The Rain of Ocean Particles and Earth’s Carbon Cycle Susumu Honjo Senior Scientist, Geology and Geophysics Department hytoplankton photosynthesis has provided “marine snow” and include pelletized feces of Earth’s inhabitants with oxygen since early small animal plankton. CO2 carbon is also trans- P life began. Without this process the ported to the deep ocean in another way: Some atmosphere would consist of carbon dioxide (CO2) plankton, such as coccolithophorids, planktonic plus a small amount of nitrogen, the atmospheric foraminifera, and pteropods produce beautiful pressure would be 60 times higher than the air we calcite and aragonite shells or tests that sink breathe, and the planet’s air temperatures would toward the seafloor when the organisms die. hover around 300°C. (Conditions similar to these Once the organic carbon and calcium carbon- are found on Earth’s close sibling Venus). ate particles reach the ocean interior at a depth of As phytoplankton grow through the process of a few kilometers, they are “stored” there and will photosynthesis, they fix CO2 in their cells as not return to the atmosphere for a relatively long organic carbon and thus absorb atmospheric CO2 period of time. This complex carbon-transporting into the upper ocean layers. Animal plankton ocean process, often called the “biological pump,” Integration of graze the phytoplankton, employing most of the is a critical mechanism in preventing what we now sediment trap based organic carbon as their energy source and oxidiz- know as the “greenhouse effect,” the collection of measurements from a ing the rest of it back to CO2, which eventually gases in the atmosphere that hinders upward variety of locations returns to the atmosphere. A small portion of this transport of heat. allows investigators photosynthetic carbon escapes the oxidation Understanding of Earth’s carbon cycle is one of to estimate the global flux of carbon to the process by settling, or “sedimenting,” through the humankind’s great scientific questions. Sediment ocean’s interior. water column in particles, which are often called traps are an important tool for studying the spatial and temporal variability of sinking particles (and Organic Carbon Export Fluxes carbon) in the ocean. The idea behind these North Pacific Ocean North Atlantic Ocean devices is very simple: Vertically settling particles Organic Carbon Flux Organic Carbon Flux (milligrams per cubic meter per day) (milligrams per cubic meter per day) are collected at a specific area during a specific 40 10 time period by providing a stable collection area at FRAM 78˚N 0.4 30 a depth along a mooring. The collected particles BERING 58 N1.8 SEA ˚ 20 10 JAN MAJAN 70˚N 0.5 are then recovered and weighed, and the vertical 10 flux of particles can be calculated as weight or 35 10 LOFOTEN 69˚N 1.4 volume per unit area in a unit of time—milligrams OKHOTSK 10 53 N 1.7 per square meter per day. Because the export flux SEA ˚ 10 E. ICELAND 65˚N 0.6 of carbon is usually highly seasonal and often STATION 10 (2.3) 50 N PAPA ˚ 1.1 episodic, a short-term measurement produces data 10 NABE 48˚N 1.0 that is only useful for limited special purposes. It is 10 EQPAC 9˚N 0.5 therefore critical to collect sediment in time series 10 NABE 34˚N 0.9 for at least a year. Though this is not as technologi- 10 EQPAC 5˚N 1.5 10 cally easy as this simple description may sound, BATS 32˚N 0.6 the time-series sediment trap array method, 10 EQPAC 2˚N 1.3 0 J F MAMJ J ASOND together with multidisciplinary ocean measure- 10 Arabian Sea ments, has recently brought large leaps in the EQPAC 0˚ 1.2 -2 -1 Organic Carbon Flux (mg m day ) understanding of basinwide dynamics of the 10 10 WAST 14˚N 1.8 biological pump in relation to such global oceano- EQPAC 2˚S 1.3 graphic phenomena as El Niño and the Asian 10 10 CAST 15˚N 1.5 EQPAC 5˚S 1.0 monsoons. Sediment trap experiments have come to be one of the principal methods for understand- 10 10 EAST 16˚N 1.6 EQPAC 12˚S 0.3 ing global CO2 cycles in the ocean. 0 0 J F MAMJ J ASOND We now have 15 years of time-series, sediment- J F MAMJ J ASOND Jack Cook 4 • FALL/WINTER 1997 DIATOMS COCCOLITHS (Silica Hard Parts) (Calcium Carbonate Hard Parts) Little Net Net Carbon Dioxide Air-Sea Uptake by Ocean Carbon Dioxide Carbon Dioxide Uptake by Ocean Exchange Photosynthesis Photosynthesis Calcification Carbon Organic Carbon Organic Bicarbonate Carbon dioxide Dioxide Carbon Dioxide Carbon + Calcium + + Oxygen + Oxygen Calcium Carbonate (Shell) Sinking Particle Flux Respiration Organic Respiration Organic Dissolution Carbon dioxide Carbon Carbon Carbon Carbon Bicarbonate + Dioxide + Oxygen dioxide + Oxygen + Calcium Calcium Carbonate (Shell) (Promotes Carbonate Dissolution) thanks Dan McCorkle) to Jack Cook (and SILICA OCEAN CARBONATE OCEAN (Nutrient Rich (Nutrient Poor Surface Waters) Surface Waters) trap data collected from the interior of the world’s organic carbon and 0.44 gigaton in inorganic Conditions that open oceans through the collaborative effort of an carbon (calcium carbonate). The global calcium in influence removal of carbon-dioxide international group of scientists, including the carbonate and silicon in biogenic opal export carbon from WHOI PARFLUX group. We are finally beginning fluxes are 2.9 and 1.4 gigatons per year, respec- atmosphere to deep to understand the pattern of basin-to-basin tively. Interannual fluctuation of global export ocean are illustrated export-flux variability and to make intelligent production is unknown, as we are far from here and discussed in the article. estimates of the flux of CO2 carbon in particulate understanding the interannual changes in overall matter to the ocean’s interior. A sediment trap primary production. collects not only carbon products but also other Our 15-year export flux data set reveals distinct kinds of particles: For example, we have found that “biogeochemical regions” where the biological the export flux of biogenic silica pump operates in two Fecal pellets are produced by plankton with significantly different modes about the size of a siliceous frustules and tests known as “Carbonate Ocean” comma (,). provides important information and “Silica (Opal) Ocean” (see for understanding another type of figure above). A Carbonate Ocean is basin-scale biological pump (see Arabian Sea defined as the condition in which sinking particles article on page 24). deliver more carbonate than silica to the ocean By integrating sediment trap based measure- interior, as well as more inorganic than organic ments (figure opposite), we estimate the global flux carbon. In a Silica Ocean, these ratios are reversed. of carbon to the ocean’s interior at 0.8 gigaton, To date, these conditions are only observational, nearly one billion tons, per year—0.35 gigaton in and we are anxious to determine the reasons for OCEANUS • 5 from the diatoms because coccolitho- phorids need no dissolved silica to grow. On the other hand, the upper layers of the sub- Arctic Pacific are enriched by the up- welling of dissolved silica. A large flux of biogenic opal particles is supplied to the relatively shallow but vast shelves and slopes in the Sea of Okhotsk and the Bering Sea and is recycled to the upper waters, generating the positive feedback loop of a “silica trap.” Thus the ecosystem is essentially occupied by diatoms, and relatively scarce coccolithophorids Susumu Honjo appear only when the Transmision electron this partition in the ocean. Global distribution of surface ocean is temporarily depleted of silica at the microscope image dissolved silica and other essential nutrients, end of the export-flux bloom. (enlarged some 3,000 times) of a section of which is controlled by very large scale ocean The biological pump operating in a Silica Ocean a fecal pellet circulation, might be the deciding factor in this removes CO2 carbon from the upper to the deep collected by a partition. The majority of present seas are Carbon- ocean in the form of particulate organic carbon on sediment trap in the ate Oceans while Silica Oceans account for less a short time scale of a few to several weeks (see North Atlantic. It than 20 percent of the world ocean. The export Arabian Sea article on page 24). The biological includes undigested, phytoplankton cell flux of organic carbon in a Silica Ocean is usually pump in a Carbonate Ocean also removes carbon organelles, including higher than that in a Carbonate Ocean because from the atmosphere and upper oceans in the form chloroplasts. The their biological pumps function differently: blue (artificially Photosynthesis of diatoms, whose shells are largely colored) areas composed of silica, is the mechanism for primary represent remains of coccoliths that were production in a Silica Ocean, and photosynthesis eaten by a zoolankter of coccolithophorids, whose shells are largely and passed composed of calcium carbonate, is the primary unaffected through producer in a Carbonate Ocean. the gut. Coccoliths At present, one of the most typical Silica are composed of ° nearly pure calcite, Oceans is the sub-Arctic Pacific north of 45 N, an the heaviest mineral area that includes the Bering Sea, the Sea of produced by marine Okhotsk, and the northern East Sea (Japan Sea). organisms, which By contrast, most of the North Atlantic is a makes a fecal pellet Carbonate Ocean. Nutrient availability is the major heavy enough to settle rapidly through oceanographic difference between these seas. the water column. Layers rich with dissolved nutrients underlie the sub-Arctic Pacific at about 600 meters; at 2,000 meters the dissolved silica maximum is reached.