Effect of Biological Processes on CO

In the previous section we observed that the deep waters of the global ocean, especially in the Pacific and Indian oceans, have a different CO2 composition from surface waters. While

surface waters can be considered close to equilibrium with the atmosphere, deep waters have substantially higher AT, CT and f (CO2). These differences are a direct result of biological fixation

of CO2 in surface waters, some of which escapes respiration until it has escaped into the deep

water reservoir, thus transporting a difference in CO2 composition into the deep layers of the

ocean.

Photosynthesis, which takes place exclusively in the euphotic zone of the surface ocean, consumes CO2 dissolved in seawater in the production of biological organic tissue. It may be

represented by the simplified equation

2 2

2

2

H

O

(CH

O)

O

CO

+

→

+

About 95 % of the CO2 reduced to organic tissue (represented by CH2O) undergoes respiration

back to CO2 in surface waters. Respiration is the exact reverse of [8.1], and the combination of

photosynthesis and respiration obviously involves no net change in the water chemistry. However, about 5 % of the carbon that is reduced by photosynthesis escapes the surface water zone of respiration by sinking into the cold, quiescent deep layers of the ocean. However, this does not mean that it escapes the ultimate fate of oxidation back to CO2. Indeed, virtually all of this

biological carbon that sinks into deep waters gets oxidized back to CO2 in the deep layer. Only a

very small fraction survives the passage through deep waters to become preserved in deep ocean sediments, some of which eventually becomes fossil fuel!

The transport of reduced CO2 in the form of sinking biological tissue onto the deep ocean

sets up a biological pump that shifts a fraction (5 %) of the products of surface water photosynthesis into deep waters. The remineralized products, i.e. CO2, are eventually returned to

surface waters as the ocean waters circulate and turn over. Thus the cycle, of which the biological pump is an integral part, is essential closed. However, the circulation process is relatively slow. Studies using radiocarbon (half-life 5780 yr) indicate that the average turnover time for oceanic deep waters is about 1600 yr. This is an important value, because it indicates the time scale over which the deep ocean and the atmosphere maintain equilibrium.

We know a lot about the action of the biological pump through the study of the vertical concentration profiles of the variuous chemical constituents of seawater, including CO2, that are

involved in the photosynthesis/respiration process. Of these, perhaps the most important are the macronutrients nitrate and phosphate, both of which are essential for photosynthesis. Nitrate is the major source of nitrogen needed for the synthesis of nitrogen-containing molecules such as amino acids, the key components of protein. Phosphate is required for the adenosine triphosphate (ATP) energy transfer system in primary organisms. Both nitrate and phosphate are in short supply in surface waters, and their low availability is often the factor that limits the total amount of net photosynthesis that can take place.

This is revealed by Figure 8.2 which shows typical vertical concentration profiles for both substances in the global ocean.

10 20 30 40 Nitrate (µmol/kg) Atlantic Pacific 5000 4000 3000 2000 1000 Depth (m) 0.5 1.0 1.5 2.0 2.5 3.0 Phosphate (µmol/kg)

Figure 8.2 Typical vertical profiles of the macronutrients nitrate and phosphate in the Atlantic and Pacific Oceans. Drawn using data from the GEOSECS Program.

In both cases, we see that surface water concentrations of these macronutrients have been reduced to almost zero because of their efficient utilization for photosynthesis. Concentrations are maximum in deep waters as a result of the biological pump. Since the rate of return of deep waters to the surface is so slow, the macro-nutrients generated in deep waters by the oxidation of

biological material sinking down from the surface accumulate to high relative concentrations in the deep water layer. Ad the deep waters return to the surface by upwelling and diffusion, they carry with them the high nutrient concentrations, thus completing the closed cycle.

The corresponding vertical profiles for CO2 and O2, both of which are involved in the

photosynthesis/respiration cycle (see [8.1]) are shown in Figure 8.3. These profiles show the features expected from the discussion given so far for the macro-nutrients. Total CO2 increases

from surface to deep waters by about 10 % in the Atlantic and 20 % in the Pacific (note that these relative increases are comparable to those seen for nitrate and phosphate in Figure 8.2). At the same time as CT increases through the respiration of the sinking biological tissue, O2 is

consumed, as required by reaction [8.1]. Indeed, at mid-depths of the Pacific ocean, O2 is reduced

in concentration to almost zero.

Examination of data such as those displayed in Figures 8.2 and 8.3 indicates that the stoichiometric ratios of the biological pump are approximately

1 PO43– : 15 NO3– : 100 CO2 : 140 O2 [8.2]

These ratios are known as Redfield ratios in honour of Arthur Redfield, the marine scientist who first suggested their use as a description of the changes in water chemistry accompanying biological growth and respiration.

50 100 150 200 250 O2 (µmol/kg) 5000 4000 3000 2000 1000 Depth (m) 2000 2200 2400 CT (µmol/kg) Atlantic Pacific

Figure 8.3 Typical vertical profiles of total CO2 and dissolved oxygen in the Atlantic and Pacific Oceans. Drawn using data from the GEOSECS Program using the same station locations as Figure 8.2. CT values are normalized to a constant salinity of 35.000.

Equation [8.1] is not the full story, however, because we still need to explain why the deep waters of the ocean have a higher alkalinity than at the surface. In its simplified form, [8.1] implies that photosynthesis and respiration merely remove or add neutral CO2 , a process that

would give rise to no change in AT. Detailed investigations show that in fact there is a small

consumption of H+ during photosynthesis, with a corresponding production of H+ accompanying respiration. Quantitatively, this amounts to about 1 H+ for every 5 CO2 molecules. Therefore, we

would predict that the 20 % increase in CT with depth as a result of the respiration of sinking

organic tissue that is seen in the Pacific Ocean (Figure 8.3) should be accompanied by a decrease of about 20/5 = 4 % in AT.

A quick glance at the results shown in Figure 8.4 shows that this is far from what is observed. In fact the trend is opposite and much larger: AT increases significantly with depth,

particularly in the Pacific Ocean where the increase is almost 10%. These changes can only be explained by an additional process accompanying CO2 involvement in photosynthesis that acts

directly on the proton balance of seawater. This process is the precipitation and dissolution of the mineral calcite (CaCO3). Calcite is used by many of the planktonic organisms in the ocean for the

construction of their mineral exoskeleton.

A large group of photosynthetic organisms that construct calcite exoskeleta is the coccolithophoridae (or coccoliths for short). These tiny unicellular plants build a network of interlocking calcite plates around the outside of the cell, rather like medieval armour. Figure 8.4 shows electron micrographs of typical coccolith plates. The shape and structure of the plates is different for each species and highly characteristic.

Figure 8.4 Electron micrograph of typical coccolith plates found in marine sediments. Calcite exoskeleta are also constructed by the class of planktonic animals known as foraminifera. These normally construct a multi-chambered microscopic cell within which the organism lives. Foraminifera feed on bacteria and other planktonic organisms, including coccoliths. As with coccoliths, the shape and structure of the foraminifera shell is highly characteristic of the species, Examples are shown in Figure 8.5.

Figure 8.5 Electron micrograph of two foraminifera: Nodoseria aspera (left) and Uvigerinella californica (right).

The precipitation of calcite by marine organisms has a simple reaction stoichiometry Ca2+ + CO3

2–

→ CaCO3 (s) [8.3]

This reaction consumes the alkaline species CO32– and therefore decreases the water alkalinity

(and CT) while the reverse process generates CO32– and increases AT (and CT). The CaCO3

components of the biological debris sinking into deep water eventually undergo dissolution deeper in the water column, thus returning the alkalinity extracted from surface waters. This provides the source of the increase in AT with water depth observed in Figure 8.6.

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