Marine chemistry

Marine chemistry, also known as ocean chemistry or chemical oceanography, is the study of the chemical composition and processes of the world’s oceans. It looks at how seawater interacts with the atmosphere, the seafloor, and living organisms. This field covers many important topics, such as the movement of elements like carbon, nitrogen, and phosphorus, and how tiny metals and gases behave in the ocean.

Marine chemistry helps us understand big natural cycles on Earth, how ocean currents move, and how human actions, like pollution and changes in the climate, affect the oceans. It is shaped by things like the movement of Earth’s plates, the flow of water, sediments, and even the balance of acids and bases in seawater.

Because the ocean connects many parts of our planet, scientists who study marine chemistry often work on problems related to physical oceanography, geology, biology, and atmospheric science. They are especially interested in how carbon moves through the ocean, which is important for understanding how the ocean absorbs carbon from the atmosphere and how this affects marine life. Other key areas include studying the chemistry of ocean water, pollution in the marine environment, and how human activities are changing the climate.

Organic compounds in the oceans

DOM, or dissolved organic matter, is very important for the ocean. It includes tiny molecules like amino acids, sugars, and lipids, and makes up about 90% of all organic carbon in the sea. Some of this carbon can stay in the ocean for hundreds of years, helping control how much carbon is stored there. Marine life creates many special compounds that are found mostly in the oceans.

POM, or particulate organic matter, consists of larger pieces of organic material like tiny plants, animal waste, and bits of dead material. As these particles sink through the water, tiny creatures break them down, releasing nutrients and carbon dioxide. Some of this material reaches the ocean floor and helps store carbon for a very long time.

Chemical ecology of extremophiles

The ocean is home to special kinds of living things called extremophiles. These organisms can live in very harsh places, such as hydrothermal vents, black smokers, cold seeps, and salty areas called sea ice brine pockets. Some scientists think life might have started near these hot underwater vents.

In places like hydrothermal vents, many extremophiles get their energy from chemicals instead of sunlight. These vents release chemicals like elemental sulfur, H₂, H₂S, Fe²⁺, and methane. Tiny organisms use these chemicals to make energy through special reactions, and they become food for other creatures, creating unique ecosystems.

Some extremophiles live in the very cold and salty pockets inside sea ice. They must adjust to big changes in salt levels when the ice melts. Many of these organisms can make their own food using sunlight, but the air around them can become too filled with oxygen, so they produce special substances to stay safe.

Plate tectonics

Seafloor spreading on mid-ocean ridges is a big system that exchanges ions. Hydrothermal vents at these spreading centers add iron, sulfur, manganese, silicon, and other elements into the ocean. Some of these elements get recycled into the ocean crust. Helium-3, a special kind of gas from deep inside the Earth, comes out of these vents and can be found in water plumes in the ocean.

The speed at which mid-ocean ridges spread changes between 10 and 200 mm each year. Faster spreading causes more reactions between basalt and seawater. This changes the amount of magnesium and calcium in the water. When spreading is fast, more magnesium is taken from the water and put into the rocks, and more calcium comes out of the rocks into the water. This makes the water favor forming a certain type of calcium carbonate called low-Mg calcite.

When spreading is slow, the opposite happens, and the water favors forming another type of calcium carbonate. The type of calcium carbonate that sea creatures build their skeletons from depends on these chemical changes in the ocean, which are controlled by how fast the seafloor spreads.

Human impacts

Marine pollution

Climate change

Further information: Effects of climate change on oceans § Chemical effects

When we burn fossil fuels, it releases a lot of carbon dioxide into the air. This gas gets absorbed by the oceans, changing their chemistry. Global warming also affects the oceans, along with changes in how salty the water is, which can impact the plants and animals that live there.

Acidification

Deoxygenation

History

Early studies of marine chemistry focused on understanding why the ocean is salty, with scientists like Robert Boyle exploring this question. Modern chemical oceanography started with the Challenger expedition from 1872 to 1876, which made the first detailed measurements of ocean chemistry. Researchers John Murray and George Forchhammer analyzed these samples, helping us learn more about important elements in seawater like chloride, sodium, and sulfate.

In the early 1900s, new tools allowed scientists to improve their measurements. Martin Knudsen invented the Knudsen Bottle to collect water from different ocean depths. In recent decades, especially since the 1970s, new instruments and computer models have transformed the field. Scientists now study tiny amounts of metals, organic materials, and isotopes with great accuracy. This helps us understand big issues like global climate change, ocean acidification, and deoxygenation, using tools like remote sensing and global observation programs.

Tools used for analysis

Scientists who study ocean chemistry use many special tools to measure the chemicals in seawater. They use tools like pH meters, conductivity meters, and fluorometers. They also use ships with equipment called CTDs to measure how salty, warm, and deep the water is. These ships can collect water samples for later testing.

They also use advanced tools like mass spectrometers to find tiny amounts of different elements and chemicals. Recently, they have started using underwater robots and satellites to watch the oceans over long periods, especially to see how the ocean is changing with things like ocean acidification.

Marine chemistry on other planets and their moons

The chemistry of the subsurface ocean of Europa might be similar to Earth's oceans. The subsurface ocean of Enceladus releases hydrogen and carbon dioxide into space.