Chemical oceanography is the study of the composition of seawater and the chemical processes that occur within the ocean environment. This interdisciplinary field draws on principles from chemistry, biology, physics, and geology to understand the global ocean. Chemical oceanographers investigate the distribution and behavior of chemical elements and compounds, from trace metals to dissolved gases, as they move through the marine system. They also study the exchange of materials between the ocean, the atmosphere, the seafloor, and the biosphere. This research provides fundamental insights into how the ocean regulates Earth’s climate and supports marine life.
Fundamental Chemical Properties of Seawater
The chemical character of the ocean is defined by its salinity, which is a measure of the total dissolved salts in the water. On average, open ocean salinity is about 35 parts per thousand (ppt). The six most abundant ions—chloride, sodium, sulfate, magnesium, calcium, and potassium—account for approximately 99 percent of all sea salts by weight.
Sodium and chloride ions are the most concentrated, making up about 85.7 percent of the total salinity. The relative proportions of these major ions remain nearly constant throughout the global ocean, a principle known as the “Rule of Constant Proportions”. This chemical balance is maintained by riverine input, biological uptake, and removal processes like mineral precipitation and hydrothermal venting.
Another defining feature is the ocean’s acidity, which is measured by pH and regulated by its alkalinity. The average pH of the ocean surface hovers around 8.1, giving it a slightly basic quality. The ocean contains a large reservoir of dissolved inorganic carbon, primarily bicarbonate and carbonate ions, which act as a powerful buffer system to resist large changes in pH.
Seawater also contains various dissolved gases exchanged with the atmosphere, including oxygen and carbon dioxide. Dissolved oxygen is necessary for the respiration of most marine organisms, with its concentration varying based on temperature, salinity, and biological consumption. Carbon dioxide is highly soluble in seawater and its presence is directly linked to the ocean’s buffering capacity and its ability to absorb atmospheric gases.
Global Biogeochemical Cycling
Chemical oceanography investigates the pathways of bioessential elements, such as carbon, nitrogen, and phosphorus, as they cycle through the Earth’s major reservoirs. The marine carbon cycle involves two main processes that sequester carbon from the atmosphere: the solubility pump and the biological pump. The solubility pump is a physical-chemical process where atmospheric carbon dioxide dissolves into surface waters and is transported to the deep ocean via circulation currents.
The biological pump involves marine organisms, particularly phytoplankton, that take up dissolved inorganic carbon through photosynthesis, converting it into organic matter. When these organisms die, their remains sink, carrying carbon to the deep sea and sediments, where it can be sequestered away from the atmosphere for millennia. This export of carbon is a significant factor in regulating global climate patterns.
Nitrogen and phosphorus are macronutrients that, along with silicon, regulate the productivity of marine ecosystems. Nitrogen, often a limiting nutrient, cycles through various forms like nitrate, nitrite, and ammonium, which are utilized by phytoplankton for growth. The nitrogen cycle also includes nitrogen fixation, which converts atmospheric nitrogen gas into usable forms, and denitrification, which returns nitrogen to the atmosphere.
Phosphorus cycles mainly as phosphate, released through the breakdown of organic matter and rapidly taken up by organisms. Silicon is primarily used by diatoms to construct their glass-like shells, linking the silicon cycle to the global carbon cycle. Understanding the ratios and availability of these nutrients, like the canonical Redfield ratio (106 Carbon: 16 Nitrogen: 1 Phosphorus), is fundamental to predicting marine primary production.
Deep-Sea Chemical Processes and Interactions
Chemical oceanographers investigate chemical processes that occur at the boundary between seawater and the solid Earth. One primary area is the chemistry associated with hydrothermal vents, geysers on the seafloor where superheated water circulates through the oceanic crust. Cold seawater seeps into cracks, reacts with hot rocks, and emerges as chemically altered fluid, often exceeding 300°C.
This process results in the removal of elements like magnesium and sulfate from seawater, while adding elements such as iron, manganese, and hydrogen sulfide, which are leached from the crustal rocks. The chemical energy from these reduced compounds fuels chemosynthetic bacteria in the dark deep ocean, forming the base of unique ecosystems. Hydrothermal circulation helps maintain the long-term chemical balance of the ocean by cycling the entire volume of seawater through the crust over millions of years.
Chemical exchange also occurs at the sediment-water interface, where the ocean floor acts as both a source and a sink for various elements. As organic matter decomposes in the sediments, nutrients like nitrogen and phosphorus are released back into the bottom water, a process known as benthic flux. This exchange is a significant factor in the ocean’s nutrient budget, particularly in coastal areas.
Over geological timescales, the slow weathering of continental and oceanic rock contributes dissolved solids to the ocean, influencing its long-term chemistry. For example, the weathering of silicate minerals on land consumes atmospheric carbon dioxide, which is a natural sink for climate regulation. Chemical oceanography tracks these deep interactions to understand the full scope of element movement throughout the planet.
Anthropogenic Influences on Ocean Chemistry
Human activities are significantly altering the fundamental chemistry of the ocean, with globally observable consequences. Ocean acidification results from the ocean absorbing a large fraction of the anthropogenic carbon dioxide released into the atmosphere. When \(\text{CO}_2\) dissolves in seawater, it forms carbonic acid, which lowers the water’s pH and reduces the concentration of carbonate ions.
Since the start of the Industrial Revolution, the average pH of the ocean surface has decreased by approximately 0.1 units. This change impacts marine organisms that rely on carbonate ions to build their shells and skeletons. Beyond \(\text{CO}_2\), the introduction of pollutants represents another major challenge, as heavy metals and persistent organic pollutants can accumulate in the marine food web, posing risks to marine life and human health.
The increasing presence of microplastics, tiny fragments of plastic debris, is a growing area of concern because these materials can absorb and transport toxic chemicals across the ocean. Furthermore, nutrient runoff from agriculture and wastewater, rich in nitrogen and phosphorus, can lead to eutrophication, especially in coastal waters. This excess nutrient load stimulates intense algal blooms. The subsequent decomposition of this organic matter by bacteria consumes oxygen and releases \(\text{CO}_2\), leading to localized acidification and the creation of hypoxic zones, or “dead zones”.
These interconnected chemical changes highlight the need for chemical oceanography to monitor and predict the future state of the marine environment. Research in this field is vital for quantifying the ocean’s uptake of carbon and understanding the implications of human activity.