Seasonal changes of magnetic minerals and their grain sizes in the Hiroshima Bay sediments

Frequent outbreaks of red tide have been reported since 1970 in the Hiroshima bay, and the red tide is caused by a bloom of dinoflagellates. Iron is an essential element for dinoflagellates, and is supplied as bivalent or trivalent ions and iron compounds from lands to sea. For damage predictions of red tide, it is important to research the distribution of iron in the bay. The acidification of seawater during summer has been also observed in the Hiroshima Bay. Increase of CO2 concentration and decrease of dissolved oxygen (DO) content in seawater cause an anoxic condition in the bay. It is known that iron oxides are dissolved and sulfides are formed in an anoxic condition. For clarifying variations of the distribution and mode of iron in sediments and bottom water in the Hiroshima Bay, we investigated kinds of iron compounds in the sediments and the amount of dissolved iron in the bottom waters. Sediment cores of 5cm in depth were taken at three sites in the Hiroshima Bay by using a multiple corer and crab sampler. Data of oceanographic observations at these sites showed that the temperature of the bottom water increased, whereas DO and pH values decreased during the sampling period. The sediment samples were composed of clayey silt. We measured dissolved iron concentration in interstitial and bottom waters filtered above 0.45 um grains, and performed magnetic hysteresis measurements and high temperature magnetometry on the sediment samples. The presence of magnetite (Fe3O4) and hematite (Fe2O3) were recognized in all analyzed samples, whereas greigite (Fe3S4) appeared at these sites with an anoxic condition in the bottom water. Magnetic grain size increased from June to August, while iron concentration increased in the bottom waters. It is suggested that magnetite and hematite were dissolved and greigite was formed, associated with the proceeding of the anoxic condition, and that the grain-size of magnetic minerals and the iron concentration of the bottom water also changed. Irons moves between sediments and seawater in the brief period, which may occur sensitively in the bottom of the Hiroshima Bay.