Data include inorganic carbon levels (TIC given in %), determined from all 39 subsamples of lake sediment short core EN18232-1. Prior measurements, freeze-dried aliquots were milled to gain a homogeneous material. TIC was measured by using an Elementar soli TOC cube. The measurement accuracy was 0.1%. Results below detection limit are given as <0.10. All measurements were carried out at the Permafrost Carbon and Nitrogen Lab (CarLa) at AWI Potsdam.