4 Results
The vegetation development in the lower part of the sampled profile shows fast transitions related to the start of peat growth. Apart from the natural factor of a rising water table, the vegetation succession at the sampling site may also have been affected by sedimentation of sand as a consequence of erosion at the nearby upslope human settlement site. Eutrophication related to the settlement may have also influenced the local species composition.
Zone I (505 – 487 cm below NAP)
In this sandy soil part of the profile, pollen was badly preserved. The zone was subdivided in two subzones, this division is mainly based on the large differences in the representation of Salvinia spores and Cyperaceae pollen. Pollen of trees, mainly Alnus and Quercus, is dominant. Pollen of Tilia is relatively abundant in the lowest sample but starts to decline afterwards. Pollen of Salix, Corylus, Ulmus and Betula occurs regularly.
Subzone Ia (505 – 497 cm below NAP)
This subzone represents the sandy subsoil, which originally (before the rise of the water table) may have represented a dry soil at the sampling site. The relatively high Tilia values indicate a local forest dominated by lime, with Polypodiaceae (monolete verrucate spores) in the understorey. In the wetter parts of the landscape Alnus and Quercus were dominant. Several aquatic taxa that characterize subzone Ib are still absent in Ia. Sclerotia of the soil fungus Cenococcum occur. Cyperaceous pollen still shows low values while Juncus seeds are absent. The high representation of Salvinia - a small floating fern (fig. 4) - may indicate the start of wet conditions with some bioturbation bringing microspores, microsporangia and macrosporangia downward into the soil. Alternatively the sandy deposit of subzone Ia may be the result of uphill erosion (human interference; see below) and fast sedimentation in wet conditions at the sampling site where Salvinia was already growing as a pioneer, while the sclerotia of the soil fungus Cenococcum may have arrived together with the eroded sand. Apophytic taxa (sensu Behre 1981, for an explanation see discussion below), Artemisia, other Asteraceae and Caryophyllaceae are present. These are indicative of plant communities on open ground and may be considered as indicators for (Mesolithic) human presence, but Cerealia-type pollen is still absent.
Subzone Ib (497 – 489 cm below NAP)
Two radiocarbon dates are present from the basal part of this zone, i.e., the 494 cm below NAP-level was dated to 5010 ± 70 BP (ca. 3950 – 3650 cal BC), while the 492cm below NAP-level was dated 5045 ± 30 BP (ca. 3950 – 3770 cal BC).
In this part of the sequence, the sediment becomes very organic. Salvinia is still present but shows a decline, while spores of Zygnemataceae (Mougeotia; Zygnema-type and some Spirogyra) indicate shallow stagnant water, at least during spring time (Van Geel 1976; Van Geel & Grenfell 1996). The algal spore Types 128A and 128B (Van Geel et al. 1989) and Bryozoa (Plumatella-type, Lophopus crystallinus) and Cladocera (Daphnia, Moina, Simocephalus; Van Geel et al. 1983) indicate wet and eutrophic local conditions. Other aquatic taxa are Utricularia and Stratiotes aloides.
The local vegetation was dominated by Juncus. Both J. effusus-type and J.-articulatus type occur with Lythrum salicaria, Alisma plantago-aquatica and Mentha aquatica/arvensis. Hydrocotyle vulgaris represents a transitional phase with alternating wet and dry conditions. The Alnus percentages decreased substantially and at the same time the first peak in Cerealia-type pollen occurs. Rumex acetosella-type shows a maximum, most probably linked to agriculture. Asteraceae generally increase, except Artemisia; other apophytic taxa (e.g., Melampyrum, Campanula/Jasione type, Brassicaceae and Caryophyllaceae), are at their highest level in this zone. Furthermore, taxa such as Polygonum persicaria type, Urtica dioica type and Fallopia are also present. In general, the botanical diversity is high: Potentilla-type, Ranunculacae, Fabaceae, Rosaceae and Lotus Spores of coprophilous fungi, such as Cercophora-type, Sordaria-type and Podospora-type (Van Geel et al. 2003; Van Geel & Aptroot 2006) show high values. Pteridium aquilinum and Riccia cf. sorocarpa (Hepaticae) spores are found regularly. Pteridium is a pioneer benefiting from new open places in a forest, especially after forest fires. It may be linked to human impact (Iversen 1973). Riccia indicates temporary water table levels below the soil surface. It may also have been linked to human habitation as it sporulates on moist arable land (Koelbloed & Kroeze 1965; Van Geel et al. 1983).
Zone II (489 - 480 cm below NAP)
The peat at 486 cm below NAP was dated 5005± 30 BP (ca. 3700 - 3940 cal BC).
The recorded leaf spines of Stratiotes aloides indicate the presence of open water. Alisma plantago-aquatica and Oenanthe aquatica point to temporarily wet conditions, however, Oenanthe can also grow on drier substrates. Sparganium and Typha are typical bank species. Both Poaceae and Cerealia-type show high percentages at the first part of the zone and a decline later on. Quercus is abundant, and Salix, Corylus and Pinus pollen is present in low amounts. Alnus shows varying levels apart from one higher peak at the base of this zone. Ericales show two small peaks. Apophytes are still relatively abundant. Asteraceae - both tubuliflorae and liguliflorae - are present. Rumex acetosella-type and Plantago lanceolata are present in low percentages. The latter may suggest grass land. Other apophytes also occur in low numbers, e.g., Urtica, Melampyrum and Campanula/Jasione type. Spores of coprophilous fungi, such as Cercophora-type (Van Geel et al. 2003; Van Geel & Aptroot 2006) are present. Juncus species, Zygnemataceae (Mougeotia, Spirogyra) and Type 128A (algal spores) show a decline and disappear at the end of the zone. The Salix pollen curve shows an increase, indicating the local development of a willow carr.
Zone III (480 – 460 cm below NAP)
The accumulation of peat continued under eutrophic conditions, with local taxa such as Alnus, Salix, Oenanthe aquatica, Lycopus europaeus, Cyperaceae and Cladocera. The Pinus values show a minimum in this zone. Corylus values also decrease but not as much as Pinus. Grasses are almost absent in the entire zone, as are other herbaceous taxa. Pollen grains of shrubs like Viburnum opulus and Rhamnus cathartica were observed as single finds. Sedges are well represented by pollen as well as by seeds. The spores of the parasitic fungus Kretschmaria deusta (formerly Ustulina deusta) occur. Kretschmaria thrives on different tree species (Van Geel & Andersen 1988).
Zone IV (460 – 432 cm below NAP)
Percentages of tree species are high, in particular high percentages of Salix pollen are recorded. Alnus is not as abundant as in the previous zone. Cyperaceae and monolete psilate fern spores (probably Thelypteris palustris) show low frequencies in the lower part, but show a maximum in the upper part of the zone. A peak of Cercophora-type occurs in the lower part of this zone. Two sub-zones were distinguished.
Subzone IVa (460 – 444 cm below NAP)
Salix pollen percentages show a strong increase to maximal values, while Alnus declines, probably indicating a rise of local water tables. A local occurrence of wet Salix forest seems likely. Such a rise is also suggested by the presence of Alisma (pollen and seeds) and relatively high values of the algae Pediastrum and Types 128A and 128B. A small peak of Equisetum indicates the temporary local presence of horsetail. Three types of waterflees (Cladocera) were recorded: Moina, Simocephalus and Daphnia. The presence of marsh and wetland plants like Lythrum salicaria, Alisma plantago-aquatica, Equisetum, Iris pseudacorus and Menyanthes trifoliata point to marshy and eutrophic to mesotrophic conditions.
Quercus shows relatively high percentages. Poaceae are still at a very low level. Cyperaceae pollen is almost absent. Small peaks of Brassicaceae and Rumex acetosella-type occur and some Cerealia-type pollen was found. High percentages of spores of coprophilous fungi (Cercophora-type) are present. One spore of Osmunda was recorded, but ferns were not present in the local vegetation. Apiaceae are relatively abundant in this subzone.
Subzone IVb (444 – 432 cm below NAP)
In the lower part of the subzone pollen of Nuphar and Nymphaea occur indicating the presence of pools with open water. Mougeotia spores also indicate shallow open water, at least during spring time. Monolete fern spore values show a sudden increase, together with the Cyperaceae pollen curve. The Alnus pollen curve shows a high peak in the middle of the zone, but no macroremains were found. Salix shows relatively high percentages, but considerably lower than in the previous zone and suggests that the local willow carr has become more open. Pollen of Alisma plantago- aquatica and Typha seeds occurs regularly. Sphagnum spores together with some spores of its fungal parasite Tilletia sphagni may indicate the nearby occurrence of Sphagnum and a decline in the availability of nutrients. Apophytes are present in very low percentages and spores of coprophilous fungi are absent. Pinus percentages are somewhat higher in this zone and some Picea pollen was recorded.
Zone V (432 – 420 cm below NAP)
The upper sample of the profile represents the base of a clay layer. Shell fragments and Foraminiferae are present and Chenopodiaceae pollen shows a sudden rise, indicating that flooding and deposition of the sediment occurred under brackish or marine conditions. The sediment in this zone was formed by material of different origin. Several taxa appear, indicating a mixed composition of the increasingly sandy, and in the upper part certainly reworked, sediment. A combination of pollen of cereals and apophytes is present, but in low percentages and without spores of coprophilous fungi. A sudden peak of Myrica indicates the local occurrence of this shrub during a phase when peat growth ended. The Myrica peak is followed by a sudden sharp increase in Poaceae pollen. A hiatus in the middle part of zone V as a consequence of oxidized or eroded peat is probable. In this zone a non-pollen palynomorph resembling Type 708 (Bakker & Van Smeerdijk 1982) was recognized. The presence of Chaetomium ascospores (Type 7A; Van Geel & Aptroot 2006) indicates the enhanced breakdown of cellulose in plant debris. In the upper sample Ericales pollen and Sphagnum spores show an increase, while leaves of S. imbricatum were recorded. These remains probably represent reworked raised bog peat that was eroded by the sea in the area between the present provinces of Noord-Holland and Friesland (Zagwijn 1986) during the Subatlantic period and finally deposited with the marine sediments.
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Figure 4 (1) Cereal-type pollen with thick annulus. (2) Glyceria-type pollen with relatively large pore and narrow annulus.(3 – 6) Remains of Salvinia natans: (3) and (4) Microsporangium with microspores. (5) Macrospore. (6) Microspore.
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