Kyushu University [Tasuku Akagi (Professor) Faculty of Sciences, Department of Earth and Planetary Sciences]

Geochemical studies of organism-mediated circulation of elements.
New geosciences incorporating interactions between organisms and environmental conditions.

Research

Research Interests

Glacial cycles and diatom activities
keyword : diatoms ocean elements environment
2013.04～2016.05.
Diatoms and material circulation in the oceans
keyword : diatoms ocean elements environment
2013.04～2016.05.
Isotopic composition of water in Sphagnum peat
keyword : water, Sphagnum, peat, oxygen isotope, hydrogen isotope
2011.03～2013.05.
Incorporation of trace metals and silicic acid complexes in diatoms
keyword : diatoms, opal, incorporation theory, metal-silicic acid complexes
2012.04～2013.03.
Dissolution kinetics of diatom opal and determination of trace metals in diatom opal.
keyword : diatoms, opal, rare earth elements, marine chemistry and geocycles of trace elements
2011.05～2013.05.
Dissolution of minerals supplied by Kosa and its influence on the biosphere
keyword : Kosa, weathering, bioorganisms
2008.04～2011.03.
The role of plants in the global material cycles
keyword : plants, weathering, carbon dioxide, silica
2000.01～2006.01Contribution of plants to material transportation.
Reconstruction of the atmospheric CO2 concentration in the past using carbon isotope ratio of sphagnum form peat-lands
keyword : atmospheric CO2 concentration, sea level, sphagnum, carbon isotope ratio, peat
2004.01～2006.01Restoration of the atmospheric CO2 concentration in the past from carbon isotopic data of peat moss..
Carbon isotope ratio of the inclusions in natural diamonds: the study of the carbon geo-cycle involving the upper mantle.
keyword : diamonds, carbon isotope ratio. inclusion, CO2
2000.01～2006.01Carbon isotopic studies of volatile inclusions in natural diamonds..
Interaction between biota and the earth environment
keyword : homeostasis, the earth environment. evolution of life, material cycles, the Gaia hypothesis, mathematical models
2002.01～2006.01Regulative involvement of organisms in the earth environment..

Academic Activities

Books

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Reports

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Papers

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1.	Tasuku Akagi, Saki Yasuda, Yoshihiro Asahara, Mariko Emoto, Kozo Takahashi, Diatoms spread a high εNd-signature in the Oceans, Geochemical Journal, 48, 2, 121-131, 2014.04.
2.	Tasuku Akagi, Rare earth element (REE)-silicic acid complexes in seawater to explain the incorporation of REEs in opal and the “leftover” REEs in surface water: new interpretation of dissolved REE distribution profiles, Geochimica et Cosmochimica Acta, 113, 174-192, 2013.03, In light of a new finding that diatom opal carries rare earth elements (REEs) at an amount exceeding that which would explain the REEs/Si ratio in open ocean columns, the complex formation of REEs with silicic acid and subsequent incorpo- ration of the complex into diatom opal has been explored. Reported complex formation constants of EuH SiO 2þ and an 34 empirical linear relationship of the constants with hydrolysis constants were employed in the species calculation. The calcu- lation reveals that REE–silicic acid complex is the dominant species of dissolved REEs especially in deep layers of the Pacific and the Atlantic Oceans. The degree of complex formation varies depending on dissolved silica concentration, pH, and pCO2. Assuming that only REEs in the form of REEH SiO 2þ, which is diffused/advected from deep water to surface water together 34 with dissolved silica, are incorporated into diatom opal, a REE incorporation theory has been developed. The theory links the intake of REEs and silica by diatoms in surface water and their concentrations in deep water and presents two distribution coefficients (Ds) of REEs in diatom opal, conventional D against surface water and columnar D against deep water as func- tions of pH and dissolved silica concentration. The theory successfully explains the significant REE concentration level (“left- over REEs”) in the surface water of the oceans and reproduces a concentration of REEs in diatom opal that is consistent with that observed in diatom opal in the North Pacific Ocean. The REE composition of oceanic deep water is largely explained by two fractionation processes: REE incorporation into opal and carbonate/oxide scavenging of REEs..
3.	Tasuku Akagi, Feng-fu Fu, Yayoi Hongo, Kozo Takahashi, Composition of rare earth elements in settling particles collected in the highly productive North Pacific Ocean and Bering Sea: Implications for siliceous-matter dissolution kinetics and formation of two REE-enriched phases, Geochim. Cosmochim. Acta, 75, 4857-4876, 2011.05, Settling particles were sampled monthly for 1 year using an automated time-series sediment trap positioned at similar depths at two sites of high diatomaceous productivity in the North Pacific Ocean and Bering Sea. The particles were analyzed for rare earth elements (REEs) by inductively coupled plasma mass spectrometry (ICP–MS) with and without chemical treatment of the bulk samples to isolate siliceous fractions. The REE composition of the bulk samples is explained largely by the contribution of two distinct components: (i) carbonate with a higher REE concentration, a negative Ce anomaly and lighter REE (LREE) enrichment; (ii) opal with a lower REE concentration, a weaker negative Ce anomaly and heavier REE (HREE) enrichment. The siliceous fractions of settling particles are characterized by high Si/Al ratios (30–190), reflecting high diatom produc- tivity at the studied sites. The La/Al ratio of the siliceous fraction is close to that of the upper crust, but the Lu/Al and Lu/La ratios are significantly higher than those of the upper crust or airborne particles, indicating the presence of excess HREEs in the siliceous fraction. Diatoms are believed to be important carriers of HREEs. The Ce anomaly, Eu anomaly, slope of the REE pattern, and RREE of the siliceous fraction vary exponentially with decreasing total mass flux. They can be well-reproduced according to the differential dissolution kinetics of elements in the order of Ce < lighter REEs (LREEs) < Eu = heavier REEs (HREEs) < Si from settling particles, where the dissolution rate is critically reduced through particle aggregation. This order is consistent with the vertical distribution of dissolved REEs and Si in oceans. The differential dissolution kinetics leads to HREE enrichment of the original diatoms and REE enrichment of dissolved diatoms. The Lu/Si ratio of the siliceous fraction of settling particles recovered from some of the highest diatom fluxes is identical to that of the two elements dissolved in deep seawater, providing further evidence for the dissolution of sili- ceous matter in deep water..
4.	S. Issei, Y. Igarashi, Y. Dokiya and T. Akagi, Two extreme types of mixing of dust with urban aerosols observed in Kosa particles: ‘after’ mixing and ‘on-the-way’ mixing, Atmospheric Environment, 44, 858-866 , 2010.07.

Presentations

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Membership in Academic Society

The Japanese Society of Limnology
Geochemical Society
The Oceanographic Society of Japan
Geochemical Society of Japan
The Chemical Society of Japan
The Japan Society for Analytical Chemistry

Awards

Tne Geochemical Society of Japan Award for Young Researchers

Educational

Educational Activities

Geochemical studies of organism-mediated circulation of elements.
New geosciences incorporating interactions between organisms and environmental conditions.
Inorganic geochemistry

Other Educational Activities

2018.06.
2013.04, Geochemistry of and environmental changes by marine diatoms.

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