Carbon and the Oceans

The Earth’s carbon cycle is strongly influenced by biological and abiological phenomenon which operate across major physical boundaries between the earth’s surface (land & ocean) and the atmosphere (Figure 1). Besides the oxygen and hydrogen contained within water, carbon is the most abundant element making up the tissues of living organisms (biomass). The biological conversions between inorganic carbon in the form of carbon dioxide (CO2) and organic carbon in the tissues of living or preserved dead material is a key process that controls the concentration of CO2 in the atmosphere. Without considering human influences, the natural biological processes that consume CO2 (primary production) have recently been in balance with the processes that generate CO2 (respiration, decomposition). However, human generation of CO2 from the burning of fossil fuels have created a situation in the last 150 years where CO2 concentrations in the atmosphere have increased at a rate more rapid than any time in the geologic past. Abiological processes that are important in the global cycling of CO2 include; volcanic gas emanation, sequestering of inorganic and organic carbon in non-living deposits, absorption or release of CO2 across the ocean surface, and the longer term cycling of CO2 in intermediate and deep water masses.

Figure 1. Global carbon cycle based upon information collected
from 1980 to 1989. Reservoir values indicated in while, fluxes indicated
in yellow, and annual changes indicated in pink. Data courtesy of NOAA.

Increases in atmospheric CO2 and the related problem of global warming may exert some important impacts upon the biological and physical characteristics of the oceans. A variety of changes within the oceans may affect the future rates of atmospheric CO2 additions and global warming either through positive feedbacks (which increase rates) or negative feedbacks (which decrease rates). In this web exercise we will look at three feedback effects which may influence the future rates of these processes including;

The surface of the ocean currently acts as a sponge, or sink, absorbing about 30% of the annual anthropogenic generation of CO2. Although, globally the ocean acts as a sink for CO2, regionally there exists areas of strong sinks (high latitude regions), and strong sources (equatorial regions). Let's try to understand why these regional differences exist. One of the most obvious differences between seawater in the high versus low latitudes is temperature. Water temperature effects the solubility of gases in seawater. As temperatures decrease the solubility of gases increase. Another inherent difference between the low and high latitude waters is the presence of thermoclines. Except for a narrow belt within the equatorial region the low latitude waters are characterized by thermoclines which serve as a density barrier for the mixing of shallow and deep water. One of the reasons for the strong sink for CO2 in the high latitude marine waters is the combination of the cold water (which can hold great amounts of CO2), and the occurrence of water mass sinking (termed thermohaline circulation) which moves the CO2 rich waters from the surface to deep below the surface. Such sinking water masses exist in both the northern and southern hemispheres in the high latitude regions. In fact some of these water masses continue to sink until they reach the bottom and then they turn and travel towards the equator crawling along at rates much slower than that found in surface currents. The process of this slow transport through the absorption of CO2 and sinking of these waters has been called the "solubility pump".

How do processes associated with the low latitude waters compare to that which occurs in the high latitudes? The warm lighter water of the lower latitudes does not accommodate as much CO2 per unit volume as the high latitude, and this warm water does not sink because it has a very low density. Because of these features vast areas within the central regions of the subtropical gyres in the 3 major basins (Pacific, Atlantic, and Indian oceans) are not strong sinks or sources of CO2 to the atmosphere, but are close to equilibrium regarding the concentration of CO2 within the waters versus atmosphere above. Along the equator another situation arises that creates a strong source of CO2 to the atmosphere. The thermohaline water masses previously described that originate in the high latitudes, complete their circuit by rising to the surface at a variety of upwelling regions. This produces a global "Conveyor belt" phenomenon regarding the transport of water. One of the regions where the cold water rises to the surface is along the equator, where trade wind belts drive surface water towards the north and south. The water that is driven away is replaced by cold CO2 laden water that upwells from below. The upwelling water is supersaturated regarding CO2 and once at the surface releases gas to the atmosphere.

Now lets begin our excercise on the physical factors which control ocean atmosphere CO2 flux. Below is a table illustrating the solubility of CO2 in seawater of differing temperature and salinity combinations.

Table 1. CO2 solubility as a function
of temperature and salinity
Modified from Weiss (1974)

Temp\Sal	32 psu	34 psu	36 psu	38 psu
-1° C	66.59	65.79	65.00	64.22
5° C	53.06	52.44	51.82	51.22
10° C	44.64	44.13	43.63	43.13
15° C	38.09	37.67	37.25	36.84
20° C	32.94	32.58	32.23	31.89
25° C	28.84	28.54	28.24	27.95
30° C	25.55	25.30	25.05	24.80

Describe the effects of temperature and salinity change upon the solubility of CO2. What would be the solubility of CO2 at conditions of 37psu and 12.5 ° C?

Access the web link below and examine the animation demonstrating the spatial and temporal
change in CO2 flux into the ocean;

Where were regions of flux of CO2 into the ocean, where did CO2 flux from the ocean into the atmosphere?

Now examine the figures (1a and 1b) below (Courtesy of NOAA) indicating longitudinal profiles of dissolved inorganic carbon (DIC) for the (a) Atlantic and (b) Pacific oceans. Note that the term DIC means all inorganic forms of carbon generated from the dissolution of CO2 in seawater.

In general are DIC concentrations higher in shallow waters or in deeper water? Can you think of a reason for this? There is a source of DIC at the waters surface (namely CO2), but is there a source under the water? Does the trend for DIC concentration changes as a function of latitude differ between the two ocean basins? What could cause this difference - explain! Finally, inspect a series of points at similar latitudes and depths in these two ocean basins - how do they compare? Why do they differ?

Below lets begin the exercise to discover how temperature increases in the higher latitudes may impact this Conveyor belt circulation, affecting the sink for CO2 and regional climate. Temperature and salinity are the two major determinants of the density of seawater. The joint influence of temperature and salinity in determining density can be illustrated in the figure below. The units of density illustrated are in s t values which equal the following;

Figure 2. T-S diagram illustrating the resultant density
in s _t values of different combinations of temperature and salinity.

Global warming as the result of the Greenhouse gas increases in the atmosphere may change both the temperature as well as the salinity of water in the higher latitudes.

2) In the table below you will find the current temperature and salinity values for the two deepest thermohaline water masses in the world, Antarctic Bottom water (AABW) and North Atlantic Deep water (NADW). Using figure 2 above carefully determine the % change in density given the following modifications to the temperature and salinity of these two water masses indicated below. In the North Atlantic there are two water masses that overlay the NADW including the North Atlantic Intermediate (NAI ; 4.0° C, 34.8 ppt) and the North Atlantic Surface (NAS; 4.5° C, 34.7 ppt). How would the future NADW orient relative to these currently shallower mater masses?

Table 2. Current and future physical characteristics
of thermohaline deepwater masses

Water Mass	Temperature	Salinity	s t value	% Change
AABW (current)	-0.8 ° C	34.7		XXXXX
AABW (future)	+1.8 ° C	34.2
NADW (current)	+1.0 ° C	34.8		XXXXX
NADW (future)	+2.0 ° C	34.0

3) If the sinking of water associated with thermohaline water masses halted, what do you expect would happen at the other end of the Conveyor belt? How would this affect global CO2 flux? Why?

One of the processes that creates dense water in the high latitudes is the cooling of surface waters in this region. In order for surface waters to be cooled heat must be transferred from the ocean surface to the atmosphere. This loss of heat chills surface waters making them dense.

4) Based upon this information are these regions of thermohaline water mass formation a sink or a source of heat to the atmosphere? If the water mass formation and the conveyor belt circulation ceased what would happen to the atmospheric climate in the area? Would this be a positive feedback to global warming?

5) During El nino events the strength of the trade winds decrease markedly. This in turn effects upwelling along the equator. How would these physical alterations to the earth’s wind and ocean circulation affect the global flux of CO2 from the ocean? Explain why!

Results from recent NOAA sponsored research of gas exchange along the equatorial Pacific.

Figure 3. Contrast of the change in CO2 concentration (water / atmosphere) in left panels and
CO2 flux in right panels, during normal (top) and El nino (bottom) years. Figures courtesy of NOAA.

The other process of carbon transport that occurs in the oceans is referred to as the biological pump. In this process production of organic material (through primary production and planktonic foodwebs) and the sequestering of carbon into skeletal matter occurs (i.e. CaCO3), and this matter eventually sinks into the deep ocean water. In the deep ocean the organic material is slowly decomposed, and if deep enough the carbonate dissolves. A basic difference then between the biological pump and the solubility pump (previously discussed) is the state of the matter being transported. In the solubility pump the matter is dissolved, in the biological pump the matter is particulate. For a variety of terrestrial plants it has been demonstrated that increased CO2 concentrations in the atmosphere will enhance plant production. However, the much higher concentrations of CO2 in seawater mean that phytoplankton are not limited by CO2 availability. However, the increasing temperature may exert an important influence upon the biological pump by changing the magnitude and extent of stratificationin the open ocean.

How would water column stratification effect the level of the biological pump? Hint: the lower latitude open ocean is stratified year round. Is annual primary production higher or lower in this region compared to the mid or higher latitudes? How does statification effect primary production?

The net annual absorption of carbon from the atmosphere by the ocean has been estimated at 1.4 gigatons (= 1.4 x 10¹⁵ g), while the downward flux to deep water of biogenic carbon has been estimated at 4 gigatons, and the transport of carbon by the souluability pump to deep water at 91.6 gigatons (Figure 1). Based upon the reservoir values indicated in figure 1, and loss rates to underlying reservoirs, what would be the turnover times of carbon in the atmosphere, in surface waters and in deep waters?

Which compartment has the shortest turnover time? What is the relevance of short versus long turnover times? What would the turnover times for carbon loss from the diffusion versus biological pump be for ocean surface waters? What is the significance of the difference in transport rates for these carbon forms from surface waters?

The great barrier reef in Austrailia is the largest structure built by a community of organisms on the planet. This structure is easily visible from the space shuttle with the naked eye. It runs for over 2000 km in length off of the north eastern coast of Australia. This testimony to the capabilities of hermatypic corals in constructing skeletal material is blostered by an understanding of the extent of coral reefs in the global ocean. It is estimated that 580,000 km2 of reef environment exists in the global ocean.

Although the growth of corals and the production of the accompanying skeletal material seems to be slow (roughly 4 mm of vertical growth per year), it is relentless and over geologic time has produced great deposits of CaCO3 that now underly many active reef communities. As an example of this it has been determined through geologic sampling using cores and drilling apparatus that many of the reefs that exist today in the Pacific ocean have between 1 to 2 km of coral skeletal material below.

2) If a reef community grows vertically at a rate of 4mm yr-1, how long would it take to produce an underlying deposit of 1 km thickness?

There is current argument regarding the role of coral reefs in the cycling of carbon dioxide. Some scientists feel that reef growth results in a net loss of CO2 into the atmosphere, while others believe that reefs sequester CO2. Most of these arguments are based upon recent measurements of several days to a year. The vast deposits of coral skeletal material that exist in the ocean however argue that over longer time scales that reefs must be an important sink for CO2. The following excercise will determine the amount of CO2 that is required to support the annual growth of coral reef skeletal material. The resultant value of sequestered CO2 will then be compared to the total uptake of CO2 by the oceans, and to the amount of CO2 generated by anthropogenic sources.

3) Describe the kind of information that you feel would be nessessary to determine the amount of CO2 required to support the annual growth of reef skeletal material?

4) The required steps nessessary to answer the question posed include in order the following:

Where V = total skeletal volume, P = porosity, D = density, and %W = percent weight of CaCO3 by CO2.

= [(580,000 km² * 4x10^-6km)*(1/.6)*(2.7x10¹⁵ g / km³)*(0.44)] = 1.1x10¹⁵ g CO2 or 3x10¹⁴ g C

In other words about 100 times the carbon is being sequestered by skeletal construction in coral reef environments compared to the average absorption of C from CO2 per similar area of ocean surface water.

(Examination of the conveyor belt curculation system and its relationship to abrupt climate change)
http://www.enn.com/ENN-News-Archive/1997/12/120997/currents.asp

(Climate rides on ocean conveyor belt)
http://www.enn.com/news/enn-stories/1999/09/092699/conveyor_5908.asp

(Global warming to enhance the indidence of coral bleaching)
http://www.enn.com/news/enn-stories/1999/05/051999/cbleach_3279.asp

(Carbon dioxide build up threatens corals)
http://www.enn.com/enn-news-archive/1999/04/040599/carbon_2488.asp

(Studying Deep Ocean Currents for Clues to Climates)
http://www10.nytimes.com/library/national/science/110999sci-climate.html

("Beatuty and the Bleach", Coral bleaching in the Florida Keys - linked to water temperature)
http://www.enn.com/enn-multimedia-archive/1999/02/020999/020999beau.ram

(Ocean matter may mitigate greenhouse effect)
http://www.enn.com/enn-news-archive/1998/02/021098/organic.asp

(Audio file on the link between global warming and the global conveyor system)
http://www.npr.org/ramfiles/970828.me.11.ram

(Audio file on the importance of the conveyor belt and its role in rapid climate change 11,500 years ago)
http://www.npr.org/ramfiles/971030.atc.05.ram

(Audio file on the ocean as a heat radiator)
http://www.seaweb.org/ram/8.21.98.ram

(Audio file on the interaction between opceans and climate)
http://www.seaweb.org/ram/4.9.99.ram

(Lessons from ancient heat surge)
http://www.nytimes.com/library/national/science/112399sci-environ-methane.html

(Arctic Ice melting - harbinger of future)
http://search3.nytimes.com/search/daily/bin/fastweb?getdoc+site+site+56386+37+wAAA+greenhouse%7Eocean

(Arctic ice melt / Conveyor interaction)
http://search1.nytimes.com/search/daily/bin/fastweb?getdoc+site+site+40105+3+wAAA+conveyor

Compartment	Reservoir	Loss rate	Turnover time
Atmosphere
Surface Waters
Deep waters