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Inorganic Geochemistry for the Biosphere

  • Tasuku Akagi, Professor
  • Jun-ichiro Ishibashi, Associate Professor
In this laboratory two topics are now tackled with.
  1. Diatom frustules: their chemistry and significance in marine sciences:
    We have recently shown that diatom frustules are not pure opal (SiO2 + H2O), as have been believed, using rare earth elements. Consequences of this new idea are so vast (upsetting/challenging existing theories) and the idea is now confronted with strong objection from marine chemists. Our further analyses of the Earth system introducing the new idea indicates that this idea seems to provide important clues to some outstanding problems including mechanisms of the glacial-interglacial cycles. Our missions are to present new pieces of evidence for the idea and to solve many outstanding problems on the Earth system.
  2. Geochemical processes of seafloor hydrothermal systems:
    A subseafloor fluid circulation system plays an extremely important role in the Earth's element cycle. Nevertheless, our knowledge about this dynamic system has been limited, mainly due to difficulties in access and sampling. We have been conducted studies focusing on a variety of geochemical processes within seafloor hydrothermal systems, under interdisciplinary projects.

1. Diatom frustules: their chemistry and significance in marine sciences

Figure 1

1.1 Chemical analysis of diatom frustules

Diatom frustules have been analytically formidable substances, because of their alteration and similarity to silicate minerals. Several different techniques are applied to reveal their real composition, focusing Al in frustules.

1.2 Impact of diatoms on marine elemental cycling

The influence of diatoms is now being detected from different angles on different time scales by data analyses using computers.

2 Geochemical processes of seafloor hydrothermal systems

Figure 2

2.1 Fluid-rock interactions below the seafloor

Physical and chemical conditions within a hydrothermal system enhance chemical reactions between seawater-originated fluid and substrates such as rocks or sediment. By elemental exchanges during these reactions, primary minerals transferred to (secondary) alteration minerals which often occur as clay minerals, whereas seawater evolves to the hydrothermal fluid. Studies on both fluids and minerals provide important keys to understand these chemical reactions ongoing beneath the seafloor.

2.2 Link between the fluid chemistry and microbial activity

Chemosynthetic ecosystems on and below the seafloor are supported by redox reactions involving inorganic substances (e.g., reduced sulfur compounds, methane, and molecular hydrogen) those are transported by hydrothermal fluids. Determination of chemical and isotopic composition of the fluid in the subseafloor environment provides an important data set, for example geochemical constraints on potential biomass. Since fluid chemistry is basically controlled by fluid-rock reactions within a specific geologic setting, it would bridge between lithosphere and biosphere.

2.3 Ore genesis of submarine massive sulfide deposits

Hydrothermal mineralization occurs as precipitation reactions of metal elements dissolved in high temperature fluid. However, it is unknown how hydrothermal minerals accumulate on or below the seafloor to form ore deposits that are economically beneficial, because it requires the duration of long time scales (>1000 years). Incorporation of computer simulation of fluid circulations with results of geochemical studies would break through the difficulty in speculating the long-term processes.