Holocene sea-level database for the Rhine-Meuse Delta, The Netherlands: implications for the pre-8.2 ka sea-level jump

Abstract

This paper provides the most-accurate sea-level index points (SLIPs) for the Rhine-Meuse Delta (RMD), The Netherlands, identified amidst a wealth of data accumulated in sixty years of research. Following documented protocols, 106 selected radiocarbon dates from peat beds from transgressed valley floor, upper estuarine and back-barrier lagoonal settings were first listed individually, then screened in ensemble on quantified age-depth position and inshore palaeotidal setting. The database contains 50 SLIPs and 56 upper limiting data points spanning a depth range of −34 to −1.5 m. The dates in the database originate from dedicated sea-level research, which kept surface elevation and sampling depth uncertainty small, and sampling of basal-peat beds was favoured, keeping compaction uncertainties small too. The SLIPs cover an age-range of 8.8–3.0 ka BP and are available from Greater Rotterdam and its near offshore. For upper limiting data points, the coverage extends offshore, tracing the Rhine palaeovalley, and reaches back to 11.5 ka BP. The age-depth data density and spatial concentration of subsets of dates allow to perform Bayesian radiocarbon calibration and to reduce temporal uncertainty by 25% for the average sample (1σ age uncertainty ≈ 75 cal. yr), when depth position and rates of relative sea-level rise (RSLR) are used to assign sequence order. Between 8.0–4.5 ka BP, rates of RSLR gradually decreased from 9 to 1 mm/yr. In the millennium before, rates of RSLR were much higher, on average 10 mm/yr, and marked by a sea-level jump between 8.45–8.2 ka BP. A two-phased nature of this jump starts to be resolved, with local magnitudes of 1.7 ± 0.6 m (1σ, first phase) and 0.2 ± 0.2 m (second phase), corresponding to globally-averaged jumps of 2.5 m ± 0.9 m for the first and 0.3 ± 0.3 m for the second phase. For the pre-9 ka BP period, only offshore upper limiting data points are currently available. Until new offshore data are gathered, trends of RSLR before 9 ka BP can only be estimated, fed with global insights in post-glacial eustasy, regional estimates regarding rates of glacio-isostatic adjustments and tectonic subsidence, and sedimentary geological interpretations of scattered basal-peat presence.