The Rain of Particles and Earth’s Cycle

Susumu Honjo Senior Scientist, Geology and Geophysics Department

hytoplankton has provided “” and include pelletized feces of

Earth’s inhabitants with oxygen since early small animal . 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 “,” Integration of graze the phytoplankton, employing most of the is a critical mechanism in preventing what we now 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 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. 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 78ûN 0.4 30 a depth along a mooring. The collected particles BERING 58 N1.8 SEA û 20 10 JAN MAJAN 70ûN 0.5 are then recovered and weighed, and the vertical

10 flux of particles can be calculated as weight or

35 10 LOFOTEN 69ûN 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 û 10 E. ICELAND 65ûN 0.6 of carbon is usually highly seasonal and often

STATION 10 (2.3) 50 N PAPA û 1.1 episodic, a short-term measurement produces data 10 NABE 48ûN 1.0 that is only useful for limited special purposes. It is 10 EQPAC 9ûN 0.5 therefore critical to collect sediment in time series 10 NABE 34ûN 0.9 for at least a year. Though this is not as technologi- 10 EQPAC 5ûN 1.5 10 cally easy as this simple description may sound, BATS 32ûN 0.6 the time-series sediment trap array method, 10 EQPAC 2ûN 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 0û 1.2 -2 -1 Organic Carbon Flux (mg m day ) understanding of basinwide dynamics of the

10 10 WAST 14ûN 1.8 biological pump in relation to such global oceano- EQPAC 2ûS 1.3 graphic phenomena as El Niño and the Asian 10 10 CAST 15ûN 1.5 EQPAC 5ûS 1.0 monsoons. Sediment trap experiments have come to be one of the principal methods for understand- 10 10 EAST 16ûN 1.6 EQPAC 12ûS 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 COCCOLITHS (Silica Hard Parts) (Calcium Hard Parts)

Little Net Net Carbon Dioxide Air-Sea Uptake by Ocean Carbon Dioxide Carbon Dioxide Uptake by Ocean Exchange

Photosynthesis Photosynthesis 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 ( 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 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 in biogenic 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 . 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 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- 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 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 , 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 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. In Author and co-editor contrast, the amount of silica in the northern of this issue Sus North Atlantic is an order of magnitude less than Honjo looks over some of the in the sub-Arctic Pacific. thousands of In the northern North Atlantic, upper ocean scanning electron waters sink due to thermal exchange and are micrographs taken in replaced with nutrient-depleted surface waters his laboratory for supplied from the south. This results in a negative particle flux and other studies over the silica mass balance in the upper layers in this area last 30 years. and allows coccolithophorids to take over the niche

Tom Kleindinst

6 • FALL/WINTER 1997 of organic carbon. However, when a molecule of calcium carbonate is formed in the upper ocean as Export Flux of Organic Carbon: Northern Hemisphere Basins part of a coccolith or planktonic foraminifer , a

molecule of CO2 is also formed by this chemical reaction. Therefore, there is no net removal of CO2 2 carbon from the upper oceans by the settling of °N ° 50 inorganic carbon as calcium carbonate. 40 1

Secretion of calcium carbonate EQ

meter per year 0 in the upper ocean by grams per square

180 organisms reduces ° alkalinity, resulting in an environment less capable of absorbing atmospheric

CO2. Therefore, a Jack Cook Carbonate Ocean does not play a large role in removal of atmospheric CO2. The significance of the biological pump in a Carbonate Ocean is principally concerned with the Organic carbon flux long term CO2 balance in Earth’s atmosphere (see measured over at figure on page 5). least one year with As soon as they leave the upper ocean where Major funding sources for Sus Honjo’s sediment trap work time-series sediment traps in the Northern they are produced, almost all of the coccoliths and have been the National Science Foundation and the Office of Naval Research. Hemisphere in grams foraminifera settle to the deep ocean floor within a per square meter per short time. There the oxidation of organic matter During his long research career at WHOI, Sus Honjo has year. This figure integrates data that begins in the upper ocean causes an increas- concentrated on understanding the processes and rates of ocean and the removal of carbon in the form of carbon derived from ingly acidic condition, which results in dissolution dioxide from the atmosphere to its deep ocean “sink.” He recalls sediment traps around the world, of the coccoliths and foraminifera. Indeed, trembling with excitement when he looked into a microscope including WHOI researchers find it strange to retrieve a large aboard R/V Knorr to examine the first sample collected by a deep PARFLUX results. quantity of perfectly preserved coccoliths and ocean sediment trap in the central North Atlantic: Abundant Note the huge export foraminifera tests from a sediment trap that has fecal pellets produced by surface organisms proved his hypothesis flux in the Arabian that the majority of ocean sediment is delivered to the deep been moored above a deep Pacific seafloor that Sea and relatively seafloor by large, rapidly settling aggregates. His global research large annual flux in consists of nothing but red clay and a few frag- efforts to link upper and deep ocean processes bridge marine the northwestern mented calcareous remains. geology, ocean chemistry, and biology and enhance understand- Pacific. The The deep ocean water, whose increased alkalinity ing of the biogeochemistry of the ocean. As we prepared this issue, “biological pump” is due to the dissolution of calcium carbonate, he was sending cruise reports from the near works efficiently in Antarctica where he was leading a team of “trappers” and these areas compared moves in such deep currents as the thermohaline seafloor sediment research groups from WHOI and other US to the mid ocean. driven circulation that runs south from the institutions in rough seas aboard NSF’s ice breaker/research northern North Atlantic, proceeds as a bottom vessel Nathaniel B. Palmer. current through the Indian Ocean, and eventually upwells in the northern North Pacific (See Oceanus The ratio of silicon to calcium in organism- Vol. 39, No. 2). This mass of formerly deep water assimilated particles collected by deep ocean Distributions of that has moved to the surface is thought to be a sediment traps is an indicator of the rate of Biogenic Silicon/Calcite major absorber of atmospheric CO2. This mecha- removal of carbon-dioxide carbon from the upper nism is often called the Alkalinity Pump (different ocean to the deep-ocean “sink.” This ratio Ratios in Export Fluxes: from the Biological Pump). Note that this process is indicates there are two types of ocean areas depending on which element is dominant (see Northern too slow to affect the CO2 cycle of today’s Earth—it figure on page 5). Hemisphere takes several centuries for the ocean’s deep-water Basins circulation system to complete a turnover. Our 2

°N present climate is greatly dependent on ° 50 foraminifera that lived, died, and 40 1 EQ dissolved when Vikings mole ratio 0

roamed the ocean! In 180 ° other words, calcium carbonate that sinks to the deep corrosive ocean is a “savings deposit”

regulating future greenhouse Jack Cook effects of planet Earth well into the next millennium.

OCEANUS • 7