Hand‐picked plant macrofossils (peat or plant remains) or marine calcareous fossils (mainly foraminifera and shell valves or their fragments) from 23 samples, as well as one wellpreserved but possibly reworked shell valve were dated using accelerator mass spectrometry 14C dating. Radiocarbon ages of samples from core GeoB17721‐1 were determined at the Poznań Radiocarbon Laboratory (Poznań, Poland), whereas radiocarbon ages from peat layers or layers of decayed organic matter in cores 09‐x were determined at the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory (University of California, Irvine) (see Table 1 for details). All radiocarbon dates were converted to calendar ages using the CALIB 7.0 program (Stuiver and Reimer, 1993; http://calib. org). The reservoir effect of the marine samples was corrected with ΔR values provided by Scourse et al. (2012) for the German Bight (mean ΔR = 83), with the restriction that the ΔR values were determined on post‐industrial samples.

Radiocarbon dates of GeoB17718‐3 were recalibrated from dates provided by Özmaral (2017).

Supplement to: Hepp, Daniel A; Romero, Oscar E; Mörz, Tobias; De Pol-Holz, Ricardo; Hebbeln, Dierk (2019): How a river submerges into the sea: a geological record of changing a fluvial to a marine paleoenvironment during early Holocene sea level rise. Journal of Quaternary Science, 34(7), 581-592