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Draft for Introduction to Blue Carbon

Carbon Biosequestration in Ocean
Introduction to biosequestration

Ocean storage refers to the use of large water bodies and marine lifeforms to capture carbon by exploiting natural and geological mechanisms. Oceans cover more than 70% of the earth’s surface, and plays a major role in helping to stabilise Earth’s climate. This presents itself as a readily available carbon sink to store and capture atmospheric carbon dioxide. Due to the solubility of carbon dioxide in water, CO2 naturally dissolves in oceanic waters to form an equilibrium. When the atmospheric concentration of CO2 increases, the position of equilibrium pushes the equilibrium in the direction such that more CO2 dissolves into the water. Utilising this mechanism, the oceans have taken up about 500 GtCO2 (140GtC) of the total 1,300 GtCO2 (350 GtC) of anthropogenic emissions released to the atmosphere over the past 200 years. As a result of the increased atmospheric CO2 concentrations from human activities relative to pre-industrial levels, the oceans are currently taking up CO2 at a rate of about 7 GtCO2 yr-1 (2 GtC yr-1). To enhance the natural mechanism of CO2 dissolving in water, several methods have been proposed by the scientific community. These include the use of iron fertilisation, urea fertilisation, mixing layers, seaweed as well as direct carbon injection into the sea floor.

Biosequestration with Iron Fertilization
Ocean iron fertilization is an example of a geoengineering technique that involves intentional introduction of iron-rich deposits into oceans and is aimed to enhance biological productivity of organisms in ocean waters in order to increase carbon dioxide (CO2) uptake from the atmosphere, possibly resulting in mitigating its global warming effects.

Iron is a trace element in ocean and its presence is vital for photosynthesis in plants, and in particular phytoplanktons, as It has been shown that iron deficiency can limit ocean productivity and phytoplankton growth. For this reason, “iron hypothesis” was put forward by Martin in late 1980s where he suggested that changes in iron supply in iron-deficient ocean-waters can bloom plankton growth and have a significant effect on the concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration.

In fact, fertilization is an important process that occurs naturally in the ocean waters. For instance, upwellings of ocean currents can bring nutrient-rich sediments to the surface. Another example is through transfer of iron-rich minerals, dust, and volcanic ash over long distances by rivers, glaciers, or wind. Moreover, it has been suggested that whales can transfer iron-rich ocean dust to the surface, where planktons can take it up to grow. It has been showed that reduction in the number of sperm whales in the Southern Ocean has resulted in a 200,000 tonnes/yr decrease in the atmospheric carbon uptake, possibly due to limited phytoplankton growth.

Planktons can take up and sequester atmospheric carbon through generating calcium or silicon-carbonate skeletons. When these organisms die they sink to the ocean floor where their carbonate skeletons can form a major component of the carbon-rich deep sea precipitation, thousands of meters below plankton blooms, known as marine snow. Nonetheless, based on the definition, carbon is only considered "sequestered" when it is deposited in the ocean floor where it can be retained for millions of years. However, most of the carbon-rich biomass generated from planktons is generally consumed by other organisms (small fish, zooplankton, etc.) and substantial part of rest of the deposits that sink beneath plankton blooms may be re-dissolved in the water and gets transferred to the surface where it eventually returns to the atmosphere, thus, nullifying any possible intended effects regarding carbon sequestration. Nevertheless, supporters of the idea of iron fertilization believe that carbon sequestration should be re-defined over much shorter time frames and claim that since the carbon is suspended in the deep ocean it is effectively isolated from the atmosphere for hundreds of years, and thus, carbon can be effectively sequestered.

Assuming the ideal conditions, the upper estimates for possible effects of iron fertilization in slowing down global warming is about 0.3W/m2 of averaged negative forcing which can offset roughly 15-20% of the current anthropogenic emissions. However, although this approach could be looked upon as an alternative to lower concentration of in the atmosphere, ocean iron fertilization is still quite controversial and highly debated due to possible negative consequences on the marine ecosystem. Research on this area has suggested that fertilization through deposition of large quantities of iron-rich dust into the ocean floor can significantly disrupt ocean’s nutrient balance and cause major complications in the food cycle for other marine organisms.

Urea fertilization
In waters with sufficient iron micro nutrients, but a deficit of nitrogen, urea fertilization is the better choice for algae growth. Urea is the most used fertilizer in the world, due to its high content of nitrogen, low cost and high reactivity towards water. When exposed to ocean waters, urea is metabolized by phytoplankton via urease enzymes to produce ammonia.

The intermediate product carbamate also reacts with water to produce a total of two ammonia molecules. In 2007 the 'Ocean Nourishment Corporation of Sydney' initiated an experiment in the Sulu sea (southwest of the Philippines), were 1000 tons of urea was injected into the ocean. The goal was to prove that urea fertilization would enrich the algae growth in the ocean, and thereby capture $$CO_2$$from the atmosphere. This project was criticized by many institutions, including the European commission, due to lack of knowledge of side effects on the marine ecosystem. Results from this project are still to be published in literature.

Another cause of concern is the sheer amount of urea needed to caption the same amount of carbon as eq. iron fertilization. The nitrogen to iron ratio in a typical algae cell is 16:0.0001, meaning that for every iron atom added to the ocean a substantial larger amount of carbon is captured compared to adding one atom of nitrogen.

Scientist also emphasize that adding urea to ocean waters could reduce oxygen content and result in a rise of toxic marine algae. This could potentially have devastating effects on fish populations, which other argue would be benefiting from the urea fertilization (the argument being that fish populations would feed on healthy phytoplankton.

Seaweed
Seaweed cultivation is one of the many measures that have been introduced for mitigating global warming through enhanced natural sinks.This method was prominent in those early ocean algae proposals to mitigate global warming using kelp farms are designed to encompass tens of thousands of square kilometres of the open ocean. In which seaweed beds act as effective sinks by drastically reducing the level of dissolved inorganic carbon (DIC).

Seaweeds fix abundant CO2 through photosynthesis, nearly 0.7 million tonnes of carbon are removed from the sea each year within commercially harvested seaweeds. Even though seaweed biomass only occupy a very small area of the coastal region, they are essential because of their biotic components, valuable ecosystem services, and high primary productivity.

Unlike seagrasses and mangroves, seaweeds are photosynthetic algal organisms and, as such, are non-flowering. These primary producers grow in much the same way as their terrestrial counterparts, assimilating carbon through photosynthesis and generating new biomass by taking up nitrogen, phosphorus, and many other essential minerals and trace substances.

Large-scale seaweed cultivation is attractive because of its decades-proven, low-cost technologies and the multiple uses that can be made of its products. Currently. seaweed farming represents approximately 25% of the world's aquaculture production and its potential has not been fully exploited.

Overall, seaweeds contribute 16–18.7% of the total marine-vegetation sink. In 2010 there are 19.2 × 106 tons of aquatic plants worldwide, 6.8 × 106 tons for brown seaweeds; , 9.0 × 106 tons for red seaweeds; 0.2 × 106 tons of green seaweeds,; and 3.2 × 106 tons of miscellaneous aquatic plants. Based on these figures, it is estimated that ∼1000 t of carbon is temporarily sequestered, making the sea as important a carbon sink as terrestrial ecosystems.

Mixing Layers
Mixing layers involve transporting the denser and colder deep ocean water to the surface mixed layer. As the temperature of water in the ocean decreases with depth, more CO2 and other compounds are able to dissolve in the deeper layers. This can be induced by reversing the oceanic carbon cycle through the use of large vertical pipes serving as ocean pumps, or a mixer array. When the nutrient rich deep ocean water is moved to the surface, algae bloom occurs, resulting in a decrease in CO2 due to carbon intake from Phytoplankton and other photosynthetic eukaryotic organisms. The transfer of heat between the layers will also cause seawater from the mixed layer to sink and absorb more CO2.

This method has not gained much traction as algae bloom harms marine ecosystems by blocking sunlight and releasing harmful toxins into the ocean. The sudden increase in CO2 on the surface level will also temporarily decrease the pH of the seawater, impairing the growth of coral reefs. The production of carbonic acid through the dissolution of CO2 in seawater hinders marine biogenic calcification and causes major disruptions to the oceanic food chain.

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