The accompanying .csv files contain the NanoSIMS counts of 21 maps for 11 specimens of Amphistegina lessonii (6 specimens) and Ammonia tepida (5 specimens). The ‘Specimen code’ with the accompanying culture conditions and average LA-ICP-MS-derived El/Ca are listed in table 1 of the accompanying article. The codes for all maps and profiles (of which there is always only 1 per NanoSIMS map) are as follows: O-beam derived maps are named ‘ElCax_y_map.csv’ or ‘ElCax_y_profile.csv’, where El can be either Na, Mg or K, x is the specimen code (1-11) and the y represents the map number (with a maximum of 3 maps per specimen). Cs-derived maps are named ‘ElOx_y_map.csv’ or ‘ElOx_y_profile.csv’, where El is either CN, P or S. The profile files consist of three columns: the first indicates the position, the middle one the average count rate and the right one the standard deviation. For the 11 specimens, one SEM