A palaeoecological and chemical study of a peat profile from the assendelver polder (The Netherlands)

Review of Palaeobotany and Palynology, 58 (1989): 231 288 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands

231

A PALAEOECOLOGICAL AND CHEMICAL STUDY OF A PEAT PROFILE FROM THE ASSENDELVER POLDER (THE NETHERLANDS) D I R K G. V A N S M E E R D I J K

Universiteit van Amsterdam, Hugo de Vries-Laboratorium, Section Palynology and Paleo/Actuo-ecology, Kruislaan 318, 1098 SM Amsterdam (The Netherlands) (Received January 21, 1988; revised and accepted June 17, 1988)

Abstract Van Smeerdijk, D.G., 1989. A palaeoecological and chemical study of a peat profile from the Assendelver Polder (The Netherlands). Rev. Palaeobot. Palynol., 58:231 288. The unique situation of the Assendelver Polder area in the western Netherlands in which both marine influences and oligotrophic bog development are detectable enables a detailed reconstruction of the local and extra-local vegetation development based on the analysis of pollen and other micro- and maerofossils. The vegetation succession series started as a Phragmites swamp (c. 2282-2177 ccal yr B.P. where ceal is calculated calibrated) and developed through a Molinia fen phase (c. 2177 2060 ccal yr B.P.) into a bog dominated by Ericaceae, Sphagnum and Eriophorum (c. 2060 1008 ccal yr B.P.)_ In contradiction to former suggestions, Myrica forms local patches of c. 15 20 m 2 within natural mires with Molinia and heathers. The regional vegetation development of the coastal dune area seems to be recorded better in the peat section than in the dunes itself. There is evidence for the presence of alder brook forest in the area of the Oer-IJ estuary, but not to a large extent. Several time scales have been calculated, using calibrated radiocarbon ages and depths, and also pollen densities. Based on calibrated radiocarbon ages and bulk densities the approximate rates of peat accumulation (in g m- 2 yr i and in cm yr- 1) were calculated. C/N ratios were determined for each sample. The C/N ratios reflect the differences in the local floristic composition of the peat and are correlated with the proportion of the coarse plant material in the peat. For the first time information concerning inorganic constituents of peat profiles in the Netherlands is given. They are compared with results from other countries. The inorganic constituents exhibit several patterns: the ash, silica, iron and potassium contents appeared to be correlated with the type of peat and the amount of clastic material in the peat. The sodium content is correlated with the decreasing marine influence. Manganese and iron are thought to reflect the increasing human influence in the dune area (i.e. increasing deforestation) in the upper half of the section. The amount of phosphorus seems to be correlated with Eriophorum roots. The content of fungal hyphae of the samples is strongly correlated with the quantities of rootlets of Ericaceae.

Introduction This

study

is p a r t

of a combined

research

present paper deals with the detailed recons t r u c t i o n (c. 2 3 2 0 - 1 1 9 5 y r B . P . ) o f t h e l o c a l a n d extra-local vegetation development of the site

project, in which palaeoecological and geochemical aspects of peat were studied. The results of the organic geochemical work, briefly mentioned in the Appendix of this

O (Fig.l) in the Assendelver Polder and with the regional forest development, particularly in the dune area. Some chemical aspects and the bulk density, pollen influx and net organic

paper, will be published separately (Van S m e e r d i j k a n d B o o n , 1987, a n d i n p r e p . ) . T h e

p r o d u c t i o n ( F i g s . 1 0 , 6, 11) w i l l b e d i s c u s s e d . The peat section studied was collected in the

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233 A s s e n d e l v e r P o l d e r ( w e s t e r n N e t h e r l a n d s ) , at a d i s t a n c e of a b o u t 60 m S W W (Fig.2) f r o m the s e c t i o n (site O/R) studied by W i t t e and V a n Geel (1985). D u r i n g the genesis of the studied deposits the s a m p l e d site (see Fig.l) h a d been p a r t of a m i r e c o m p l e x b e t w e e n the f o r m e r c r e e k s y s t e m of the O e r - I J e s t u a r y a n d a v a s t a r e a of o l i g o t r o p h i c p e a t g r o w t h (Zagwijn, 1971; B a k k e r and V a n Smeerdijk, 1982; Vos, 1983). The first p e a t g r o w t h in the A s s e n d e l v e r Polders d a t e s from c. 4200 yr B.P. (Vos, 1983) a n d r e v e a l s the p r e s e n c e of a P h r a g m i t e s s w a m p d e v e l o p e d on clay deposits, b e l o n g i n g to the late Calais IV A a n d e a r l y Calais IV B t r a n s g r e s s i o n phases. M o r e i n f o r m a t i o n on the geology a n d r a d i o c a r b o n d a t i n g s c a m e available, and a modified i n t e r p r e t a t i o n of the h i s t o r y of the Oer-IJ e s t u a r y could be m a d e (Vos, 1983 a n d P.C. Vos, pers. commun.). The P h r a g m i t e s s w a m p e x t e n d e d to the w e s t e r n p a r t of the A s s e n d e l v e r P o l d e r b e t w e e n 2950 and 2650 yr B.P. A wider c o n n e c t i o n b e t w e e n the O e r - I J e s t u a r y and the sea c a u s e d a l o w e r i n g of the w a t e r table, w h i c h in t u r n c a u s e d t h a t the b o a r d e r s of the P h r a g m i t e s p e a t were no longer in c o n t a c t with e u t r o p h i c water, and an o l i g o t r o p h i c p e a t d e p o s i t i o n could s t a r t (site Q: 2650 2550 yr B.P.). Afterw a r d s a c r e e k s y s t e m could d e v e l o p due to an increased tidal amplitude. The eastern b r a n c h e s of this c r e e k s y s t e m d r a i n e d the b o a r d e r s of the p e a t a r e a b e t w e e n 2550 a n d 2450 yr B.P., w h i c h r e s u l t e d in i n t e r r u p t e d p e a t growth. In the d r a i n e d p e a t a r e a E a r l y I r o n Age s e t t l e m e n t s w e r e found at site Q at a b o u t the 2500-2450 yr B.P. level. T h e i n c r e a s i n g tidal a m p l i t u d e and the s e t t i n g of the p e a t were at l e a s t r e s p o n s i b l e for the flooding of the w e s t e r n p a r t of the p e a t area, a n d t e r m i n a t e d the h a b i t a t i o n at site Q. F r o m t h a t time

o n w a r d it w a s not possible to build h o u s e s on the p e a t s u r f a c e b e c a u s e of the sogginess of the p e a t complex. A f t e r c. 2250 yr B.P. the m a r i n e influence d e c r e a s e d in the O e r - I J e s t u a r y , and the tidal flats got c o v e r e d w i t h v e g e t a t i o n . The n a t u r a l d r a i n i n g of the a r e a was t h r o u g h the c r e e k system. In the A s s e n d e l v e r P o l d e r a new h a b i t a t i o n p h a s e (of L a t e I r o n Age) started, b u t n o w on the s a n d y levees a l o n g the creeks. At a s h o r t d i s t a n c e (c. 500 m) from the f o r m e r tidal flats the P h r a g m i t e s s w a m p still existed at site O / R and lasted until c. 2390 y r B.P., subseq u e n t l y to develop into an o l i g o t r o p h i c raised bog (Witte and V a n Geel, 1985). At a b o u t 1960 yr B.P. the o l i g o t r o p h i c p e a t g r o w t h was d i s t u r b e d by the R o m a n I r o n Age h a b i t a t i o n phase. F r o m 250 A.D. o n w a r d the o l i g o t r o p h i c p e a t d e v e l o p m e n t r e s t a r t e d , and v a s t a r e a s w e r e c o v e r e d with raised bog. In the Assendelv e t P o l d e r the m e d i e v a l r e c l a m a t i o n of the p e a t s t a r t e d in the e a r l y e l e v e n t h centuTy ( B e s t e m a n and G u i r a n , 1986). A m e d i e v a l c h u r c h (1000 1200 A.D.) was b u i l t on the bog, and h a d p u s h e d the u n d e r l y i n g p e a t downwards. The u n d e r l y i n g p e a t was well p r e s e r v e d and e n a b l e d a s t u d y of the r a t h e r y o u n g bog development. Material and methods S a m p l i n g a n d sample p r e p a r a t i o n

T h e s e c t i o n A s s e n d e l f t - K E R K was collected at 29-9-1983 at site O in the A s s e n d e l v e r P o l d e r s (52°28'26"N, 4°45'36"E), c. 15 k m N N W from A m s t e r d a m d u r i n g an a r c h a e o l o g i c a l e x c a v a t i o n of m e d i e v a l s e t t l e m e n t s by the A l b e r t Egges v a n Giffen I n s t i t u u t v o o r Praeen P r o t o h i s t o r i e (IPP). T w o plastic boxes (56 x 14 x 12 cm) w e r e p u s h e d into the exposed

Fig.1. Palaeographical maps of the former Oer-IJ estuary c. 2350 yr B.P. (B) and of the area (C) studied. After P.C. Vos. Legend: 1 = coastal barriers, covered with dunes; 2= emergent sandy estuarine deposits, beach plains; 3= emergent intertidal region of the Dunkirk I estuary (clay and sandy tidal flats, high marshes, levees and basins); 3a = creek and natural levee deposits; 3b = lagoonal and back swamp deposits; 3c = back swamps; 4= reed swamps; 5= raised bogs; 6 = still functioning tidal channels of the Dunkirk I system; 7= tidal inlet; 8= actual coastline; 9= palynological sites of"Jelgersma et al. (1970); 10= archeological sites surveyed; 11= archeological sites excavated; 12= site Assendelft O.

234 profile just below the foundation of a medieval church, and cut out. The boxes were immediately sealed in plastic foil. In the laboratory the peat was cut into slices of about one cm thick. The peat from the upper box was cut into 59 slices, that of the second box was cut into 56 slices. The lowest 2 cm of the upper monolith, which had an overlap with the top of the lower monolith, were not analysed,

Degreeof decomposition

The analysis of pollen and other microfossils

pounds were not collected. The ratios of the fractions (macrofossils and fractions of 11 190 ~m) give an insight into the

Subsamples of a known volume were taken at random from the slices by means of a cork bore (ID 5.3 mm). The peat was boiled mildly in a 10~o KOH solution, and sieved on a 240 ~m sieve. The filtrates were collected in glass tubes containing Lycopodium spores. For each subsample two tablets of Lycopodium spores (11,300+400 spores per tablet; cf. Stockmarr, -

1971), dissolved in diluted hydrochloric acid to remove the chalk, were used. The subsamples were prepared f ur t her according to Erdtman (Faegri and Iversen, 1975). Small aliquots of the residues were mounted in permanent slides and all recognisable palynomorphs were counted under a microscope using a 400 x magnification. The relative low representation of arboreal pollen in the peat made it necessary to count up to 5 slides per subsample to reach a minimum of 150 arboreal pollen grains for every subsample,

Macrofossil analysis A known volume (c. 10 cm 3) of peat from each slice was boiled mildly in a 5°//o KOH solution for a few minutes, and rinsed with water using a metal sieve with meshes of 190 ~m, till the filtrates (A) became colourless. The residues from the sieve (= macrofossils) were resuspended in water and studied under a stereomicroscope, using a 10-40 x magnification. The recorded quantities of the various remains are estimations given as volume percentages or countings given as number per 10 cm 3 (Fig.5 and TableD.

After having been studied the macrofossils were collected and dried at 105°C till constant weight. The collected filtrates (A) were filtered again, using a SS 595 paper filter (retention 11 pm). The newly obtained residues (= fractions 11 190 ~m) were dried at 105°C till constant weight. The remaining dark brown stained filtrates (B) containing the base-soluble corn-

decomposition of the peat and are shown in Fig.12. It is not possible to measure the total dry weight and dry weight of the fractions of the same subsample. Therefore a second piece of peat (c. 10 cm 3) from each slice was used for the measurements of wet weight, wet volume and bulk density (dry weight at 105°C). The volume of the peat was measured with a 25 ml pyknometer, because the peat fell apart in some cases. Super-demineralised water was used to avoid contamination.

Chemical analysis and ash content For the determinations of ash content and some element analysis the above-mentioned "second" pieces of peat were used. After drying the peat was incinerated at 750°C for three hours till constant weight. The ash was dissolved in 5 ml 2 N HC1 solution and deluted to 100 ml with super demineralised water. Atomic absorbtion was used for the determination of total calcium, magnesium, potassium, sodium, iron and manganese, by means of a PERKINELMER 2380 Atomic absorbtion spectrophotometer. Colorimetric methods were used for the determination of total phosphorus as a molybdate-complex (Stewart et al., 1974) and sulphate as a methylthymolblue-complex, by means of a TECHNICON Auto-analyser II. The results of these analyses are shown in Fig.10. For the determination of the total nitrogen, carbon and hydrogen content portions of the untreated peat were frozen in liquid nitrogen, dried in vacuo for 24h and ground in a

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254 255 256 275 276

1 1 2 1 I 1 1 1

277 278

I 2 2

279 281 282 283 286 287 291 294 295 296 297 298 299 300 302 303 304 305

2

1 2 1

1 1 2 1 2 1 1 3 5 2

2 1 1

2 4 3 1

2

7 1

1 1

1 1

1 1 2 1

+ 1

+

1 1

3

1

252

B R A U N Msl 6635 micro-dismembrator. Aliquots of 2 mg were i n c i n e r a t e d using a PERK I N - E L M E R 240 e l e m e n t analyzer. D u p l i c a t e or t r i p l i c a t e samples were r u n t h r o u g h o u t . The a n a l y t i c a l e r r o r s are 0-10 mg g 1 for C and 0-0.85 mg g-1 for N.

Diagrams The t o t a l of a r b o r e a l pollen and pollen of Hippophae, Sambucus and Ilex were lumped to c o n s t i t u t e the pollen sum (Z-pollen). The r e l a t i v e frequencies of the individual taxa were c a l c u l a t e d as p e r c e n t a g e s of the Z-pollen. F o r plotting the results a modified plotting

tablets used for pollen c o n c e n t r a t i o n measurements.

Radiocarbon dating Eight samples from the section AssendelftK E R K were dated at the C e n t r e for Isotope R e s e a r c h of the U n i v e r s i t y of G r o n i n g e n (Professor W.G. Mook). The following results were obtained:

GrN-no.

Depth (cm below NAP)

Conventional 1'C age (yr B.P.) 1195+45

Calibrated age

(yr B.P.)

p r o g r a m for the H e w l e t t P a c k a r d c o m p u t e r (HP 87) was used (De Vries and Wijmstra, 1986). Different scaling options were used to

13031

195 195.9

1133 (1177 1059) 1158

13925

203.3-204.2 1210_+50

m a k e the diagrams (Figs.3-5) as short as possible. The main scale is plotted at the top of each c u r v e in three ways. In some cases a second scale is plotted just below the main scale. This second scale shows m a n y variations, due to the plotting procedure, which was p r o g r a m m e d in such a way t h a t the curves were not cut off, unless done by hand. The

13926

229.9 230.8

1305_+35

13927

252 253

1685_+35

13928

264 265

1875_+30

13929

280 281

2000_+40

13930

295 296

2150_+40

2218

plotted curves b e l o n g i n g to the second scale h a v e sometimes a misleading a p p e a r a n c e due to the above m e n t i o n e d plotting procedure, but the second scale is always ten times the main scale. In both scales the c a l i b r a t i o n units are 5 (short bars) and 10 (long bars), F o r more i n f o r m a t i o n of the microfossils o t h e r t h a n pollen (Types) in the diagrams the r e a d e r is r e f e r r e d to following literature: Van Geel (1978), Pals et al. (1980), V a n Geel et al. (1980, 1981), B a k k e r and V a n Smeerdijk (1982) and Van Geel et al. (1983). - - M o s t of the pollen of the genus Plantago was of P. lanceolata, but in a few cases the P. major~media-type was observed too. - - The c u r v e of the h u m a n influence i n d i c a t o r s includes pollen of cereals, Plantago, Rumex and Artemisia. - - T h e c u r v e of trilete psilate spores (dark brown; c. 40-50 ~tm) is given a t the end of the microfossils diagram (Fig.4). A p p a r e n t l y t h e y are a c o n t a m i n a t i o n from the Lycopodium

13032

304 305

2320+40

2346 (2354 2337)

(1256 1063)

1271 (1288 1182) 1580 (1689-1541) 1822 (1876-1743) 2033 (2040 1899) (2300 2071)

In the last c o l u m n the c a l i b r a t e d ages in c a l e n d a r years B.P. are given a c c o r d i n g to S t u i v e r and B e c k e r (1986); between b r a c k e t s the upper and lower r a n g e at sigma one. Sample GrN-13930 was the only one which had two c a l i b r a t e d ages (2284 and 2151 cal yr B.P.). The m e a n v a l u e (2218) is listed in the table. GrN-13031 had 7 possible c a l i b r a t e d ages. The m e a n value (1133 cal y r B.P.) is listed in the table. For t h a t r e a s o n the last date was not used for f u r t h e r dating purposes. Results and discussion

Palynological zonation The p a l y n o l o g i c a l z o n a t i o n is based on d a t a from m i c r o - a n d macrofossil analyses (Figs.3-

253 5). As far as possible the fossil stands of vegetation have been classified according to the recent syntaxonomical system of Westhoff and Den Held (1969). The duration of the particular vegetation zones is given in calculated calibrated years B.P. (ccal yr B.P.) For a discussion on dating, see the chapter '~Dating, pollenconcentration and polleninflux" (p.17).

Local vegetation development Zone A." 305-296 cm below N A P (Dutch ordnance level). This zone corresponds with the lower part of the section consisting of strongly decomposed peat and deposited from 2282 to 2177 ccal yr B.P. The peat of this zone must have been formed under different ecological conditions. Two subzones are distinguished, After a description of these subzones, the interpretation is given for the entire zone A.

Subzone AI: 305-301 cm below N A P (2282-2236 ccal yr B.P.). The yellowish peat consists mainly of vegetative remains of Phragmites australis and seeds of Eleocharis palustris and Mentha aquatica. The pollen curve of the Cyperaceae shows a maximum. The presence of Hystrichosphaeridae and Foraminifera in combination with high values for the pollen of Chenopodiaceae and Asteraceae tubiliflorae points t o marine infuences. The maxima of spores of Sphagnum and ferns and pollen of Ericales and Myrica probably indicate that these microfossils represent allochtonous material. The top of this subzone (302 cm) shows maxima of such microfossils as Botryococcus, Type 128A, Type 128B, Cymatiosphaera (Type 116), Chenopodiaceae, Solanum dulcamara, Malvaceae and Symphytum, together with Scirpus maritimus/lacustris seeds.

Subzone A2: 301-296 cm below N A P (2236-2177 ccalyr B.P.). The greenish peat consists mainly of remains of Phragmites australis. This subzone starts with maxima of Cyperaceae and Mentha pollen together with Mentha aquatica seeds, followed by maxima of the microfossils of Ophioglossum and Lotus, and seeds of

Lycopus europaeus. Subsequently maxima of seeds of Hypericum tetrapterum, Lychnis floscuculi, Viola palustris, pollen of Caryophyllaceae and Asteraceae tubiliflorae (including Aster-type, Anthemis-type, Cirsium.type and Helianthus.type) and Type 731 occur. The pollen of Thalictrum, Galium and Calystegia sepium is present throughout this subzone. Indicators of open water (for example Pediastrum) disappear at the end of this subzone. Towards the end of this subzone Iris pseudacorus (pollen and seed) and Valeriana played a role in the local stands of vegetation. Zone A represents a sequence of vegetation types that can be interpreted in the following way: Subzone A1 represents stands belonging to the Phragmitetalia resembling best a Scirpeturn maritimi, characteristic of shallow brackish to fresh eutrophic water, with a slightly fluctuating water table. The top of this subzone (sample 302) may represent plant communities of the Convolvuletalia sepium, a not very well defined syntaxonomic unit, of which the plant communities always are mixed up with communities of the Phragmitetalia and Molinietalia. Most of the micro- and macrofossils ofsubzone A2 belong to taxa from the Molinietalia. This subzone represents a combination of vegetation types resembling the Calthion palustris, mainly represented by local communities in which Hypericum tetrapterum and Lychnis floscuculi and Ophioglossum vulgatum dominate, respectively. The presence of Phragmites australis, Lycopus europaeus and Iris pseudacorus point to one of the communities of the Phragmition. Stands with Iris pseudacorus are often found in contact with the Calthion palustris (Westhoff and Den Held, 1969). The synecology of this subzone is rather complex: a wet to very wet, nitrogen rich, rather nutrient rich environment, with a fluctuating water table. In recent times the Calthion palustris occurs as natural and as anthropogenic vegetation types. In the last situation they occur as substitutes for the Alnion glutinosae (Westhoff and Den Held, 1969).

254

Zone B: 296-265 cm below N A P (2177-1815 ccal yr B.P.). The transition between zone A and zone B is mainly characterised by a nearly complete disappearance of the micro- and macrofossil assemblage of zone A, a maximum in the pollen curve ofApiaceae, a very distinct maximum in the pollen curve of the Poaceae, and the first occurrence of rootlets of Ericaceae and epidermal fragments of Molinia caerulea. The vegetation development reveals a detailed transition from an eutrophic to an oligotrophic environment. The influence of the groundwater on the vegetation cover was decreasing. The vegetation types become more and more dependent on rainwater. In this zone three plant communities dominated the local vegetation sand successively. A more detailed description of the vegetation development of zone B, based on a division in subzones, will be given below. The vegetation types of these subzones are not strictly separated from each other, especially subzone B2 is an intermediate phase between subzone B1 and subzone B3. Subzone B1: 296- 286 cm below NAP(2177-2060 ccal yr B.P.). The sediment of this subzone consists of a moderately to strongly decomposed peat, with epidermal fragments of Molinia caerulea and many roots of monocots (cf. Molinia caerulea). The subzone starts with decreasing percentages of the pollen of Poaceae. Many microfossils occur for the first time in the diagrams, such as the pollen of Gentiana pneumonanthe, fungal Type 10 (an indicator for the presence of Calluna) and Type 12, the newly recognised Types 494, 495, and 496 reaching maxima at a depth of 292-290 cm. The curves of Sphagnum spores, TiUetia (Type 27) and Cercophora-type (Type 112) show pronounced maxima only at the depth 291 cm. The monolete psilate spores have also a maximum at depth 291 cm. From depth 291 cm onward the pollen curve of Ericales increases strongly. The fungus Type 55A, already present from the start of subzone A2 onward shows two maxima in the beginning of subzone B1, and may point to drier local conditions. Anthoceros spores and Type 3A ascospores only occur between

depths 291 and 286. Type 3A is indicative of dry local conditions in the peatbog (Van Geel, 1978). Anthoceros, in particular Anthoceros punctatus is known from humid trenches in arable land on loamy soils and from humid sandy soils (Koelbloed and Kroeze, 1965; Westhoff et al., 1973; Landwehr, 1980). These environments were not present in the mire, so that an alternative explanation can be postulated: (a) the spores had been transported by wind from the arable land, probably situated on the high sand deposits in the northern part of the Oer-IJ estuary and in the humid valleys in the dunes along the west coast, (b) the spores are from one of the many species of Anthoceros, that grew on the mire itself in a habitat that is not well known. The very high percentages of Cercophora-type spores may point to a local enrichment with dung. The rise of the pollen curve of Ericales from depth 291 cm onward shows that Ericales became a dominant element in the vegetation. This is also reflected in the presence of some macrofossils of Calluna and Erica. Rootlets of Ericaceae and the associated fungus (Type 10) in the lower part actually do not belong to this subzone but must have penetrated from above. The local stand of vegetation was related with another community of the Molinietalia, viz., the Cirsio-Molinietum, that consists of tussocks of Molinia cerulea with Gentiana pneumonanthe, Drosera, Succisa pratensis and Dryopteris cristata (producing monolete psilate spores) on the more lowly situated bare spots between the tussocks. Sphagna with their parasite Tilletia were growing at the very wet spots. The available ecological information on Molinia vegetation points to relatively high water tables in winter and low water tables in summer. Probably the above mentioned ecological conditions also prevailed in the Assendelver Polders peat bogs dominated by Molinia (cf. Van Geel, 1978; Van Geel and Dallmeijer, 1986).

Subzone B2." 286-279 cm below N A P (2060-1978 ccal yr B.P.). The sediment in this subzone consists of moderately to strongly decomposed peat. Epidermal fragments of Molinia are

255 absent. Molinia caerulea was still part of the stands of vegetation, which can be deduced from the presence of Molinia seeds, but apparently it was not a strictly local element, The curves of Poaceae and the fungal spore types Type 10, Type 12 and Meliola cf. M. uiessliana (Type 14, a parasite on Calluna, showing a very distinct maximum at depth 286 cm) decline and show minimum values. The pollen percentages of Ericales reach a first maximum at depth 286 cm and decline towards the end of this subzone. Macrofossils of Erica

tetralix, Oxycoccus palustris, Calluna vulgaris and Aulacomnium palustre are rather abundant and mainly found in the first part of this subzone. Curves of pollen of Cyperaceae and of Rhynchospora alba and ferns, and also of the rhizopods Amphitrema flavum (Type 31A) and Assulina (Type 32), of the alga Cylindrocystis (Type 100), and of the fungus Helicoon pluriseptatum (Type 30) reach maxima in the first part of this subzone. The increase of the pollen percentages of Cyperaceae and the presence of some seeds and epidermal fragments and the presence of the fungus Anthostomella fuegiana (Type 4) are indicative of the presence of Eriophorum in the stand (Van Geel, 1978). The spores of the fungus Type 16A enters this subzone with low percentages. In the upper part of this subzone the moss layer consisted of

Dicranum scoparium, Drepanocladus revolvens, Cirriphyllum spec. and Hypnum cupressiforme, The vegetation cover of the top of subzone B1 and of subzone B2 resembles an initial phase of the Oxycocco Sphagnetea (cf. Ericetum tetralices), in which Erica tetralix, Oxycoccus palustris, Sphagnum and Rhynchospora alba point to somewhat wetter local conditions and Cal-

luna vulgaris, Aulacomnium palustre, Pleospora (Type 3B), Hypnum cupressiforme and probably the other mosses too point to somewhat drier local conditions. Synecology: a humid to wet, meso-oligotrophic environment with varying water tables,

Subzone B3: 279-265 cm below N A P (1978-1815 ccal yr B.P.). The sediment consists of moderately to strongly decomposed peat, rich in

roots of monocots. The pollen percentages of the Ericales rise and attain a second maximum at depth 271 cm and maintain a rather high level till the end of the subzone. Among the ericaceous taxa Erica tetralix dominated. The curve of Sphagnum spores shows a very distinct maximum at depths 276-271 cm. Macrofossils of Sphagnum (e.g., section Cuspidata) show the local presence of peat mosses. The preservation conditions were apparently better than they were in subzone B2, where only Sphagnum sporangia and very few, highly decomposed leaves were found. The Cyperaceae pollen percentages show a maximum again. The pollen percentages of Myrica reach high values from depth 280 cm onward and stay very high throughout the whole subzone B3, together with macro remains of Myrica gale. The curve of the fungal spore Type 16A, already started at depth 282 cm, reaches very high percentages in this subzone. The curves of the fungi Type 18 and Type 12 increase at the beginning of this subzone and reach maxima at the levels 271 267 cm. The local stand of vegetation resembles a Myrica variety of the Ericetum tetralices and indicates somewhat drier and slightly minerotrophic conditions. Myrica gale is most common at sites where the water-table is relatively close to the surface (less than 10 cm) in midsummer. The root system is shallow, with well developed laterally spreading secondary roots (Schwintzer and Lancelle, 1983).

Zone C: 265-253cm below NAP(1815 1674ccal yr B.P.). Zone C is very poor in macrofossils. The sediment consists of strongly decomposed peat, mainly consisting of roots of monocots (cf. Molinia). Epidermal fragments and seeds of Molinia caerulea are found in the upper part of this zone. The zonation is based on the distribution of microfossils. The pollen curves of Poaceae and Lysimachia increase sharply and attain maxima at the depths 259-257 cm, then decline sharply and stay very low till the end of zone C. The pollen percentages of Myrica decrease sharply. Halfway this zone the pollen percentages of Ericales increase and

256 show a third maximum at depth 255 cm. The curve of monolete psilate spores has a sharp maximum at the end of zone C. Other curves of microfossils such as those of Cercophora (Type 112), Type 55A, Gelasinospora (Type 1) and the new Type 495 show remarkable successive maxima. Cercophora (Type 112), Type 55A, Gelasinospora (Type 1) are indicative for local dryness. The maxima of pollen percentages of Typha angustifolia, T. latifolia and seeds of Typha spec. point to open water in the vicinity of the sampled site. The slight rise of the pollen curves of Asteraceae tubiliflorae, Chenopodiaceae (+ Chenopodiaceae seeds), Filipendula, Ranunculus, Cyperaceae (+ Carex fruits) and Brassicaceae point to a somewhat ruderal vegetation. The very high percentages of the spores of the fungus Herpotrichiella (Type 22) - - living on wood, for example of Calluna (Van Geel, 1978) - - may point to a high degree of decomposition of the woody material. Characteristic human influence indicators, such as Plantago, Rumex and Artemisia are not better represented than they are in zones A and B. The percentages of Cerealia" pollen are significantly higher than in zones A and B and point to human activity, such as crop processing, Human influence is discussed later. The local stands of vegetation was another phase of the Oxycocco-Sphagnetea probably dominated by Molinia tussocks with Apiaceae, Lysimachia,

Gentiana pneumonanthe, Rhynchospora alba, Sphagnum and Potentilla-type (+Potentilla erecta seeds) between the tussocks, Charcoal and the fungus Gelasinospora (Type 1) point to local fires which could have favoured the growth of Molinia. Molinia tussocks are wet and very resistant to fires, Probably as a consequence of an increase of the water-table Molinia disappeared at the transition to zone D.

Zone D: 253-195 cm below N A P (1674-1008 ccal yr B.P.). The sediment consists of less to strongly decomposed peat, mainly consisting of rootlets of Ericaceae and remains of Calluna

vulgaris, Erica tetralix, Eriophorum vaginatum and Sphagnum. A more detailed description of

the vegetation development of zone D is given below.

Subzone DI: 253-218 cm below N A P (1674-1415 ccal yr B.P.). The vegetation cover was dominated by Eriophorum vaginatum, Erica tetralix, Calluna vulgaris while Pohlia nutans and Sphagna sect. Acutifolia dominated the moss layer. Local stands of Oxycoccus palustris, Andromeda polifolia, Scirpus cespitosus and Aulacomnium palustre completed the picture of one of the communities of the Erico Sphagnion. At the top of this zone (depth 226-220 cm) Sphagnum section Cuspidata occurred in the stand. The synecology points to an oligotrophic, relatively wet environment.

Subzone D2." 218-201 cm below N A P (1265-1104 ccal yr B.P.). Several curves of microfossils increase and attain conspicuous maxima successively: Myrica, Helicoon pluriseptatum (Type 3 0 ) , Cyperaceae, Ericales, Tilletia sphagni (Type 27), Amphitrema flavum (Type 31A), Sphagnum, Assulina (Type 32), ArceUa and Anthostomella fuegiana (Type 4). Many macrofossils of Myrica gale throughout the whole subzone point to the presence of Myrica gale in the local stand of vegetation. Macrofossils of Oxycoccuspalustris attain high values at depth 202 cm. The local stand resembles a community of the Erico-Sphagnion, but with local growth of Myrica gale. At the depths 207-201 cm a less decomposed Sphagnum section Acutifolia layer was recognizable in the peat profile. This peatmoss layer represents a wet stage in the peat development, but not of strictly local occurrence as it extended over several hundreds of square meters (see Fig.2, profiles). Probably it is indicative of a more regional change in the water table.

Subzone D3: 201 195cm below N A P (1104 1008 ccal yr B.P.). Eriophorum vaginatum macrofossils predominate in this subzone. Most of the macrofossils of subzone D2 have disappeared. From the beginning of this subzone curves of the microfossils Gelasinospora (Type 2), Anthostomella fuegiana (Type 4), Type 55A, Pleo.

257

spora (Type 3B) and Type 18 successively attain maxima. The pollen curve of Rumex has a very remarkable maximum, and the pollen curve of Myrica reaches its third maximum. The pollen curve of Poaceae, showing an increase from the middle of subzone D1 on, now reaches a third maximum, The local stand of vegetation is interpreted as a community of the Erico-Sphagnion, in which Eriophorum vaginatum is somewhat over-represented. At depths 197 195 cm, a less decomposed Sphagnum palustre layer, pointing to more mesotrophic conditions, was recognizable in the peat profile. The high values for Poaceae and Rumex may point to increasing human influence in the vicinity of the sampled section, Extra.local vegetation development In the literature the interpretation of the local vegetation types varies with the scale of the area that is considered for an investigation of the vegetational development (Janssen, 1973). The local vegetation development of the section Assendelft-KERK is mainly based on the distribution of macrofossils in the peat section studied. The macrofossils mainly represent the plants grown at the sampled spot. Together with some data from the microfossil analyses, a detailed reconstruction of the local vegetation type could be made. In the present study an attempt was made to describe the extra-local vegetation development, which concerns the vegetation cover of the peat complex in a few hectares around the sampling site. Arguments pleading in favour of this are: the ancillary information available from an extensive geological survey in the area conducted by Vos (1983), from many drawings made of the soil profiles (a few profiles are depicted in Fig.2B) in the excavation pits at site O, and from palynological investigation (Witte and Van Geel, 1985). Profile I (48 m) is north-south orientated, and profile II (44 m) is e a s t- wes t orientated, and intersects profile I about 50 cm n o r t h of the section AssendelftKERK. In the centres of the profiles the oligotrophic peat is very well represented

because of the medieval church which had pushed the underlying peat downwards. At the ends of the profiles already much of the oligotrophic peat is absent caused due to erosion during and after the Middle Ages. Background information of the dates used in this chapter will be discussed in a separate chapter (p.275 and Fig.9). Initially the whole area within the ranges of the profiles was covered with reed swamps, in shallow brackish to fresh eutrophic water. There was some influence of the tidal flats to the west of it. These reed swamps lasted till c. 2320-2358 ccal yr B.P. at site O/R and till c. 2177-2258 ccal yr B.P. at site O and subsequently developed into a transitional fen. In the west (site O) a greater number of representatives of the Convolvuletalia sepium occurred among the micro- and macrofossils, because the influence of the eutrophic water on the vegetation development was somewhat stronger than at site O/R. An increasing drainage of the area causing a substantial (temporary) lowering of the water-table in the peat together with an increasing acidification and oligotrophication favouring the development of a Molinia-dominated vegetation. This Molinia vegetation covered a r a t h e r large area of the mire surface, and in t hat case the underlying ecological factors were of a more regional character. The Molinia vegetation lasted till c. 2270 cca! yr B.P. at site O/R and till c. 2060-2150 ccal yr B.P. at site O. The oligotrophication of the mire led to the development of heather-dominated raised bog vegetation (Oxycocco-Sphagnetea). At site O a somewhat drier variety could establish itself than at site O/R. Afterwards Myrica codominated the vegetation. Lateral water movements from more elevated spots towards the more low-lying boundary in the bog system could have been the main factor for the establishment of Myrica in the stand. Ratcliffe (1964) is of the opinion that in Scotland the recent Molinia-Myrica mires are evidently anthropogenic, but in former days natural processes may have favoured the establishment of Myrica as well. Peat deposits rich in

258

Myrica remains are found in large areas in the western part of The Netherlands. This peat is mostly found in transitional situations between eutrophic and oligotrophic peats (Bakker and Van Smeerdijk, 1982, Van Smeerdijk, unpubl., P.C. Vos, pers. commun.). The profiles | and II (Fig.2B) show that the Molinia-Myrica peat occurred as patches of 15-20 m 2. In the Assendelver Polders stands of Myrica are thought to have formed part of open raised bog vegetation, dominated by heaths, with scattered patches of Myrica and Molinia tussocks, Later on, from 1759-1847 ccal yr B.P. at site O/R and from 1735 1815 at site O, Myrica disappeared from the stand, and a Molinia grassland remained. The increasing human influence (the Roman Iron Age habitation period), probably grazing cattle on the bog and burning the bog surface, must have been the main reason for the disappearance of Myrica out of the vegetation (Ratcliffe, 1964). The Roman Iron Age habitation horizon is not well discernible in the peat profiles. At site O it was represented by a Molinia peat layer, associated with ditch-like features (Fig.2B) about 5 m to the north and 7 m to the east. These ditch-like features probably were the places where the Typha species had been growing. Farther along the profiles, the Roman Iron Age habitation horizon is probably represented by the transition between Molinia peat and Ericaceae-Sphagnum-dominated peat. At site O/R the habitation horizon is visible as a charcoal layer, Subsequently the Assendelver Polders became covered with an oligotrophic raised bog, which formed a part of the raised bog complexes at that time, covering vast areas of the western Netherlands. In the section Assendelft-KERK this oligotrophic peat is well developed. The upper part again represents a Myrica variety of the Oxycocco Sphagnetea (cf. zone D2), but this vegetation is not so very well developed in the profiles. Remarkable features are the thin layers consisting of Sphagnum section Cuspidata and S. section Acutifolia respectively, which extended at least 9-28 m across in the profiles. Presumably they represent very wet stages in the bog development,

Regional processes such as a storm surge on 26th December 838 A.D. and numerous indications of river floods (Gottschalk, 1971) in the 9th century A.D. could have effected rises of the water table in the bog areas. If this is true, at least the water table must have been high enough for a certain period of time to allow a visible reaction of the vegetation development. On top of the above-mentioned Sphagnum section Acutifolia layer there is a 6 25 cm thick deposit of oligotrophic peat in the profiles. At site O/R (Witte and Van Geel, 1985) this oligotrophic peat became eroded during the medieval occupation period. At site O there is no clear evidence for any disturbance, and the oligotrophic peat apparently is a natural in situ deposit, dominated by Eriophorum vaginatum. From archaeological information (Besteman and Guiran, 1986) we know that turves (dutch: plaggen) were used for the floor in the church and in the grave yard, visible in the profiles as short Sphagnum bands. Concentrations of chips of wood just outside the northern foundation of the church may also indicate a superficial disturbance of the peat. The topmost 2 cm (Sphagnum palustre)from site O are probably part of a turf. The correlation described above between site O and site O/R is based on palaeobotanical records. When comparing the depths and ages of the particular zones, then we are confronted with some problems. Comparable zone borders at site O lay c. 28-40 cm below those of site O/R. The peat layers in the profiles (Fig.2B) show a slant of c. 30-40 cm. Differences in depth (below NAP) of the zones do not really pose a problem when it is taken into account that the church on top of site O pressed the underlying peat farther down than the medieval house at site O/R. Differences in age (c. 130 160 yr) of the corresponding zone borders of both sites probably mainly reflect that the vegetation succession showed minor shortdistance differences in time and space. More to the west (site O) the eutrophic and mesotrophic stands of vegetation persisted longer than at site O/R.

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267

Regional vegetation development Pollen spectra always consist of a mixture of pollen of (strictly) local and of regional origin, The ratio of these groups is not always the same during the period covered by the section investigated. In the present study the macrofossils and the bulk of the herbaceous pollen quantities reflect the local and extra-local vegetation development. The area between the coastal dunes and the developing raised bog in the east (e.g., the Assendelver Polder), the actual Oer-IJ estuary, was partly covered by almost treeless reed swamps (Zagwijn,1971; Vos, 1983). Local stands of Salix in the reed swamps can be deduced from in situ finds of Salix wood at site O/R (J.C. Besteman, pers. commun.). In the present study the vegetation of the Oer-IJ estuary is left out of the discussion, because too little is known about the former types of vegetation in that area. The fluctuations in the tree-pollen quantities as recorded in bogs reflect the regional forest development. The coastal dune area to the west and southwest is thought to be the main source of the tree-pollen. Palynological investigations from sites in the western Netherlands (Jelgersma et al., 1970; Witte and Van Geel, 1985) have the disadvantage that the tree-pollen frequences are very low. Pollen diagrams were constructed by using different pollen sums with arboreal and non-arboreal elements, which renders a comparison of the pollen diagrams rather complicated. In spite of the scarcity of tree-pollen in the samples a traditional tree-pollen sum (EAP> 150) is used in the present study. The catchment area for treepollen is wider in the Assendelver Polders than in the dune area as already suggested by Jelgersma et al. (1970), which may mean that general trends in the forest development are better registered in the hinterland (e.g., the Assendelver Polder) than in the dune area itself. The most important forest elements (see Fig.3, pollen percentages diagram and Fig.8, pollen concentration diagram) were Alnus, Quercus and Fagus. The relatively low pollen frequences of Betula and Pinus indicate that both trees did not play an important role in the

forest development. The high percentages of Fagus pollen (< 5~/o) and a continuous curve for Carpinus pollen place the whole section in the Subatlanticum, which dating is supported by the radiocarbon dates. Different pollen sums have been used for the construction of a series of '~Iversen diagrams" (Figs.6A D), in which the relation between the principal palynologically defined vegetation types is shown. The main trends in Figs.6A, B are the increasing raised-bog development, somewhat obscured by the very local contribution of pollen of Molinia and Myrica, on the one hand, and the regional forest development in three phases on the other. In Figs.6C and D two and three theoretically distinguishable forest types respectively are represented by their pollen frequences. Both forest types, the oak forests from the rather dry soil and the alder brook forest from the wet soils, contributed more or less equal to the total of the forest elements, and their contributions do not change much along the profile. The first phase in the forest development (depths 305-296 cm) is characterised by rather high percentages of Quercus pollen (25-43%) and increasing ones of Alnus pollen (15-40°/O). Pollen of Pinus is over-represented because it is easily transported by water. In this phase there is some evidence for alder forest, but at a rather great distance from the sampling site. Probably an open oak and beech forest was present on the Older Dunes. This phase is comparable with zone V2b of the work of Jelgersma et al. (1970). The second phase (depths 294-265 cm) in the forest development is characterised by an increase of Alnus pollen up to c. 65%, and a decrease of Quercus pollen to c. 20% ; the values of Fagus fluctuate between 4 and 10°/'o. The maximum extension of the forest elements such as Alnus, Quercus, Fagus, Betula and Pinus (see Figs.6A, B and Fig.8) in this phase is between the depths 286 and 273 cm. There is no good correlation between the palynological zonations of the coastal dune area and the Assendelver Polder. The oak forest established itself and from 2150 yr B.P. onward an alder forest developed. In the westernmost part

268

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269 of the coastal dune area (Velzen Vormenhal, see Jelgersma et al., 1970) after 1910 yr B.P. an alder brook forest developed (zone V3). The development of alder brook forest recorded in the Assendelver Polders had taken place somewhat earlier, probably on the beach plains bordering the east side of the- most western Older Dunes. An extension of the alder brook forest on the beach plains implies r a t h e r high fresh water ground water tables in the beach plains, and as a consequence the marine influence was decreasing in the beach plain area. At the end of the second phase, c. 1875 yr B.P. the pollen curve of Alnus decreases and in the ~Iversen diagrams" (Figs.6A, B) the curve of the forest elements attain minimum values, None of the other tree-pollen percentages curves show a decline, but the pollen concentration curves of Betula, and less clearly those of Quercus and Fagus also fall off. The conclusion must be an increasing human influence (forest clearance, in particular the alder brookforest, and at the local level an extension of the Molinia mire). The third phase (depths 253 195 cm) in the forest development started about 1685 yr B.P. with an increase of the pollen percentages of Alnus, Fagus and Carpinus, and a decrease of the pollen that of Pinus. The curves of Betula and Quercus at first stay at the same level and subsequently decrease slightly. The high percentages of Fagus (up to 28~o) and Carpinus (up to 8~o) are very conspicuous and point to re-afforestation. In the coastal dunes from the Roman Period onward there are indications of re-afforestation (Jelgersma et al., 1970), this agrees very well with the results from the Assendelver Polder. Towards the top of the section the total forest elements decrease, which point to an increasing human influence. Jelgersma et al.

(1970) found t hat upon the whole the pollen percentages of Quercus are higher than those of Alnus, except in the situation where wood peat is involved (Velzen Vormenhal). In the present study of the Assendelver Polders the percentages of Alnus pollen are always higher than those of Quercus, except in seven samples from the bottom of the section. In the upper part of the section the figures of Fagus pollen are often higher than these of Quercus pollen. Fagus was an important element in the dune forest, and attained its greatest extension in the dunes southwest of Haarlem after 1360 yr B.P. (Jelgersma et al., 1970). The high pollen percentages of Fagus (10 28%), originated from an area c. 15 km SW from the Assendelver Polder, are very important for the interpretation of the Alnus pollen percentages (40 6 8 ° ) . The alder brook forest must have been situated closer to the sampling site than the beech and oak woods, for example on' the beach plains, but not so close to the bog area. The alder brook forest could not have covered vast areas, and this implies that the whole area of the OerIJ estuary was very poor in alder forest. This conclusion substantiates the idea about the scarcity of trees in this area (Zagwijn, 1971; Vos, 1983). The main conclusion from these results is t hat in the Assendelver Polder a forest development in an extensive area has been recorded, particularly as regards the forest on the dry soils. Extremely low quantities (20%) of Alnus pollen are found at depths 204 206 cm. The pollen concentration of Fagus is higher than that of Alnus in these samples (the ratio Alnus:Fagus pollen is c. 0.9). This all points to dramatic changes in the alder forest. Depth 204.2 cm has been dated at 1210___50 yr B.P., this is 694-887 cal A.D. (Stuiver and Becker,

Fig.6. Series of "Iversen diagrams", showing the relation between the main vegetation types in the section AssendelftKERK. A: Trees (all), Myrica, Poaceae, Cyperaceae, Ericales. B: Regional forest (all trees excl. of Alnus and Salix), subregional forest (Alnus), swamp (Poaceae, Cyperaceae, Salix), shrubs (Myrica), raised bog (Ericales, Sphagnum, Rhynchospora, Drosera). C: Quercetea (Quercus, Fagus, Carplnus, Ulmus, Tilia, Acer, Fraxinus, Corylus, Pinus, Betula), Alnetea (Alnus). D: Querceto robori-petraeae (Quercus, Fagus, Betula, Pinus), Querco-Fagetea (Quercus, Fagus, Carpinus, Ulmus, Tiha, Acer, Fraxinus, Corylus), Alnetea (Alnus).

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271 1986). In the n i n t h c e n t u r y A.D. one s t o r m s u r g e and n u m e r o u s i n d i c a t i o n s of r i v e r floods ( G o t t s c h a l k , 1971) m i g h t h a v e b e e n r e s p o n s i b l e for the p a r t i a l d i s a p p e a r a n c e or d a m a g e of the a l d e r b r o o k forest, in p a r t i c u l a r on the b e a c h plains w h i c h w e r e m o r e low lying t h a n the dunes. If this is true, a q u i c k r e c o v e r y of the a l d e r forest a f t e r this c a t a s t r o p h e , as the p o l l e n c u r v e s of Fig.3 suggest, m u s t h a v e t a k e n place. It is u n c e r t a i n if this h a p p e n e d indeed,

Dating, pollen concentration and pollen influx F o r a detailed r e c o n s t r u c t i o n of v e g e t a t i o n p h a s e s in the m i r e d e v e l o p m e n t , and e s p e c i a l l y for a n e s t i m a t i o n of t h e d u r a t i o n , a reliable d a t i n g s y s t e m is required. T h e best s y s t e m for p e a t at the m o m e n t is r a d i o c a r b o n dating. The c o n v e n t i o n a l r a d i o c a r b o n d a t e is b a s e d on the a s s u m p t i o n t h a t the 14C level h a s b e e n cons t a n t in the past. H o w e v e r , p a s t a t m o s p h e r i c 14C levels h a v e fluctuated, and as a r e s u l t a r a d i o c a r b o n age is o n l y a n a p p r o x i m a t i o n of the h i s t o r i c a l age expressed in c a l e n d a r years, T h e r e f o r e a single 14C age m a y c o r r e s p o n d w i t h s e v e r a l c a l i b r a t e d ages (Stuiver, 1982), w h i c h is to s o m e i n v e s t i g a t o r s a r e a s o n to r e j e c t c a l i b r a t e d r a d i o c a r b o n ages ( J e l g e r s m a et al., 1970), o t h e r w o r k e r s on the o t h e r h a n d , are of the o p i n i o n t h a t c a l i b r a t e d ages give a b e t t e r i n s i g h t into e r r o r s of the r e c o r d e d r a d i o c a r b o n a g e s (Van H e e r i n g e n , 1986). In the p r e s e n t s t u d y the r a d i o c a r b o n a g e s of b o t h sites (O and O/R) h a v e b e e n ( r e ) c a l i b r a t e d a c c o r d i n g to S t u i v e r a n d B e c k e r (1986), by m e a n s of a c o m p u t e r p r o g r a m for r a d i o c a r b o n age c a l i b r a t i o n ( S t u i v e r a n d Reimer, 1986). T h e d a t e d s a m p l e s a r e not a l w a y s at the b o r d e r s of the p a l y n o l o g i c a l z o n a t i o n s , so t h a t an interpolation of the c a l i b r a t e d r a d i o c a r b o n ages m u s t be made. As a l r e a d y m e n t i o n e d , some radio-

c a r b o n a g e s c o r r e s p o n d with m o r e t h a n one c a l i b r a t e d age. As a rule one uses the m e a n age for f u r t h e r purposes. F o r c a l c u l a t i n g the age of zone borders or a n y o t h e r depth, different r e l a t i o n s (linear or n o n - l i n e a r ) b e t w e e n d e p t h s and c a l i b r a t e d ages h a v e been tested u s i n g the statistic computer program STATWORKS available ontheMacintoshcomputer(Rafferty et al., 1985). Two l i n e a r r e g r e s s i o n s , one w i t h six c a l i b r a t e d r a d i o c a r b o n ages (not shown), a n d one with s e v e n c a l i b r a t e d r a d i o c a r b o n ages (Fig.7A), were used for p l o t t i n g c a l i b r a t e d age a g a i n s t depths. T h e c o r r e l a t i o n coefficients are 0.984 and 0.983 respectively, so t h a t on this score a c o n s t a n t p e a t a c c u m u l a t i o n r a t e c a n be a s s u m e d to h a v e occurred. F i g u r e 9, c o l u m n s D and F, shows the c a l c u l a t e d ages b a s e d on the l i n e a r r e l a t i o n s h i p . T h e studied p e a t section r e v e a l s different types of v e g e t a t i o n ( a l t h o u g h the p r i n c i p a l v e g e t a t i o n type is a raised bog) and different p e a t a c c u m u l a t i o n r a t e s m a y be expected. A s m o o t h c u r v e (in this case a thirdd e g r e e r e g r e s s i o n line) t h r o u g h the s a m e a b o v e m e n t i o n e d d a t a p a i r s p r o b a b l y gives a b e t t e r r e p r e s e n t a t i o n of the r e l a t i o n b e t w e e n calib r a t e d r a d i o c a r b o n ages and d e p t h s (see Fig,7B for a plot of s e v e n cal. ages a g a i n s t depth). The c o r r e l a t i o n coefficients a r e 0.999 and 0.999. F i g u r e 9, c o l u m n s C and E, shows the calculated ages b a s e d on the t h i r d - d e g r e e regressional r e l a t i o n s h i p . T h e r e are no g r e a t differences in the c a l c u l a t e d ages b e t w e e n the series of six and the series of s e v e n c a l i b r a t e d ages, if one type of r e g r e s s i o n line is used (see Fig.9, c o l u m n s C/E a n d D/F). In the u p p e r p a r t of the section a d a t i n g of the s a m p l e s is not possible if a n o n - l i n e a r r e l a t i o n b e t w e e n c a l i b r a t e d radioc a r b o n ages and depths is assumed. T h e r i g h t h a n d p a r t of Fig.9 shows the rec a l c u l a t e d ages of site O/R, to m a k e t h e m c o m p a r a b l e w i t h t h o s e from site O. W i t t e and

Fig.7. A and B. Calibrated ages plotted against depth, excluding GrN-13031, for GrN-13930 the mean age is used. A. linear regression (corr. coef. 0.983), B. third degree regression (corr. coef. 0.999). D and C. Idem calibrated ages plotted against cumulative accumulated arboreal pollen over a sedimentary column of 1 cm2. C. linear regression (corr. coef. 0.992), D. third degree regression (corr. coef. 0.995). E and F. Cumulative accumulated arboreal pollen over a sedimentary column of 1 cm2 plotted against depth. E. linear regression (corr. coef. 0.995) and F. third degree regression (corr. coef. 0.999).

272

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273

Van Geel (1985) based the dating of the zone borders on partially older calibrated ages (than used now) in combination with pollen-density dating. P ar ticu lar l y the deeper zone borders appear to be much older. Only the present dates concerning the Roman Iron Age (zone C) correspond with those in their paper. The influence of the Roman Iron Age habitation in this particular area must be perceptable at both sites at the same time. F o r t u n a t e l y at both sites the influence of the Roman Iron Age habitation started c. 1800 yr B.P. Middeldorp (1984) worked out a method for indirect dating in a peat bog ecosystem based on pollen concentration. Fluctuations in the pollen co n cen tr at i on are caused primarily by fluctuations in pollen production and also by the fluctuations in the accumulation rate of the sediment. De Vries and Wymstra (1986) developed a computer program that calculates the relation between accumulated pollen and time, and subsequently the relation between accumulated pollen and depth in order to calculate the relation between depth and age. In the present paper the regionally produced arboreal pollen is used for the determination of

the pollen time function. Two regressions, one linear and one third-degree, were plotted as a function of accumulated arboreal pollen numbers through the same calibrated radiocarbon dates as for Figs.7A, B (Figs.7C, D). The correlation coefficients are 0.992 and 0.995, respectively. The calculated ages based on these regressions are shown in Fig.9, columns B and A. To check the relation between accumulated pollen and depth anot her two regressions were plotted (see Figs.7E, F). The correlation coefficients are 0.995 and 0.999 respectively. Since the time scale in Fig.9, column F, is thought to be the most useful one, all further calculations in which calibrated ages are involved are based on this time scale (mean 11.7 yr cm 1). The total of arboreal pollen (1,134146) from the 110 cm of this section fallen in a time span of 1280 years, gives a mean pollen influx of 886 arboreal pollen per cmZ/yr. The conclusions to be drawn from this chapter are: (a) there is a linear relation between time and depth, i.e., a linear peat accumulation rate in the present case of the peat deposits of site O. Hence the pollen concentration diagram (Fig.10) is also a pollen

Fig.8. A and B. Calculated ages based on accumulated arboreal pollen and calibrated radiocarbon dates excluding GrN13031. For GrN-13930 the mean age is used. A. Third-degree regression function is used. Correlation coefficient 0.995 (see Fig.7D). B. Linear regression function is used. Correlation coefficient 0.992 (see Fig.7C). C and D. Calculated ages based on depth and calibrated radiocarbon dates excluding GrN-13031 and GrN-13930. C. A third-degree regression is used. Correlation coefficient 0.999. D. A first-degree regression is used. Correlation coefficient 0.984. E and F. Calculated ages based on depth and calibrated radiocarbon dates excluding GrN-13031. For GrN-13930 the mean age is used. E. A third-degree regression is used. Correlation coefficient 0.999 (see Flg.7B). F. A first-degree regression is used. Correlation coefficient 0.983 (see Fig.7A). G and H. Calculated ages based on all recalibrated radiocarbon dates of site O/R. G. A first-degree regression is used. Correlation coefficient 0,998. H. A third-degree regression is used. Correlation coefficient 0.997. I and J. Calculated ages based on recalibrated radiocarbon dates of site O/R, excluding GrN-11585, GrN-11581 and GrN11579. I A first-degree regression is used. Correlation coefficient 0.983. J. A third-degree regression is used. Correlation coefficient 1.000. amean age (range of calibrated ages 1093 1170); GrN-13031. hmean age of the two calibrated ages 2151 and 2284; GrN-13930. Cmean age (range of calibrated ages 1420 1517); GrN-11585. dmean age (range of calibrated ages 2184 2300); GrN-11581. Cmean age (range of calibrated ages 2404 2708); GrN-l1579. NB: The ages from column F are used for the calculations of peat accumulation etc.

274

influx diagram, and (b) there is a linear relation between the total arboreal pollen accumulation and the depth, so that fluctuations in the individual arboreal pollen concentration curves reflect fluctuations in pollen production and thus record structural changes in the vegetation cover,

Dry weight, peat accumulation, ratio macrofossils and fine fraction and ash content The amount of peat was not sufficient to carry out separate elemental analyses or analyses in du(tri)plicate. The analyses of K, Na, Ca, Mg, Fe, So 4 and total P were performed with the same samples used for the measurements of wet weight, dry weight and ash content (see F ig . l l for the results of these analyses). The ash content is usually measured after incineration at 550°C (Chapman, 1964), but, since in the present study incineration at 550~C left a black residue, empirically a minimum incineration temperature of 750~C was assessed. The ash was dissolved in diluted HC1, and in nearly all the samples a white residue remained. This was interpreted as SiO 2 (Chapman, 1964). Only in the deeper subsamples of the section could the amount of SiO 2 be measured by weighing. The curve of the bulk density (Fig.ll) indicates a fair constancy of the values t h r o u g h o u t the section (mean value 0.17g cm ~), despite the changes in vegetation type. Bulk density values in peat of c. 0.17 point to high rates of decomposition (Clymo, 1983). Decay of the organic m a t t e r mainly occurs at the surface of the peat above the water-table, and is much slower in the water-logged peat. Drainage of the peat in recent times may have caused a secondary decomposition of the peat in the superficial layers. In the Assendelver Polder the secondary decomposition is negligible, because the peat was completely waterlogged, the top of the section lying 195 cm below NAP (the local water table lies at 175 cm below NAP). The bulk density not only depends on decomposition but also on production

and compaction. After the peat reaches the water-saturated zone, the compaction is stronger in peats formed under very wet conditions (Sphagnum peat) than in peats formed under dryer conditions (Ericaceae-rich peat). This implies that the bulk densities of these peats can have more or less the same values. In the present study compaction of the peat must have been caused by the pressure of the medieval church built on top of the peat surface. The assessment of the peat accumulation depends on a reliable and sufficiently detailed time axis (see the preceding chapter). In this study a mean age of the peat samples (11.7 yr cm 1) __ the number of years the sample encompasses - - is assumed. Figure 10A shows the peat accumulation profile expressed as g m -2 yr ~, and there are no great differences between the samples (mean value 148 g m 2 yr ~). The topmost samples gave somewhat higher values, but one must keep in mind ~o~ ~a~ I 40

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pp.275-276

Assendelft-KERK Pollen concentration

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Myrica

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Sphagnum

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Ericaceae/Sphagnum peat

Fig. 10. Pollen concentration diagram, Assendelft-KERK.

Plot unit is 100 pollengrains per cm3

279

that the dating of these samples was not satisfactory. In Table II the total and average peat accumulation for each vegetation type, the main constituents and the duration in calendar years of each local phase, and the average amounts of macrofossils and strongly decomposed material of each vegetation are given. The peat accumulation rates of the different vegetation phases, recorded as cm yr 1 has the same value (0.085 am yr l) because of the assumed linear relationship between depth and time (see preceding chapter). Only a crude comparison with other data on peat accumulation can be made. Tallis (1983) gives a very general impression of the peat accumulation rates (0.018-0.136; mean 0.045 cm yr-1), Middeldorp (1984) and Witte and Van Geel (1985) give peat accumulation rates of several vegetation types from peat sections (0.02-0.17; mean 0.06 cm yr 1, and 0.017 0.195; mean 0.08 cm yr -1 respectively). As could be expected the present results correlate best with those of Witte and Van Geel (1985). There are several methods to estimate the degree of decomposition of peat. When a comparison between the methods is made, there is an overall general agreement, but in details the agreement is poor (Clymo, 1983). Nevertheless, if used in a comparative way, the methods may yield useful insights into some of the processes of peat formation and decomposition. In the present study none of these

methods was used, but an attempt was made to quantify the size fractions that reflect some aspects of the decomposition. Figure 12 shows the relative contribution of the macroscopic, microscopically unidentifiable fine and basesoluble fractions of the peat. The three frac-

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21022o~ 230zd ~ ~ 240z~ ~ ~ 2sorv

c .

~ z6o27028oe903oo3o5.

~~_~_

="

Fig.12. Relative c o n t r i b u t i o n of macrofossils (fraction > 190 pm) and t h e fine fraction ( < 190 pm) to the peat mass after t r e a t m e n t with a 5% KOH solution. Left: macrofossils, middle: fine fraction, right: KOH soluble fraction.

TABLE II Total and average a c c u m u l a t i o n of peat and of peat fractions and the d u r a t i o n in years recorded for different types of v e g e t a t i o n in the Assendelver Polders, section A s s e n d e l f t - K E R K Peat

Local Depth below Main constituent zone NAP (CA) D3 D2 D2 D1 C B3 B2 B1 A2 A1

201-195 207-201 218-201 253-218 265-253 279-265 286-279 296-286 301-296 305-301

Eriophorum, Sph. palus~e Sph. s. Acutifolia Sph: s. Acutifolia, Ericac., Myrica Calluna, Erica, Eviophorunl, Pohlia Ericaceae, Molinia Erica, Myrica Erica, Calluna, Bryophyta Molinia Phragmites Phragmites

Peat fractions >190g <190p_ Number Total dry Mean dry cm y.l Mean dry Mean dry of years weight weight weight weight g m_2 y_l g m_2 y_l g m_2 y_l g m_2 y_l 58 1265 211 0.103 38 46 70 692 115 0.086 37 59 199 2692 158 0.085 29 69 409 5265 135 0.085 29 66 141 1658 138 0.085 22 65 163 2068 148 0.086 23 53 82 1043 149 0.085 20 64 117 1393 139 0.085 27 66 59 761 152 0.085 13.5 69 46 752 188 0.087 11 72

280

tions together are taken to represent the total amount of organic matter of a sample. From Fig.12 it is very clear that the macroscopic remains (> 190 9m), in volume constituting the major part, on a weight basis appear to be the minor part. The fine fraction (11-190 pm), including fragments of roots, bark, leaves, stems, seeds, fungal hyphae, pollen grains and spores, is usually the main fraction on a weight basis. This fraction does not contain any retained KOH. High values often correlate with an abundancy of ericaceous material in the samples. The KOH-soluble fraction is not measured but is assumed to contain the remainder of the material, because the other two fractions together do not reach the 100% (= total dry weight of the sample). This fraction also contains a mixture of products, such as humic acids and compounds liberated from the plant tissues by the KOH treatment. Figure 11 gives the amounts of total peat, macrofossil fraction and fine fraction expressed as g cm 2 yr ' Inorganic constituents and carbon content In the Netherlands there is no information available concerning the inorganic constituents in deep peat-profiles. Several profiles from other countries have been studied and discussed by Clymo (1983). In the present inquiry a first step was made to obtain a better insight into the content of inorganic constituents of peat-profiles. Figure 11 shows the concentrations of sodium, potassium, magnesium, calcium, iron, manganese, sulphate and phosphorus, all expressed as total amount (promiles-mg g 1) of the dry weight, and the ash and silica content, expressed as total amount (in percentages) of the dry weight. The ash content (the mean was 6.6% in the upper 104 cm) is rather uniform throughout the profile except in the lowermost six samples, where the ash content is much higher (mean 24.9%). The high values in the basal part of the profile are caused by the high concentration (mean 17%) of silica (SiO2), which in turn is derived from the reed fragments and the clastic material

(only visible in the pollen slides). The ash content of the peat in the Assendelver Polder is about 2-3 times greater than that of profiles described by Chapman (1964), Clymo (1983) and Middeldorp (1984). The Assendelver Polder section is situated much closer to the sea (c. 12 km) than the sites studied by the above mentioned authors. The relatively high ash contents in the Assendelver Polder are thought to be derived from dust from the dune area and the sea spray. Particularly the influence of sea spray can be deduced from the concentrations of magnesium, sodium and chlorine (Clymo, 1983). The latter element was not measured in the present study. The curve of magnesium reveals a rather constant (mean 7 mg g- 1) concentration profile, and in comparison with sodium (see below) magnesium is not indicative of marine influence; on the other hand, the curve of sodium shows a concentration profile which is rather constant (mean: 4.5 mg g 1) during the vegetation zones A, B and C, and declines gradually towards the top of the section (4.9-1.1 mg g 1). From the palaeoecological and geological data we know that the peat development in zones A-C took place at a time when the distance to the areas with marine influence was shorter than it was in zone D. The potassium curve has high values in zone A, and declines gradually towards the top of the section (c. 3 0.3 mg g '). Potassium has more or less the same, but less conspicuous pattern as sodium. The high concentrations of potassium(c. 3 m g g ') and iron (c. 1.3mgg -1) in the basal part of the section are probably attributable to the clastic material in the peat. The main portion of the curve of iron shows a rather constant concentration profile (mean: 0.38 mg g-l). From depth 224 cm onward the curve increases from 0.45 to 1 mg g 1 after which a steep increase occurs from depth 204 cm onwards to the top of the section (mean: 3.1 mg g-l). The curve of manganese has a rather constant concentration profile (mean: 0.02 mg g 1) in zones A and B and afterwards the curve increases, with some fluctuations, from 0.02 to 0.1 mg g-1. From depth 204 cm onward the curve increases rather steeply to the top of

281

the section (mean 0.19 mg g- 1). The curves of iron and manganese show more or less the same trends, both elements are thought to be derived from soil dust (Peirson et al., 1973). Probably these elements reflect an increasing human influence in the dune area (decreasing of dune forests, resulting in a more open vegetation type). The curve of calcium shows the same trend as that of magnesium in zones A, B and C (mean: 6.5 mg g-l), subsequently to increase slightly and to reach its maximum (16.5 mg g- ~) at depth 208 cm. At depth 205 cm the curve shows a steep decrease to 5 mg g-1 and to the top of the section the curve shows a low concentration profile (mean: 7 mg g-l). The curve of sulphate is rather constant in zones A-C (mean: 14 mg g-1), subsequently to increase slightly towards the top of the section, The curve of phosphorus show a slight increase (c. 0.29 mg g 1) in zone A2, which is correlated with the vegetation types of this zone, already pointing to a more nutrient-rich environment, The curve shows a constant concentration profile (mean: 0.16 m g g -1) in zones B and C. Interesting is that in zone C, the Roman Iron Age period, no increase of the phosphorus content of the peat is found. At the beginning of zone D the curve rises to attain values up to 0.46 mg g- 1 and declines slightly towards depth 207 cm (0.14 mg g 1). From that depth onward the curve increases to the top of the section (max. 0.55 mgg-1). The high phosphorus concentrations coincide with the levels containing roots of Eriophorum. Eriophorum is thought to be a main contributor to the relocation of phosphate from greater depths (Clymo, 1983), and may be responsible for the differences in phosphorus concentrations in the peat section studied, The curve of nitrogen shows high concentrations in zone A (1.7-2.2%) and declines to a minimum (0.9%) value in zone B1, and exhibits a nearly constant concentration profile (mean: 1.3%) in zones C and D. The curve of the carbon content shows a slight decline from c. 50°/'0 in the top of zone A to c. 45% in the top of the section. In zone A the carbon content attains a minimum value of 38°/0, in those

samples in which the ash content is very high. The C/Nratio increases from c. 23 in zone A to 54 at depth 289 cm in the centre of zone B1, afterwards to decline slightly to 25 at depth 275 cm, and subsequently to increase rather abrupt to 32 at depth 273 cm and subsequently to increase very slightly to c. 45 at depth 222 cm. The values for C/N in the upper part of the section lay between 33 and 36, only at the depths 206 to 203 the C/N ratio attaining higher (max. 45)values. Generally speaking a decreasing C/N ratio reflects an increasing rate of decomposition. On the other hand the C/N ratio depends also on the type of peat, particularly Phragmites peat deposits show a high content of organic nitrogen as a result of the higher availability of organic carbon and subsequent immobilization by micro-organisms (Clymo, 1983). The interpretation of the C/N curve is given below: the low values in zone A primary reflect the type of peat (Phragmites peat); the high values in zone B1 are attributable to the low nitrogen concentrations, the peat in this zone containing rather high amounts of coarse plant material, mainly Molinia remains (Fig.12), fungal hyphae (Fig.11) and spores (Fig.4). Although there is evidence for decomposition (the presence of fungal hyphae and fine grained plant material), the C/N ratio seems to be determined by the type of peat (Molinia peat). The lower values for the C/N ratio in subzones D2 and D3, caused by the slightly higher N-concentrations, are probably related with the macrofossils of Myricagale. The micro-organisms in the roots nodules of Myrica can fix appreciable amounts of nitrogen, and it is generally accepted that this contributes substantially to the nutrient status of mires inhabited by Myricagale (Dickinson, 1983). A similar phenomenon took place in subzone B3. The higher values for the C/N ratio in the top of subzone D2 are correlated with the Sphagnum layers exhibiting a low degree of decomposition. The general trend of a C/N ratio decreasing with depth can be observed, but it does not necessarily reflect an increasing rate of decomposition, the local plant species contributing

282 to the peat primarily determining the C/N ratio.

Fungal hyphae Fungal hyphae are found in the slides used for pollen countings and the amount of fungal hyphae was determined for every spectrum by estimating the combined length of all hyphae in 12 fields of vision of the microscope (400 x magnification) in combination with the counting of Lycopodium spores. The ratios of counted and total Lycopodium spores added were used for the calculation of the total amount of hyphae. In the last column of Fig.10 the amounts of fungal hyphae are presented as m cm 3 fresh peat. The distribution of the fungal hyphae is strongly correlated with the type of peat, particularly with the distribution of the rootlets of Ericaceae. Ericaceae point to relatively drier conditions in the peat. The fungal hyphae may be of mycorrhizal origin, This is confirmed by the curve of Type 10, a fungus associated with mycorrhizal roots of Calluna. In zone D, dominated by Ericaceae and Eriophorum, the amount of hyphae is 88 930m cm -3, the amount of hyphae in the peat where the peat mosses predominate is 40 98 m cm -3, on the transition of zone B1 and B2, dominated by Ericaceae, the amount of hyphae is 24 253 m cm 3, in the other zones the amount of hyphae is low (3 50 m cm 3). Human influence An extensive archaeological excavation program of the IPP, carried out in the Assendelver Polders in the years 1978-1983 (Brandt et al., 1987), clearly showed the influence of human activities during the settlement periods in this area. The aim of the present study was to investigate the natural peat development in the Assendelver Polder. The most important indicators of human influence in zones A-C are occurrences of pollen of Plantago, Rumex and Artemisia (Fig.3). These plants will not have grown on the mire surface itself, but probably on the sandy ridges along the creek system in

the western part of the area. Particularly the

Plantago pollen curve attains r a t h e r high frequencies. In zone C (1815 1674 ccal yr B.P.) the pollen curve of cereals attain higher values than in zones A and B. At depth 259 cm the curve reaches the highest value (12%) of the whole section. No other characteristic human influence indicator showed a corresponding higher abundancy. This zone contained charcoal, and because the peat is strongly decomposed, it is thought to represent the Roman Iron Age occupation layer. The pollen of cereals is thought to be derived from crop processing in the neighbourhood of the section, cultivation of cereals on the mire is most unlikely. During the archaeological excavation at site O/R the remains of a Roman Iron Age house were found (Besteman, 1981; Therkorn, 1981). Witte and Van Geel (1985) had no palaeoecological indications of human activities in the corresponding peat layer, other than the strongly decomposed peat and a charcoal layer. The beginning of the influence of the Roman Iron Age habitation is recognisable at both sites more or less at the same time (c. 1800 ccal yr B.P.). The Roman Iron Age habitation period has ceased to extend before c. 1588-1674 ccal yr B.P. at site O. From depth 239 cm (c. 1511 ccal yr B.P.) onward the curve of cereal pollen is constant at c. 1%, and from depth 228 cm (c. 1383 ccal yr B.P.) to the top the values fluctuate between 2 and 10°/0. Secale pollen is an important contributor to the cereal pollen curve from depth 239 cm onward. This may point to cultivation of Secale on the bog surface in the area. Final conclusions For the present study a good deal of valuable, ancillary information was available on the geology (Vos, 1983; P.C. Vos, pers. cornmun.), the palaeoecology (Witte and Van Geel, 1985) and on archaeology (Besteman and Guiran, 1986; T h e r k o r n and Abbink, 1987). T hat enables a study of the peat development in an unique situation in which both marine

283 influences are detectable and an oligotrophic bog developed. The studied section showed a detailed succession series ranging from eutrophic reed swamps through mesotrophic Molinia fen to an oligotrophic raised bog and covering a period of about 1200 years. In a parallel section a short distance (c. 60 m) away differences in age of c. 130-160 years were found for the borders of corresponding vegetation horizons, which gives a good impression of the development of the bog system in time and space. In particular the Molinia fen, as a transitional fen between the reed swamps and the raised bog, could be described and interpreted in detail. Also the presence of Myrica in mires is now better understood. Its growth was not primarily initiated by human activities as suggested by Witte and Van Geel (1985). It forms local patches of 15-20 m 2 within natural mires with Molinia and heathers. This vegetation type covered vast areas in the western Netherlands. In the peat described above developmental changes in vegetation had taken place within 100 years, and in this situation the radiocarbon dating is the limiting factor for accurate dates of vegetation phases. The period 2500 yr B.P. to recent times in particular gives many difficulties in the calibration of the radiocarbon datings (Stuiver and Becker, 1986). In the present study a time scale (Fig.9, column F) based on a linear relation between depth and time (mean age of the peat 11.7 yr cm-1) was chosen. One must bear in mind that all further results involving ages are based on the abovementioned linear relation between time and depth. The peat accumulation rate is c. 0 . 0 8 5 cm yr i or 148g m 2 yr 1, and there are no great differences between the vegetation types. The bulk density is nearly constant despite the changes in the stands of vegetation (mean value 0.17 g cm 3), which points to a high rate of decomposition of the peat). Aspects of decomposition have been visualised. The fine fractions on a weight basis form a greater portion of the total peat than do the macrofossils. The C/N ratios show a general trend of values decreasing with increasing depths, but

this trend does not reflect an increasing rate of decomposition. The local plant composition of the peat primarily determines the C/N ratio. Low values in reed peat (C/N ratio: 23) coincide with a high organic N-content as a result of a higher microbiological activity, and in Myrica-containing peat (C/N ratio: 33-36), owing to a higher organic N-content derived from the nitrogen-fixing microorganisms in the root nodules. Very high values are found in the peat mainly consisting of Molinia remains. Fungal hyphae are strongly correlated with the presence of rootlets of Ericaceae (indicative of relatively drier conditions), which points to a mycorrhizal relation, and do not influence the overall C and N metabolism in the peat. From the results of the analyses of inorganic constituents some interesting conclusions may be drawn: (1) the ash content (mean: 6.6~/o) is about 2 3 times higher than it is in other profiles (Chapman, 1964; Clymo, 1983; Middeldorp, 1984) which is attributable to a higher influx of sea spray and dust from the dune area, (2) the sodium concentration profile reflects the decreasing influence of sea spray during the bog development, (3) the concentration profiles of manganese and iron are thought to reflect the increasing human influence in the dune area (increasing deforestation), (4) there is no correlation between the inorganic constituents and the botanical composition of the peat, except that in the raised bog higher phosphorus concentrations coincide with the presence of roots of Eriophorum. Appendix As already mentioned in "the introduction" other chemical aspects of the peat have also been studied (Van Smeerdijk and Boon, 1987; Van Smeerdijk and Boon, in press). Briefly the results of this work will be discussed. Curie-point pyrolysis-mass spectrometry (Py-MS) has been applied for fingerprinting of the total peat from the same samples as used for the chemical analysis. Principal component analysis of these pyrolysis-mass spectrometric

284

PLATE I

i~31~'~i~!~'~!!!~!~' !ii '¸'~'¸'~~"'?~¸

;

;:

731b

~ld

492a

492 b

~

492 c

'P

494 a

Y

494 b

494 d

495 a

//

495 b

49,5 c

495 d

496

498a~

i

498b

497

494 e

285 data expressed as the first discriminant scores revealed a depth profile with varying distributions of carbohydrates and lignin. The P y - M S spectra of the oligotrophic peat (zone D) are dominated by pyrolysis products derived from carbohydrates, whereas the spectra of the peat of zones A-C, are dominated by pyrolysis products derived from lignins. Minor differences in the discriminant scores between the samples must be interpreted as differences in the botanical composition of the peat. Pyrolysis-gas c h r o m a t o g r a p h y - m a s s spectrometry ( P y - G C - M S ) was applied for structural identification of the pyrolysis products of a few total peat samples, rootlets of Ericaceae and leaves of Sphagnum. The P y - G C - M S data consist of evaporation products of absorbed compounds and a large number of pyrolysis products from remnants of the plant macromolecular systems. The pyrolysate of Sphagnum mainly consists of compounds derived from polysaccharides, including anhydro sugars from xylose, mannose, galactose and glucose. These anhydro sugars together with furans, pyranones, cyclopentanones and cyclopentenones indicate t ha t the Sphagnum remains in the peat consist almost entirely of nonbiodegraded polysaccharides. The total peat and Ericaceae rootlets yielded a wide range of components such as pyrolysis products derived from carbohydrates and lignins and many aliphatic and alicyclic compounds. The suberin of the rootlets of Ericaceae is postulated to be an important source of aliphatic hydrocarbons, methylketones (C17-C31), u n s a t u r a t e d methylketones (C19 C29), 2,4-diketones (C25 C31), long-chain alcohols (Cl5-C25), fatty acids (C8 C24), 24-ethylcholesterol, 24-ethylcholes-

tanol, 24-ethylcholestanone, and a number of triterpenoids with friedooleanan skeletions (e.g. taraxerol and taraxerone). Acknowledgements The E art h Science Foundation (AWON), branch of the Dutch Organisation for the Advancement of Pure Research (ZWO), is gratefully acknowledged for the financial support of this research project. I like to t hank Dr. B. van Geel for his critical discussion of the manuscript, and Drs. B. de Vries for his help with the computer programs. Mrs. E. Beglinger and Mrs. A. Philip and Drs. R. Bregman are thanked for their technical assistance. I also wish to t hank The Limnologisch Instituut Nieuwersluis, and particular Mr. A. Dekker for the facilities for the C, N and H measurements, Professor A.D.J. Meeuse for his correction of the English text, Mr. B. Donker (IPP, Amsterdam) and Mr. H. Snethlage (RAAP, Amsterdam) for drawing the profiles and Mr. R. Vermeij for the finishing touch to some of the figures. Appendix -- Description of some previously undescribed and less known microfossils During the microfossil analysis some newly recorded microfossils with a characteristic morphology were registered and identified to at least a palynological Type-level.

Type 492."(Plate I, 492a-c) Globose hyaline microfossil, 24 ~m in diameter, including the evenly distributed protuber-

PLATEI 731a-e. 792a c. 793. 794a e. 495a d. 496a, b. 497. 498a, b.

Type731 ( x 1000; depth 298 a: convex side, b: fiat side, c and d: side view, e: flat side with protruding wall). Type792 (× 1000; low, high focus, depth 294). Type 793 ( x 1000; depth 294). Type494: fungal spores (494a-b: x 1000; depth 292; 494c e: x 1000; depth 289). Type495: fungal spores (495a-c: x 1000; low, high, equatorial focus, depth 289; 495d: x 1000; depth 289). Type496: fungal spores (× 1000; depth 292). Type 497: algal spore ( x 1000; depth 275). Type498: conidiosporesof Dictyosporiurn(498a: x 1000; depth 251; 498b: x 1000; depth 277).

286 ances, c. 2 Fm apart. The solid protuberances are more or less circular at the bottom and 3.7-4.4 ~m in diameter, and 3 ~m high with a rounded top. Type 492 is r a t h e r rare in the present section and has mainly been observed in strongly decomposed peats (in zones A and C).

Type 493."(Plate I, 493) Smooth, dark brown, broadly pear-shaped microfossil, 35-39 x26-29 ~m. A slightly protruding pore at the apical side, 5 6 pm in outer diameter, c. 2 ~m inner diameter. Wall c. 2 ~m thick. Type 493 only occurs at the transition of zone A and subzone B1 and in the lower part of subzone B1.

Type 496."(Plate I, 496a, b) Globose hyaline fungal spores, 14-16 ~m in diameter, including the evenly distributed protuberances. The solid protuberances, are very close at their circular base 6 pin in diameter and 2 3 ~m high with a rounded top. Occurring solely and in clusters of 2 to more t han 20 spores. Type 496 is only found in r a t h e r large amounts in subzone B1, where roots of monocots (cf. Molinia) and Molinia epidermal remains are found. Type 496 is thought to be a fungus associated with Molinia tussocks. These fungal spores had earlier been observed in the section Assendelft O/R (H.J.L. Witte, pers. commun.).

Type 497."(Plate I, 497) Type 494: (Plate I, 494a-e) Fungal spores, 24-33 ~m x 14 18 ~m, threeto five-celled. The three-celled form is the most common. The septa of the central cell with a pore of c. 1.2 ~m. The central cell, and often the two bordering cells are brown. If present, the top cells are hyaline. The size of the cells decreases from the middle to the tops. Type 494 only occurs in r a t h e r large amounts of subzone B1, where roots of monocots (cf. Molinia) and Molinia epidermal remains are found. Type 494 is t hought to represent a fungus associated with Molinia tussocks.

Type 495."(Plate I, 495a-d) Globose fungal spores, 6-10 ~m in diameter, including the evenly distributed, 1 pm long spines, the latter c. 0.6 pm apart. One large pore c. 2-3 ~m, with an annul ar thickening, Around the pore a hyaline raised cell (3 ~m wide), open at the outside. Mostly occurring solely, sometimes in small clusters, Type 495 is only found in appreciable numbers in subzone B1 and in the top of zone C, and seems to be associated with epidermal remains of Molinia.

Tetragonal algal spores, 37 47 x 32-39 ~un. Wall densely pitted, pits c. 0.7 ~m wide and 1.5-3 pm apart. Wall 1-1.3 ~ thick, thickened (2 pm) at the corners. These algal spores are rat her similar to the Type 356 spores described by Van Geel et al. (1980, 1981). Type 497 occurs in subzones B1 and B2, and particular at the transition of these subzones, co-occurring with fruits of Rhynchospora alba. This type 497 is indicative of wetter local conditions in the peat.

Type 498."(Plate I, 498a, b) Conidia, flattened in one plane, 26 x 15 pm. Three rows of cells and three to four cells per row, cells 3-4 x 5.5-8 ~m. Rows of more or less the same length, up to 25 ~m long. Cap cell of about the same size as the other cells. This type of coniditun occurs in the genus Dictyosporium of the dematiaceous Hyphomycetes (Ellis, 1971), but too few conidia became available to enable a reliable identification. Dictyosporium species occur on dead and decaying wood and dead herbaceous stems. In the present section Type 498 is not frequently found, and only in the raised bog peat.

287

Type 731: (Plate I, 731a-e) T y p e 731 h a s a l r e a d y b e e n d e s c r i b e d b y B a k k e r a n d V a n S m e e r d i j k (1982). A d d i t i o n a l i n f o r m a t i o n o n its m o r p h o l o g y will be g i v e n h e r e . T h e c r u m b l e d a r e a i n t h e c e n t r e of t h e flat side (731b) is a p r o t r u d i n g p a r t of t h e w a l l , c. 12 m m i n d i a m e t e r a n d 4 - 6 m m h i g h (731e). T h e w a l l a t t h e t r a n s i t i o n b e t w e e n t h e flat a n d t h e c o n v e x s i d e s is t h i n n e r a n d f o r m s a f i s s u r e e x t e n d i n g across, at least h a l f the circumfere n c e of t h e m i c r o f o s s i l , T y p e 731 o c c u r s i n t h e p r e s e n t s e c t i o n o n l y i n s u b z o n e A2, a n d does n o t c o i n c i d e w i t h o n e p a r t i c u l a r micro- or macrofossil. The synecology of t h i s s u b z o n e is r a t h e r c o m p l e x : a w e t to v e r y wet, n i t r o g e n r i c h , r a t h e r n u t r i e n t r i c h e n v i r o n m e n t , with a f l u c t u a t i n g water table. T y p e 731 p r o b a b l y is a n i n d i c a t o r of w e t mesoto e u t r o p h i c c o n d i t i o n s , as a l r e a d y s u g g e s t e d b y B a k k e r a n d V a n S m e e r d i j k (1982).

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