The Ocean

Out in the open ocean, the store ofliving carbon as plants is tiny—less than in a desert on land. The floating cells of phytoplankton have lifetimes of only a few days before they sink and die or are eaten, so biomass cannot build up near the top of the ocean. The material that rains down from the surface into the deep ocean slowly rots and disperses into the water as it sinks, in a journey that may take a month. Often it clumps together as it sinks into what is aptly named "marine snow". What reaches the sea floor thousands of meters below lends to be the most inert, indigestible material that bacteria and animals find difficulty making use of. It forms a loose gelatinous material that coats the sediment surface. No-one is quite sure how much carbon is held in the occans as this flufl'on the ocean floor, or in the form of organic molecules dissolved in the seawater. but it might rival the amount stored in soils on land.

There is another much vaster store of carbon in the occan water, which forms an integral part of the carbon cycle. This is inorganic carbon in the form of C02

dissolved in the water. Rather than just existing like most gases would in the form of molecules floating around in solution, CO: actually chemically reacts with water to form an acid, known as carbonic acid, with the chemical formula FI2C03.

It forms by this reaction:

CO2 also reacts with carbonates to form bicarbonate, dissolved as ions in the ocean water. So, for example, if CO2 reacts with calcium carbonate, an insoluble substance on the sea floor:

CO, | II20 + CaCO}(solid) « Ca(IiC03)2(dissolved) <=> Ca2+ + 2HN03"

(Note that in both cases the arrows point two ways, because the reaction is easily reversible. Both carbonic acid and bicarbonate can easily break down to yield C02 again if conditions shift.)

Oceanographers have chosen to call the dissolved bicarbonate and carbonate forms of carbon "alkalinity", although the term does not have much to do with pi I and almost seems designed to confuse any newcomer to the subject! The oceans essentially control the C02 level of the atmosphere by storing most of the world's C02 in the form of this dissolved alkalinity. If the amount of CO: in the atmosphere suddenly goes up, the oceans will gradually dissolve most of it when it reacts with carbonate in the ocean and 011 the sea floor, forming bicarbonate so that only about one-eighth of the original amount is left in the atmosphere, like the end of an iceberg poking above the waterline when most of it is below. If. on the other hand, the CO? level in the atmosphere decreases, bicarbonate and carbonic acid break up to yield C02 and the oceans release carbon, pushing the atmosphere's C02 content back up again (Figure 7.2a, b). So, the oceans with their huge capacity to store and release carbon act as a very effective buffer against any changes in C02 causcd by living

HCO.

HCO.

HCO.

H2co3 co2

HCO.

HCO.

co2 h2co3

HCO.

Figure 7.2. A huge amount of C02 is stored in the form of both bicarbonate and dissolved C02 in the ocean, (a) If the C02 concentration in the atmosphere becomes low. C02 will leave these reservoirs to top up the atmosphere, often depositing calcium carbonate on the sea floor as the bicarbonate breaks up to yield C02. (b) If C02 concentration in the atmosphere increases, this ocean reservoir will tend to soak up more C02 until most of it has been taken out of the atmosphere—often dissolving more carbonate from the sea bed to make the bicarbonate.

organisms, by volcanoes or by anything else. When land plants act to alter the CO: level in the atmosphere, they are always working against this massive buffer which rather limits how much they can change the composition of the atmosphere on the timcscalc of thousands of years. An increase or decrease in carbon storage in vegetation or soils may produce temporary changes lasting a few decades, but those changes will tend to be evened out by oceans taking up or releasing carbon over centuries and millennia. Only if the land plants work relentlessly over millions of years will they finally be able to overcome the effect of this big ocean reservoir and cause major changes to the C02 content of the atmosphere.

7.2 PLANTS AS A CONTROL ON C02 AND 02

Since the beginning of photosynthetic life on earth, plants have likely had a big influence on the C02 level in the atmosphere. Green and (especially) blue-green bacteria, the precursors and distant cousins of modern-day green plants, began to spread through the oceans about 3.5 billion years ago. They were a source of oxygen, pouring out this highly reactive corrosive gas, which gives life to us but acts as a poison for many of the more primitive bacteria. At the same time, these photosynthc-sizers actcd as a trap for carbon, but not in terms of standing biomass as in today's forests, for there would have been very little living carbon stored at any one time in all the ureen and blue-green bacteria in the world. Instead they left carbon in debris, dead cells buried in sediment that added up over all the millions of years to a huge amount of C02 taken out of the atmosphere. Still dispersed through the world's sedimentary rocks is a vast store of organic carbon, put there mainly by marine algae. This all adds up to an amount of carbon many times greater than the amount in C02 presently in the atmosphere. As the deep buried carbon reservoir increased in size over time, oxygen concentrations in the atmosphere would have risen. This is because carbon not buried in rocks tends to accumulate as CO> in the atmosphere, holding oxygen as well as carbon. When more carbon is buried, the oxygen is left behind. If all the dead carbon fixed by plants had quickly been able to oxidize back into CO:, the oxygen left behind in photosynthesis could not have built up in the atmosphere— because when the dead plant cells decayed and oxidized back to C02, this would have taken up exactly the same amount of 02 as was initially released in photosynthesis. Balanced by only the living biomass of plants and the dead carbon in soils at the surface, the oxygen concentration in the atmosphere would be far lower: much less than one percent. As it is, with most of the organic carbon out of reach below the surface, oxygen has accumulated to very high levels—a fifth of the atmosphere.

It is likely that the buried organic carbon reservoir in rocks has also undergone significant fluctuations, sometimes storing up extra carbon and sometimes releasing it. Unlike changes in C'02 brought about by volcanic output and weathering (sec below), this variability in the organic carbon reservoir would have been paralleled by changes in oxygen concentration, because organic carbon released from rocks will always tend to react with oxygen in the atmosphere to form C02. So, carbon released from rocks uses up oxygen from the atmosphere. Some calculations have it that about

"Time (million years)

Figure 7.3. Estimated C02 concentrations in the atmosphere over the last several hundred million years. Concentrations are calculated from balancing changes in volcanic activity (a source of C02) against rock-weathering and burial of plant carbon (sinks of CO:), inferred from the sediments surviving from each time. There are broad bands of uncertainty on this (the upper and lower lines). The vertical lines represent more detailed studies on the carbon balance of the rocks and atmosphere for particular timeframes, and the range of uncertainties on these. From: Schwartzman, after Berner (permission Columbia University Press).

"Time (million years)

Figure 7.3. Estimated C02 concentrations in the atmosphere over the last several hundred million years. Concentrations are calculated from balancing changes in volcanic activity (a source of C02) against rock-weathering and burial of plant carbon (sinks of CO:), inferred from the sediments surviving from each time. There are broad bands of uncertainty on this (the upper and lower lines). The vertical lines represent more detailed studies on the carbon balance of the rocks and atmosphere for particular timeframes, and the range of uncertainties on these. From: Schwartzman, after Berner (permission Columbia University Press).

450 million years ago the oxygen concentration in the atmosphere was only 15%. instead of the present 21 %, because there was so little organic carbon held in the sorts of rocks that existed at that time. At that time the C02 concentration in the atmosphere would have been something like 15 to 20 times what it is now (Figure 7.3). Over the tens of millions of years that followed, land plants evolved from seaweeds and spread across the continents. Where they formed the first forests in swampy river deltas, they laid down undccayed carbon as peats. Some of these peat layers were compressed into coal, while others were washed away and incorporated as fragments into the sediments of deep ocean floors. Geologists who study the chemical balance of rocks (geochemists) suggest that the huge amount of carbon taken out of the atmosphere by undecayed parts of land plants was enough to cause atmospheric C02 levels to plunge down to levels similar to the present. For instance, in the environment in general at that time there was a big decrease in the abundance of the carbon-12 isotope, which is preferentially taken up by plants during photosynthesis (see Chapter 8). This suggests that plants were sucking away a lot of carbon—especially carbon-12—and that it was ending up held in undccayed organic material. In contrast, the oxygen level between around 300 and 150 million years ago might have stood at 25%. or even 30% because so much of the oxygen split off in photosynthesis was unable to rejoin with carbon in decay. Fluctuations in oxygen level would have had all sorts of interesting effects on life at the surface. For instance, they would have affected the ease with which fires could start and spread on plant material. At 15% oxygen, it is hard to sustain a lire even on dry material, whereas at 30% oxygen even moist plant tissues will burn. Some geologists have claimed to find evidence of these fluctuations in oxygen in the form of changes in the frequency of charcoal layers in rocks laid down during the past several hundred million years. For example, the coal swamp forests that existed around 350 million years ago may have at least partially burned every 3 or 4 years. Others suggest that there are too many complexities affecting the likelihood of preservation of charcoal to reach any meaningful conclusions about fire frequency. It is also rather difficult to explain the existence of forests at times when the atmospheric oxygen level was supposedly around 30%. At this sort of concentration, a single lightning strike in even the moistest forest would cause it all to be consumed by fire, and it is rather unlikely that forest anywhere in the world would be able to grow and reach maturity. Yet, throughout the past 350 million years there is evidence of forests having existed; so. we can at least say that some of the uppermost estimates of past oxygen concentration are probably wrong. On the other hand, some independent evidence that oxygen levels were at least somewhat higher around 300 million years ago comes from the existence of huge flying insects, such as dragonflies with 70cm wingspans. Calculations suggest that in our present atmosphere of 21% oxygen, such insects could not exist because they would not be able to get enough oxygen for their active lifestyle, due to the limits of how fast the breathing tubes (called tracheids) in their bodies can supply oxygen to their muscles. More oxygen around in the atmosphere would also have increased the density of the air, making it easier for such huge winged insects to hold themselves aloft.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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