New data from terp excavations on sea-level index points and salt marsh sedimentation rates in the eastern part of the Dutch Wadden Sea (original) (raw)
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Netherlands Journal of Geosciences, 2021
In the early 20th century, archaeological research in the terp (artificial dwelling-mound) region of the northern Netherlands focused, besides settlement history, on natural salt-marsh dynamics and sea-level rise. In particular Van Giffen used salt-marsh deposits under dated terp layers to reconstruct the rate of sedimentation of the developing salt marsh and relative sea-level rise. This line of research in archaeology was rekindled during excavations in the terp of Wijnaldum-Tjitsma between 1991 and 1993. Since then, geology has become an integral part of archaeological research in the terp region. This paper focuses on the northwestern part of the province of Friesland (Westergo), where most archaeological terp research during the past three decades has been carried out, owing to several research programmes by the Province of Friesland. The primary aim of the geoarchaeological research is to better understand the interaction between human inhabitants and the salt-marsh landscape. The sedimentary record exposed in the excavation trenches makes it possible to collect data on the development of the coastal environments of the Wadden Sea prior to habitation, including data on sea-level rise. The sea-level data collected in the geoarchaeological studies in Westergo are the topic of this paper. The measured levels of the tidal-flat/salt-marsh boundary underneath the terps make it possible to reconstruct palaeo-Mean High Water (palaeo-MHW) levels. Such sea-level index points (SLIPs), based on marine shell data points from 12 locations, now make it possible to establish a palaeo-MHW diagram for this part of the Wadden Sea, for the period between 1200 BC and AD 100. In this period the palaeo-MHW in the Westergo region rose from c.1.8 m to 0.3 m −NAP: a mean sea-level rise of c.0.12 m per century. We discuss the fact that elevation of the palaeo-MHW SLIP is not only determined by relative sea level (RSL), but also by the magnitude of the tidal amplitude. The tidal range, and therefore the MHW elevations in a tidal basin, can change from place to place and also in time. Also in a single tidal basin the tidal range is variable, due to the distortion of the tidal wave as a result of the morphology of the tidal system. Such local tidal range fluctuations-not related to sea-level rise-influence the palaeo-MHW curve of Westergo and other tidal basins in the Wadden Sea and need to be taken into account when interpreting the curve. In this paper, we will go into the causes of changes in palaeotidal ranges in meso-and macro-tidal systems, analyse the tidal range variations in recent and subrecent basins and estuaries and discuss the implications of these changes on the sea-level curve of the Westergo region in NW Friesland.
Holocene relative sea-level change and crustal movement in the southwestern Netherlands
Marine Geology, 1995
Sea-level data collected by Jelgersma (1961) from Zeeland in the southwestern Netherlands show an anomalously high time-depth position with respect to sea-level curves established for the rest of the Netherlands. Until now, it was not clear whether this was related to differential crustal movements, a higher tidal range and/or to groundwaterinfluenced peat growth above contemporaneous sea level. New radiocarbon dates from the base of the basal peat in Zeeland and the adjacent estuarine flood plain of the River Schelde in Belgium have a considerably lower time-depth position than the original data of Jelgersma (1961). Comparison of the lowest mean sea-level (MSL) timdepth points from the study area with a reference MSL error band for the western Netherlands yields a maximum crustal uplift of Zeeland relative to the western Netherlands of about 0.17 m/1000 yr over the last 8000 yrs cal B.P. A regional MSL envelope for the southwestern Netherlands has been constructed by correcting the reference MSL error band with this maximum rate of relative uplift. The new regional maximum MSL error band plots significantly below the Zeeland curve and the Zeeland data points of Jelgersma (1961, 1966) but is still higher than MSL curves established for the rest of the Netherlands. The remaining vertical distance of the data of Jelgersma to the new Zeeland MSL error band is attributed to early basal peat growth above contemporaneous MSL, primarily due to a groundwater-gradient effect on the gently inclined Pleistocene subsoil. The sea-level index-points with the lowest time-depth position all come from locations close to or within the Late Pleistocene palaeovalley of the River Schelde. This was probably due to the slope of the regional groundwater table towards the palaeovalley, resulting in better drainage conditions on the valley sidewalls. Peat could only develop under the direct influence of the river-water level, which was controlled by sea-level rise. This suggests that comparable palaeovalleys on other coastlines of the North Sea or elsewhere offer considerable potential for sea-level research, if the topography of the compaction-free substratum is well known.
Holocene sea-level change and crustal movement in the southwestern Netherlands
Sea-level data collected by Jelgersma (1961) from Zeeland in the southwestern Netherlands show an anomalously high time-depth position with respect to sea-level curves established for the rest of the Netherlands. Until now, it was not clear whether this was related to differential crustal movements, a higher tidal range and/or to groundwater-influenced peat growth above contemporaneous sea level.New radiocarbon dates from the base of the basal peat in Zeeland and the adjacent estuarine flood plain of the River Scheide in Belgium have a considerably lower time-depth position than the original data of Jelgersma (1961). Comparison of the lowest mean sea-level (MSL) time-depth points from the study area with a reference MSL error band for the western Netherlands yields a maximum crustal uplift of Zeeland relative to the western Netherlands of about 0.17 m/1000 yr over the last 8000 yrs cal B.P. A regional MSL envelope for the southwestern Netherlands has been constructed by correcting the reference MSL error band with this maximum rate of relative uplift. The new regional maximum MSL error band plots significantly below the Zeeland curve and the Zeeland data points of Jelgersma (1961, 1966) but is still higher than MSL curves established for the rest of the Netherlands. The remaining vertical distance of the data of Jelgersma to the new Zeeland MSL error band is attributed to early basal peat growth above contemporaneous MSL, primarily due to a groundwater-gradient effect on the gently inclined Pleistocene subsoil.The sea-level index-points with the lowest time-depth position all come from locations close to or within the Late Pleistocene palaeovalley of the River Scheide. This was probably due to the slope of the regional groundwater table towards the palaeovalley, resulting in better drainage conditions on the valley sidewalk. Peat could only develop under the direct influence of the river-water level, which was controlled by sea-level rise. This suggests that comparable palaeovalleys on other coastlines of the North Sea or elsewhere offer considerable potential for sea-level research, if the topography of the compaction-free substratum is well known.
Quaternary International, 2005
The younger (post 4000 cal BC) part of the water-level curve for the eastern Flevo area, central Netherlands runs below the relative mean sea-level (MSL) curve for the western and northern Netherlands (Mededelingen Rijks Geologische Dienst 36(1) (1982) 93pp). We investigated if this difference can be attributed to (i) root rejuvenation of the bulk-dated basal peat samples on which the curve for the eastern Flevo area is based and/or (ii) underestimation of the water depth in which the dated peat accumulated. It appears that these potential sources of error did not influence significantly the results obtained by Roeleveld and Gotje´(De Holocene laagveenontwikkeling in de randzone van de Nederlandse kustvlakte (Noordoostpolder), unpublished Ph.D. Thesis, Vrije Universiteit Amsterdam, 1993, pp. 76-86). On the basis of new and recently published water-level data from the eastern and southwestern Flevo area we confirm, refine, and tentatively extend the water-level rise reconstruction by Roeleveld and Gotje´and establish a relative MSL-trend curve for the central Netherlands. The systematic age differences of our radiocarbon dates on various fractions of four new basal peat samples from the eastern Flevo area support the interpretation by Roeleveld and Gotje´that the younger part of the 1982-MSL curve may be based on basal peat samples (all from the Rotterdam area), that have been dated 100-200 yr too old. If this interpretation is correct, the fact that the relative MSL curve for the central Netherlands lies below the 1982-MSL curve cannot be interpreted to indicate less crustal subsidence for the Rhine-Meuse delta, as predicted by geophysical modeling (
Netherlands Journal of Geosciences, 2010
We present a revised relative mean sea-level (MSL) curve for the Rhine-Meuse delta, western Netherlands, for the period 7900-5300 cal yr BP. The revision is based on a series of new and previously unpublished local groundwater-level index data from buried Late Glacial aeolian dunes in the lower Rhine-Meuse delta, and reinterpretation of existing data.The new index data consist of (AMS and conventional) radiocarbon dates of samples, collected from the base of peat formed on dune slopes, near Vlaardingen (21 index points), Hillegersberg (one index point), and Hardinxveld-Giessendam (10 index points). The Vlaardingen data represent the coast-nearest Rhine-Meuse delta local water-level record, which therefore is highly indicative for sea-level change. Pollen and macrofossil analysis, and dating of paired samples was carried out to assess the reliability of the groundwater-level index data.The revision of the MSL curve involves: (1) a significant (0 to >1 m) upward adjustment for the ...
Holocene deposits at the lower shoreface and inner shelf of the Dutch coast
Ocean and Coastal Management, 2022
To further detail insights into the composition and distribution of Holocene and late Pleistocene deposits at the surface and in the shallow subsurface of the lower shoreface of the Dutch coast, vibrocores were collected in three coastal sections. The study areas Noordwijk, Terschelling and Ameland Inlet represent contrasting settings: closed Holland coast vs. segmented Wadden coast (Noordwijk – Terschelling) and lower shoreface of a barrier island vs. lower shoreface of an ebb-tidal delta (Terschelling – Ameland Inlet). Six different depositional environments were distinguished: 1. the active layer, 2. seabed deposits, 3. lower-shoreface deposits, 4. ebb-delta channel deposits, 5. tidal channel deposits, and 6. alluvial (river) channel deposits. The several dm-thick active layer forms the mobile top of the seabed-, lower-shoreface- and ebb-delta channel deposits. Ebb-delta channel deposits (probably grading into terminal-lobe deposits) are restricted to the Terschelling and Ameland Inlet areas, fluvial deposits to the Noordwijk site. The front of the Ameland Inlet ebb-tidal delta is steep and consists of material supplied by the main ebb channel. These ebb-delta channel deposits are reworked by waves and currents, they grade seawards into seabed deposits. The low-gradient shoreface of the Terschelling site consists of a thin active layer on top of ebb-delta channel deposits. At the Noordwijk site fluvial deposits with incised bodies of tidal channel sand underlie a steep shoreface and a ridge-swale topography farther offshore. The ebb-delta channel deposits at the shoreface of Terschelling are similar to those at the front of the ebb-tidal delta of Ameland Inlet. Moreover, the tidal channel deposits that are common in the Noordwijk area, occur in only one core at Terschelling. This indicates that the deposits underlying the shoreface of Terschelling were formed in the ebb-tidal delta of a precursor of Ameland Inlet and not in the transgressive setting of a retreating barrier island. These deposits were possibly formed as part of the ebb delta of the Middelzee, a large medieval predecessor of the Ameland tidal basin, this needs to be confirmed by dating. Reworking of the shoreface of the prograded Subboreal beach barriers at Noordwijk at water depths of 12.5–13.5 m produced a 1.1-m-thick series of fining-upwards storm beds, including the active layer. At the shoreface of Terschelling storm beds are missing at these depths and only an active layer 0.2 m thick occurs. This suggests that the largest part of reworked sediment at Terschelling is carried off, which implies large-scale erosion of the shoreface. This needs further investigation.
Holocene relative mean sea-level changes in the Wadden Sea area, northern Netherlands
Journal of Quaternary Science, 2018
Although the Netherlands has a long tradition of sea-level research, no Holocene relative sea-level curve is available for the north of the country. Previous studies hypothesized that the relative sea-level reconstruction for the western Netherlands is also valid for the northern part of the country. However, glacial isostatic adjustment (GIA) models predict a lower and steeper relative sea-level curve because of greater postglacial isostatic subsidence. Long-term data of relative sea-level change are important to inform GIA models and understand postglacial vertical land motion related to the rebound of Fennoscandia and neotectonic activity. We compiled and evaluated a set of basal peat radiocarbon dates to reconstruct the Holocene relative mean sea-level rise in the Dutch Wadden Sea area. For the early Holocene, this reconstruction is lower than the western Netherlands curve. After 6400 cal a BP, the curve for the Wadden Sea is statistically indistinguishable from that for the western Netherlands, a result that conflicts with GIA model results. It remains to be investigated whether the problem lies with the GIA model predictions or with the quality of the available data. Additional basal peat radiocarbon dates from suitable sites should be collected to further resolve this problem.
Holocene depositional sequences in the Dutch Wadden Sea south of the island of Ameland
Mededelingen Rijks Geologische Dienst, vol. 57 (1996), p. 41-68
Over a hundred cored boreholes form the basis for a reconstruction of the Holocene depositional history of the Dutch Wadden Sea south of the island of Ameland. The general stratigraphic sequence shows basal peat grading into reed clays and mud-flat deposits of Atlantic age, erosively overlain by Subatlantic, sandy channel-point-bar deposits. Radiocarbon dating of the peat shows that the area has been below mean sea level since 7100 BP. Subboreal deposits were completely eroded by shifting Subatlantic channels. However, an almost complete sequence of mud and peat, deposited in the distal part of this tidal basin, ranging in age from Late Atlantic to Late Subatlantic, has been preserved onshore, in the present-day province of Friesland. An idealized channel-point-bar sequence was constructed from 14 box cores. It shows cross-bedded to parallel-laminated sands that grade upwards into ripple-bedded sands with intercalated flaser bedding. Comparison of this reference sequence with the channel deposits in the cores from the boreholes shows that the sequences found in the latter are incomplete. A gradual lateral transition from marine sandy deposits to brackish muddy deposits existed in the Atlantic. In the present-day Wadden Sea this gradient no longer exists. The parts of the basin at the landward side have been reclaimed and the dikes form abrupt boundaries between land and sea. Net deposition of fine-grained sediments is limited to a small part of the area.
Morphodynamic development and sediment budget of the Dutch Wadden Sea over the last century
The availability of nearly 100 years of bathymetric measurements allows the analysis of the morphodynamic evolution of the Dutch Wadden Sea under rising sea-level and increasing human constraint. The historically observed roll-over mechanisms of landward barrier and coastline retreat cannot be sustained naturally due to numerous erosion control measures that have fixed the tidal basin and barrier dimensions. Nevertheless, the large continuous sedimentation in the tidal basins (nearly 600 million m3), the retained inlets and the similar channel-shoal characteristics of the basins during the observation period indicate that the Wadden Sea is resilient to anthropogenic influence, and can import sediment volumes even larger than those needed to compensate the present rate of sea-level rise. The largest sedimentation occurs in the Western Wadden Sea, where the influence of human intervention is dominant. The large infilling rates in closed-off channels, and along the basin shoreline, rather than a gradual increase in tidal flat heights, render it likely that this sedimentation is primarily a response to the closure of the Zuiderzee and not an adaptation sea-level rise. Most of the sediments were supplied by the ebb-tidal deltas. It is, however, unlikely that the sediment volume needed to reach a new equilibrium morphology in the Western Wadden Sea can be delivered by the remaining ebb-tidal deltas alone.
In May 2009, Orson van de Plassche sadly passed away. In a paper of which parts, especially the discussion section, were written after his death, new data and a revision of an existing sea-level curve are presented for the Rotterdam area (Van de Plassche et al., 2010). This comment concerns two topics addressed in the discussion section: 1) connection of their revised Rotterdam relative sea-level curve for the period 7900-5300 cal yr BP (MSL-R2; Jelgersma, 1961; Van de Plassche, 1982; 1995; Berendsen et al., 2007; Van de Plassche et al., 2010) to the sea-level curve for the same area for the period 9000-7500 cal yr BP (MSL-R1; Hijma & Cohen, 2010); 2) The role of the river gradient on the calculation of the magnitude of a sea-level jump that occurred between 8450-8250 cal yr BP (Hijma and Cohen, 2010). -- We [...] apologize for commenting on a posthumous paper [...] We regard it very important to unify sea-level rise datasets and concepts from the drowning estuarine Holocene delta base with that of the overlying back-barrier protected setting. This comment is written in that spirit.