3 Methods
Two undisturbed sequences of peat and sandy subsoil were sampled in metal boxes (50 x 15 x 10 cm and 50 x 5 x 5 cm respectively). A 1 x 1 m wide sampling trench was dug for this purpose at the southern slope of the sand dune, subsequent to the stratigraphic mapping by means of coring. The sample boxes were packed in plastic bags after their extraction in order to prevent contamination and desiccation. Detailed sampling for microfossil and macrofossil analysis was conducted in the laboratory. The sub-samples for macro- and microfossils were collected from the same depths. Microfossil samples were 1cc, macrofossil samples were ~ 7cc (1 cm thick). Thirteen sub-samples were taken at 4 cm intervals from the upper box and twelve sub-samples were taken from the lower box. In the middle part of this box, in an area labeled as Zone II, sample distance was 2 cm. The depth of the sub-samples given in Figure 3 is in cm below NAP.
The sampling location has some implications for the interpretation of the pollen and macrofossil records. Since the site is located on a slope, the sample contains both localized vegetation and also remains from upslope and down slope. The conditions on top of the dune would have been dry, while the sampling site had already become influenced by the rising water table. The local vegetation change along the slope would thus include the effects of the rising water table.
Microfossil analysis
next sectionFor the study of microfossils, twenty-five sub-samples from the 85 cm high profile were prepared according to Faegri & Iversen (1989). Pollen, fern spores, fungal spores and other palynomorphs were recorded with a magnification of 400x (and 1000x when necessary). Pollen identification was performed with help of an extensive reference collection and with the keys and illustrations by Moore et al. (1991) and Beug (2004).
During the initial analysis no distinction was made within the Cerealia-pollen type (measuring more than 38 µm; diameter of annulus more than 8 µm), because relatively large pollen grains of some wild grass species (e.g. Glyceria and Bromus) are difficult to distinguish from pollen of Cerealia (Beug 2004). Cereals and wild grasses however, grow in different habitats. Glyceria usually grows in moist to wet habitats, while cereals are usually cultivated on dryer soils. Therefore, at a later stage, all large Poaceae pollen grains were measured again (total diameter; size of pore and annulus) and in addition to the size, the ratio between pore diameter and annulus width was also measured because relatively large pores are also an indication for wild grass pollen (Tweddle et al. 2005). If the pore diameter was twice the width of the annulus, the pollen most likely belonged to Glyceria. Within the Cerealia-type both ‘real’ Cerealia (diameter more than 38 µm and a pore smaller than two times the annulus width) and wild grasses with relatively large pollen grains could be distinguished (fig. 4). The ratio between cereal-pollen and Glyceria-type pollen within the Cerealia-type was ca. 50:50.
The plan was to count a pollen sum (Σ-pollen, used for percentage calculations) of 400 pollen grains, excluding grasses and local taxa such as aquatics and wetland species. In most of the samples this number was reached (fig. 3), but microfossils in the upper and lower parts of the sampled profile were badly preserved, making counting and identification rather difficult. In those samples a pollen sum of 400 was not achieved; 250 grains of arboreal pollen was then set as a minimum. The groups and taxa included in the pollen sum are listed in table 1. The pollen taxa not included in the pollen sum and also the non-pollen palynomorphs are expressed as percentages based on the pollen sum.
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Trees |
Shrubs |
Herbaceous taxa |
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Alnus |
Ericales |
Artemisia |
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Betula |
Myrica |
Asteraceae liguliflorae |
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Corylus |
Rhamnus cathartica |
Asteraceae tubiliflorae |
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Fagus |
Rhamnus frangula |
Brassicaceae |
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Fraxinus |
Viburnum |
Campanulaceae |
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Picea |
Caryophyllaceae | |
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Pinus |
Chenopodiaceae | |
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Quercus |
Circaea | |
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Salix |
Fallopia | |
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Tilia |
Melampyrum | |
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Ulmus |
Plantago lanceolata | |
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Plantago major/media | ||
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Polygonum persicaria-type | ||
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Rumex acetosella-type | ||
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Succisa pratensis Urtica | ||
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Table 1 Taxa included in the pollen sumAnalysis of macroremains | ||
Samples for the analysis of microfossils and macroremains were taken at corresponding depths; volumes of c. 7 cc were kept cool and sealed until the analysis of macrofossils started. Preparation of samples was done according to Mauquoy & Van Geel (2007). Samples were gently boiled in 5% KOH solution and then rinsed in a metal sieve with a mesh size of 160 μm. Residues were flushed into glass beakers with demineralised water. The residue was poured into a petri dish and placed under a binocular microscope. Volume percentages of charcoal and sand were estimated; the remaining part was categorized as organic debris. The species composition of the organic debris could not be identified in many of the samples, but in general the material consisted of a combination of leaf and root material. Fruits, seeds and other objects that could be identified were put in china cups with glycerin. A reference collection for fruits and seeds was used in addition to existing literature (Birks 2007; Körber-Grohne 1964; Mauquoy & Van Geel 2007). For practical reasons all fruits and seeds are referred to as seeds, despite the fact that this might not always be the correct botanical name. Samples that were selected for 14C dating were stored in millipore water. The record of macrofossils, shown as bars in Figure 3, represents counted numbers per sample of c. 7 cc.
Combined micro- and macrofossil diagram
In the diagram (fig. 3), a combination of pollen percentages and macrofossils (numbers) is given. Macrofossils are shown by bars and pollen percentages by lines connecting percentage values. Calculations and visualizations were made using the Tilia software (Grimm 1990). If pollen or seeds were very rare, finds are visualized by a plus (+) symbol. Pollen types with low percentages but with an indicative value were exaggerated five times. This is indicated by an open curve on top of a solid curve. Zonation of the diagram is derived from the CONISS calculations (Grimm 1987). These calculations are based on the taxa in the pollen sum only. The CONISS figure is displayed at the end of the upper half of the diagram.
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Figure 3 Microfossil and macroremains diagram from site Schokkerhaven-E170.
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Dating
Three samples from the profile, at 494, 492 and 486 cm below NAP, were selected for radiocarbon dating. A sample containing seeds from non-aquatic plants was chosen to prevent contamination with younger root material and to avoid reservoir effects. Due to lack of material only one of the three samples (494 cm below NAP) was composed of seeds (total 0,596 mg dry weight). The other two were bulk peat samples. Results are shown in table 2.
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cm –NAP |
14C BP |
Laboratory number |
material |
95,4% (2σ) age intervals after calibration |
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486 |
5005 ± 30 |
GrA-39737 |
bulk |
3812 - 3703 cal BC |
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3939 - 3859 cal BC | ||||
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492 |
5045 ± 30 |
GrA-39696 |
bulk |
3952 - 3768 cal BC |
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494 |
5010 ± 70 |
GrA-39695 |
seeds |
3685 - 3660 cal BC |
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3954 - 3690 cal BC | ||||
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Table 2 Radiocarbon dates of three samples with range in calendar years BP derived from Calib 5.0 ( Stuiver & Reimer 1993 ) using the INTCAL 08 calibration curve. | ||||