The data was produced during a 16 day in-situ incubation experiment in the Baltic Sea. In order to assess the potential for enhanced benthic calcite weathering as a ocean alkalinisation and thus negative emissions strategy, a Biogeochemical Observatory (BIGO). was deployed at 54° 34.432 N, 10° 10.776 E, at 22 m water depth between 7/12/2025 and 7/29/25. The BIGO is equipped with two benthic chambers that were lowerd to the sea floor. In chamber two, 29 g of fine calcite powder were added to the bottom water. 7 Samples were taken via an automatted glassyringe sampler from each chamber and the ambient bottom water.