Influence of acidification on carbonate sediments in Majuro Atoll, Marshall Islands was studied by chemical and mineralogical methods coupled with synchrotron-based X-ray mu-CT. We found that the lowering of soil pH near the surface layer caused decrease of Mg concentration and increase of porosity of foraminifera (Calcarina spp.) due to the selective dissolution of magnesian calcite in Calcarina spp.test.