セメント物質に着目したビーチロックの形成メカニズムに関する考察

Abstract

Beachrocks are coastal deposits cemented mainly by calcium carbonate cement; these deposits are found in the tidal zone of sandy beaches in tropical and subtropical regions. Manmade beachrocks have the potential to inhibit coastal erosions; considering this important application, we performed field investigations and laboratory tests to understand the formation mechanisms of beachrocks in Okinawa and Ishikawa, Japan. We performed a needle penetration test, determination of viable count, elemental analyses and mineral analysis. Our investigation of the formation mechanisms of the beachrocks showed that in Okinawa, the evaporation of seawater and/or urease activity of the microorganisms may have resulted in the precipitation of high Mg calcite (HMC), leading to the formation of beachrocks. On the other hand, in Ishikawa, beachrocks and sand were present near a spring. Here, the pH value of the spring was in the range 4.7 and it has a higher concentration of Al3+. The mixing of spring water with seawater could have led to the precipitation of the cement containing Al and Si between sand particles and, thus resulting in the formation of the beachrock. Therefore, we have interpreted the formation mechanisms of beachrocks.