Yukihiro Nojiri

Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects

Based on about 940,000 measurements of surface-water pCO2 obtained since the International Geophysical Year of 1956–59, the climatological, monthly distribution of pCO2 in the global surface waters representing mean non-El Nino conditions has been obtained with a spatial resolution of 4°×5° for a reference year 1995. The monthly and annual net sea–air CO2 flux has been computed using the NCEP/NCAR 41-year mean monthly wind speeds. An annual net uptake flux of CO2 by the global oceans has been estimated to be 2.2 (+22% or ?19%) Pg C yr?1 using the (wind speed)2 dependence of the CO2 gas transfer velocity of Wanninkhof (J. Geophys. Res. 97 (1992) 7373). The errors associated with the wind-speed variation have been estimated using one standard deviation (about±2 m s?1) from the mean monthly wind speed observed over each 4°×5° pixel area of the global oceans. The new global uptake flux obtained with the Wanninkhof (wind speed)2 dependence is compared with those obtained previously using a smaller number of measurements, about 250,000 and 550,000, respectively, and are found to be consistent within±0.2 Pg C yr?1. This estimate for the global ocean uptake flux is consistent with the values of 2.0±0.6 Pg C yr?1 estimated on the basis of the observed changes in the atmospheric CO2 and oxygen concentrations during the 1990s (Nature 381 (1996) 218; Science 287 (2000) 2467). However, if the (wind speed)3 dependence of Wanninkhof and McGillis (Res. Lett. 26 (1999) 1889) is used instead, the annual ocean uptake as well as the sensitivity to wind-speed variability is increased by about 70%. A zone between 40° and 60° latitudes in both the northern and southern hemispheres is found to be a major sink for atmospheric CO2. In these areas, poleward-flowing warm waters meet and mix with the cold subpolar waters rich in nutrients. The pCO2 in the surface water is decreased by the cooling effect on warm waters and by the biological drawdown of pCO2 in subpolar waters. High wind speeds over these low pCO2 waters increase the CO2 uptake rate by the ocean waters. The pCO2 in surface waters of the global oceans varies seasonally over a wide range of about 60% above and below the current atmospheric pCO2 level of about 360 ?atm. A global map showing the seasonal amplitude of surface-water pCO2 is presented. The effect of biological utilization of CO2 is differentiated from that of seasonal temperature changes using seasonal temperature data. The seasonal amplitude of surface-water pCO2 in high-latitude waters located poleward of about 40° latitude and in the equatorial zone is dominated by the biology effect, whereas that in the temperate gyre regions is dominated by the temperature effect. These effects are about 6 months out of phase. Accordingly, along the boundaries between these two regimes, they tend to cancel each other, forming a zone of small pCO2 amplitude. In the oligotrophic waters of the northern and southern temperate gyres, the biology effect is about 35 ?atm on average. This is consistent with the biological export flux estimated by Laws et al. (Glob. Biogeochem. Cycles 14 (2000) 1231). Small areas such as the northwestern Arabian Sea and the eastern equatorial Pacific, where seasonal upwelling occurs, exhibit intense seasonal changes in pCO2 due to the biological drawdown of CO2.

Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

The decline and fate of an iron-induced subarctic phytoplankton bloom

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the oceans interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

Basin scale estimates of sea surface nitrate and new production from remotely sensed sea surface temperature and chlorophyll

The highly variable nature of T-N relationships in oceanic waters has restricted nitrate (N) measurements from remotely sensed sea surface temperature (SST) to small time and space domains. Here we show that if changes in T-N relationships resulting from phytoplankton (chlorophyll a) are taken into account, remote sensing can be exploited to provide high resolution maps of sea surface nitrate (SSN) that are valid over much larger scales than has been previously possible. We illustrate the potential of the method for monitoring basin scale, interannual variations in SSN in the north Pacific Ocean using co-registered imagery of SST and chl a and demonstrate the usefulness of such data for estimating basin scale annual new production.

Distribution of methyl iodide, ethyl iodide, bromoform, and dibromomethane over the ocean (east and southeast Asian seas and the western Pacific)

Ambient concentrations of four marine-derived halocarbons (methyl iodide, ethyl iodide, bromoform and dibromomethane) and two man-made halocarbons (trichloroethylene and tetrachloroethylene) were measured during western Pacific cruises and east and southeast Asian cruises. Ethyl iodide was detected in the atmosphere for the first time and was identified as an atmospheric iodine source compound. Bromoform concentrations were positively correlated with those of dibromomethane, and methyl iodide showed variations similar to those of ethyl iodide. However, there was no correlation between the bromocarbons and the iodocarbons. The concentrations of methyl iodide and ethyl iodide changed more markedly, possibly owing to higher rates of photodecomposition of iodocarbons.

Correlations and emission ratios among bromoform, dibromochloromethane, and dibromomethane in the atmosphere

[1] Bromoform (CHBr 3 ), dibromochloromethane (CHBr 2 Cl), and dibromomethane (CH 2 Br 2 ) in the atmosphere were measured at various sites, including tropical islands, the Arctic, and the open Pacific Ocean. Up to 40 ppt of bromoform was observed along the coasts of tropical islands under a sea breeze. Polybromomethane concentrations were highly correlated among the coastal samples, and the ratios CH 2 Br 2 /CHBr 3 and CHBr 2 Cl/ CHBr 3 showed a clear tendency to decrease with increasing CHBr 3 concentration. These findings are consistent with the observations that polybromomethanes are emitted mostly from macroalgae whose growth is highly localized to coastal areas and that CHBr 3 has the shortest lifetime among these three compounds. The relationship between the concentration ratios CHBr 3 /CH 2 Br 2 and CHBr 2 Cl/CH 2 Br 2 suggested a large mixing/ dilution effect on bromomethane ratios in coastal regions and yielded a rough estimate of 9 for the molar emission ratio of CHBr 3 /CH 2 Br 2 and of 0.7 for that of CHBr 2 Cl/CH 2 Br 2 . Using these ratios and an global emission estimate for CH 2 Br 2 (61 Gg/yr (Br)) calculated from its background concentration, the global emission rates of CHBr 3 and CHBr 2 Cl were calculated to be approximately 820(±310) Gg/yr (Br) and 43(±16) Gg/yr (Br), respectively, assuming that the bromomethanes ratios measured in this study are global representative. The estimated CHBr 3 emission is consistent with that estimated in a very recent study by integrating the sea-to-air flux database. Thus the contribution of CHBr 3 and CHBr 2 Cl to inorganic Br in the atmosphere is likely to be more important than previously thought.

A one-dimensional ecosystem model applied to time-series Station KNOT

A vertically one-dimensional ecosystem model is applied to Station KNOT (Kyodo North Pacific Ocean Time series; 44°N, 155°E). This model has 15 compartments, including two categories of phytoplankton (diatoms and non-diatom small phytoplankton) and three categories of zooplankton (small, large and predatory zooplankton). Observed seasonal cycles of ecosystem dynamics at Station KNOT, such as surface nutrient concentrations and column-integrated chlorophyll-a, are successfully reproduced by the model. Observed significant seasonality of a total primary production is also reproduced, but its amount is overestimated by more than 50%, especially during summer. The simulated spring diatom bloom seems to occur 1 month earlier than in reality, considering dramatic decreases in the observed surface silicate in May and June, but it is impossible to determine the time of the actual bloom more precisely because of lack of data in April. Sensitivity studies for several important parameters are described. The Ekman upwelling velocity strongly affects the amount of the total primary production. Stoichiometry of silicon to nitrogen for diatoms strongly determines the amount of the primary production by small phytoplankton. Maximum photosynthetic rate for diatoms contributes to set both the timing and strength of the spring diatom bloom. Maximum rate of grazing on diatoms by large zooplankton controls the timing of the end of the spring diatom bloom and the strength of the autumn diatom bloom. While most of the parameters can be set to the same values as those for Station A7 (41.5°N, 145.5°E), in the Northwestern Pacific like Station KNOT, some values need to be modified. The modification of the parameter values may be partly caused by the difference in the ecosystem dynamics between Stations KNOT and A7. Observations of the mixed-layer depth and surface nutrient concentrations in March and April, when they reach their maxima, are strongly required to reduce uncertainty of the parameter values.

Production of methane from alasses in eastern Siberia: Implications from its 14C and stable isotopic compositions

[1] The radiocarbon ( 14 C) and stable isotopic ( 13 C and D) compositions of methane and carbon dioxide from alasses (typical landforms in permafrost terrain, consisting of lakes and wetlands) were measured around Yakutsk, eastern Siberia, where few isotope studies have been done. The carbon isotopic compositions of methane and carbon dioxide were used to study the pathways of methane formation in alasses. The mean value of methane and of carbon dioxide in each alass ranged from --63.9 to ∼58.2‰ and from -16.7 to -0.6‰, respectively. In small alasses, where the supply of fresh organic matter from surrounding wetland ecosystems is large, methane was produced almost equally from acetate fermentation and CO 2 reduction pathways. In larger alasses the CO 2 reduction pathway slightly dominates over acetate fermentation, owing to a smaller supply of labile organic matter. The 13 C enrichment in CO 2 in large lakes indicates depletion of methane precursors. Gas-bubble methane is depleted in deuterium (the mean 6D value from lakes is -360 ± 14%o and from shores is -380 ± 20%o), which reflects the deuterium-depleted environmental water (from -136 to -117‰) of alasses. Lake methane relatively enriched in D is interpreted to be the result of depletion in the hydrogen pool in lake sediments. Methane in shallow lakes is somewhat enriched in 14 C relative to modern biogenic methane and is produced from fresh, recently fixed organic matter. The 14 C content of methane from deeper lakes is 10-20% less than that of shallow lake methane, indicating a greater contribution from older methane derived from deeper parts of the sediment. The mean 13 C, D, and 14 C of methane from the alasses in general are estimated to be -61.1 ± 4.4‰, -363 ± 20%o, and 104 ± 6 pMC, respectively. This corresponds to the reported mean isotopic composition of methane from wetlands for δ 13 C, but shows lower value for δD and 14 C content.

Comparison of spatial distributions of argon species number densities with calcium atom and ion in an inductively coupled argon plasma

Abstract Spatial distributions of calcium atom and ion in an atmospheric pressure argon inductively coupled plasma ICP have been observed in absorption and emission measurements. The number densities of metastable argon and ground state calcium atom and ion in an ICP are estimated from the results. The spatial distributions of temperatures, electron number density, metastable argon number density, and calcium species number densities, are compared, and excitation mechanisms for argon and Ca + and Ca are discussed. Although the ionization temperature for argon is higher than other temperatures, the metastable argon number density agrees with that obtained by assuming an equilibrium at the ionization temperature corrected by taking into account the ionizations of hydrogen and oxygen. The excitation temperature is reduced almost to the gas temperature which is caused by the high collisional rate in the ICP. The thermal and non-thermal excitation mechanisms for analyte have been revealed with the spatial profiles of the emission intensities and absorbances of calcium atom and ion.

Simultaneous multi-element analysis by inductively-coupled plasma emission spectrometry utilizing micro-sampling techniques with internal standard

A micro-sampling technique, which requires sample volumes of less than 100 μl, is used for multi-element analysis by i.c.p. emission spectrometry. One drop of sample solution in a teflon cup is nebulized through a capillary tube. Internal standardization with yttrium improves the precision of measurement. The method is applied to the analysis of serum and whole blood samples, after dilution or digestion.