Shigeto Nakabayashi

Photo-reduction of Fe(III) by dissolved organic substances and existence of Fe(II) in seawater during spring blooms

Abstract Fe(III) in seawater was readily reduced to Fe(II) in sunlight in the presence of hydroxycarboxylic acids, such as sugar acids. Tartaric, gluconic, glucaric acids and glucaric acid-1,4-lactone gave higher reduced Fe(II) concentrations and a more rapid increase for a short illumination time than other acids. The reduced Fe(II) concentration was found to be greater at 5°C than at 20°C. In sunlight the reduction of Fe(III) in seawater occurred even at low concentration of the hydroxycarboxylic acids (1 μM). From field observations, 0.02–0.04 μM of dissolved Fe(II) appeared in oxic surface seawater during spring blooms in Funka Bay (Japan) during 1987, 1988 and 1989 as a result of vertical water mixing by which Fe(III) from sediments was transported to the surface. These observations may result from photo-reduction of Fe(III) in the presence of hydroxycarboxylic acids which were possibly released by phytoplankton during spring blooms.

Photoreduction of Fe(III) by hydroxycarboxylic acids in seawater

Abstract The photoreduction of Fe(III) in seawater (pH 8.1) has been investigated over a range of natural sunlight intensities at 5, 10 and 15°C in the presence of hydroxycarboxylic acid (HCA), such as sugar acids. The Fe(III) photoreduction abilities of HCA were in the order: glucaric acid-1,4-lactone = glucaric > tartaric > gluconic >> citric > glyceric = malic > glucuronic acids. The order probably depends on the complexation ability of HCA with Fe(III) in seawater and the photo-activity of Fe(III)-HCA complex. Stable soluble Fe(III)-glucaric complex systems indicated a reversible photochemical reaction The photoreduction rate constant (kred) was estimated from the oxidation rate constant (kox) of Fe(II) in the presence of glucaric form and steady state Fe(II) concentration assuming first-order processes. The photoreduction rate constant increases linearly with increasing light intensity and is independent of temperature. The photoreduction mechanism is probably a photoinduced ligand to metal charge transfer (LMCT) reaction. For colloidal hydrous ferric oxide-HCA systems, the net photoreduction rate probably depends on the surface concentration of adsorbed HCA on colloidal ferric oxide (surface complexation), the back oxidation rate of photoproduced Fe(II), and hydrolytic precipitation rate of reoxidized Fe(III) in seawater remaining HCA. These above interpretations are supported from the observed shift in the Fe(III)HCA absorption spectra measured before and after each photo-experiment. The photoreduction of Fe(III)HCA complex systems is probably promoted by the absorption at longer ultraviolet (u.v.) wavelengths (290–400 nm) of sunlight by the second absorption band centered at 310 nm observed in the spectra of soluble Fe(III)HCA complexes in seawater.

Dissolution rate and solubility of colloidal hydrous ferric oxide in seawater

Dissolution rate constants and solubilities of colloidal hydrous ferric oxide in seawater over the pH range 5.5–8.1 at 20°C were experimentally determined by dialysis techniques involving γ-activity measurements of 59Fe. The Fe(III) dissolution rate was defined as a first-order reaction in proportion to the concentration of particulate Fe(III) in seawater. These rate constants and solubilities within the pH range 6.8–8.1 were independent of pH with values: 0.015 ± 0.002 day−1 and 1.05 ± 0.05 × 10−8 mol l−1, respectively. This result probably indicates the existence of Fe(OH)30 as well as Fe(OH)2+ in the normal pH range of seawater. At lower pHs, the logarithmic dissolution rate constants and solubilities increased linearly with decreasing pH with a slope of −0.55 and −1.0, respectively. The Fe(III) dissolution rate of colloidal hydrous ferric oxide at the normal pH of seawater can be estimated from 0.015 × [Fe (III) p] (conc. day−1) by the concentration of particulate Fe(III) in seawater.

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.

Control on dissolved iron concentrations in deep waters in the western North Pacific: Iron(III) hydroxide solubility

[1]The vertical profiles of nutrient, AOU (apparent oxygen utilization), and Fe(III) hydroxide solubility (Fe(III) solubility) in the western North Pacific commonly show that their concentrations increased with depth below the surface mixed layer and have strong gradients around the North Pacific Intermediate Water (NPIW) in a potential density (s q ) range of 26.7‐27.0. They had the maximum values at as q range of 27.0‐27.5, and then rapidly decreased with depth at highers q than 27.5. The similarity of their profiles versuss q suggests that they are controlled by similar processes with an intrinsic timescale such as deep-ocean circulation. The vertical profiles of dissolved Fe were also characterized by mid-depth maxima and, subsequently, a slight decrease with depth, which is markedly similar to nutrient and Fe(III) solubility depth profiles. This implies that the major source of dissolved Fe in the deep ocean is release during the remineralization of biogenic organic matter. In the present study, we attempted to confirm that the dissolved Fe depth profiles are controlled by the sinking particulate organic matter (POM), the production of dissolved Fe from POM during carbon remineralization, the scavenging of dissolved Fe and the temporal fixed Fe(III) solubility depth profile. Using the production rate (a )o f dissolved Fe (1/a =5 ‐10 years), the calculated depth profile of dissolved Fe is in remarkable agreement with the observed profile of dissolved Fe with mid-depth maxima. Therefore we concluded that dissolved Fe concentrations in deep ocean waters are controlled primary by the Fe(III) solubility. INDEXTERMS:4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4807 Oceanography: Biological and Chemical: Chemical speciation and complexation; 4842 Oceanography: Biological and Chemical: Modeling; 4875 Oceanography: Biological and Chemical: Trace elements;KEYWORDS:iron(III) hydroxide solubility, dissolved iron, deep water, western North Pacific Ocean Citation:Kuma, K., Y. Isoda, and S. Nakabayashi, Control on dissolved iron concentrations in deep waters in the western North Pacific: Iron(III) hydroxide solubility,J. Geophys. Res.,108(C9), 3289, doi:10.1029/2002JC001481, 2003.

Trace determination of sugar acids (gluconic acid) in sea water by liquid chromatography

Abstract A liquid chromatographic (LC) method with flurescence detection was developed with sufficient sensitivity to determine organic acids in sea water and sediments. Sugar acids, except for uronic and α-keto acids, have not previously been measured in sea water because of their low concentrations. After desalting with a column, gluconic acid was esterified with 9-anthryldiazomethane. The reaction mixture containing the gluconic acid derivative was directly chromatographed by LC using an octadecylsilane reversed-phase column and fluorescence detection. The peak areas were linearly related to gluconic acid concentration. The detection limit was less than 10 nM. The reproducibility and recovery were 8% at 200 nM gluconic acid and 99.8%, respectively.

Behavior and dynamic balance of manganese during spring bloom in Funka Bay, Japan

Abstract The distribution of manganese was investigated during a spring bloom in Funka Bay by chemically segregating dissolved, soluble and refractory Mn. The concentration of dissolved Mn changed slightly owing to assimilation by phytoplankton and dissolution of aerosol particles. A simple mass balance was adopted in this bloom period to clarify the dynamics of Mn in the euphotic zone. From this result, the assimilated Mn was estimated to be 10.6 μmol m−2 day−1. However, it was observed that the total Mn increased significantly despite the removal of Mn by settling. This Mn increase was attributable to atmospheric transport of fine particles from land close to this bay.