Iron fertilization
Iron fertilization is the intentional introduction of iron-containing compounds to iron-poor areas of the ocean surface to stimulate phytoplankton production. This is intended to enhance biological productivity and/or accelerate carbon dioxide sequestration from the atmosphere. Iron is a trace element necessary for photosynthesis in plants. It is highly insoluble in sea water and in a variety of locations is the limiting nutrient for phytoplankton growth. Large algal blooms can be created by supplying iron to iron-deficient ocean waters. These blooms can nourish other organisms.
Ocean iron fertilization is an example of a geoengineering technique. Iron fertilization attempts to encourage phytoplankton growth, which removes carbon from the atmosphere for at least a period of time. This technique is controversial because there is limited understanding of its complete effects on the marine ecosystem, including side effects and possibly large deviations from expected behavior. Such effects potentially include release of nitrogen oxides, and disruption of the ocean's nutrient balance. Controversy remains over the effectiveness of atmospheric sequestration and ecological effects.
Since 1990, 13 major large scale experiments have been carried out to evaluate efficiency and possible consequences of iron fertilization in ocean waters. A study in 2017 considered that the method is unproven; the sequestering efficiency was low and sometimes no effect was seen and the amount of iron deposits needed to make a small cut in the carbon emissions would be in the million tons per year. Since 2021, interest is renewed in the potential of iron fertilization, among other from a white paper study of NOAA, the US National Oceanographic and Atmospheric Administration, which rated iron fertilization as having "moderate potential for cost, scalability and how long carbon might be stored compared to other marine sequestration ideas"
Approximately 25 per cent of the ocean surface has ample macronutrients, with little plant biomass. The production in these high-nutrient low-chlorophyll waters is primarily limited by micronutrients, especially iron. The cost of distributing iron over large ocean areas is large compared with the expected value of carbon credits. Research in the early 2020s suggested that it could only permanently sequester a small amount of carbon.
Process
Role of iron in carbon sequestration
Iron is a trace element in the ocean and its presence is vital for photosynthesis in plants such as phytoplankton. So, adding iron to deficient areas promotes phytoplankton growth. For this reason, the "iron hypothesis" was put forward by Martin in late 1980s where he suggested that changes in iron supply in iron-deficient seawater can bloom plankton growth and have a significant effect on the concentration of atmospheric carbon dioxide by altering rates of carbon sequestration. 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. 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 shown 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.
Carbon sequestration by phytoplankton
is photosynthetic: it needs sunlight and nutrients to grow, and takes up carbon dioxide in the process. Plankton 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.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. Most of the carbon-rich biomass generated from plankton is generally consumed by other organisms 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.
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.
Efficiency and concerns
Assuming the ideal conditions, the upper estimates for possible effects of iron fertilisation 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. This approach, which stimulates phytoplankton growth by introducing iron into nutrient-poor regions of the ocean, could be seen as a potentially easy and scalable method to reduce atmospheric levels. While it offers a theoretical means of mitigating climate change, ocean iron fertilisation remains highly controversial and debated due to its potential negative impacts on marine ecosystems.Research in this field suggests that introducing large amounts of iron-rich dust into the ocean can significantly disturb the ocean's nutrient balance. These disruptions can create serious issues within the food chain, threatening the survival of marine organisms that rely on stable nutrient cycles. Excessive iron may also alter the structure of plankton communities, potentially favouring certain species over others, thereby reducing the diversity vital for a healthy marine ecosystem.
Iron fertilisation can trigger expansive phytoplankton blooms, which, as they decompose, could create hypoxic or anoxic zones in the ocean, posing severe risks to marine life and biodiversity. In some cases, iron fertilisation has been linked to harmful algal blooms, which can produce toxins detrimental to marine organisms and humans. For example, trials in the Southern Ocean, including the SOFeX experiments, demonstrated that iron fertilisation can lead to the rapid growth of harmful algae, with potential consequences for local ecosystems and food chains.
In addition to ecological concerns, there are challenges related to the effectiveness and long-term stability of carbon sequestration through iron fertilisation. While phytoplankton can capture and sink to the ocean floor, a significant portion of this carbon may eventually be released back into the atmosphere due to various oceanic processes, diminishing the technique's long-term effectiveness. Recent research indicates that the success of carbon sequestration is highly variable, influenced by factors such as ocean currents and temperature. Feedback mechanisms, such as alterations in the ocean's biogeochemical cycles or changes in marine species populations, may weaken the overall effectiveness of iron fertilisation as a climate change mitigation strategy.
Methods
There are two ways of performing artificial iron fertilization: ship based direct into the ocean and atmospheric deployment.Ship based deployment
Trials of ocean fertilization using iron sulphate added directly to the surface water from ships are described in detail in the [|experiment section] below.Atmospheric sourcing
Iron-rich dust rising into the atmosphere is a primary source of ocean iron fertilization. For example, wind blown dust from the Sahara desert fertilizes the Atlantic Ocean and the Amazon rainforest. The naturally occurring iron oxide in atmospheric dust reacts with hydrogen chloride from sea spray to produce iron chloride, which degrades methane and other greenhouse gases, brightens clouds and eventually falls with the rain in low concentration across a wide area of the globe. Unlike ship based deployment, no trials have been performed of increasing the natural level of atmospheric iron. Expanding this atmospheric source of iron could complement ship-based deployment.One proposal is to boost the atmospheric iron level with iron salt aerosol. Iron chloride added to the troposphere could increase natural cooling effects including methane removal, cloud brightening and ocean fertilization, helping to prevent or reverse global warming.
Experiments
Martin hypothesized that increasing phytoplankton photosynthesis could slow or even reverse global warming by sequestering in the sea. He died shortly thereafter during preparations for Ironex I, a proof of concept research voyage, which was successfully carried out near the Galapagos Islands in 1993 by his colleagues at Moss Landing Marine Laboratories. Thereafter 12 international ocean studies examined the phenomenon:- Ironex II, 1995
- SOIREE, 1999
- EisenEx, 2000
- SEEDS, 2001
- SOFeX, 2002
- SERIES, 2002
- SEEDS-II, 2004
- EIFEX, A successful experiment conducted in 2004 in a mesoscale ocean eddy in the South Atlantic resulted in a bloom of diatoms, a large portion of which died and sank to the ocean floor when fertilization ended. In contrast to the LOHAFEX experiment, also conducted in a mesoscale eddy, the ocean in the selected area contained enough dissolved silicon for the diatoms to flourish.
- CROZEX, 2005
- A pilot project planned by Planktos, a U.S. company, was cancelled in 2008 for lack of funding. The company blamed environmental organizations for the failure.
- LOHAFEX, 2009 Despite widespread opposition to LOHAFEX, on 26 January 2009 the German Federal Ministry of Education and Research gave clearance. The experiment was carried out in waters low in silicic acid, an essential nutrient for diatom growth. This affected sequestration efficacy. A portion of the southwest Atlantic was fertilized with iron sulfate. A large phytoplankton bloom was triggered. In the absence of diatoms, a relatively small amount of carbon was sequestered, because other phytoplankton are vulnerable to predation by zooplankton and do not sink rapidly upon death. These poor sequestration results led to suggestions that fertilization is not an effective carbon mitigation strategy in general. However, prior ocean fertilization experiments in high silica locations revealed much higher carbon sequestration rates because of diatom growth. LOHAFEX confirmed sequestration potential depends strongly upon appropriate siting.
- Haida Salmon Restoration Corporation, 2012 - funded by the Old Massett Haida band and managed by Russ George - dumped 100 tonnes of iron sulphate into the Pacific into an eddy west of the islands of Haida Gwaii. This resulted in increased algae growth over. Critics alleged George's actions violated the United Nations Convention on Biological Diversity and the London convention on the dumping of wastes at sea which prohibited such geoengineering experiments. On 15 July 2014, the resulting scientific data was made available to the public.