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Coaslal dunes in western Netherlands Zagwijn & Van Staalduinen (1975) Jelgersma<?foi(J970)

Marine transgressions and regressions Roelevcld ( 1 'J74)

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2000 BP

3000 BP-

5000 BP

6000 HP

7000 UP-

KillII) HI'

9000 BP

10,000 BP

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Crusaders

Aran Moslem

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Iron

Middle Bron/c Age

Early Bron/e Age

Chalcoltlhic

Poltciy NcolilliicA+fJ

Pre-pollen Neolithic B

P re-pollen Neolilliie A

warm-dry Linie Ice Age

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warm-dn o E

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cold warm cold

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cold-humid «anu-dn warm-dij

Dunkirk IIIB

Hgnsipm

Dunkirk llfX Holland vm

Dunkirk. IIA

Holland vil

Dunkirk I B

Dunkirk IA

Dunkirk I) [Holland

Caláis IVB

Holland IVA

Calais IV. A

Holland llf

Calais 111

Holland II

Holland I

Dunkirk IIIB

Hgnsipm

Dunkirk llfX Holland vm

Dunkirk. IIA

Holland vil

Dunkirk I B

Dunkirk IA

Dunkirk I) [Holland ma

Caláis IVB

Holland IVA

Calais IV. A

Holland llf

Calais 111

Holland II

Calais U

Holland I

Calais i

Du ilk irk HI

Dunkirk II

Dunkirk I

Dunkirk 0

Calais 111

Calais III

Calais 1

No beach deposits or dunes known

Fig. 2.6. Paleo-hydrology and geography of the Netherlands.

of old and new dunes, with paleo-soils and peat layers sandwiched between them.

Sea floor and barrier deposits of the Calais transgressions overlay the Pleistocene age sand in the western coastal part of the Netherlands. The layers of the lowest Calais I transgression are found only in the facies of the inner shore deposits of peat and lagoonal beds. The Calais II transgressions are present as tidal flat deposits, while those of Calais III and Calais IV form coastal barrier ridges. The dating of the peat layers, which were deposited simultaneously with the barrier ridge of Calais III, shows that this transgression took place c. 4.8kaBP. Late Neolithic settlements, found on top of the eolian deposits above Calais III, show that the phase of the Calais III transgression had ended before 4.2kaBP. Therefore, the Calais IV transgression started a little before 4.2kaBP and ended c. 3.5 BP.

The deposits of the Calais transgressions are overlain along the present coast by a series of sand dunes, called the Old Dunes. This section of dunes is interspersed with thin soil and peat layers, showing that the dunes were deposited during a few phases of transgressions. These are correlated with the transgressional Dunkirk phases. The complex of Old Dunes is covered by younger dunes, which began to be deposited c. 1.2kaBP. According to Jelgersma et al. (1970), the formation of the barrier ridges - as well as the dune complexes of the Netherlands - was dependent on the supply of sand. During the lower part of the Holocene, this supply mainly came from the sea floor. Therefore, every transgression led to the destruction of the former barrier ridges and caused redeposition of barrier ridges along new shorelines. A small percentage of the sediments came from rivers.

The sea level stopped rising c. 5 ka BP, at which time the coastline was located 10 km east of the present one. As the coastline shifted westward, coastal barriers and three tidal inlets were formed. The construction of coastal ridge barriers was completed c. 3 kaBP. The formation of the Old Dunes, overlying the coastal ridge barriers, ended c. 2 ka BP, namely during the Roman period. Jelgersma et al. (1970) believe this was caused by depletion of the sand. They do not consider a regressional phase at this period to be the reason for termination of the dune formation, which is the explanation I would favour. The renewal of dune deposition, the Younger Dune phase, started c. 0.8kaBP. Again, this is not linked to a transgressional phase but to processes of shore erosion, which may be connected with the Little Ice Age. They also link the formation of the dunes to the cutting down of the forests. The processes that formed the sand dunes may have caused shore erosion through removal of the sand along the shore. Yet, Jelgersma et al., do recognize the fact that the dunes reveal cyclic variations between over blowing and soil formation. The soil formation phase is indicative of wet conditions whereas sand removal indicates dry conditions. These variations occurred simultaneously over the whole area, suggesting a common cause. However, the anthropogenic reason for formation of the Young Dunes does not hold for the Old Dunes, since humans did not inhabit all the land on which these dunes were deposited. Moreover, the cyclicity of the dune formation is similar to that observed in the alternation of transgressive and regressive phases in the coastal area. Any explanation must also cover cyclicity in the formation of peat layers in the hinterland of the dunes. Jelgersma et al. (1970) suggested a mechanism that would account for all these simultaneous phenomena, relating to changes in precipitation, which influences the frequency of high floods in the rivers and thus the flooding of the estuaries. In periods of higher precipitation, gale activity and force increase, affecting the shore environment. Another hypothesis relates to the over blowing of the sand during dry phases, perhaps leading to the blockage of some tidal inlets, resulting in regression in the hinterland.

Jelgersma et al. (1970) quote some studies in Dutch which suggested that transgressions and regressions in the Netherland coastal areas resulted from the actual breaking up of the coastal barrier complex. Vegetation also played a role in the process of dune formation. The vegetation was less dense during the dry than during the wet phases. Thus, over blowing began as the vegetation cover opened up locally as a consequence of reduced precipitation and soil drying.

In conclusion, Jelgersma et al. (1970) suggest three factors responsible for the processes of coastal dune building in the western Netherlands:

• formation of coastal barriers and the subsequent coastal development;

• climate (wind and precipitation);

• vegetation cover: a function of human as well as climatic interference.

They do not suggest eustatic transgressions and regressions as factors in the formation of either the coastal ridge barriers or the coastal sand dunes. Van der Woude (1983) investigated the Holocene paleo-environmental evolution of the peri-marine fluvi-atile area in the western Netherlands. In this area, fluviatile clastic beds (clay and sand deposits) alternate with peat layers. These overlie loamy river deposits and river dunes of the late Weichselian Early Holocene age. The history of the region during the Holocene can be deduced from analysis of boreholes and profiles utilizing lithology, pollen analysis and 14C dates. From these data, it was found that the rise in the level of Holocene groundwater brought about the development of moist conditions in the region from c. 7.4kaBP. After an initially slow organic lacustrine deposition of peat, and a precursory fluvial clay deposition, extensive fluvial deposition of clay and sand took place in a fluvio-lagoonal environment. At c. 6.1 ka BP, there was a fall in the absolute water level and the region became covered by swamp forests, which persisted in many places despite the continuation of the Holocene sea-level rise on a global scale. From c. 5.3kaBP onwards, clay deposition took place. From c. 4.1 kaBP, extensive fluviatile deposition occurred, synchronous with transgressive marine phases. Around 3.8 ka BP, shallow open-water conditions persisted for several centuries. The complete covering by swamp forest first occurred in the downstream area c. 3.3 kaBP and reached the upstream area c. 2.7kaBP. These conditions persisted up to c. 2kaBP.

Evolution of the central part of the coast of the Netherlands in the beach-barrier area, during the Mid to Late Holocene was investigated by Van der Valk (1992). In his opinion, the main factors affecting this evolution were not paleo-climatic but rather the dynamics of a coastal sedimentary system, controlled, on one hand, by a gradual rise in sea level and, on the other, by the processes of progradation, related to storm wave action.

A synthesis of the studies carried out on the Holocene's paleo-environments in the western part of the Netherlands offers the following sequence of events. During the transition period, from the Weichselian Late Glacial to the Early Holocene, both fluvial deposits and the eolian river dune sands were deposited. From c. 7kato c. 5.6 ka BP, not much fluvial activity is in evidence. The deposits were mainly of humic clays and peat. The paleo-botanical evidence points to quiet shallow water conditions. From c. 5.6 ka to c. 5.1 ka BP, a levee-forming stream, developing a stream ridge, flowed through this area. To the south, in the basin area, aquatic conditions existed, causing a significant influx of clastic material. From c. 5.1 ka to 4.1 kaBP, there was a prolonged phase of reduced fluvial activity. There was a marshy environment in which peat and organic clays were deposited. During a short interval around 4.1 ka BP, a major branch of the river Rhine formed a sandy levee containing some gravels. From c. 4.1 ka to 3.45 ka BP, fluvial conditions were again reduced and deposition of organic material persisted in a semi-terrestrial environment. It is supposed that the area was flooded during winter and spring, while dry conditions prevailed during summer and autumn. From c. 3.45 ka to 3.15 ka BP, fluvial conditions prevailed and clays were deposited. These conditions may be related to the fact that a major branch of the Rhine was situated not far to the northeast. Later on, the basin area was characterized by aquatic conditions. From c. 3.15 ka to 2.1 kaBP, mainly shallow aquatic conditions, with abundant vegetation, prevailed. Between 2.1 ka and 1.7kaBP, deposition of fluvial clay in the basin area occurred. From 1.7ka to 1.6kaBP, semi-terrestrial to very shallow aquatic conditions existed. From 1.6ka to 1.5kaBP, the area became wetter. Later on, significant fluvial sedimentation occurred.

The northern region

Roeleveld (1974) investigated the Groningen coastal area in the northern region of the Netherlands (Fig. 2.6). He distinguished two major types of sediment: clastic material and peat. He maintained that the clastic material was deposited as a result of tidal action while the peat was formed along the inland margins of the basins in which the clastic materials were deposited. Roeleveld argued that the classical division of the Holocene along the northwestern coast of Europe (northern France, Belgium and the Netherlands) was over simplified. This division was based on a lower clastic layer - Calais deposits - an intermediate peat layer - Holland peat - and an upper clastic layer - Dunkirk deposits. The data from the northern Netherlands and the North Sea coastal district in Germany revealed the existence of a rather complex alternation of clastic layers and peat. Moreover, it showed that a continuous but gradually slowing rise in the level of the sea took place during the Holocene, while cyclic variations, with a periodicity of c. 500 years, occurred at the same time. These variations were interpreted as signs of transgressions and regressions. According to Roeleveld (1974), the mechanism behind these cycles is not certain. However, the transgression and regression phases were dated and correlated with archaeological periods, as well as with other divisions of the Holocene in other parts of the Netherlands.

Roeleveld constructed a curve of the rising sea level in the Groeningen area. It was based on the relationship between the radiocarbon dates, representing phases of regressive maxima, and correlated with the curve proposed by Louwe Kooijmans (1974). He found an agreement between the two curves, thus reinforcing their validity.

The overall picture of the geological evolution of the Groeningen coastal area according to Roeleveld's curve is as follows:

1. A general sea-level rise, reflected in an overall transgressive development;

2. A decrease in the rate of sea-level rise during certain periods, especially around 3.6kaBP;

3. The supra-regional transgressive and regressive oscillations recorded in the chronological regularity of the occurrence of transgressive and regressive phases in the area of Groningen.

Louwe Kooijmans (1980) correlated the archaeological sites with the coastal changes of the Netherlands, which were governed by the rise and retreat of sea level. During transgression phases, es-tuarine creek systems gradually became extended and flat tidal areas were enlarged. This was followed by periodic sedimentation, causing the creek systems to fill up with silt, and the tidal flats to change into salt marshes. This sedimentation phase was included in the transgression part of the cycle, but it represented, in essence, the first part of a regression phase, which culminated in widespread peat formation.

Casparie (1972) investigated the stratigraphy and development of the Late Glacial and Holocene peat deposits in the northeastern part of the Netherlands. His pollen analyses show that the transition zone from the Pleistocene to the Holocene, between c. 11.8 ka and 10.9 ka BP, was characterized by an increase in arboreal pollen

(.Juniperus, Salix, Betula and Pinus spp.). From c. 10.9 ka to c. 10kaBP, there was a marked phase of loess blowing into the region, forming mud deposits. During the same period, there was a decrease in arboreal pollen, especially Pinus, and an increase in Cyperaceae and other heliophilous plants. Towards 10.3 kaBP, the predominant vegetation of this loess deposit was a small birch tree. Later, a non-ferruginous fen peat was formed in depressions, which gradually extended until c. 7.5kaBP. During this period, there was a drying process of the peat bog, which continued until c. 7kaBP. The quantity of arboreal pollen also increased and Corylus, Ulmus and Quercus spp. appeared.

Shortly before 7 ka BP the area became moist again. Fen-wood peat containing Alnus and Betula developed quickly but then stopped when the supply of water decreased at c. 6.5kaBP. At this time, seepages of ferruginous water developed in the eastern part of the region, again causing the formation of ferruginous peat deposits. The supply of seepage water increased c. 6.5kaBP to such an extent that it flooded and covered the non-ferruginous fen-wood peat area for a few centuries. Later, c. 6kaBP, highly humified peat of Sphagna spp. established itself on a large scale in the Pinus forest, and this caused the decline of the forest. By c. 6 ka BP, only a few trees remained from the previous uninter-ruptedpine forest. By c. 5.1 ka BP, the seepage had stopped and the peat bog had dried out. This allowed the Pinus woods to reestablish in areas where the iron content was not too high. Within a short time, the area became moist again, and within 150 years the highly humified sphagnum peat again overgrew the Pinus forest. About 4.5kaBP, part of the sphagnum peat drained via drying cracks in the seepage peat. Then, c. 4 ka BP, highly humified peat growth took place. At c. 3.5 ka BP, an elongated lake with no outlet was formed in the region. The moisture content of the peat continuously increased until, c. 2.5kaBP, the water spilled over to form a rivulet and many lakes and pools emptied, causing extreme erosion.

Dupont (1985) studied the paleo-ecology of the raised bog system in the northeastern region of the Netherlands (Meerstalblok; Fig. 2.6). The study involved the analysis of pollen assemblages, the determining of pollen density as a function of time, the identification of macro wood remains, as well as the determination of deuterium to hydrogen isotopes ratios in the cellulose of the peat. The systematic 14C dating of the research profiles enabled the authors to calibrate the 14C dates with the dendrochronologi-cal time scale suggested by Klein et al. (1982). The stratigraphy was established on the basis of two sections. The comparison between them showed that layers of the same age lay about 40 cm lower in one section than in the other (distance between sections about 3 m). They concluded that one was deposited on a hummock while the other was in a hollow. The last section showed a hiatus in the sedimentation, which started c. 2.5 kaBP (dendrochrono-logical age, c. 2.4kaBP on the radiocarbon time scale (14CBP))

and lasted for about 400 years. Dupont explained that this was a result of an increase in rainfall to a level above the bog's retention capacity, causing a drainage rivulet to be formed. The influence of humans can be traced from c. 5.5 kaBP dendrochronological years (c. 4.8 ka years BP: the start of the cultures named Funnel Beaker and Protruding Foot Beaker).

The analysis of all the components mentioned above allowed a temperature and humidity curve for the Holocene to be constructed. The main conclusion to be drawn from this curve is that cold periods equate to more humidity because of an increase in precipitation. For example, this was the reason for the hiatus of deposition at 2.5 kaBP dendrochronological years (c. 2.4ka 14CBP).

Detailed work on the paleo-hydrological changes in the Netherlands during the last 13 ka was carried out by investigating the changes in both fluvial and mire environments, as expressed by changes in the paleo-vegetation assemblages (Bohncke and Van-derberghe, 1991). It appears that the beginning of the last deglaciation, following the Weichselian pleniglacial period, was charac-terizedby a superfluous supply of eolian material owing to lack of vegetation cover and low precipitation. At c. 13 ka BP, as temperatures and precipitation rose, the bare soil became covered by tundra type grasses and herbs, thus reducing erosion and enabling the soils to stabilize. From c. 12 ka BP, evapo-transpiration increased, which caused a fall in the groundwater table. This, in turn, resulted in local dune formation. A severe cold period occurred between c. 10.85 ka and 10.5 kaBP, while the period between 10.5 ka and 10.25 ka was dry. The establishment of a more oceanic humid climate c. 8.3kaBP led to a general rise in the groundwater table. After the climatic optimum (c. 8.5ka to 6kaBP), the effective precipitation increased.

The paleo-environment and paleo-hydrology of the Holocene in the Netherlands was derived from a detailed study of the paleo-flora in the sediments of Mekelermeer, a small lake in the northern part of the country. During its history, the lake went through a sequence of changes in its level, which, in the opinion of Bohncke (1991), was a function of the changes in humidity during these periods. During wet periods (with abundant precipitation), the level of the lake and that of the groundwater in its vicinity rose, while during dry periods they fell. These fluctuations are represented in Fig. 2.6.

Bohncke (1991) did not investigate the correlation between humidity and temperatures. Yet, from several correlations, one can deduce that wet periods during the Holocene were coincident with cold periods, while dry periods were the result of a warm climate. For example, the decline in the abundance of Corylus started c. 2.6 ka BP, during a period of a high water table. This decline in Corylus is a well-known feature in many northwestern European pollen profiles and is generally interpreted as being caused by the climate getting colder and wetter. (See references in Bohncke,

1991.) A correspondence also exists between the period of high water table c. 0.7 ka BP and low temperatures recorded in England during that period (Aaby, 1976). Moreover, a predominant positive relationship was found between the start of transgression phases and phases with low lake levels, recorded in the Noordoost polder region in the northern part of the Netherlands by Van de Plassche (1982). In this study, a time relative mean sea-level curve for the period c. 7.5 ka to 2 ka BP was constructed. This curve was obtained from the re-evaluation of earlier curves suggested by various authors, as well as from new data derived from investigating past fluctuations in sea level revealed by studying ancient beach plain sediments bordering the former Old Rhine estuary. The groundwater level curve was derived from basal peat data and the paleo-altitude of river dunes (donken). Van de Plassche (1982) found that the beach plain curve was more or less parallel with the revised mean sea-level curve: the curve for the river dunes lying slightly above the other until c. 6.7 ka BP and later converging with it. According to Van de Plassche, this is because, until this date, the morphological features of the area were affected mainly by the river gradient, while later the flood plain influence became dominant. In accordance with Van der Woude (1983), this author found a positive correlation between the sea-level fluctuations and the levels of the peat deposits, which are dependent on the level of the groundwater. He also found that the fluctuations which he observed in the sea-level curve corresponded to the transgressive and regressive intervals.

Steenbeek (1990) investigated the stratigraphy of two sites in the delta region of the rivers Rhine and Meuse. Deposition started from 7 ka BP in the more western sites and from 6.1 ka BP in the more eastern site, at a distance of about 23 km to the east.

The onset of vertical accretion thus reflected the adjustment of the profile along the river to the rising sea level, causing a progressive shift inland, as well as an intensified rate of deposition. The latter was not merely a function of increased river discharge but was primarily caused by a rise in sea level. The levels of the levee and basin, simultaneously deposited, show an altitudinal difference of 1.5 m. Thus, a general picture emerges of fluvial deposition during the Holocene in this region governed mainly by the rise in sea level, progressing upstream. Fluvial clastic sediments were deposited in the area in the form of fluvial clays and gyttjas (organic clays), while reduced fluvial activity allowed peat formation. The rate of deposition of the fluvial material in the peri-marine areas could not counterbalance the process of accretion brought about by the rise in the regional water level. This caused the formation of large areas in which fluvio-lagoonal and fluvio-lacustrine conditions prevailed. However, in more terrestrial zones, the gradients remained much steeper and differences between channel and basin levels persisted.

The factors that could have caused the various changes in the physical and human environment observed during the period 1000 to 1300 AD were discussed and analyzed in a special symposium held at the University of Amsterdam in 1983 (Berendsen and Zagwijn, 1984). The main changes observed for this period were as follows. Increased river recharge occurred, especially from 850 to 1000 AD, when increased precipitation was reported, and from 1250 to 1400 AD. A relative increase in river flooding was observed for periods during the ninth, eleventh, thirteenth, fifteenth and sixteenth centuries AD. However, the tenth century was relatively dry. The number of severe storm surges seems to have increased a little after 1200 AD and reached a maximum in the sixteenth century. In the twelfth and thirteenth centuries, many storm surges in the northern and southern parts of the Netherlands led to the widening of the tidal inlets and to the enlargement of the estuaries. Thus, tidal influences reached further upstream. A number of authors believe that a sea-level transgression occurred between 800 and 1000 AD. Some of these authors believe that this transgression was related to the formation of the Younger Dunes, which received their material from the submarine erosion of the coastal barrier. These phenomena were related to coastal erosion and the coastal profile becoming steeper. In the dune area, peat growth ended between 900 and 1000 AD. Under relatively dry hydrologic conditions, the dune belt widened considerably from about 1000 to 1180 AD. Then, from 1180 to 1330 AD, the water table in the dune area rose and there was an increase in vegetation. These conditions continued until 1600 AD.

In northern Netherlands, clays were deposited under brackish conditions between 1150 and 1250 AD. The population of the area increased between 1100 and 1300 AD. During this time, forests were cleared, peat was extracted for fuel and salt, and the following three rivers were dammed: the Kromme Rhine (1122 AD), the Ijssel (1285 AD) and the Linge (1304 AD).

Van Geel and Renssen (1998) suggested that a cold period occurred between 850 and 760 calendar years BC (2.75 ka to 2.45 ka 14C BP) because of reduced solar activity.

Conclusions

Regarding the relationships between the fluctuations of sea level and groundwater table, one can see that there is a difference between the peri-marine environments described by Van der Woude (1983) and Van de Plassche (1982) and the inland areas described by Bohncke (1991). In the peri-marine environments, the warm climate (which correlates with high sea levels) caused a rise in groundwater level and the formation of fresh-water peat layers. At the same time, further inland, the warm climate caused lowering of the groundwater table and drying of the peat bogs.

As most of the proxy-data time series of the Netherlands are concerned with local changes, where local circumstances may have played an important role in deciding the variations, correlation with the sequence of the paleo-climates of the Levant is rather difficult. Moreover, the moderating influence of the sea may have blurred the impact of minor climate changes. However, when the major changes are compared (Fig. 2.6), such as, for example, the warm period of the MB of the Levant (from c. 4kaBPto c. 3.5 BP), the Netherlands experienced a low groundwater table and low lake levels, while during most of the cold EB period in the Levant (5ka to 4kaBP), the water levels in the Netherlands were high. During the Moslem-Arab warm period from c. 1200 to c. 1000 BP (800 to 1000 AD), water levels in the Netherlands were again low. During this period, a transgression occurred that caused the invasion of young dunes. During the Little Ice Age, there was a rise in the groundwater table. The regressive conditions during this period stopped the supply of sand to the coastal dunes, which caused their fixation by vegetation.

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