Tsuneo Ono

Temporal increases of phosphate and apparent oxygen utilization in the subsurface waters of western subarctic Pacific from 1968 to 1998

30-years time series of AOU and phosphate in the Oyashio sub-surface domain showed an increasing trend superimposed on bidecadal oscillation. AOU and phosphate trend on isopycnals between 26.7 and 27.2 σθ increased by an average of 0.9±0.5 and 0.005±0.003 µmol/kg/y, respectively. Salinity on these isopycnals also showed an average linear increase of 0.0008 psu/y. Salinity and density of winter mixed layer, on the other hand, was found to have decreased during the observation period. Observed bidecadal oscillation (average period 20±1 y) in subsurface AOU negatively correlated with that of North Pacific Index (r=−0.88±0.06). As the cause of the linear increase of subsurface phosphate and AOU, we speculated that vertical water exchange in the upper layers of the subarctic North Pacific might have been diminished during this period. A decreasing trend of salinity and density of winter mixed layer in Oyashio that was observed during the same period supported this speculation.

Increase in Total Carbonate in the Western North Pacific Water and a Hypothesis on the Missing Sink of Anthropogenic Carbon

Sea water samples were collected from various depths in the North Pacific (40–21°N) along 165°E in 1991. Their total carbonate (total dissolved carbonate species) contents were determined with random errors less than 0.2% by a coulometric method. The preformed carbonate contents defined by Chen (1982) were calculated from the obtained data and other observed data including potential temperature, salinity, dissolved oxygen and total alkalinity. The same calculation was done for the GEOSECS data obtained in nearly the same region in 1973. The difference between the two data sets reveals that the preformed carbonate has increased by 180±41 gC/m2 during the last 18 years. This value is comparable or somewhat larger than 150 gC/m2 obtained in the case that the ocean uptakes 3 GtC/yr for 18 years and distributes it equally among the world oceans. Based on the results, a hypothesis on the missing sink for the anthropogenic carbon dioxide is presented, in that the missing sink is the intermediate waters formed in the northern North Pacific and the Southern Ocean besides the deep waters formed in the North Atlantic and the Southern Ocean.

Probability of a reduction in the formation rate of the subsurface water in the North Pacific during the 1980s and 1990s

Comparing the apparent oxygen utilization (AOU) and apparent CFC tracer ages (τ) between extensive decadal reobservation data along 47°N (’85–’99) and 165°E (’87–’00) lines, we found that both AOU and τ markedly increased over the North Pacific between 26.4–27.4 σθ. The observed AOU increase was almost consistent with the AOU increase calculated from observed change of τ. Based on a linear trend of increasing AOU over 30 years (’68–’98) in the subpolar region [Ono et al., 2001], we concluded that the formation rate of the subsurface water in the North Pacific has continuously reduced at least during the last fifteen years. In the North Pacific, the recent uptake rate of oceanic anthropogenic carbon was also estimated as reduced by as much as 10% from the efficiency of anthropogenic carbon absorption in the middle of 1980s.

A preliminary study of carbon system in the East China Sea

In the central part of the East China Sea, the activity of CO2 in the surface water and total carbonate, pH and alkalinity in the water column were determined in winter and autumn of 1993. The activity of CO2 in the continental shelf water was about 50 ppm lower than that of surface air. This decrease corresponds to the absorption of about 40 gC/m2/yr of atmospheric CO2 in the coastal zone or 1 GtC/yr in the global continental shelf, if this rate is applicable to entire coastal seas. The normalized total carbonate contents were higher in the water near the coast and near the bottom. This increase toward the bottom may be due to the organic matter deposited on the bottom. This conclusion is supported by the distribution of pH. The normalized alkalinity distribution also showed higher values in the near-coast water, but in the surface water, indicating the supply of bicarbonate from river water. The residence time of the East China Sea water, including the Yellow Sea water, has been calculated to be about 0.8 yr from the excess alkalinity and the alkalinity input. Using this residence time and the excess carbonate, we can estimate that the amount of dissolved carbonate transported from the coastal zone to the oceanic basin is about 70 gC/m2/yr or 2 GtC/yr/area-of-global-continental-shelf. This also means that the rivers transport carbon to the oceans at a rate of 30 gC/m2/yr of the coastal sea or 0.8 GtC/yr/ area-of-global shelf, the carbon consisting of dissolved inorganic carbonate and terrestrial organic carbon decomposed on the continental shelf.

Size dependence of iron solubility of Asian mineral dust particles

[1] Asian mineral dust was sampled at Hokkaido, northern Japan, in spring 2004 and 2006. Iron solubility of the bulk aerosol, the size-segregated aerosol (0.45 4.7 μm). We suggest that an iron solubility of around 0.4% is typical for Asian mineral dust of large particles transported to Hokkaido. In the high-nutrient low-chlorophyll region of the western subarctic North Pacific near the Asian continent, where the mineral dust deposition is dominated by large particles, the iron solubility of the mineral dust entering the ocean is around 0.4%.

Distribution of total carbonate and related properties in the North Pacific along 30°N

Cross sections of the total carbonate (TC), titration alkalinity (TA), and pH were observed during the WOCE/JGOFS-P2 cruise (30°N, 130°E –122°W). The increases in TC in the years since 1973 are calculated by the comparison of TC data observed during P2 and those observed during the Geochemical Ocean Section Study. At stations east of 170°W, TC has increased proportionally to the atmospheric increase of CO2 in water shallower than the σθ = 26.20 density surface, while at stations west of 180°E TC had increased proportionally to the atmospheric increase only in water shallower than the σθ = 25.40 density surface. Increase in the column inventory of TC is 143±32 gC/m2 on average and does not significantly differ with longitude.

Southwest intrusion of 134Cs and 137Cs derived from the Fukushima Dai-ichi nuclear power plant accident in the Western North Pacific.

Enormous quantities of radionuclides were released into the ocean via both atmospheric deposition and direct release as a result of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. This study discusses the southward dispersion of FNPP-derived radioactive cesium (Cs) in subsurface waters. The southernmost point where we found the FNPP-derived (134)Cs (1.5-6.8 Bq m(-3)) was 18 °N, 135 °E, in September 2012. The potential density at the subsurface peaks of (134)Cs (100-500 m) and the increased water column inventories of (137)Cs between 0 and 500 m after the winter of 2011-2012 suggested that the main water mass containing FNPP-derived radioactive Cs was the North Pacific Subtropical Mode Water (NPSTMW), formed as a result of winter convection. We estimated the amount of (134)Cs in core waters of the western part of the NPSTMW to be 0.99 PBq (decay-corrected on 11 March 2011). This accounts for 9.0% of the (134)Cs released from the FNPP, with our estimation revealing that a considerable amount of FNPP-derived radioactive Cs has been transported to the subtropical region by the formation and circulation of the mode water.

Increase in the uptake rate of oceanic anthropogenic carbon in the North Pacific determined by CFC ages

We propose an approach to estimate the rate of increase of oceanic anthropogenic carbon inventory with CFC11 dating technique. This approach relies on the elapsed time from when the water lost contact with atmosphere as determined by CFC age. Furthermore, the assumption is made that it remains constant over a decadal time scale. Finally, we consider only the increase in anthropogenic carbon from one decade to another and not the entire change from the pre-industrial period to the present. The advantages and disadvantages of our approach are discussed. Using this approach, the spatial distributions of the rate of increase of the anthropogenic carbon inventory and the uptake rate of anthropogenic carbon in the North Pacific were obtained. The western North Pacific subtropical region exhibited a maximum in the rate of increase of the anthropogenic carbon inventory of more than 8 g C m -2 year -1 during 1988-1998, which was equivalent to 34% of the total uptake rate in the entire North Pacific. The net total uptake rate of anthropogenic carbon in the whole North Pacific increased with time and was 0.55 ± 0.09 Pg C year -1 during 1988-1998 indicating that the North Pacific absorbs 24% of the whole oceanic uptake of anthropogenic carbon.

Impacts of elevated CO 2 on particulate and dissolved organic matter production: microcosm experiments using iron-deficient plankton communities in open subarctic waters

Response of phytoplankton to increasing CO2 in seawater in terms of physiology and ecology is key to predicting changes in marine ecosystems. However, responses of natural plankton communities especially in the open ocean to higher CO2 levels have not been fully examined. We conducted CO2 manipulation experiments in the Bering Sea and the central subarctic Pacific, known as high nutrient and low chlorophyll regions, in summer 2007 to investigate the response of organic matter production in iron-deficient plankton communities to CO2 increases. During the 14-day incubations of surface waters with natural plankton assemblages in microcosms under multiple pCO2 levels, the dynamics of particulate organic carbon (POC) and nitrogen (PN), and dissolved organic carbon (DOC) and phosphorus (DOP) were examined with the plankton community compositions. In the Bering site, net production of POC, PN, and DOP relative to net chlorophyll-a production decreased with increasing pCO2. While net produced POC:PN did not show any CO2-related variations, net produced DOC:DOP increased with increasing pCO2. On the other hand, no apparent trends for these parameters were observed in the Pacific site. The contrasting results observed were probably due to the different plankton community compositions between the two sites, with plankton biomass dominated by large-sized diatoms in the Bering Sea versus ultra-eukaryotes in the Pacific Ocean. We conclude that the quantity and quality of the production of particulate and dissolved organic matter may be altered under future elevated CO2 environments in some iron-deficient ecosystems, while the impacts may be negligible in some systems.

Basin-scale pCO2 distribution using satellite sea surface temperature, Chl a, and climatological salinity in the North Pacific in spring and summer

[1] An empirical method is presented for the estimation of basin-scale distribution of partial pressure of carbon dioxide (pCO 2 ) in the North Pacific using satellite-derived sea surface temperature (SST), chlorophyll-a concentrations (chl a), and climatological sea surface salinity (SSS). In this approach, multiple regression equations were developed to compute mixed layer dissolved inorganic carbon (DIC) based on SST, SSS and Chl a, whereas mixed layer total alkalinity (TA) was linearly regressed with SSS. The DIC-SST relation exhibited three different slopes at SST 27.5°C. Therefore data have been grouped with reference to SST. Regression equations were developed for two seasons (spring and summer). The regression errors for DIC and TA were 10.5 and 5 μmol kg -1 , respectively. The pCO 2 was computed from the estimated DIC and TA using dissociation constants given by Mehrbach et al. (1973), refit by Dickson and Millero (1987). The derived pCO 2 agreed with the shipboard pCO 2 observations within an error of 17-23 μatm. The sensitivity test on the regression equations for DIC estimation indicated that SSS is the most influencing parameter, followed by SST and Chl a. Using the monthly average SST and Chl a fields derived from the Advanced Very High Resolution Radiometer (AVHRR) and SeaWiFS (Sea-viewing Wide Field of view Sensor), respectively, and climatological SSS, monthly basin-scale pCO 2 fields were computed. The statistical model derived pCO 2 results are in agreement with underway pCO 2 in the North Pacific. This study strongly suggests that satellite-based techniques are promising tools for estimation of pCO 2 fields on a basin scale but the associated error bars are larger than required to study anthropogenic carbon uptake by the oceans. Incorporation of more in situ shipboard data may help in refining the estimating equations and reducing the errors further.