Holocene drowning of the paleo-Scheldt delta: An isotope-based salinity reconstruction

Sea-level rise is one of the most problematic global warming effects that the Netherlands, and coastal areas worldwide, have to face. Therefore, it has become increasingly important to understand the effects and dynamics of the sea-level rise resulting of climatic and environmental change. This study presents a high-resolution reconstruction of the Early to Middle Holocene salinity evolution within the West-Brabant area, influenced by the paleo-Scheldt River and relative sea-level rise. The salinity reconstruction was achieved by using a sediment core, obtained near Calfven, the Netherlands. Furthermore, the Calfven core provides a new locality of the lower Kreekrak Formation, consisting of organic clays deposited by the freshwater paleo-Scheldt. Research on the Kreekrak Formation is limited as it is known from only a few locations in the Dutch subsurface. With the aim of reconstructing the salinity, 93 measurements of stable isotope composition of carbon (δ18O and δ13C) were performed. In addition, 13 radiogenic strontium isotope analyses were executed, which are virtually
unaffected by biological isotopic fractionation. The latter is especially suitable for determining water provenance in estuarine waters (Ingram & Sloan, 1992; Van der Lubbe et al., 2017). The Calfven core shows an evolution from a freshwater bog/fen in the late Early Holocene/early Middle Holocene to a fluviolacustrine environment with mesohaline salinity.  The tidal influence increases to the top of the fluviolacustrine paleo-Scheldt sediments of the of the Kreekrak Formation. The paleo-Scheldt delta drowned as a result of the relative sea-level rise. A shift to polyhaline conditions occurs at the boundary between the Kreekrak Formation and the Wormer Layer. The tidal influence reaches its maximum around 5800-5300 ka cal. BP, which is reflected in the salinity reconstruction and the macro/microfossils (Kiden, 2006). The depositional environment has transitioned into a tidal channel, which fills up.
The maximum tidal influence is followed by a gradual salinity decrease towards the top of the record. The final tidal channel infill is represented by a gradual transition into peat, which forms due to the closure of the Dutch coast in the Subboreal (Beets et al., 1992). This study provides new constraints of the Early Holocene to Middle Holocene salinity evolution in the tidal-Scheldt estuary and offers new reference points for better recognition of the Kreekrak Formation, which enables a better understanding of this understudied formation.