Evaluation of environmental change in the sediments under hypoxia revealed by non-destructive speciation of elements.

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Hypoxia is water mass with little dissolved oxygen. This research aims at clarifying the scale of past hypoxia by the vertical analysis of sediments, assuming that the history of hypoxia is recorded on the bottom sediment. The dredged trench off Makuhari in Tokyo Bay has the maximum area and depth, and strong hypoxia has been observed there. Therefore, this area is suitable for an estimation of past hypoxia.The core sediments were collected from dredged trenches and non-dredged seabed as a reference site. We have applied Moessbauer spectroscopy and X-ray absorption fine structure, which are non-destructive state analysis, to every layer of the sediments obtained according to the sedimentation age. As a result, it is proved that the sedimentation environment of dredged trenches was specific and that the scale of hypoxia was decreasing. Moreover, paying attention to a relative concentration change of redox-sensitive elements, redox conditions at the time of deposition was evaluated.