Masashi Kusakabe

Interdecadal variability in dissolved oxygen in the intermediate water layer of the Western Subarctic Gyre and Kuril Basin (Okhotsk Sea)

Using dissolved oxygen (DO) concentration at isopycnal surfaces we have observed an oscillation in the intermediate water layer of the Western Subarctic Gyre (WSG) and the Kuril Basin of the Okhotsk Sea with a period of about 20 years. The interdecadal oscillation in DO is strongly correlated with the intensity of the Aleutian Low pressure cell with a 2-year time lag. The interdecadal variations in DO in the intermediate water layer of the WSG and the Okhotsk Sea may be a result of oscillation in the atmospheric forcing, leading to variations in the isopycnal fluxes of DO at the boundaries of the Okhotsk Sea-Western Subarctic Gyre-subtropical region, the isopycnal- surface replacement in the WSG, and in changes in the intermediate water formation rate in the Okhotsk Sea.

Vertical distributions of iron(III) hydroxide solubility and dissolved iron in the northwestern North Pacific Ocean

Detailed vertical distributions of Fe(III) hydroxide solubilities and dissolved Fe concentrations, which are strongly related to the concentration and affinity of natural organic Fe(III) chelators in seawater, were measured at three typical stations in the northwestern North Pacific Ocean. Iron(III) hydroxide solubility in the surface mixed layer was generally high and variable (0.3-2.4 nM), corresponding with the depth of high chlorophyll a concentrations, and the solubility minima (0.2-0.4 nM) occurred at 75-125 m depth. The vertical profiles of Fe(III) hydroxide solubility in mid-depth and deep waters are characterized by mid-depth maxima (0.6-0.7 nM) and, subsequently, a slight decrease to 0.4-0.5 nM with depth, which is markedly similar to nutrient and dissolved Fe depth profiles. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters are controlled primarily by the Fe(III) complexation with natural organic ligands, which were released through the oxidative decomposition and transformation of biogenic organic matter in mid-depth and deep waters.

Radiocarbon of sediment trap samples from the Okinawa trough: lateral transport of 14C-poor sediment from the continental slope

Abstract Radiocarbon of carbonate (PIC) and of organic carbon (POC) in sediment trap samples from the Okinawa trough was measured by AMS. Concentrations of 14 C in PIC and POC ( Δ 14 C -PIC and Δ 14 C -POC) ranged from approximately +40‰ to −80‰ and average over the entire 2 years was approximately −32‰. These values are much lower than Δ 14 C values of dissolved inorganic carbon ( Δ 14 C -DIC) in the upper 200 m of the water column (+100‰ on average). Δ 14 C -PIC and Δ 14 C -POC showed seasonal variability over 2 years with lower values in winter and higher values in summer. In the 1994–1995 period, Δ 14 C -PIC was also low in spring. Variations in Δ 14 C -PIC and Δ 14 C -POC were positively correlated with concentrations of inorganic and organic carbon, respectively, and negatively correlated with concentration of Al. This suggests that variability in Δ 14 C -PIC and Δ 14 C -POC were associated with the input of lithogenic materials. Assuming Δ 14 C -PIC and Δ 14 C -POC for two end members (settling particles produced in the overlying water column, and laterally transported materials produced outside of the overlying water column, which originated from the continental slope of the East China Sea), contributions of laterally transported materials to the sediment trap samples were estimated for each collecting period. The contribution of laterally transported materials ranged from approximately 50%–90% and the annual average of flux of old carbon was ca. 5 mg m−2 day−1 in 1993 and 10 mg m−2 day−1 in 1994–1995.

Evidence for the offshore transport of terrestrial organic matter due to the rise of sea level: The case of the East China Sea Continental Shelf

Accelerator mass spectrometry (AMS) 14 C dating and analyses of the stable isotope ratio of organic carbon (δ 13 C org ) were carried out on two piston cores obtained from the continental slope and the Okinawa Trough in the East China Sea (ECS). These cores covered the last ca. 15 14 C kyr BP (17 to 18 cal kyr BP). Mass and organic carbon (org. C) accumulation rates decreased gradually after the last deglaciation period. On the other hand, δ 13 C org showed distinct depletion from ca. 14 to 12 14 C kyr BP (17.5 to 13.2 cal kyr BP). The depletion relates to the large supply of terrestrial org. C from the ECS shelf triggered by the seas intrusion onto the shelf area because of the rise in sea level.

Biogeochemical cycles in the East China Sea: MASFLEX program

As a part of Japanese Joint Global Ocean Flux Study (JGOFS) of IGBP/SCOR, marginal sea flux experiments in the West Pacific (MASFLEX) were performed in the East China Sea. This project was also a part of Japan–China Joint Program on material flux in the East China Sea (MAFLECS) and the first land–ocean interactions in the coastal zone (LOICZ) study within IGBP. The project began in April 1992 and continued for 5 years under the support of Science and Technology Agency of Japan (STA). Its main goal was to clarify a role of marginal and coastal seas in the biogeochemical cycles of carbon, nitrogen and other substances relating to global change. A multidisciplinary approach made it possible to elucidate the highly complex nature of biogeochemical cycle of materials in the marginal sea.

137Cs concentration in zooplankton and its relation to taxonomic composition in the western North Pacific Ocean

To study the role of zooplankton in the transport of (137)Cs in the ocean, zooplankton samples were collected in October 2005 and June 2006 in the western North Pacific Ocean. The peak zooplankton biomass was observed in the surface layer, and gelatinous plankton was more abundant in October 2005 than in June 2006 reflecting exchange of water masses. The concentrations of (137)Cs in zooplankton varied from 11 to 24 mBq kg wet(-1) and were higher in October 2005 than in June 2006. The elevated abundance of gelatinous zooplankton probably led to higher concentration of (137)Cs in zooplankton in October 2005. Annual export fluxes of (137)Cs by ontogenetic vertical migrant copepods were estimated to be 0.8 and 0.6 mBq m(-2) year(-1) at 200 and 1000 m depths, respectively; this suggested that transport of (137)Cs by zooplankton may be no trivial pathway.

Effects of the Anticyclonic Eddies on Water Masses, Chemical Parameters and Chlorophyll Distributions in the Oyashio Current Region

Data from the R/V Mirai cruise (May–June 2000) have been examined to discover how mesoscale processes associated with eddy dynamics direct affect the water masses, the distributions and the vertical fluxes of the dissolved oxygen, nutrients and dissolved inorganic carbon in the western subarctic Pacific. Using maps of the temperature, salinity, dissolved oxygen, nutrients, chlorophyll and sea-air pCO2 difference we show that the boundaries of the anticyclone eddies in the study region were composed of high productivity coastal Oyashio water. The coastal waters were wrapped around the anticyclone eddies (thus creating a high productivity belt) and intruded inside of them. Using SeaWifs data we demonstrate that temporal variations in the position and the strength of anticyclone eddies advected the Kuril island coastal high productivity waters to the pelagic part, resulting in temporal variations of the chlorophyll in the Oyashio region. Computed vertical fluxes of the dissolved oxygen (DO), inorganic carbon (DIC) and silicate show that the anticylonic eddies in the Kuroshio-Oyashio Zone are characterized by enhanced vertical fluxes of the DO and DIC between the upper (σθ = 26.7–27.0) and lower (σθ = 27.1–27.5) intermediate layer, probably due to the intrusions of the Oyashio waters into the eddies.

Methane in Izena Cauldron, Okinawa Trough

Methane in the deep water of Izena Cauldron (maximum depth: ca. 1650 m) at the east side of mid-Okinawa Trough was studied by casting a CTD system with 12 Niskin bottles for water sampling at 11 stations inside and outside the cauldron. The water contained much methane up to 706 nmoles/l. The depths of maximum concentration varied widely from station to station, indicating the existence of a considerable number of vents emitting methane and heat. The waters containing less methane formed a straight line in theT-S diagram, while those containing more methane were more largely deviated from the line. The temperature anomaly was virtually proportional to the methane concentration, suggesting that the oxidation rate of methane inside the cauldron is negligibly small and methane can be used as a tracer of the cauldron water. The relation and the estimated vertical diffusivity gave the following fluxes. The emissions of methane and heat out of the bottom below 1450 m turn out to be 1400 moles/day and 7×1010 cal/day, respectively. The total emission rates inside the cauldron are presumed to be about twice the above values. The turnover time of methane has been estimated to be 240 days, which is also that of heat generated from the bottom and probably that of the bottom water.

Bomb radiocarbon invasion into the northwestern North Pacific

Invasion of the bomb radiocarbon was investigated in the northwestern North Pacific, including the subarctic (Oyashio region), the subtropical (Kuroshio region), and the subarctic/subtropical mixing (transition) areas. After the GEOSECS Pacific expedition in 1973, the bomb radiocarbon decreased in the Oyashio region and increased in the Kuroshio region. The former was caused by rapid ventilation of the Oyashio water. The latter was owing to accumulation of the bomb radiocarbon in the intermediate water of the Kuroshio region. The reverse trends in the temporal variation of radiocarbon between the Oyashio and the Kuroshio regions lead to virtually small temporal change of the bomb radiocarbon in the transition area where the two waters mix. The temporal changes of the bomb radiocarbon suggest that the North Pacific Intermediate Water is formed in the northwestern North Pacific.

Plutonium isotopes and 241Am in surface sediments off the coast of the Japanese islands before and soon after the Fukushima Dai-ichi nuclear power plant accident

The concentrations of 239+240Pu, 241Pu and 241Am in the sediments at the coast of Japanese islands from 2008 to 2011 varied widely from one sampling site to another and were generally lower in sandy sediments at shallower sites, and higher in clayey sediments at deeper sites. In contrast, there seemed to be no temporal variation in the concentrations during the survey period. The 241Am/239+240Pu activity ratio was relatively higher than the global fallout presumably owing to the ingrowth of 241Am from the ancestor, 241Pu originating mainly from the hydrogen bomb explosion tests in the Marshall Islands in the 1950s.