Oceans

What is ocean sequestration?

Ocean sequestration involves the storing of carbon in the ocean. Carbon is naturally stored in the ocean via two pumps, solubility and biological, and there are analogous man-made methods, direct injection and ocean fertilization, respectively.  Eventually equilibrium between the ocean and the atmosphere will be reached with or without human intervention and 80% of the carbon will remain in the ocean. The same equilibrium will be reached whether the carbon is injected into the atmosphere or the ocean.  The rational behind ocean sequestration is simply to speed up the natural process.

source: Carbon sequestration by direct injection into the deep ocean involves the capture, separation, transport, and injection of CO2 from land or tankers

"The solubility pump arises from the circulation of water in the oceans.  Surface water flowing towards the poles cools, thus lowering pCO­₂ and causing an uptake of CO­₂ from the atmosphere.  At high latitudes the cold, dense, CO­₂ rich water sinks to form bottom water.  The north Atlantic (Greenland Sea) and the Antarctic are the major sites for this bottom water formation process.  The CO­₂-rich bottom water formed in this way eventually wells up to the surface after a period of up to 1500 years, the Equatorial Pacific being the major site for such upwelling.  As the upwelled water is heated to tropical temperatures (25-30°C), the result is high pCO­₂ value and loss of CO­₂ to the atmosphere through outgassing." [1]

"The biological pump is the net downward transport of carbon through food-web processes (photosynthetic production of organic matter from dissolved inorganic carbon and nutrients in surface waters, grazing of the organic matter by zooplankton, downward rain of organic debris and/or downward biomass migration, with subsequent remineralization/respiration of the organic carbon back to inorganic carbon at depth). This natural carbon sequestration process not only affects the levels of CO₂ in the atmosphere but also structures the organic and inorganic pools of C, N, P, O, and many other chemical species in the oceanic water column. One model simulation has shown that atmospheric CO₂ would rise by over 150 ppmv without an operating biological pump [34]. Current estimates indicate that the biological pump is 10-20% efficient at moving photosynthetically fixed carbon below 100 m, and only several percent efficient below 1,000 m." [2]  

source: Herzog 5/24/2001 p.15

Direct injection involves putting the carbon dioxide directly into the ocean to dissolve rather than going through the atmosphere as an intermediate.  Ocean fertilization requires adding iron, the limiting nutrient for many biological organisms in the ocean, to the ocean to increase the photosynthesis fixation of carbon dioxide.  As mention above, because this website is dedicated to sequestration of carbon as a chemical and not in connection with living organisms, we will not go into detail about ocean fertilization.

The facts

• 1/3 of CO₂ emitted a year already enters ocean (2 of 6 PgC/yr)
• The ocean/air exchange is 90 GtC/year (with a net uptake of 2 GtC/year).
• Oceans have 50X more C than does the atmosphere.
• The amount of carbon the would cause the concentration of carbon dioxide in the atmosphere to double would only raise the concentration in the ocean by less than 2%.
• There has not been a full-scale field experiment as of yet so all information and predictions with regard to carbon residence time and environmental impacts come from models. [3]
• Capacity (based only on physical view) larger than fossil reserve of 5000-1000 GtC on
environmental view (i.e. taking into account pH) unknown - 1300GtC increase pH by .3 [4]
•  "The concept of storing carbon dioxide in the ocean can be traced to a 1977 paper by Cesare Marchetti of the International Institute for the Applied Systems Analysis in Laxenburg, Austria, who suggested that carbon dioxide could be piped into the waters of the< Mediterranean Sea at Gibraltar, where it would naturally flow out into the Atlantic and be< carried to the deep ocean. Even today building a pipe along the ocean floor to transport carbon dioxide to an appropriate depth remains one of the most realistic options for carbon
sequestration.  " [5]
• DOE Center for Research on Ocean Carbon Sequestration (DOCS) established July 1999
and is centered at LBNL and LLNL [6]
• Inorganic sinks for CO_2­ emissions to the atmosphere[9] are CO₂ absorption by surface ocean ~1 yr, mixing to deep ocean ~300 yr, carbonate dissolution ~6000 yr, and silicate-rock weathering ~300,000 yr (note: this is the natural mineral sequestration discussed earlier)

- CO₂ Properties [7]

- Oceanic Carbon Fluxes [8]

Advantages

Herzog and Edmond list the following reasons to use the ocean [10]

• It is vast, covering 70% of earth's surface with an average depth 3800 m.  In addition,  "The deeper layers are highly unsaturated in CO₂ compared to the solubility of CO­₂ at the temperature (~2°C) and pressure (>100 atm) prevailing in those layers."

• The total carbon content of the ocean is 38,000 GtC versus the 750 GtC in the atmosphere.   If all anthropogenic carbon entered the ocean, the carbon content of the ocean would only increase 0.016%/year.
• The deep ocean is separated from atmosphere by thermocline, hindering mixing of the layers creating residence times for the carbon from 50 years to 1000 years.
• Ecological perturbations should be limited to area of release, which will be a small percentage of ocean volume.
• Many fossil fuel plants on coast or along river waterway aiding transportation of the carbon dioxide to the disposal site via pipes, barges, or tankers.

source: Herzog 5/24/2001 p.16

Disadvantages

Right now the biggest disadvantage facing ocean sequestration is ignorance and the lack of hard data available.  The environment's response to direct injection has not been well characterized and all information is based on models.  The models right now are inadequate and sometimes one model will predict the opposite of another model.  Until more substantial data can be gathered it is unsafe to proceed with ocean sequestration because secondary effects and such are not know.  We could end up creating more problems than we solve.  "Many people are wary of ocean sequestration, including some authors of this chapter, because it is known that small changes in biogeochemical cycles may have large consequences, many of which are secondary and difficult to predict. Nevertheless, ocean carbon sequestration is occurring on a large scale today, and entrepreneurs are already trying to commercialize these technologies. Therefore, it is imperative to conduct research to better understand the risks as well as the opportunities. The ocean plays an important role in sustaining the biosphere, so any change in ocean ecosystem function must be viewed with extreme caution. At present, we do not have enough information to estimate how much carbon can be sequestered without perturbing marine ecosystem structure and function." [11]

Benson et al pose an example of a secondary problem that we must be aware of. "Both the biological and physical pumps may be changed in this century. Coupled atmosphere ocean simulations suggest that anthropogenic emissions of CO₂ will lead to increased stratification in the upper ocean and a lowered surface pH [35]. Both effects would reduce the efficiency of both pumps, thereby leaving a larger fraction of anthropogenically produced CO ₂ in the atmosphere." [12]

As we see it, another problem with ocean sequestration is that although it is merely the speeding up of a natural process, the equilibration of the atmospheric and oceanic carbon, it does not take into account that humans have increased the amount of carbon entering into the equilibrium.  Equilibrium may be reached, but it will be equilibrium with a higher content of carbon.  What will the extra carbon do to the environment?  The other sequestration techniques discussed here simply reduce content of carbon in the atmosphere/ocean system thereby the steady state of the planet before mankind's intrusion is better maintained.