Ocean iron fertilization, or the “iron ocean,” is a proposed geoengineering technique. It involves deliberately introducing iron into specific marine areas to influence marine life and the global carbon cycle. This method aims to alter natural oceanic processes to achieve environmental goals.
The Natural Role of Iron in Oceans
Iron functions as a fundamental micronutrient for marine phytoplankton, which are microscopic plant-like organisms, particularly in certain oceanic expanses known as “high-nutrient, low-chlorophyll” (HNLC) regions. These regions, such as the North Pacific, Equatorial Pacific, and Southern Ocean, possess ample macronutrients like nitrate and phosphate but exhibit low phytoplankton biomass due to limited iron availability. When iron is sufficiently present, phytoplankton engage in photosynthesis, a process where they convert sunlight, water, and carbon dioxide from the atmosphere into organic matter and oxygen.
The natural iron cycle introduces this element into oceans through various pathways. Dust storms, carrying iron-rich particles from land, deposit them onto the ocean surface. Hydrothermal vents on the seafloor also release dissolved iron. However, despite these sources, iron remains scarce in vast oceanic areas, thus restricting phytoplankton growth. This natural scarcity in HNLC regions underpins the hypothesis that adding iron could stimulate marine productivity.
How Iron Addition Aims to Capture Carbon
The proposed mechanism for ocean iron addition as a carbon sequestration method involves stimulating large phytoplankton blooms in HNLC regions. When iron is introduced into these iron-poor waters, it acts as a limiting nutrient, allowing phytoplankton populations to expand significantly. These expanded blooms then absorb substantial amounts of carbon dioxide from the atmosphere during photosynthesis, drawing it into the ocean’s surface waters.
This process aims to enhance the “biological pump,” a natural oceanic mechanism where carbon is transferred from the surface to the deep ocean. As phytoplankton in these blooms die or are consumed by zooplankton, their carbon-rich organic matter sinks. A portion of this sinking material can reach the deep ocean, effectively sequestering carbon from the atmosphere for extended periods, potentially hundreds to thousands of years.
Ecological Changes from Ocean Iron Addition
Large-scale ocean iron addition could induce significant changes within marine food webs. While it might initially benefit some species, such as certain zooplankton and fish that feed directly on the increased phytoplankton biomass, it could also cause shifts in species composition. Different phytoplankton species respond uniquely to iron enrichment, potentially leading to the dominance of less desirable or even harmful algal blooms. Such shifts could disrupt the delicate balance of existing ecosystems.
A major concern is the potential for oxygen depletion in deeper waters. As large phytoplankton blooms eventually die and sink, their decomposition by bacteria consumes dissolved oxygen, which can lead to areas of hypoxia (very low oxygen) or even anoxia (no oxygen). These low-oxygen zones can severely impact marine life, including fish and other organisms that require oxygen to survive, potentially harming local fisheries and overall marine biodiversity. Furthermore, nutrient cycling could be altered, with increased nutrient consumption in fertilized areas potentially leading to “nutrient robbing” downstream, thereby reducing productivity in distant ecosystems, including coastal fisheries.
Ongoing Research and Governance
Ocean iron fertilization currently remains largely an experimental concept rather than a widely implemented solution. Numerous small-scale research experiments have been conducted since the 1990s to assess its effectiveness and potential impacts.
Despite these studies, considerable scientific uncertainties persist regarding the long-term effectiveness of carbon sequestration, the permanence of carbon storage, and the full range of ecological consequences. International discussions and regulatory frameworks, such as the London Protocol, govern such large-scale geoengineering experiments due to their transboundary nature. In 2013, parties to the London Protocol agreed to limit ocean fertilization for research purposes, though this amendment is not yet legally binding for all parties. This field continues to be an area of active scientific investigation and policy debate.