The file contains data from the Marine Carbon System. It gathered parameters from the inorganic carbon and incorporate the organic alkalinity as a main contributor to the sea surface microlayer (SML) compared to the Underlaying Water (ULW). Data was collected during Mesocosm Study from 18-May to 17-July 2024 in the Sea sURface Facility (SURF), Institute for Chemistry and Biology of the Marine Environment, Wilhelmshaven, Germany. Discrete samples to measure Dissolved Inorganic Carbon (DIC), Total Alkalinity (TA) and Organic Alkalinity (OA) were collected. For SML data, DIC, TA and OA was collected every third day following the glass‑plate technique (Harvey and Burzell, 1972). The ULW data, DIC, TA and OA were collected every day using a suction system to collect the sample from 0.4 m depth. Discrete samples were transported to the laboratory for further analysis; DIC was determined coulometrically (CM5017, UIC, IL, USA), and TA concentration was directly measured by high-precision closed-cell potentiometric titration (916 Ti-Touch, Metrohm, Switzerland). OA concentration was determined directly after TA was measured, using the same sample (from which all carbonate species had been purged), denoted as back titration. The dataset includes quality flags 0-4 with flags 1 and 2 are ready for use. See metadata for more information.