Holocene water-level indicator database for the Dutch coastal plain
Authors: de Wit Kim , van de Wal Roderik S.W. , Cohen Kim M. 
Published: March 2024

The evolution of the Holocene coastal plain in the Netherlands is strongly influenced by global sea-level rise and regional subsidence patterns. Added up these components are known as relative sea-level rise (RSLR), and explain the coastal plain build-up and accommodation space. Due to RSLR, geological indicators of gradual-drowning formed, such as basal peat layers. These indicators have been sampled and dated from different depths and locations across the coastal plain and are used to document rising coastal sea levels and inland groundwater levels. Databasing and spatial-temporal analysis of the large set of indicators (N=~720) serves to assess local and regional variabilities in RSLR.Collection of geological water level indicators in the Netherlands started as early as the 1950ies. It was carried out for various purposes: RSLR reconstruction, geological mapping of the coastal-deltaic plain, wetland paleoenvironmental reconstructions. Full formal overview of this data did not exist, as past reviews and data compilations (N=50-300) were subregion restricted and usage specific. Regional differences within the Netherlands, e.g. greater RSLR in the north than in the SW, are also long noticed, and mostly attributed to&#160; differential subsidence as caused by glacial isostatic adjustment (GIA: Scandinavian forebulge collapse, at non-linear rate) and longer-term North Sea Basin tectono-sedimentary subsidence (at a linear rate).Here, we present a uniform database of Holocene coastal plain water level indicators for the Netherlands, using the HOLSEA workbook format. By compiling a database of geological water level indicators, with an explicit and consistent &#160;standardized treatment of dealing with vertical uncertainties, age uncertainties, and indicative meaning of each indicator (e.g. does it resemble former inland&#160; groundwater level, or former sea-level), we enable more accurate break down of differential subsidence and its source components.Database compilation included documentation of all vertical corrections applied, such as for water depth, (paleo-)tides, long-term background land motion and for compaction, as well as the propagation of uncertainties associated with these corrections. &#160;The ~720 indicators are further categorized into sea level index points (SLIPs), sea-level upper limiting data (ULD) and sea-level lower limiting data (LLD). ULD data is further categorized to separate tidally, river gradient and local-hydrology influenced indicators. Vertically corrected relative sea-level positions and relative groundwater-level positions are reported separately.Spatial-temporal analysis of the Holocene water level data allowed for an interpolated reconstruction of Holocene RSLR, resulting in map-output that has continuous coverage of the Dutch coastal plain. Furthermore, this data-driven RSLR reconstruction is used to further disentangle components of RSLR: the Holocene water level rise part versus the two main land subsidence parts, independently from global sea-level analysis, basin-geological subsidence reconstructions and geophysical GIA-modelling&#160; output.&#160; We&#160; compare our reconstructed sea level plains to the RSLR output of glacio-isostatic adjustment modelling, which incorporate ice sheet deglaciation history and Earth-rheological models. This enhances our ability to quantify the contributions of GIA and basin subsidence to past and ongoing RSLR and subsidence in the Netherlands.