active carbon gases, other than CO2, produced by biological activity in the oceans. They are both greenhouse gases of some significance (Table 1.2).

Methane is a product of anaerobic decay, that is, bacterial decay in the absence of oxygen, for example:

There are several other chemical pathways by which alcohols or carbon dioxide may be reduced to methane. The ocean contributes only 1-2% of the total methane input to the atmosphere. Oceanic methane production may be associated with regions of high phytoplankton biomass. The oxygen minimum below the surface waters (§3.4) due to biological utilization may then be sufficiently severe that the reducing environment necessary for methane-producing decomposition exists. Such a situation, encountered in the Arabian Sea in September 1986, is shown in Fig. 4.8.

bacteria

2CH2O

Fig. 4.8. Percentage saturation of dissolved methane, with depth, along a transect through the Arabian Sea at the end of the summer monsoon period. Stations 7-16 are in part of the upwelling region off the Arabian coast at this time of year. [Reprinted with permission from Nature, 354, Owens et al, pp. 293-5. Copyright 1991 Macmillan Magazines Limited.]

Fig. 4.8. Percentage saturation of dissolved methane, with depth, along a transect through the Arabian Sea at the end of the summer monsoon period. Stations 7-16 are in part of the upwelling region off the Arabian coast at this time of year. [Reprinted with permission from Nature, 354, Owens et al, pp. 293-5. Copyright 1991 Macmillan Magazines Limited.]

There is a large flux of carbon monoxide to the atmosphere from the ocean (Table 3.2). Some of this will have been formed by photochemical oxidation of methane (see §3.6) but much of it is produced by microbiological activity. Carbon monoxide is a product of incomplete respiration, that is, oxidation when there is an inadequate oxygen supply for complete respiration to produce CO2, for example:

4.2.2 Nitrogeneous compounds

Nitrous oxide, N2O, and ammonia, NH3, are the principal nitrogen-based gases given off by marine biological processes. During the oxidation of organic material by phytoplankton some of the nitrogen is converted to N2O rather than nitrate, that is,

rather than

This is seen in Fig. 4.9, which shows a clear peak in concentration at the oxygen mininum. The conversion rate is small: only one N atom in a thousand is converted to N2O rather than nitrate. In some areas of the ocean, where the oxygen level can be taken to zero by biological uptake, nitrate and nitrous oxide become the source of oxygen and so some of the gas can be re-cycled within the ocean. The northwest Indian Ocean, the sub-tropical east Pacific, and the deep waters of the Bering Sea show this behaviour.

Fig. 4.9. Variation of

dissolved oxygen (shown by triangles) and nitrous oxide (N2O; open circles)

140 180 220 260 300 O2 mol/kg)

concentration with depth in the northwestern Atlantic at

39°N, 62°W in July 1972. Note the correspondence of the oxygen minimum and N2O maximum near 1000 m. The vertical scale changes at 1000 m; note also that the concentration of oxygen is in 10-6 mol/kg while N2O is in 10-9 mol/kg. [Using data from Broecker and Peng, 1982.]

North Atlantic 39°N-62°W July 1972

140 180 220 260 300 O2 mol/kg)

Nitrous oxide is also produced on land as a by-product of combustion and aerobic bacterial activity in soils. The ocean is also a source of N2O, however; it contributes about a fifth of the net input to the atmosphere each year. Nitrous oxide is a minor, but increasing, greenhouse gas - see Table 1.2 and

Ammonia is produced by cell protein decomposition, in both aerobic and anaerobic conditions. While it is present in the atmosphere at very low concentrations it has a limited lifetime of a day or so. There seems to be approximate equilibrium for ammonia exchange across the air-sea interface. We saw in §4.1.1 that ammonia can be utilized as a nitrogen source by some phytoplankton, particularly in its dissolved form, as the ammonium ion:

Ammonia is a weak greenhouse gas, as well as being a supply of nutrient for marine biological activity.

4.2.3 Sulphureous compounds

Hydrogen sulphide, H2S, and dimethyl sulphide, (CH3)2S, are the two principal sulphureous gases produced by marine biological processes that have climatic influence. Hydrogen sulphide is a major oxidation product of anaerobic decay. H2S can be photochemically oxidized very rapidly to sulphuric acid, H2SO4, in air:

The sulphate particles from this acid can then contribute to cloud condensation nuclei (§3.5.1). However, as H2S is oxidized so rapidly in both air and water (see §3.6), it is unlikely that significant quantities escape from the ocean.

Dimethyl sulphide, or DMS, by contrast, has been observed in considerable concentrations in the marine atmosphere during plankton blooms (2.5 x 10-7 gm-3). It is excreted by plankton during oxidation of its precursor, dimethylsulphoniopropionate, DMSP (CH3C2H4CO2SCH3). DMS is destroyed within the ocean by several mechanisms. These include biological consumption, biologically and photochemically-aided oxidation eventually leading to dissolved sulphate ions, and adsorption onto particles. However, sufficient gas survives these processes to allow transfer of up to a few tens of micromoles of DMS per square metre per day to the atmosphere.

Within the atmosphere DMS can be oxidized via reaction with hydroxyl ions to form sulphur dioxide or methane sulphonoic acid, MSA (CH3SO3H). Both sulphate, the product of oxidation of SO2 (see reaction (3.11)), and MSA form particles that can act as sub-micron cloud condensation nuclei (§3.5.1). The oxidation pathways are

Pathway (4.8a) is the predominant one - 80% of DMS is oxidized this way -except in air with low concentrations of NOx species. In this case pathway (4.8b) dominates. The precise distribution of aerosols derived from SO2 and MSA for a particular air mass is therefore poorly known. The differing properties of aerosols derived from these two sources makes for difficult assessment of the impact of DMS on climate (§4.4).

DMS release seems to be associated with cell destruction. Oceanic concentrations are greatest not at the height of a spring bloom, when the plankton population is at its highest, but shortly thereafter, when the zooplankton grazing begins to dominate the primary production cycle. The potential importance of DMS for climate means that full discussion of this biological product is reserved for §4.4.

4.2.4 Iodic compounds

Methyl iodide, CH3I, is another product of algal cell destruction. There is a net flux to the atmosphere, but the main climatic significance of this gas stems from its reaction with dissolved chloride ions to form methyl chloride, CH3Cl:

The ocean appears to be the most prolific source for methyl chloride, with biomass burning also important. This gas is a natural source of chlorine, the element of concern in the decay of the ozone layer in the stratosphere (§7.2.1).

Organic iodide vapours are now also seen to be the origin of some cloud condensation nuclei, in a similar fashion to DMS. Seaweeds can emit easily photolysed compounds such as ethyl iodide, CH2I2. Gaseous iodine is produced photochemically and this reacts with ozone and other atmospheric oxidants to produce iodine oxide particles. Particularly in coastal locations, this reaction sequence can be a potent source of aerosols. At the very least, these emissions must be considered as more biologically-created aerosols but they may be a significant source globally that is currently missing from many climate models.

4.3 Bio-geochemical cycles

All the natural elements cycle between the atmosphere, ocean, biosphere, and geosphere (§1.3.1). A schematic illustration of possible links within a typical bio-geochemical cycle is shown in Fig. 4.10. There are exchanges between the different reservoirs, whose natural equilibrium may be balanced, or strongly for exchange in one direction or the other. In addition there may be exchanges within a reservoir, as is particularly true of the ocean. The ocean plays an important role in these cycles through biological processing, and its deposition of sediments (and thus elements) to the underlying geosphere. Several of the major cycles vital for the existence of life on Earth strongly involve the ocean and its ecology. A full discussion of these cycles is beyond the scope of this book but the following brief overview of the role of marine organisms in several of these cycles summarizes the interactions that have appeared in several other parts of the book.

4.3.1 The carbon cycle

The greenhouse gas carbon dioxide is a component of the carbon cycle, the mechanism by which carbon moves between the various chemical reservoirs of the Earth. This was described in §3.3, focusing in detail on the chemical exchange between the ocean and atmosphere, and is shown schematically in Fig. 3.4 (this can be compared with Fig. 4.10). Marine organisms play a small but important part in this cycle. Phytoplankton fix carbon from dissolved carbon dioxide through photosynthesis (reaction (4.1)). When these organisms, and the larger species which use them as a food source, die, their remains sink to the sea floor, adding to the sediment. This falling material is known as detritus. The cascade of detritus transfers carbon from the surface waters to the geosphere reservoir. It is estimated that without this natural sink of carbon the equilibrium carbon dioxide concentration before the Industrial Revolution would have been 450 ppm rather than 270 ppm.

There are several complications to this simple picture. Not all the detritus sinks to the bottom of the ocean. A large proportion is re-cycled within the ocean, either through direct consumption by other organisms, or by mixing of the upper ocean in the mixed layer and use of part of the detritus, after bacterial decay, in new production. This re-use is modified by seasonal changes in the mixed layer depth. In winter the mixed layer is deepened by wind mixing and thermal reduction of the upper ocean stratification. This deepening entrains water rich in nutrients, and carbon. In addition the fall-out of carbon from the mixed layer does not provide an escalating sink of carbon dioxide because the biological activity is controlled by a number of physical and chemical properties of the ocean. These have been discussed in §4.1. In the absence of changes to

Fig. 4.10. Schematic illustration of a typical bio-geochemical cycle. Each main component consists of several sub-components, within which cycling can occur, as well as that between each main component. The dotted links indicate direct volcanic input from deep beneath the Earth's surface. Some of the other links are drawn uni-directional; this means only that the major exchange is almost invariably in that direction and not that it is the only route. Boxes with some shading are those in which significant biological activity can occur.

these properties the detritus sink cannot contribute to reducing the impact of anthropogenic increase in atmospheric carbon dioxide. In §7.2.4, however, we will see how there will be feedbacks between physical climatic changes expected from greenhouse warming, some of which are important in controlling marine biological activity.

4.3.2 The nitrogen cycle

Micro-organisms control the oceanic component of the nitrogen cycle to a greater extent than for any other geochemical cycle. In coastal regions a large part of the annual budget of nitrogen comes from the land, through run-off. In the deep ocean, however, most nitrogen is cycled within the upper layers, as production is driven by the release of ammonia from dead algae. Some biologically usable nitrogen is mixed upwards from deeper in the ocean, and a small part is added by rainfall from the atmosphere, but these are both relatively small contributions. The most abundant form of nitrogen is the gas, N2, but most cycling involving marine organisms is between organic and inorganic forms of the element. The global total of coastal and deep oceanic vertical fluxes may be of similar total magnitude despite the smaller (10%) surface area of the coastal ocean.

4.3.3 The phosphorus cycle

The chemical composition of organic soft tissue is relatively constant with the ratio of phosphorus:nitrogen:carbon being, on average, 1:15:105. This ratio is known as the Redfield ratio, in honour of the scientist who first demonstrated the ratio's approximate invariance. The ratio of phosphorus to nitrogen in sea water also obeys this ratio in what is thought to be a biologically driven balance. Even in the deep ocean, well away from active microbiological consumption, this ratio holds. Some research, however, suggests that the C:N ratio within the Redfield ratio may not be as reliable as previously thought. Evidence has been presented of carbon consumption, relative to nitrogen, in both coastal and deep-sea waters significantly in excess (approximately double) of that predicted by the Redfield ratio. This means that current estimates of oceanic uptake of carbon dioxide may be too low. This would have significant consequences for the climatic feedbacks discussed in §7.2.4 and the speed of the climate's response to anthropogenic emissions of CO2.

The phosphorus cycle is similar to the nitrogen cycle in that much of the exchange within the ocean involves re-processing of material, rather than input from, and output to, outside the system. A small amount of phosphorus enters the sea from land, assisting the rich biological productivity of the coastal waters. Very little enters from the atmosphere. Some is lost to marine sediments, but more than 99.9% of the cycling is between the marine biosphere and the ocean waters.

4.3.4 The oxygen cycle

The ocean acts as a sink for oxygen because of its use in respiration by marine organisms. It is also a source through photosynthesis and the release of oxygen during the chemical changes associated with the deposition of marine sediments. Much of the oxygen is re-cycled but the sediment deposition release represents a small leak to the atmosphere which it is estimated would double the atmospheric oxygen in four million years. The excess oxygen is used in the atmosphere during weathering of surface materials to maintain the balance in oxygen levels.

4.3.5 The sulphur cycle

Sulphur is a necessary trace element for biological activity. It is provided to marine organisms through re-cycling of dead organic material, dissolution of atmospheric sulphur dioxide, input of sulphates from rivers and precipitation, and anaerobic decay of organic material (see Fig. 4.11 in §4.4). Much of the net sulphur that enters the ocean each year is deposited in sediments. The remainder that is lost from the ocean leaves as biologically produced sulphureous gases, such as DMS. The potential climatic importance of the sulphur cycle, through DMS, merits a separate discussion, given in the next section, §4.4.

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