HOLSEA-NL: a 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, rather than for the entire Netherlands. In this paper, we present a dataset 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 (https://doi.org/10.5281/zenodo.11098446, De Wit and Cohen, 2024). This format was expanded to allow for the registration of basal-peat-type geological indicators, the documentation of Dutch-setting-specific parameters and their accompanying uncertainties, the assessment of the indicative meaning, and the appropriate correction of 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 to their research and report RSLR uncertainties transparently. With respect to the indicators, 59 % were collected in 1950–2000, mainly in academic studies and survey mapping campaigns; 37 % were collected in 2000–2020 in academic studies and archaeological surveying projects; and 4 % were collected in this study, with the latter mainly being collected in previously undersampled central and northern regions of the Netherlands. Prominent regional differences exist in the vertical position and abundance of the indicators. Older indicators in our dataset are primarily located in the deeper seaward area of the Netherlands. These indicators correspond well with previous transgression reconstructions partly based on the same data. The younger, landward set of indicators in the Rhine–Meuse delta inland and Flevoland regions corresponds to the transgression phase reaching further inland, from 8000 cal BP onwards. Northern indicators of Middle Holocene age (8–5 ka cal BP) generally lie 2–3 m lower than those in the south. This difference is less for younger data, showing spatial and temporal variation in RSLR throughout the Netherlands.