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 erosion; 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, determined viable count, and conducted elemental and mineral analyses. Our investigation of the formation mechanisms of the beachrocks showed that in Okinawa, the evaporation of seawater and/or urease activity of the microorganisms might 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 water was 4.7, and the spring water 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, resulting in the formation of the beachrock. Thus, we have interpreted the formation mechanisms of beachrocks.

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