The Rain of Ocean Particles and Earth's Carbon Cycle

By Honjo, Susumu | Oceanus, Fall-Winter 1997 | Go to article overview

The Rain of Ocean Particles and Earth's Carbon Cycle


Honjo, Susumu, Oceanus


Phytoplankton photosynthesis has provided Earth's inhabitants with oxygen since early life began. Without this process the atmosphere would consist of carbon dioxide (C[O.sub.2]) plus a small amount of nitrogen, the atmospheric pressure would be 60 times higher than the air we breathe, and the planet's air temperatures would hover around 300 [degrees] C. (Conditions similar to these are found on Earth's close sibling Venus).

As phytoplankton grow through the process of photosynthesis, they fix C[O.sub.2] in their cells as organic carbon and thus absorb atmospheric C[O.sub.2] into the upper ocean layers. Animal plankton graze the phytoplankton, employing most of the organic carbon as their energy source and oxidizing the rest of it back to C[O.sub.2], which eventually returns to the atmosphere. A small portion of this photosynthetic carbon escapes the oxidation process by settling, or "sedimenting," through the water column in particles, which are often called "marine snow" and include pelletized feces of small animal plankton. C[O.sub.2] carbon is also transported to the deep ocean in another way: Some plankton, such as coccolithophorids, planktonic foraminifera, and pteropods produce beautiful calcite and aragonite shells or tests that sink toward the seafloor when the organisms die.

Once the organic carbon and calcium carbonate particles reach the ocean interior at a depth of a few kilometers, they are "stored" there and will not return to the atmosphere for a relatively long period of time. This complex carbon-transporting ocean process, often called the "biological pump," is a critical mechanism in preventing what we now know as the "greenhouse effect," the collection of gases in the atmosphere that hinders upward transport of heat.

Understanding of Earth's carbon cycle is one of humankind's great scientific questions. Sediment traps are an important tool for studying the spatial and temporal variability of sinking particles (and carbon) in the ocean. The idea behind these devices is very simple: Vertically settling particles are collected at a specific area during a specific time period by providing a stable collection area at a depth along a mooring. The collected particles are then recovered and weighed, and the vertical flux of particles can be calculated as weight or volume per unit area in a unit of time - milligrams per square meter per day. Because the export flux of carbon is usually highly seasonal and often episodic, a short-term measurement produces data that is only useful for limited special purposes. It is therefore critical to collect sediment in time series for at least a year. Though this is not as technologically easy as this simple description may sound, the time-series sediment trap array method, together with multidisciplinary ocean measurements, has recently brought large leaps in the understanding of basinwide dynamics of the biological pump in relation to such global oceanographic phenomena as El Nino and the Asian monsoons. Sediment trap experiments have come to be one of the principal methods for understanding global C[O.sub.2] cycles in the ocean.

We now have 15 years of time-series, sediment-trap data collected from the interior of the world's open oceans through the collaborative effort of an international group of scientists, including the WHOI PARFLUX group. We are finally beginning to understand the pattern of basin-to-basin export-flux variability and to make intelligent estimates of the flux of C[O.sub.2] carbon in particulate matter to the ocean's interior. A sediment trap collects not only carbon products but also other kinds of particles: For example, we have found that the export flux of biogenic silica produced by plankton with siliceous frustules and tests provides important information for understanding another type of basin-scale biological pump (see Arabian Sea article on page 24).

By integrating sediment trap based measurements ([ILLUSTRATION OMITTED] opposite), we estimate the global flux of carbon to the ocean's interior at 0. …

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