Susumu Honjo

A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals

Abstract In simulation studies of the oceans role in the global carbon cycle, predicting the depth-distribution for remineralization of particulate organic carbon (POC) is of particular importance. Following Sarmiento et al. (Global Biogeochemical Cycles 7 (1993) 417), most simulation models have the power-law curve of Martin et al. (Deep-Sea Research 34 (1987) 267) for this purpose. The Martin et al. curve is an empirical fit to data, most of which is from shallow floating sediment traps. Using such a fit implies that all the information necessary for prediction is contained in the carbon flux itself, so that the organic-carbon flux F OC ( z ) at any depth z can be predicted from the flux of organic carbon F OC ( z 0 ) at some near-surface depth z 0 . Here, we challenge this basic premise, arguing that fluxes of ballast minerals (silicate and carbonate biominerals, and dust) determine deep-water POC fluxes, so that a mechanism-based model of POC flux must simultaneously predict fluxes of both POC and ballast minerals. This assertion is based on the empirical observation that POC fluxes are tightly linked quantitatively to fluxes of ballast minerals in the deep ocean. Here, we develop a model structure that incorporates this observation, and fit this model to US JGOFS EqPac data. This model structure, plus the preliminary parameter estimates we have obtained, can be used to explore the implications of our model in studies of the ocean carbon cycle.

The distribution of oceanic coccolithophorids in the Pacific

Abstract Horizontal and vertical distributions of oceanic coccolithophorids were investigated along five traverses in the North and Central Pacific. The population was characterized by: 1. (1) a high standing crop in high latitude, 2. (2) an abrupt decrease in the temperature and subtropical areas except in the Kuroshio Water, and 3. (3) a significant increase in the equatorial and Kuroshio ares. When most abundant, there were as many as 1·7 × 10 5 individuals per liter of water in the −5 m level at 50°N 155°W, but at some deeper localities in the photic layer there were no cells at all, particularly in the south Philippine Sea. Approximately 90 species of the Coccolithophoridae were identified. Six coccolithophorid zones were established in the surface water along the 155°W meridian based on the distribution pattern of characteristics species: Subarctic, Transitional, Central North, Equatorial North, Equatorial South and Central South. The Subarctic Zone corresponded to the Pacific Subarctic Current, where the specimens were almost exclusively a subarctic variety of Emiliania huxleyi. Emiliania huxleyi , a cold variety and Rhabdosphaera clavigera were dominant in the Transitional Zone, which coincided with the North Pacific Current. The Central zones were dominated by Umbellosphaera irregularis . The North Equatorial Current and Equatorial Countercurrent comprise the Central North Zone where the largest variety of species was observed. The Equatorial zones includes the South Equatorial Current and were characterized by an abundance of three placolith-type species: Gephyrocapsa oceanica, Cyclococcolithina leptoporus and Cyclococcolithina fragilis . The Central South Zone coincides with the northern portion of the South Pacific circulation. Three vertical coccolithophorid zones were designated in the 200 m water column of the Transitional and Central zones, while the Subarctic and Equatorial zones had one and two vertical zones respectively. Umbellosphaera irregularis and Rhabdosphaera clavigera preferred shallow water while Umbellosphaera tenuis and Cyclococcolithina fragilis were mainly observed in the middle euphotic waters. Florisphaera profunda and Thorosphaera flabellata occurred only in lower euphotic waters. Water temperature and light intensity appeared to be most influential factors in the distribution of coccolithophorid species, but currents also delimited the distribution of some species.

Sedimentation of biogenic matter in the deep ocean

The major constituents of biogenic particles that settle through the water column of the ocean are carbonate tests, opaline shells, and particulate organic matter (cellular and amorphous). This paper describes the quality and quantity of such fluxes to the deep ocean and compares them with biogenic remains in the bottom sediment. Studies of samples collected during PARFLUX sediment trap experiments in the Atlantic and Pacific have shown that 60 to 90% of the total particulate flux is of biogenic origin; the contribution of biogenic materials decreases with increasing depth. Carbonate hard parts accounted for 30 to 60% and were the largest constituents in pelagic sediments at temperate and tropical areas. Combustible components ranged from 10 to 30% with zooplankton remains and fecal pellets accounting for the bulk of the organic flux. Amorphous fine particles were also significant in the organic flux. The atomic ratios of organic constituents in settling particles deviated systematically from the ‘Redfield ratio’ of 106:16:1 (C:N:P) for marine plankton. In the mesopelagic depths the ratio was 200:21:1, and in the bathypelagic depths the ratio was 300:33:1, with a wide range of variation in phosphorus. The residence time of biogenic particles in the deep-ocean water columns is relatively short and the particles can be expected to arrive at the abyssal floor without major dissolution and remineralization (excepting small opal particles). Fine organic particles such as cell remains and pigmented granules are a major source of organic carbon and nitrogen at deep traps. Microscopic study suggests that the fine organic particles were once included in larger but loosely-formed particles that settled rapidly. Such particles dispersed from settling fecal pellets or similar particles are reintroduced into the water column resulting in ‘secondary’ suspended particles. Such particles may eventually be remineralized while in suspension. The decomposition rate of organic carbon in the deep water was estimated to be about 2.2 mg C m−2 day−1, a rate consistent with rates of oxygen consumption estimated by other investigators. The bottom sediment is enriched in refractory lithogenic particles because of the remineralization of biogenic matter. A hypothetical benthic transition layer serves as a reservoir of benthic activity and can exist under certain conditions at the abyssal floor.

Export production of particles to the interior of the equatorial Pacific Ocean during the 1992 EqPac experiment

Abstract Twenty-four time-series, moored sediment traps were deployed between 2/2/92 and 1/27/93 along 140°W at 9°N, 5°N, 2°N, 0°, 2°S, 5°S and 12°S at water depths of approximately 1200 m and 2200 m, and 700 m above the bottom. The opening/closing of the traps was synchronized at 17-day periods, for 21 events, covering a total of 357 days. The average annual particle flux in the oceans interior (2.2 to 4.4 km deep) from 5°N to 5°S was 28.5 g m−2 year−1, with 34.8 g−2 year−1 the maximum annual flux at the equator. Sixty-six per cent of settling particles were carbonate; 24% biogenic SiO2 and 5% organic carbon. The onset of tropical instability waves, marking the years El Nino/post-El Nino boundary, was associated with a succession of intervals with greater organic carbon and opal at 5°N, 2°S and 5°S that occurred synchronously with a meridional oscillation of instability waves, while net carbon flux during El Nino and post-El Nino periods did not change. Although organic carbon flux increased at 5°N, 2°S and 5°S during the post-El Nino period, it was counterbalanced by decreases at the upwelling stations (2°N and the equator), resulting in no net carbon flux increase across the 5°N to 5°S region. In February/March 1992, only 0.34% of the organic carbon fixed by primary production over the 5°N to 5°S zone arrived in the oceans interior. In August/September that year, zonal average of organic carbon flux increased slightly to 0.5% of primary production. Very little carbon reached the interior depths of the upwelling stations; however, the fraction of export was higher at the 5°N, 2°S and 5°S stations. The pattern of variability of particle flux at the shallow depths was observed also in deeper traps, without temporal offsets, suggesting a settling particle residence time shorter than the 17-day timeseries resolution during most of this experiment.

Annual biogenic particle fluxes to the interior of the North Atlantic Ocean; studied at 34°N 21°W and 48°N 21°W

Abstract In order to clarify the annual quality, quantity and export processes of biogenic particles from the euphotic zone to the deep ocean interior, an array of automated time-series sediment traps were deployed for 1 year from 4 April 1989 to 17 April 1990 at 34°N 21°W and 48°N 21°W as part of the Joint Global Ocean Flux Program (JGOFS) North Atlantic Bloom Experiment (NABE). Three sediment traps with 13 time-series sediment collectors were placed at both stations approximately 1 and 2 km below the surface and 0.7 km above the bottom. They collected settling particles during 26 14-day intervals for 376 days with an 20-day hiatus in September-October 1989 for changeover of the trap moorings. The collection periods of the six traps were synchronized, forming a spatio-temporal matrix of 156 samples. The annual mass flux at about 2 km deep during this experiment was 22 and 27 g m−2 y−1 at the 34 and 48°N stations, consisting of biogenic particles with traceable quantities of lithogenic particle flux. The spring particle bloom, characterized by the sedimentation of particles relatively enriched by Norg, began in January at the 34°N station and in March at the 48°N station. The bloom continued for 4.5 and 3 months and provided 62 and 50% of the annual biogenic particle mass flux at 2 km at the 34 and 48°N stations. The surface bloom penetrated to the ocean interior within a few weeks, with apparently accelerated settling speed at deeper layers. The order of susceptibility of biogenic elements to mineralization while settling in the 0.7–1 km a.b. water column was, from least to most resistant: P, Norg, Corg, Si and Ca. The C/N/P ratio at 0.7 km a.b. was 154:18:1 at the 34°N station and 148:18:1 at the 48°N station.

Coccoliths: Production, transportation and sedimentation

Abstract The presence of coccolith ooze on the deep-sea floor and the well preserved suspended coccoliths in the undersaturated water column is explained by accelerated and communal sinking of coccoliths and coccospheres in small zooplanktons fecal pellets. The community structure in the euphotic layer will be replicated in the underlying thanatocoenosis with high resolution without significant time-lag between the production and the deposition. The euphotic biocoenosis of coccoliths drift only a few hundred kilometers at the maximum while they descend at the rate of more than 150 m per day through 5,000 m of water column where the variance of advection is approximately 5 km a day. At an Equatorial Pacific station it was estimated that 92% of coccoliths produced in the euphotic layer were thus being transported to the deep-sea bottom. Coccolithophore production provides several grams of calcite to a square meter of sea floor per year. Coccoliths do not undergo significant change during passage through copepods and other planktonic grazers. The membrane which covers a copepods fecal pellet is coherent particularly in cold water. It protects contents from spilling and from dissolution and accelerates sinking rates by providing a smooth surface. The membrane is biodegraded rapidly in warm water and its contents exposed. As such naked pellets sink, coccoliths are shed and suspended in aphotic water. The majority of freed coccoliths will be dissolved in the undersaturated water column before arriving at the bottom. Approximately 8% of coccoliths produced were estimated to be remineralized in the water column at the above-mentioned station if Petersons dissolution rate of calcite is applicable.

Biogenic barium fluxes to the deep sea: Implications for paleoproductivity reconstruction

Dymond et al. (1992) have recently proposed an algorithm to reconstruct paleoproductivity from biogenic Ba (bio-Ba) accumulation rates in sediments. Their equation is based on sediment trap data which indicate that Corg/bio-Ba ratios in settling particles are higher in the western Atlantic compared to the Pacific. From this observation they have suggested that the flux of bio-Ba to the seafloor may depend on dissolved Ba concentrations in intermediate and deep waters which are significantly higher in the Pacific compared to the Atlantic. Accordingly, they have introduced a factor related to dissolved Ba concentration in their equation as a variable which strongly influences paleoproductivity estimates. In an attempt to confirm the proposed dependency of bio-Ba fluxes to the seafloor on dissolved Ba concentrations in seawater we have compiled additional data on organic carbon and bio-Ba fluxes in the deep sea. These data confirm Dymond et al.s findings that settling particles have significantly higher Corg/bio-Ba in the western Atlantic compared to the Pacific. However, we also found lower ratios in traps deployed in the North Atlantic, similar to those found in the Pacific, while in the Panama Basin we found ratios as high as those in the western Atlantic. From these observations we conclude that dissolved Ba concentration is not an important factor in regulating the flux of bio-Ba to the seafloor. Instead, we propose that high Corg/bio-Ba ratios found in the western Atlantic, the Panama Basin, the Arabian Sea, and some stations in the Nordic Seas result from the addition of refractory organic carbon from nearby continents, shelves, or slopes. If that is confirmed, the algorithm proposed by Dymond et al. (1992) could be simplified and could provide a powerful means to estimate paleoproductivity. In addition, deviations from the Corg/bio-Ba ratios in settling particles could be used to estimate the input of continental or shelf-derived refractory organic matter into the deep sea.

Large aperture time-series sediment traps; design objectives, construction and application

Sediment traps with 0.5 and 1.15 m2 apertures which are capable of collecting 12–25 samples at programmed intervals, typically weekly or bi-monthly, during one continuous semi- to interannual deployment have been developed. They utilize a number of new synthetic materials and stable metallic components which ensure reliable, long-lasting performance at any oceanic depth. The key component of the trap is a set of sequentially rotating samplers which is driven by a microprocessor-controlled electronic stepping motor. The electronic power controller controls sampler exchange with a high degree of flexibility and precision, as well as independently recording the executed sampling events. Each sampling bottle is sealed from ambient water during the time samples are stored before recovery. After continuous improvement and modification during 29.5 deployment-years of application in deep ocean experiments since 1982, we are convinced that these sediment traps can provide a relatively large quantity of settling particles in time-series with high experimental reliability.

Comparison of isotopic composition of planktonic foraminifera in plankton tows, sediment traps and sediments

Planktonic foraminifera from plankton tows, sediment traps and sediments in the central North Atlantic were studied in order to understand what determines their oxygen and carbon isotope composition. A clear separation of species and genera on a δ18O vs. δ13C plot for all samples suggests that their isotopic composition is controlled to a certain degree by biological factors. Within a species population, the globorotaliids show a positive linear correlation between δ18O and δ13C, while the shallow-dwelling spinose species (mostly Globigerinoides species) do not show a definite trend. The latter species, when collected in plankton tows, often show slight negative deviations from isotopic equilibrium with respect to oxygen. All species deviate from carbon isotope equilibrium by −1.5 to −6‰. These deviations from equilibrium are probably caused by incorporation of isotopically light metabolic CO2 into the skeleton, which is enhanced by the activity of symbiotic algae. During their ontogeny the average weight per individual of most species increases which indicates that calcification continues to a depth of about 100 m. This additional skeleton (roughly 50% by weight) is isotopically heavier because temperatures are lower and photosynthesis of symbiotic algae stops below the photic zone. Therefore, the skeleton of foraminifera collected in sediment traps below 400 m has an overall oxygen isotope composition that seems to be in equilibrium for CaCO3 deposited in the upper 100 m.

Seasonal changes in the isotopic composition of planktonic foraminifera collected in Panama Basin sediment traps

Abstract Isotopic analyses have been made on five species of planktonic foraminifera collected in two deployments of PARFLUX Mark II time-series sediment traps in the Panama Basin. The automated sampling system on the traps provided 4 one-month samples from 29 July to 16 November 1979 and 6 two-month samples from December 1979 to November 1980. The δ18O values of Globigerinoides ruber and Globigerinoides sacculifer in this region are primarily affected by a low-salinity surface layer that forms during the early winter. These species each show a 1‰ total range in δ18O. The δ18O values of the colder-water species Globorotalia menardii, Neogloboquadrina dutertrei, and Globorotalia theyeri show smaller seasonal changes in δ18O. The δ13C values of G. ruber and G. sacculifer exhibit small seasonal changes (0.35 and 0.4‰ respectively) despite large seasonal changes in surface water productivity. The colder-water species exhibit only slightly larger changes in δ13C (up to 0.55‰) throughout the year. All colder-water species exhibit minimum or near minimum δ13C values during February and March, which is the period of maximum upwelling and primary productivity. Seasonal variations in the flux of foraminifera in the water column at this location will have only a small effect on the isotopic composition of the sediment assemblage; extreme values of δ18O and δ13C do not occur during the periods which are associated with the high flux of foraminiferal tests.