Diatom frustules and radiolarian tests picked from the core RAPiD-21-03K were dissolved in 0.5mL 1.0M potassium hydroxide solution (KOH) for quantification of Si-28 using inductively coupled plasma - mass spectrometry. To test for the background Si-28 in the KOH, blank samples were prepared and the Si-28 quantified. In this dataset, 'sample id' is the arbitrary name of the blank run, 'Si 28 concentration' is the measured concentration of silicon-28 in each blank in µg/L.