Shinichiro Noriki

Compositional change of settling particles with water depth in the Japan Sea

Abstract Settling particles at five different water depths (890–3240 m) and a box core sample were collected in September 1984 in the Japan Sea (40°49.4′N, 138°40.7′E; 3350 m deep), and analyzed for major components and 25 elements. According to the vertical changes in elemental concentrations (Me) and elemental concentration ratios to aluminum (Me/Al), these elements are grouped into four types: (1) the elemental concentration increases with water depth and the Me/Al ratio remains almost constant vertically (Al, Sc, La, Th, Hf, V, Ta, K, Rb, and Cs); (2) both the elemental concentration and the Me/Al ratio decrease with depth (I, Ba, Ca, Sr); (3) both the elemental concentration and the Me/Al ratio increase with depth (Mn); and (4) the elemental concentration remains almost constant or increases a little with depth and the Me/Al ratio decreases with depth (As, Sb, Se, and Ag). These four types are named refractory, biogenic, scavenged, and biogenic-scavenged, respectively; the fourth type is recognized here for the first time. Fe, Co, Zn, and Br show maximum Me/Al ratios within the water column but are grouped as scavenged (Fe and Co) and biogenic-scavenged (Zn and Br) elements. The concentration changes of elements in the settling particles at 3240 m relative to the surface sediment suggest that elements except those of the refractory type are regenerated or liberated on or near the sea floor.

Particulate fluxes and major components of settling particles from sediment trap experiments in the Pacific Ocean

Abstract Total particulate fluxes in the deep water column, as measured with sediment traps, were 6–82 mg m−2 day−1 in the subtropical central Pacific of Hawaii and the eastern Pacific off California, 300–420 mg m−2 day−1 in the western Pacific off Japan, and 790–1200 mg m−2 day−1 in the Antarctic Ocean. The fluxes of CaCO3 particles observed at the five stations did not vary widely from station to station. The opal contents, on the other hand, increased with the total particulate fluxes, with the highest observed opal content of the settling particles being 80% in the Antarctic Ocean. Assuming that the clay fraction in the settling particles is refractory, our results show that the regeneration of biogenic particles occurs mainly in the bottom water and at the sediment surface.

Removal of trace metals from seawater during a phytoplankton bloom as studied with sediment traps in Funka Bay, Japan

Abstract To study biological effects on the particulate removal of chemical elements from seawater, sediment trap experiments were carried out successively ten times throughout the spring phytoplankton bloom in Funka Bay. Sediment traps were deployed every one to two weeks at 1, 40 and 80 m depths. The settling particles obtained were analyzed for trace metals, phosphate and silicate. The propagation of diatoms in spring results in larger particulate fluxes than that of dinoflagellates. The biogenic silicate concentration is higher in the earlier period, when diatoms are predominant, than in the subsequent period, when dinoflagellates are predominant. The concentrations of aluminum, iron, manganese and cobalt in the settling particles comprising largely biogenic particles are lower during phytoplankton bloom. The concentration of copper is not reduced by the addition of biogenic particles, and its vertical flux is approximately proportional to the total flux, indicating that its concentration in the biogenic particles is nearly equal to that in the non-biogenic particles. The results for nickel and lead show the same tendency as for copper. Cadmium is more concentrated in biogenic particles than in non-biogenic particles, and the concentration of cadmium in the settling particles decreases with depth, similarly to phosphate and organic matter. Thus, metals in seawater are segregated by biological affinities, and the degree of incorporation into biogenic particles is in the order Cd > Pb, Ni, Cu > Co > Mn, Fe, Al. Biogenic particles are the most important agent controlling the vertical distribution of metals in the ocean. They remove the metals from the surface water, transport them through the water column, and regenerate them in the deep.

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%.

Organic matter fluxes and the sites of oxygen consumption in deep water

Abstract Sediment trap experiments at various stations in the Pacific and Antarctic Ocean compare observed particulate organic carbon fluxes with those obtained indirectly from vertical profiles of dissolved oxygen in the Pacific deep water. The observed carbon fluxes are characterized by large spatial variation and small vertical variation. The organic carbon fluxes at the 1000 m level ranged from 2 mg C m −2 d −1 in the subtropical ocean to more than 100 mg C m −2 d −1 in the highly productive subpolar sea, and decreased by 25 ± 10% at intervals 1000 m in depth. These results suggest that much particulate matter is transferred rapidly to the bottom of comparatively small areas of the polar, subpolar, hemipelagic and coastal seas and degraded there, and that the bottom water imprint resulting from the effects of degradation of particulate matter is transported fairly quickly to the pelagic ocean by isopycnal mixing and advection. Many unsolved phenomena occurring in the deep ocean can be explained by this suggestion.

Isotopic composition and morphology of living Globorotalia scitula: a new proxy of sub-intermediate ocean carbonate chemistry?

Abundance, isotopic composition and morphological imprints of the planktonic foraminifera Globorotalia scitula (Brady) were closely examined for possible use as a novel reconstruction tool of chemical environments in sub-intermediate depth seawater in the past. Based on the MOCNES plankton tow observation of dwelling depths of G. scitula and the isotopic compositions together with hydrochemistry data, the empirical relations between isotopic disequilibria in carbon (Δδ13C=δ13CG. scitula−δ13CDIC) and oxygen (Δδ18O=δ18OG. scitula−δ18Ow) isotopes in the carbonate tests and the seawater δ18O and δ13C of dissolved inorganic carbon (DIC), respectively, are introduced. The morphological information such as pore density and porosity is also examined for significant relations to carbonate chemistry. Shell porosity is strongly correlated saturation state of calcite. The dissolution of living G. scitula tests may promote the observed isotopic differences as well as the increases in porosity. Δδ18O of G. scitula is found effectively to be linear function of both water temperature and calcite saturation state (Ω), and thereby temperature equation for G. scitula is provided, while Δδ13C of G. scitula is a linear function of only calcite saturation state. The equation was validated by using Globorotalia scitula collected by a sediment trap in intermediate water depths. Satisfactory agreements were found between observed and calculated Δδ18O from the empirical equations based on temperature and hydrochemistry data at sediment trap deployment site, indicating that the equation may be useful in paleo-environmental reconstruction of sub-intermediate water. The sediment trap observation further suggests that the abundance of G. scitula does not necessarily correspond to surface water productivity and to POC flux, but instead, it correlates well with the supply of fine organic matter, which appears to be a result of water convection. Thus, G. scitula may be an unambiguous and excellent paleo-environmental recorder for carbonate chemistry and for fine organic matter transport to the depths, if isotopic and morphological observations are combined.

Particulate flux of A1, a component of land origin, in the western North Pacific

The particulate fluxes of A1 are generally greater in the western North Pacific than in the central and eastern North Pacific, Atlantic and Antarctic oceans. For instance, sediment trap data reported in this paper show the Al flux in the northern part of the Japan Trench is 12.7 mg m−2 day−1 at 5.2 km depth, 130 times greater than that in the deep Antarctic, even though total particulate fluxes are similar. The particulate fluxes of A1 extrapolated to the ocean surface layer roughly equals the observed A1 flux occurring at the ocean-atmosphere interface, suggesting that particulate A1 is atmospheric in origin. Excess A1 fluxes in the subsurface water probably indicate horizontal transport from the continental margin. This is indicated by the different Mg/K ratios of settling particles between the western and eastern North Pacific.

Dynamics of fatty acids in newly biosynthesized phytoplankton cells and seston during a spring bloom off the west coast of Hokkaido Island, Japan

Abstract 13C uptake experiments were carried out at a station off the west coast of Hokkaido Island, Japan, during the period between late winter and spring bloom in 1997. The composition of newly biosynthesized particulate fatty acids was determined using a 13C tracer and gas chromatography/mass spectrometry (13C GC/MS), and compared with that of particulate matter (seston). Two fatty acid compositional ratios revealed that diatoms biomass (16:1(n−7)/16:0, ∑C16/∑C18) were strikingly high in new cells compared with seston on 17 April, coinciding with the dominance of fucoxanthin. In this study, the Polyunsaturation Index (a measure of the percentage of C16 fatty acids that are polyunsaturated) in new cells is generally close to that in seston, while there was a notable discrepancy between new cells and seston in surface (10 m) waters on 17 April. This large difference between new cells and seston is ascribed to: (1) an increase of newly synthesized storage lipids (mainly 16:0 and 16:1) induced by nitrogen limitation in the top 10 m of the water column; (2) an addition of earlier synthesized phytoplankton cells (high Polyunsaturation Index at logarithmic growth stage) and non-phytoplankton components (detritus and bacteria and zooplankton) into seston. The highest Polyunsaturation Index of C16 fatty acids in seston (33%) occurred at 100 m water depth. This may be caused by “settling planktonic aggregates” or “diatom resting spores”. In the present study, it is reasonable to suggest that the Polyunsaturation Index of C16 fatty acids can be a useful indicator for the ecophysiological state of marine diatom populations.

Large but variable particulate flux in the antarctic ocean and its significance for the chemistry of antarctic water

Settling particles were collected at 1,460 m and 3,760 m depth in the Antarctic Ocean with sediment traps of time series type. The total deployment period of 40 days was divided into four terms of 10 days each. Seawater samples were collected both at deployment and retrieval of the traps at each site. During the 42 days the concentration of silicate in the surface water decreased by 32%, whereas those of nitrate and phosphate decreased by only 4–5%. The total particulate flux in the Antarctic Ocean is the largest among those hitherto observed in the world ocean. The time variation of the particulate flux at 1,460 m depth almost coincided with that at 3,760 m. The settling particles were comprised roughly of 80% biogenic silica, 15% organic matter and 5% other substances including sea salt. The clay fraction was only 0.05% at 1,460 m depth. The settling flux of biogenic silica agrees fairly well with the calculated rate of change in the concentration of silicate in the surface 100 m. Thus it is concluded that preferential propagation of diatoms reduces the concentration of silicate prior to other nutrients in the Antarctic Ocean.

Modeling of the spring bloom in Funka Bay, Japan

Abstract Effects of vertical stability and concentration of nutrient on the diatom bloom in Funka Bay, south Hokkaido, in spring were investigated using one-dimensional and three-dimensional ecosystem models. In the model, six compartments: two for phytoplankton (diatom and dinoflagellate), two for nutrients (silicate and nitrate), one for zooplankton and one for detritus were considered. Vertical stability depended on the net heat flux through the sea surface. Calculated results are compared with the observational results for 1981 (Tsunogai and Watanabe, Journal of the Oceanographical Society of Japan 39 (1983) 231–239). This comparison shows that the rapid spring diatom bloom corresponds to the timing when the net heat flux through the sea surface changes from cooling to heating. This result suggests that the stability of the water column due to warming play an important role in the onset of the diatom bloom in Funka Bay. In addition, the limiting factor of the diatom bloom during the spring bloom is not the silicate, but nitrate. The model can reproduce the observed data by changing the nutrient uptake rate. This suggests a dramatic increase in the assimilation rate of silicate after the consumption of nitrate.