Using 14C-Dated Peat Beds for Reconstructing Subsidence by Compression in the Holland Coastal Plain of the Netherlands

Subsidence in the Holland coastal plain of the Netherlands was reconstructed from the vertical displacement of Holocene peat layers below their reference groundwater levels at the time of peat formation. This quantifies the part of subsidence that is due to compression processes and allows specification of the current state of peat compression in a map. 14C-dating of peat layers found intercalated in the Holocene sequence were used in the reconstruction. This dataset was combined with results from a recent coastal-deltaic plain wide three-dimensional (3D) interpolation of reference palaeogroundwater levels, at which the intercalated peats are thought to have formed before they were buried, compressed, and vertically displaced. Empiric relations between reconstructed displacement and the thickness of overburden were determined and deployed in a national 3D geological subsurface model to establish a subsidence map with continuous cover of the coastal plain. The resulting maps show compressed peat layers under urbanized areas with 1 to 8 m of natural and anthropogenic overburden have subsided 1 to 5 m below the original level of formation. In the agricultural area of the coastal plain, where overburden is merely decimetres thick, consisting of fluvial flood- and sea-ingression deposits, peat generally experienced less than 1 m subsidence. The reference-level reconstruction method is deployable over large coastal plain areas to reconstruct subsidence caused by postdepositional vertical displacement of intercalated peat layers. It could therefore serve as an alternative approach for methods based on soil mechanics, which require input often not available for coastal plains on regional scales.