HOLSEA-NL: Holocene water level and sea-level indicator dataset for the Netherlands

Abstract. Deltas and coastal plains worldwide developed under the influence of relative sea level rise (RSLR) during the Holocene. In the Netherlands, Holocene RSLR results from both regional sea-level rise and regional subsidence patterns, mainly caused by glacial isostatic adjustment (GIA: Scandinavian forebulge collapse) and longer-term North Sea Basin tectono-sedimentary subsidence. Past coastal and inland water levels are preserved in geological indicators marking the gradual drowning of an area, for example basal peats. Such geological water-level indicators have been used in the Netherlands for varying types of research. However, uniform overviews of these data exist only for smaller local subsets and not for the entire Netherlands. In this paper we present a data set of 712 Holocene water-level indicators from the Dutch coastal plain that are relevant for studying RSLR and regional subsidence, compiled in HOLSEA workbook format. This format was expanded to allow for registering basal-peat type geological indicators, documenting Dutch-setting specific parameters and accompanying uncertainties, to assess indicative meaning, and to appropriately correct the raw vertical positions of the indicators. Overall, our new, internally consistent, expanded documentation provided for the water-level indicators encourages users to choose the information relevant for their research and report RSLR uncertainties transparently. From the indicators, 59 % was collected in 1950–2000, mainly in academic studies and survey mapping campaigns; 37 % was collected in 2000–2020 in academic studies and archaeological surveying projects, 4 % was newly collected (this study), the latter mainly in previously under sampled central and northern Netherlands regions. Prominent regional differences exist in the vertical position and abundance of the indicators. Older indicators in our data set are mostly located in the deeper seaward area of the Netherlands. These indicators correspond well with previous transgression reconstructions, that are partly based on the same data. The younger, landwards set of indicators in the Rhine-Meuse central and Flevoland regions corresponds with the transgression phase reaching further inland, from 8000 cal. BP onwards. Northern indicators of Middle Holocene age (8–5 ka cal. BP), in general lie 2–3 meters lower compared to those in the south. For younger data this difference is less, showing spatial and temporal variation in RSLR throughout the Netherlands.