A Subsidence Legacy

Abstract

Relative sea-level rise (RSLR) poses a major threat to low-lying deltas worldwide. In the Netherlands, global sea-level rise is amplified by local land subsidence. While human-induced subsidence (e.g., gas extraction and soft soil compaction due to water extraction) can be mitigated by changing land-use policies, natural subsidence is an unavoidable geological legacy. The natural subsidence is mainly driven by Glacial Isostatic Adjustment (GIA). During the last ice age, the mass of the Scandinavian and British ice sheets pushed the area around itself up, forming a forebulge. The Netherlands was located on a forebulge. When the ice sheets melted the forebulge collapsed, the area subsided, a process that still continues until today at lower rates. Because the natural subsidence rates in the Netherlands are small (0.1–1 mm/year) compared to the human induced subsidence, it can be difficult to isolate them directly using modern geodetic techniques. Therefore, this thesis uses Holocene paleo-sea-level reconstructions to determine the GIA component within the RSLR signal. This was done by compiling the standardized HOLSEA-NL database, containing 658 water level indicators, primarily from basal peat layers in the subsurface. And using a 3D spatiotemporal interpolation over the last 10,800 years, the study further isolates the natural subsidence signal. The results show that GIA was the primary driver of RSLR during the Middle and Late Holocene. Although GIA rates have decelerated over time, current natural subsidence remains stronger in the north (Waddenzee) than in the southwest. Evaluated against geophysical models, current GIA-induced subsidence is estimated at 0.2 mm/yr (SW) to 0.7 mm/yr (N). This amounts to 2–7 cm of subsidence by 2100, contributing 5% to 16% of total projected RSLR under low-emission scenarios (KNMI). Quantifying this smooth, unavoidable, natural background signal is essential for improving future coastal sea-level projections.