Improving Methodology for High-Resolution Reconstruction of Sea-Level Rise and Neotectonics by Paleoecological Analysis and AMS 14C Dating of Basal Peats

Sea-level research in several submerging coastal regions has traditionally been based on 14C dating of basal peats that overlie a compaction-free substratum and can be related to paleo-(ground)water levels. Provided that an unequivocal relationship between (ground)water level and sea level can be assumed, this approach contains two sources of uncertainty: (1) the paleoenvironmental interpretation of samples is usually based on inherently inaccurate macroscopic descriptions in the field, and (2) 14C ages of bulk peat samples may be erroneous as a result of contamination. Due to the uncertainties in both the altitude and the age—the two crucial sources of evidence necessary to arrive at accurate sea-level curves—sea-level index points are therefore represented by considerable, but typically not quantified, error boxes. Accelerator mass spectrometry (AMS) opens new perspectives for this type of sea-level research, as illustrated by a paleoecological and AMS 14C study of basal peats from a small study area in the Rhine–Meuse Delta (The Netherlands), where previous (conventional) work revealed highly problematic results. A detailed macrofossil analysis has two purposes: (1) an inferred paleoecological succession indicates a relatively accurate level of paludification of the site, and hence rise of the (ground)water level; (2) suitable macrofossils from that specific level are then selected for AMS 14C dating. In spite of very small sample sizes, our results are consistent and indicate that this approach can constitute a step forward in high-resolution reconstruction of sea-level rise. The new results further enable a revision of Holocene (ground)water gradient lines for the Rhine–Meuse Delta. A knickpoint in these gradient lines can be related to the effect of faulting. This approach therefore also has considerable potential to unravel and quantify neotectonic activity in submerging coastal settings.