Evidence for a weakening ‘dead zone’ in Tokyo Bay over the past 30 years

Abstract

Weakened hypoxia in the past 30 years at a dredged area in Tokyo Bay was proven by the existence of amorphous hematite (α-Fe2O3) in sediments. The chemical states of iron in sediments can become a proxy for the scale of anoxia at the time of sedimentation. In 2009, we collected core sediments from a dredged area off Makuhari in Tokyo Bay that is very strongly anoxic in the summer. Every layer of the sediments was analysed by ⁵⁷Fe Mössbauer spectroscopy and excess 210Pb dating, and amorphous hematite was identified in the sediments from the 1976–1979, 1986–1989 and 2006–2009 layers. Using an estimate based on the Eh-pH diagram optimized for the sedimental environment, the existence of hematite in the dredged area proves that the scale of hypoxia/anoxia is decreasing, and these results agree well with the observed dissolved oxygen level of the seawater mass.