Ocean Sequestration

Ocean is considered to be a great storage facility for the carbon sequestration. Although the feasibility of this technique is not assured, there are two ocean sequestration methods under study: i) direct injection of CO2 and ii) the iron fertilization.

The ocean carbon "intake" is considered to be around 2±0.8 Gigatons Carbon (GtC) per year. There are thoughts that in the next 1000 years 90% of the today's carbon emissions will be diffused into oceans. Although the oceans' biomass represents almost 0.05% of the whole, it transforms annually, almost 50 GtC of inorganic carbon to organic. This process is often referred to as biological pump.

16.3.1 Direct injection

Direct injection starts with the CO2 capture, continues with the carbon transportation, in tubes or tanks, and it ends up to the ocean sequestration. Carbon can be efficiently sequestrated for a period of time of several hundreds of years. In order to achieve that, carbon must be liquefied. Carbon deposition takes place in depths beneath the thermocline; that is to say, depths greater than 1000 m. The technology is mature in order to proceed into a commercial implementation. There is a lack of knowledge of the impact on the climate change and the diversity of the biochemical cycle of the ocean.

There are several techniques in the implementation of the direct injection. There is medium-depth sequestration (1000-2000 m), high-depth sequestration (over 3000 m), sequestration on the bottom of the ocean, or sequestration at the undersea earths' layer. Carbon sequestration can be done through dissolution or CO2 hydrate formation, and its efficiency is depending on the actual depth and the injection point. The deeper the injection, the more efficient the sequestration (Fig. 16.2).

The dominant environmental impact is the lowering of the ocean pH, resulting from the CO2 reaction with the ocean water, something that will affect the organisms that live in depths beneath 1000 m. The impact scale depends on the pH variation and the exposure duration. Additionally, there might be an impact on the microbial ecology of the ocean that may deflect biochemical reactions relative to the natural ocean carbon cycle. Regionally, these impacts could be minimized if the injection system is designed in a way that the injected carbon will disperse in a wider area and not to the injection point. The ocean sequestration is presented in Fig. 16.3.

Fig. 16.3 Basic flow of ocean sequestration.

16.3.2 Reinforcement of natural sequestration - iron fertilization

In the natural ocean carbon cycle, the phytoplankton sequestrates carbon into ocean. It is estimated that 70-80% of the carbon is recycled on the upper layer of the ocean while the rest is transported to the deep ocean where it is mineralized. Ocean fertilization with substances such as iron, nitrogen, and phosphorus is considered a carbon minimization technique due to the reinforcement of the biological cycle. At ocean areas with high nitrogen and phosphorus content, iron contents' increase or decrease can additionally increase or decrease the phytoplankton biomass. Initial studies have shown that iron fertilization on the upper ocean layer resulted in the phytoplankton increase. For instance, IRONEX I and IRONEX II experiments (US Department of Energy) have shown that the delivery of 500 kg of iron into 72 km in the Pacific Ocean resulted in a 30 times increase of the phytoplankton biomass. Low-scale iron fertilization is already used in the fish cultivation industries.

On the other hand, large-scale fertilization is not an easy task. There are technical and biological constraints in the implementation of this method, related to the lack of knowledge in specific areas. For instance, large-scale fertilization impact on the structure and operation of the seas' ecosystem is unknown. The alteration of the phytoplankton may affect the food net, and additionally, the eutro-phication of the ocean is a possibility. If these restraints could be resolved, iron fertilization could be very promising for the future.

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