Yutaka W. Watanabe

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.

Tritium in the Japan Sea and the renewal time of the Japan Sea deep water

Watanabe, Y.W., Watanabe, S. and Tsunogai, S., 1991. Tritium in the Japan Sea and the renewal time of the Japan Sea deep water. Mar. Chem., 34: 97-108. In 1987 in the Japan Sea, the mean concentration of tritium below 200 m depth was !.2 T.U. (Tritium Unit= 10 ~8 times of the molar fraction of tritium in ordinary hydrogen (T/H X 10 iS) ), which was one third of that in the surface water from 0 to 200 m depth (3.6 T.U.). Tritium was even found near a depth of 2000 m at two stations; 0.5 and 0.9 T.U., values which were outside the analytical error ( _+0.3 T.U.). These results indicate that vertical mixing is more rapid in the Japan Sea than in the Pacific Ocean where the concentration of tritium decreases steeply with depth and is less than the detection limit for water below 1000 m depth. By introducing these data and the previously obtained 226Ra data into a three-box model, the turnover time of the Japan Sea deep water and the residence time of the water within the Japan Sea were calculated to be about 100 years and 1000 years, respectively. The exchange coefficient of CO2 at the air-sea interface was also estimated to be 1.4--3.3 m day-~ by coupling these data with the 14C data in the three-box model. This value which is smaller than the world mean value is probably due to the fact that the region producing the Japan Sea deep water of the northern Japan Sea near the Siberian coast has rather low windspeeds.

Consistency and synthesis of Pacific Ocean CO2 survey data

Between 1991 and 1999, carbon measurements were made on twenty-five WOCE/JGOFS/OACES cruises in the Pacific Ocean. Investigators from 15 different laboratories and four countries analyzed at least two of the four measurable ocean carbon parameters (DIC, TAlk, fCO2, and pH) on almost all cruises. The goal of this work is to assess the quality of the Pacific carbon survey data and to make recommendations for generating a unified data set that is consistent between cruises. Several different lines of evidence were used to examine the consistency, including comparison of calibration techniques, results from certified reference material analyses, precision of at-sea replicate analyses, agreement between shipboard analyses and replicate shore based analyses, comparison of deep water values at locations where two or more cruises overlapped or crossed, consistency with other hydrographic parameters, and internal consistency with multiple carbon parameter measurements. With the adjustments proposed here, the data can be combined to generate a Pacific Ocean data set, with over 36,000 unique sample locations analyzed for at least two carbon parameters in most cases. The best data coverage was for DIC, which has an estimated overall accuracy of ∼3 μmol kg−1. TAlk, the second most common carbon parameter analyzed, had an estimated overall accuracy of ∼5 μmol kg−1. To obtain additional details on this study, including detailed crossover plots and information on the availability of the compiled, adjusted data set, visit the Global Data Analysis Project web site at: http://cdiac.esd.ornl.gov/oceans/glodap.

Chlorofluorocarbons in the central North Pacific and southward spreading time of North Pacific intermediate water

From August to October 1992, cross sections of Chlorofluorocarbons (CFCs) trichlorofluoromethane (CFC-11) and dichlorofluoromethane (CFC-12) were obtained between 48°N and 15°S along 175°E in the central North Pacific. On the basis of the distribution of CFCs, apparent ages of the North Pacific Intermediate Water (NPIW) along 175°E were determined for the isopycnal horizons of 26.40 to 27.40 σθ. The apparent age of NPIW was usually older toward the south and with increase of σθ along the section. The oldest NPIW on all isopycnal horizons in the North Pacific was located near 10°N. North of 20°N, NPIW on isopycnal horizons less than 26.80 σθ outcrops in winter in subpolar regions and appears to have formed after 1970. It is suggested that NPIW on isopycnal horizons less than 26.80 σθ is laterally well mixed between subpolar and subtropical regions on timescales of a few decades. NPIW on isopycnal horizons greater than 26.80 σθ near 37°N was older than its surroundings. It is suggested that new NPIW formed in the subpolar region is not transported directly southward and that the major current of NPIW is zonal in the central North Pacific, flowing eastward in the subpolar region and then turning more to the south, flowing toward 10°N near the eastern boundary. Thus we also estimated the zonal mean apparent ages of NPIW. The mean ages of NPIW suggested that the apparent southward spreading rates and the apparent southward spreading times of NPIW from the subpolar region to 10°N were approximately 80–170 km yr−1 and 20–45 years, respectively. It is concluded that the southward spreading time of NPIW is a few decades at most.

Size and taxonomic plankton community structure and carbon flow at the equator, 175‡E during 1990–1994

Abstract Size and taxonomic structure of plankton community carbon biomass for the 0.2–2000 μm equivalent spherical diameter range were determined at the equator at 175°E in September 1990–1993 and April 1994. Total biomass of the plankton community ranged from 1944 to 3448 mg C m −2 . Phytoplankton, zooplankton and bacteria carbon biomasses were 604–1669 mg C m -2 , 300–797 mg C m 2 , and 968–1200 mg C m -2 , and the percentages were 31–54%, 15–26%, and 29–54%, respectively. Biomass of heterotrophic bacteria was always the largest fraction and Prochlorococcus biomass was second. Heterotrophic and autotrophic flagellates and dinoflagellates in the nanoplankton size range and copepods (adults and copepodites) in the mesoplankton range were also high. Relatively small biomass was observed in the microplankton size range. The differences in integrated biomass of plankton community for El Nin˜o type oligotrophic conditions of September 1990–1993 and non-El Nifio type mesotrophic conditions of April 1994 were generally small compared with the interannual difference during 1990–1993. However, the percentage of Prochlorococcus in phytoplankton carbon biomass was larger in non-El Nin˜o year. Biomasses of cyanobacteria, diatom, dinoflagellates, nauplii of copepods, and crustaceans other than copepods were larger in the non-El Nin˜o year. Primary production increased significantly from El Nin˜o to non-El Nin˜o years. Carbon flow through the plankton food chain was estimated using the plankton carbon biomass data, primary production measurements, and published empirical relationships.

Dynamics of the Japan Sea Deep Water Studied with Chemical and Radiochemical Tracers

Abstract Chemical and radiochemical tracers have been used to make clear the dynamics of the Japan Sea deep water including its chemical alteration. They are the Si-O combination, tritium, CFCs (trichlorofluoromethane and dichlorodifluoromethane), C-14 and Ra-226. The Si-O diagram method has revealed a small areal variation of the Japan Sea deep water except water in a small basin surrounded by the Yamatotai Rise and its coastal upwelling off Japan. The tritium and Ra-226 combination method is applied to a box model with three layers and four boxes, which is based on the vertical profiles of tracer components including conservative and nutrient elements. The result shows that the turnover time of the Japan Sea water for vertical mixing is about 100 yrs and the residence time of the whole Japan Sea water except the warm Tsushima current water is about 1000 yrs, indicating that the Tsushima current flowing through the Japan Sea makes little interaction with the Japan Sea Proper Water. The difference between the apparent C-14 age of the Japan Sea deep water of 300 years and the turnover time is originated in the delay in the air-sea gas exchange equilibrium of carbon dioxide. The gas transfer velocity has been calculated to be 1.5 m/day. The vertical profiles of CFCs also support the estimated water exchange rates between the reservoirs, but our present data are insufficient to explain their small apparent gas exchange rates less than 0.1 m/day. The oxygen consumption rate of the Japan Sea deep water is nearly equal to that of the Pacific deep water indicating that the Japan Sea is not fertile, while the regeneration rate of silica in the Japan Sea is much larger than that in the Pacific.

Adsorption—desorption control of phosphate in anoxic sediment of a coastal sea, Funka Bay, Japan

Abstract Sediment core samples were taken once a month from July 1980 to September 1981 at a station in Funka Bay (92-m depth) for the determination of phosphate, silicate and alkalinity in interstitial water. A remarkable seasonal variation was found for interstitial phosphate, that is, distinct maxima appeared in spring (March—April), just after a phytoplankton bloom which brought a large amount of settling particles to the bottom, and in summer (July—August) when the water was stratified and the dissolved oxygen content of the bottom water decreased due to the decomposition of organic matter. The high interstitial phosphate concentration was always accompanied by a sharp increase in alkalinity, indicating sulfate reduction. This large seasonal variation in interstitial phosphate cannot be explained by in situ decomposition of organic matter and/or the diffusive loss of interstitial phosphate. A more likely explanation is adsorption and desorption of interstitial phosphate coincident with the depth of the active sulfate reduction layer.

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.

Recent increase of DIC in the western North Pacific

Abstract The temporal variation of the total dissolved inorganic carbon (DIC) content in the western North Pacific is investigated by comparing the DIC distribution obtained from the data sets of three different periods, the GEOSECS data observed in 1973, the CO 2 dynamics Cruise data observed in 1982, and recent Japanese data sets observed during the early 1990s. The overall feature of the signal of temporal DIC change during 1973 and early 1990s agreed with that of former studies, and did not significantly change with the calculation scheme (the grid-selection method vs. the multiple regression method). The observed increase in DIC among the different time scales showed a good inner consistency, which also indicates the stability of the method used in the DIC change calculation. The apparent rate of increase of the DIC inventory in the upper 1000 m water column, however, differed significantly by the data set used for the calculation: It was 5.6±2.4 g C/m 2 /year, based on the data comparison between 1982 and the early 1990s, while it became 7.6±2.4 g C/m 2 /year when based on the data between 1973 and the early 1990s. This result provides us an information about the data-dependency on the former estimation of temporal DIC change.

Tritium in the northwestern North Pacific

Vertical profiles of tritium in seawater were determined for samples collected during the period from 1988 to 1990 at fourteen stations in the northwestern North Pacific (the Oyashio region) including the Okhotsk Sea and the Bering Sea. The profiles usually had a maximum in the surface layer and decreased gradually with depth down to 1,000 m. The water column inventory of tritium averaged 63% of the total atmospheric input in this region.The horizontal distribution of tritium showed a maximum in the region facing the Okhotsk Sea near 45°N for every isopycnal surface ofσ0 ranging from 26.60 to 27.40. The ages of the intermediate water were calculated for the respective isopycnal surfaces in the maximum region. This calculation assumed that the intermediate water was formed by the isopycnal mixing of two water masses—the Okhotsk Sea and the Bering Sea Component Waters, which had been produced in wintertime by the diapycnal mixing of the surface and the deep waters in the respective marginal seas. The results show that the intermediate water in this region was formed in the late 1980s for the water which hasσ0 of 26.60 to 26.80 and about 1970 for the water which hasσ0 of 27.00 to 27.40. Although we have estimated the mean ages of the intermediate water, the horizontal profile of dissolved oxygen suggests that the Okhotsk Sea Component Water is younger than the mean age.