Ocean Biogeochemistry

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Ocean Biogeochemistry

Biogeochemistry is the study of the interactions of the biology, chemistry, and geology of the Earth. In the case of a large body of water such as the ocean, biogeochemistry can be thought of as a huge experiment or set of reactions. Instead of happening in a clean glass beaker, the reactions have the ocean floor as the container.

The surface of the water is open to the air, and every day more dust and dirt from land blows over the ocean and falls in. Moreover, the surface of the water contains many small plant forms that are continually growing and being consumed by animals that are themselves consumed by other animals.

As this life and death drama continues, the scraps and leftovers drift downward towards the ocean floor like a snowfall; hence the name "marine snow." (See the photograph on page 130.) Around the edges of the ocean, rivers empty water and sediment . Deep in the ocean, mud-dwelling creatures await the arrival of their next meal from the falling biological debris (marine snow). These events are linked to each other, to the history of life on Earth, and to variations in Earth's climate.

Cycles

Scientists who study biogeochemistry usually consider the cycling of materials through the different parts of the system. To do this, they deal with reservoirs of materials and the fluxes of a substance from one reservoir to another. For example, they examine reservoirs such as the surface ocean water versus the deep ocean water, or the transfer of masses of materials per unit time (fluxes). An example of this kind of approach to biogeochemical cycles in the ocean can be seen in the following figure, where the reservoirs represented are the atmosphere, lithosphere , terrestrial (land-based) biosphere, surface ocean, phytoplankton , and deep ocean. The figure shows the global carbon cycle, a network of interrelated processes that transports carbon between different reservoirs on Earth.

Most scientific study has focussed on the carbon cycle. Carbon, after all, is the basis of life on Earth, and its gaseous form, carbon dioxide, is linked to the greenhouse effect and changes in Earth's climate over time. For these reasons, understanding the carbon cycle has been the focus of several large research programs supported by the U.S. government. Three examples include:

the U.S. Global Change Research Program (USGCRP): a joint project to design a carbon cycle research program; funded by the Department of Energy; the National Aeronautic and Space Administration, the National Oceanic and Atmospheric Administration, National Science Foundation, and U.S. Geological Survey;
Global Ocean Ecosystems Dynamics (GLOBEC): a major research program funded by the National Science Foundation to determine how global change affects the marine ecosystem and what the feedbacks to the physical climate system will be; and
the Global Carbon Program (GCP): a study funded by the NationalOceanic and Atmospheric Administration to improve scientists' ability to predict the fate of human-derived carbon dioxide and future concentrations of atmospheric carbon dioxide.

Other substances also have well-studied cycles. Water, of course, is constantly moving into, through, and out of the ocean. Some of the atmospheric gases such as oxygen and carbon dioxide are vitally important to life. Nutrient elements such as nitrogen, phosphorus, and silicon are necessary to the phytoplankton, and form the basis for the oceanic food web .

A Cycling Example.

The presence of life forms on Earth is tremendously important in the cycling of elements through the major reservoirs. Consider the ocean as an example: If one focuses on the impact of a single diatom on the ocean, the following story emerges.

Diatoms are a group of algae living by the millions in each cubic centimeter of surface ocean water. There each alga has access to the sunlight needed for photosynthesis; the CO₂ (carbon dioxide), N (nitrogen), and P (phosphorus) needed to make its soft tissue; the Si (silicon) needed for its shell-like covering; and a number of rare or trace substances in sea water, including Cu (copper) and Fe (iron). To reproduce, it undergoes cell division. Its life processes produce O₂ (oxygen) that can be used by other organisms; organic tissue that becomes food for the next higher creatures in the food web; and often an exudate or slime.

Once the diatom has been consumed by an animal (a copepod, for example), its life is over, but its effect on the ocean is not. The copepod digests and derives energy from the diatom's soft tissue, then packages the remains into a fecal pellet that is discharged as waste to become part of the falling debris (marine snow) headed for the ocean floor.

The pellet lands on the ocean floor, forming a site for bacteria to live as well as food for them to consume. The inorganic part of the diatom that remains (the silica shell) will begin to dissolve on the way to the ocean bottom, and Si taken out of the surface water is returned to deeper water as the shell dissolves. Decomposition of sinking organic matter by bacteria returns N, C, and P to the water and removes dissolved O₂.

Carbon.

Ocean water itself is changed by life processes. During the growth of diatoms and the consumption of diatoms by zooplankton , carbon is removed from ocean water and in turn from the atmosphere as the diatoms use it to grow. The transfer of this carbon toward the ocean floor and its partial burial in the sediments is often referred to as the carbon pump; it is one of the processes that slow the accumulation of CO₂ in the atmosphere.

Silicon.

The silicon (Si) used in the diatom shell enters the ocean from rivers, from the hot springs along mid-ocean ridges and by diffusion from deep-sea sediments. Diatoms remove Si so efficiently from the ocean surface water that it is a very scarce element there, and mixing and upwelling processes are necessary to redistribute enough Si back to the surface to provide for diatom growth. For that reason, Si as well as N, P, and other biologically important elements are in low concentration in surface water of the ocean, and increase with depth, as shown in the figure above.

Oxygen.

Another consequence of ocean biogeochemistry can be seen in the distribution of O₂ (oxygen) with depth (see figure above). The oxygen content at the surface is relatively high (about 6 milliliters per liter) and is replenished from the air. Deeper in the water, the O₂ content begins to decrease with depth, until at about 1,000 meters (3,082 feet), the value reaches a minimum. The reason for the decrease is the consumption by bacteria of the rain of organic debris (marine snow) falling through the water. The process requires O₂, and below the surface there is no immediate source to return the O₂ being used up.

The exact amount of O₂ at the O₂ minimum varies with location in the ocean; below the minimum, O₂ content begins to increase again with depth. The increase is related to water circulation in the ocean. The deep water in the ocean starts out at the surface in polar regions, where it becomes very dense because of the extreme cold, and sinks to great depths in the ocean, carrying with it dissolved oxygen from the surface waters. This cold, dense, deep water flows along the ocean floor close to the bottom, well beneath the depths of the O₂ minimum. These factors combine to give the observed shapes of O₂ profiles in the ocean.

Hydrothermal Processes.

There are other processes that play a role in determining the nature of the ocean. For example, hydrothermal activity at mid-ocean ridges results in significant changes in the chemistry of ocean water. The water that comes out of these hot springs comes from normal deep-ocean water that runs down into deep cracks on the ocean floor alongside the ridges. As the water penetrates into the oceanic crust, it becomes heated to very high temperatures, and reacts with the rocks. The water that comes out of the vents is very hot; contains sulfide (S⁻) instead of sulfate (SO₄²⁻); contains no Mg (magnesium) or O₂ (oxygen); and contains large amounts of Si (silicon). Because the entire volume of the ocean circulates through the mid-ocean ridge system every 10 million years, these changes are of great significance to the oceans and the organisms that live in them.

see also Hot Springs on the Ocean Floor; Mid-Ocean Ridges; Ocean Chemical Processes; Oceanography, Biological; Oceanography, Chemical; Oceanography, Geological; Sea Water, Physics and Chemistry of; Volcanoes, Submarine.

Martha R. Scott