Quaternary Science Journal - Some Results and Consequences

171 — 181 Eiszeitalter

u.

Gegenwart

30

Hannover

8 fig.

1980

C o m p a c t i o n a n d Other S o u r c e s of Error in O b t a i n i n g Sea-Level D a t a : S o m e R e s u l t s and C o n s e q u e n c e s ORSON

VAN DE PLASSCHE *)

C-14-dating, sea level, data processing, peat, compaction, contamination, section, Pleistocene North Sea, Netherlands (Hillegersberg, Rijkswegdonk) A b s t r a c t : This paper describes observations and results of radiocarbon assays relating to the effect of several sources o f error on the time/depth position of data-points. It is shown (a.o.), that the comparability of compaction-free, time/depth data-points from the base of peat beds, representing the onset of peat growth at different levels, can be negatively influenced by compac­ tion of the sampled peat, root contamination, and contamination by older material.

[Kompaktion und andere Fehlerquellen bei der Gewinnung von Seespiegeldaten: einige Ergebnisse u n d Folgerungen] K u r z f a s s u n g : Diese Arbeit beschreibt Beobachtungen und Ergebnisse zu RadiocarbonAltersbestimmungen, die mit der Auswirkung verschiedener Fehlerquellen auf die Zeit/Tiefen­ position von Datenpunkten zusammenhängen. Es wird (u. a.) gezeigt, daß die Vergleichbarkeit setzungsfreier Zeit/Tiefen-Datenpunkte von der Unterfläche von Torflagen, die das Einsetzen von Moorwachstum in verschiedenen Höhenlagen anzeigen, durch Setzung des beprobten Torfes, durch Wurzelkontamination und durch Kontamination mit älterem Material negativ beeinflußt werden kann.

1.

Introduction

As a c o n t r i b u t i o n to P r o j e c t 6 1 o f t h e I . G . C . P . , a large n u m b e r o f n e w

time/depth

d a t a - p o i n t s h a v e been o b t a i n e d from t h e m i d - w e s t e r n Dutch c o a s t a l p l a i n . O f t h e t o t a l o f 8 5 s a m p l e s , all but o n e f r o m peat beds, 4 0 d e r i v e from 2 e a r l y - h o l o c e n e r i v e r dunes ( s o - c a l l e d d o n k e n ) near R o t t e r d a m : the d o n k o f H i l l e g e r s b e r g a n d t h e R i j k s w e g d o n k ; 1 9 f r o m t h e base o f the L o w e r P e a t on t h e M i d - N e t h e r l a n d s P l a t e a u b e t w e e n U t r e c h t a n d T h e H a g u e ; a n d 2 6 , i n c l u d i n g 1 s a m p l e o f b i v a l v e d shells, f r o m the c o a s t a l b a r r i e r a r e a b e t w e e n T h e H a g u e a n d t h e f o r m e r e s t u a r y o f the O l d e r R h i n e ( V A N D E P L A S S C H E 1 9 7 9 b ) . T h e p o s i t i o n o f t h e s a m p l i n g a r e a s is i n d i c a t e d in figure 1 .

T h e u l t i m a t e a i m o f t h e i n v e s t i g a t i o n h a s b e e n to establish a r e p r e s e n t a t i v e

relative

sea-level c u r v e f o r the ( m i d - ) w e s t e r n N e t h e r l a n d s . I n order t o refine a n d / o r t o p r o v i d e an e x p l a n a t i o n for differences b e t w e e n a n d 1954;

V A N STRAATEN

1 9 5 4 ; JELGERSMA

with

existing s e a - l e v e l graphs

1 9 6 1 , 1 9 6 6 ; LOUWE

KOOIJMANS

(BENNEMA

1974, 1976;

R O E L E V E L D 1 9 7 4 ) much a t t e n t i o n has b e e n p a i d t o the a c c u r a c y a n d r e l i a b i l i t y o f the d a t a ; e i t h e r b y a v o i d i n g o r r e d u c i n g as m u c h as possible the effect o f several sources o f error, or, in c e r t a i n cases, b y c o d a t i n g s u s p e c t e d o r c o n t a m i n a t e d samples t o o b t a i n an idea o f t h e o r d e r o f m a g n i t u d e

of the error involved. S o m e o f the observations

and

results m a d e a n d o b t a i n e d in this respect a r e t h e s u b j e c t o f this p a p e r .

*) Address o f the author: Drs. O. v a n d e P l a s s c h e , Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, 1081 H V Amsterdam, Netherlands.

172

Orson van de Plassche

Fig. 1 : Map showing the position o f the sampling areas mentioned in the text, in relation to their general geological/geomorphological context. [Based on maps from HAGEMAN ( 1 9 6 9 ) , JELGERSMA et al. ( 1 9 7 0 ) , P O N S & BENNEMA ( 1 9 5 8 ) , and

VERBRAECK

(1974)].

Compaction and Other Sources of Error in Obtaining Sea-Level Data

173

2. Sources of error O f t h e m a n y possible causes f o r o b t a i n i n g a n i n a c c u r a t e o r u n r e l i a b l e value f o r t h e a l t i t u d e and, in p a r t i c u l a r , for t h e age o f a s a m p l e , o n l y the f o l l o w i n g will be discussed: 1. D i a c h r o n o u s b o u n d a r i e s 2. C o m p a c t i o n o f t h e s a m p l e d p e a t 3. R o o t c o n t a m i n a t i o n 4. Contamination by older material 5. C o m p a c t i o n o f u n d e r l y i n g deposits 2.1.

Diachronous

boundaries

I n sea-level studies it is n o t u n c o m m o n t o m a k e use o f r a d i o c a r b o n dates, which o r i g i n a l l y h a v e been o b t a i n e d for s t r a t i g r a p h i c a l p u r p o s e s o n l y . F o r i n s t a n c e , the a g e o f a s a m p l e from the b a s e o f a p e a t bed t h a t has b e e n l o w e r e d b y c o m p a c t i o n o f u n d e r l y i n g deposits, can be c o n n e c t e d t o t h e a l t i t u d e o f t h e b e d w h e r e it is f o u n d t o rest on n o n — o r much less c o m p a c t a b l e sediments. I n h e r e n t in t h i s p r o c e d u r e is t h e a s s u m p t i o n t h a t t h e b a s e o f t h e peat b e d is isochronous. As is i n d i c a t e d b e l o w , this a s s u m p t i o n , if n o t sup­ p o r t e d b y e v i d e n c e , m a y lead t o c o n s i d e r a b l e e r r o r s .

F o r e x a m p l e : p e a t g r o w t h f o l l o w i n g a p e r i o d o f sediment d e p o s i t i o n c a n begin (much) l a t e r o n r e l a t i v e l y high a n d w e l l a e r a t e d f e a t u r e s such as n a t u r a l l e v e e s w h e n c o m p a r e d t o n e a r b y b a c k s w a m p s ; when t h e deposition o f a c l a y bed a r o u n d a d o n k is f o l l o w e d b y a t e m p o r a r y l o w e r i n g o f the ( l o c a l ) w a t e r l e v e l , p e a t f o r m a t i o n is m o s t l i k e l y to begin a t s o m e distance f r o m t h e dune w h e r e c o m p a c t i o n o f t h e (thicker) u n d e r l y i n g deposits c a n b e e x p e c t e d to reach t h e highest v a l u e s . A c c u m u l a t i o n o f p e a t on t h e d u n e slope does n o t o c c u r until t h e w a t e r level has risen a g a i n . T h i s s e q u e n c e o f e v e n t s has been d e p i c t e d s d i e m a t i c a l l y in figure 2 . A final e x a m p l e m a y be g i v e n . T h e base o f a n o l i g o t r o p h i c p e a t l a y e r , embedded in a b o d y o f e u - / m e s o t r o p h i c p e a t (fig. 3 ) , has b e e n d a t e d 4 4 0 0 ± 6 0 B P

Fig. 2 : Schematic representation of events resulting in a diachronic lower boundary of a peat layer (for explanation see taxt).

( G r N 8 9 2 4 ) . I t reaches the d o n k s u r f a c e at an e x t r a p o l a t e d height o f — 1 . 8 ± 0 . 1 m N A P ( D u t c h O r d n a n c e D a t u m ) . A s a m p l e from the u n d e r l y i n g Phragmites-Carex peat, t a k e n a t t h e p e a t / d u n e s a n d i n t e r f a c e at a d e p t h o f — 3 . 1 0 m N A P , g a v e a n a g e o f 4 3 9 0 ± 5 0 B P ( G r N 7 8 3 2 ) . T h e s e datings p r o v e t h e base o f t h e o l i g o t r o p h i c p e a t t o b e diachronous. T h e e r r o r f o r the a l t i t u d e o f t h e t i m e / d e p t h d a t a - p o i n t , h a d the age o f 4 4 0 0 B P been c o u p l e d t o t h e v a l u e o f — 1 . 8 ± 0 . 1 m N A P , w o u l d h a v e b e e n 1.3 ± 0 . 1 m ( t o o h i g h ) , a n d w o u l d a l s o h a v e led to an erroneous c o m p u t a t i o n o f t h e l o w e r i n g o f the base o f t h e o l i g o t r o p h i c p e a t bed, and f r o m it the degree o f c o m p a c t i o n o f t h e u n d e r l y i n g c o l u m n o f p e a t a n d clay.

174

Orson van de Plassche

Fig. 3 : Section perpendicular to the southern slope o f the donk of Hillegersberg, showing the diachronous character o f the lower boundary of the oligotrophic peat layer.

T h e s i m u l t a n e o u s g r o w t h o f e u - / m e s o t r o p h i c v e g e t a t i o n at a n d close to the d o n k , a n d o f a s u r r o u n d i n g girdle o f o l i g o t r o p h i c p l a n t s ( E r i c a c e a e , Eriophorum sp.), will h a v e t o be taken i n t o c o n s i d e r a t i o n w h e n e v a l u a t i n g t h e height r e l a t i o n s h i p o f the level o f p e a t g r o w t h (i.e. t h e g r o u n d - w a t e r l e v e l ) at t h e d o n k t o the w a t e r l e v e l in the w i d e r sur­ rounding. T h e o l i g o t r o p h i c v e g e t a t i o n , w h e n in an a d v a n c e d s t a t e o f d e v e l o p m e n t , w i l l h a v e f o r m e d a slightly raised d o m e , and in b e t w e e n t h e dune a n d this low c u s h i o n t h e g r o u n d - w a t e r level m a y w e l l h a v e been raised a little r e l a t i v e t o t h e w a t e r level o u t s i d e the o l i g o t r o p h i c peat b o d y .

2.2.

C o m p a c t i o n

of

peat,

sample

position

and

T h e p e a t o n the d o n k o f H i l l e g e r s b e r g a n d b e l o w a d e p t h o f c a 8 o n the R i j k s w e g d o n k , has b e e n subject t o c o n s i d e r a b l e c o m p a c t i o n due t h e o v e r l y i n g deposits a n d a r t i f i c i a l d r a i n a g e o f the surroundings. T h i s t h e d r y a n d c o m p a c t n a t u r e o f t h e p e a t a n d t h e difficulty o f p e n e t r a t i n g open t u b e b o r e r .

thickness m below surface to the w e i g h t o f is a p p a r e n t f r o m it with a p o i n t e d

In o r d e r t o determine t h e r e j u v e n a t i n g effect o f c o m p a c t i o n o n t h e age o f a s a m p l e in r e l a t i o n t o its thickness, a s a m p l e p a i r f r o m t h e base o f t h e p e a t (see core 1 in fig. 4 ) has been d a t e d . F r o m t h e v e r y insignificant a g e difference b e t w e e n sample 1.1 a n d 1.2 it is c o n c l u d e d t h a t the a g e - r e d u c i n g effect o f a b o u t 8 8 °/o c o m p a c t i o n (see b e l o w ) o n t h e a g e o f a s a m p l e 1.1 + 1.2 w o u l d h a v e been e n t i r e l y negligible. T h e e x p l a n a t i o n f o r this, r a t h e r surprising, result is n o t t o b e found in a m i s d a t i n g o f o n e o f t h e t w o peat s a m p l e s , but, as will be argued b e l o w , in t h e high r a t e o f p e a t a c c u m u l a t i o n t h a t o c c u r r e d a t t h e time/depth interval concerned.

LEGEND

core 1

2

v

•V

3

4 AAA

A M 469

•V-

(7845) 5400+40

571 (7844) 5440+35 J , v : • 575 v: v: : v{

(7846) 5560+40

eg

I

-V •904

(8929) 6495+40

;v:

cm

:v: •

:v: :v : v : :v :v: :v: :v: :v: tH : v : CO : v :

.V.

(7855)

I -v- • : x : : 594

-

v. v. v: v: v: v: -

(8928) 6670+35

y :v .v :v :v '.v

• V

906 :v .v :v :v " '.v.

'-v 910

: v: . v . :v

x;

: x::

68

69

I

.1.2

peat peat sand v e r y

peaty

sand

peaty

humic

( 7 8 4 3 ) GrN 7 8 4 3

54.10 _l

570 •

peat peat peat

: : v ; : sand p e a t y sand s l i g h t l y

:*:.*.'

V W

55 _l

1030 ~1

1 3.1

67.10 I i I

I

66 I I

3

-

67

I

900

66.10

_5L j4.2

2

910 -

1040

580 • 590

V 1030

CO

589 : : 590 : v : :v :v:: v • :v: ;\

V. ".V

.V. (7853) 6665+40

CM <N

.V

585

I 2-11

I

I

'

E . 2 1050 D e p t h s i n cm b e l o w

N.A.P.

600 Time i n c o n v e n t i o n a l Fig. 4 : Time/depth plot of sample pairs from cores taken at the donk of Hillegersberg (cores 1, 2, and 3) and at the Rijkswegdonk (core 4 ) .

radiocarbon years

176

Orson van de Plassche

I n 3 cores f r o m the d o n k e n ( n o . 2 , 3, a n d 4 in fig. 4 ) t h e t r a n s i t i o n f r o m the p e a t , v i a ( v e r y ) s a n d y p e a t a n d ( v e r y ) p e a t y sand, t o s a n d , w a s t o o g r a d u a l t o ignore t h e p o s s i ­ b i l i t y t h a t t h e v e r y p e a t y s a n d in f a c t r e p r e s e n t e d t h e e v e n t w e w i s h e d to date, n a m e l y t h e onset o f p e a t g r o w t h on t h e o r i g i n a l d u n e slope at t h a t spot. I n v i e w o f t h e h i g h d e g r e e o f c o m p a c t i o n o f t h e p e a t , t h e v e r y p e a t y sand m a y o r i g i n a l l y h a v e b e e n v e r y s a n d y peat, in w h i c h case o n e w o u l d s a m p l e t o o high i f the b o u n d a r y between t h e v e r y p e a t y sand a n d t h e v e r y s a n d y p e a t in t h e c o r e is t a k e n t o r e p r e s e n t the f o r m e r d u n e surface. W h i l e this w o u l d n o t s e r i o u s l y effect t h e a l t i t u d e o f the d a t a - p o i n t , it m i g h t h a v e a n o t i c e a b l e influence on the a g e o f it. In each o f these three cases, t h e r e f o r e , t w o s a m p l e s , i m m e d i a t e l y a b o v e each o t h e r ( o r v e r y n e a r l y s o ) , h a v e been r a d i o m e t r i c a l l y d a t e d . T h e results o f t h e a g e d e t e r m i n a t i o n s a r e given in figure 4 .

T h e age differences for t h e s a m p l e pairs o f c o r e 2, 3, a n d 4 a r e 1 6 0 , 1 7 0 , a n d 1 7 5 r a d i o c a r b o n y e a r s r e s p e c t i v e l y . A l t h o u g h these a g e differences a r e n o t significant a t the 9 9 . 7 °/o c o n f i d e n c e level, t h e r e m a r k a b l e i n t e r n a l c o n s i s t e n c y , t o g e t h e r with t h e

low

1-sigma values f o r t h e v e r y p e a t y s a n d samples ( i n d i c a t i n g high c a r b o n c o n t e n t ) , suggest t h e age difference between t h e s a m p l e s o f each p a i r t o be q u i t e m e a n i n g f u l . T h u s , in c a s e o f a high d e g r e e o f c o m p a c t i o n a n d a t r a n s i t i o n a l p e a t - s a n d b o u n d a r y , the effect o f t h e c o m p a c t i o n s h o u l d be fully t a k e n i n t o a c c o u n t w h e n d e t e r m i n i n g t h e v e r t i c a l p o s i t i o n o f t h e sample a n d t h e width o f t h e s a m p l e i n t e r v a l . I n case o f d o u b t , it is a d v i s a b l e t o h a v e a p a i r o f s a m p l e s dated.

C o m b i n i n g t h e dating results o f c o r e 1 a n d c o r e 2 , which h a v e b e e n t a k e n o n l y 1 m a p a r t , the f o l l o w i n g r e c o n s t r u c t i o n c a n be m a d e . G i v e n the s a m e a g e f o r the s a m p l e s 2 . 2 a n d 1.2, a n d assuming a ( s u b - ) h o r i z o n t a l s u r f a c e o f the p e a t a r o u n d the d o n k o f H i l l e ­ gersberg at t h a t t i m e , the p r e s e n t ( " c o m p a c t e d " ) t i m e / d e p t h p o s i t i o n o f the s a m p l e s (see fig. 5) c a n b e e x p l a i n e d b y a r e d u c t i o n o f t h e p e a t t o a b o u t l / 8 t h o f its original thickness ( c a 8 8 ° / » c o m p a c t i o n ) . In figure 5 this v a l u e has been applied in r e c o n s t r u c t i n g t h e o r i ­ g i n a l a l t i t u d e o f t h e t o p o f t h e samples. A c u r v e c o n n e c t i n g t h e f o u r t i m e / d e p t h b o x e s 5600

p

1

M

5500 1

5 4 0 0 C y BP 1

1

1

1

1

. / ,^

1

/ 1

)

1t

,

/ 4

1 -580

^

*

1

1.2

1.1

I - 6 0 0 l— cm NAP

Fig. 5: Time/depth graph showing a distinct increase in the rate of the water level rise (at Hi legersberg). The increase represents the beginning o f a Calais I I - B transgression sub-phase.

s h o w s t h a t b e t w e e n 5 4 7 5 a n d 5 4 0 0 B P the w a t e r level rose much f a s t e r t h a n in t h e p e r i o d b e t w e e n 5 6 0 0 a n d 5 4 7 5 B P . T h i s sudden i n c r e a s e in the r a t e o f t h e w a t e r level rise r e ­ presents t h e o n s e t o f a C a l a i s I I - B transgression sub-phase. T h e r a p i d rise in

water

level has b e e n a c c o m p a n i e d b y a high rate o f p e a t a c c u m u l a t i o n , a n d this e x p l a i n s w h y , despite the h i g h degree o f c o m p a c t i o n (possible b e c a u s e o f the h i g h p o r e space a n d w a t e r

Compaction and Other Sources of E r r o r in Obtaining Sea-Level D a t a

177

c o n t e n t ) , t h e age difference b e t w e e n s a m p l e 1.1 a n d 1.2 is so s m a l l . W h i l e it is e v i d e n t t h a t in t h e case o f a h i g h l y c o m p a c t e d , b u t r a p i d l y a c c u m u l a t e d p e a t t h e w i d t h o f t h e s a m p l e i n t e r v a l has l i t t l e effect o n the age, it does h a v e a r e l a t i v e l y s t r o n g effect on the h e i g h t r a n g e o f the d a t a - p o i n t .

2.3.

Root

c o n t a m i n a t i o n

I n f o l l o w i n g t h e f r a c t i o n a t i o n e x p e r i m e n t b y S R E I F ( 1 9 7 1 ) , all H i l l e g e r s b e r g samples have been carefully seperated into a root- a n d a restfraction with the help o f a binocular m i c r o s c o p e . As it p r o v e d t o be v e r y t i m e - c o n s u m i n g t o r e m o v e a l l r e c o g n i z a b l e r o o t l e t s , it w a s decided t o s u b j e c t m o s t o f t h e o t h e r s a m p l e s t o a s l i g h t l y less r i g o r o u s p r e t r e a t m e n t .

F r o m these, w i t h o u t using t h e b i n o c u l a r m i c r o s c o p e , l o o s e bits o f w o o d , all o r all l a r g e r w o o d y r o o t s a n d r o o t l e t s , Phragmites rhizomes, and (sub-)vertically positioned roots and stems h a v e been r e m o v e d . T h e i m p o r t a n c e o f this m i c r o - o r m a c r o s c o p i c p r e t r e a t m e n t , p a r t i c u l a r l y w h e n d e a l i n g w i t h s l o w l y a c c u m u l a t e d p e a t , has been stressed b y the a u t h o r ( V A N D E P L A S S C H E 1 9 7 9 a ) , a n d is well i l l u s t r a t e d in t h e f o l l o w i n g e x a m p l e .

M o s t o f the samples in t h e c o a s t a l b a r r i e r s a m p l i n g a r e a h a v e b e e n c o l l e c t e d close t o t h e f o r m e r estuary o f t h e O l d e r R h i n e (i.e. in t h e n o r t h e r n p a r t o f t h e b e a c h - p l a i n s ) , as t h e r e t h e levels o f s e d i m e n t a t i o n a n d p e a t g r o w t h w i l l h a v e been m o s t d i r e c t l y d e t e r ­ m i n e d b y t h e w a t e r l e v e l s in t h e estuary itself. T o e x a m i n e t h e effect o f the f a c t o r d i s t a n c e t o e s t u a r y o n t h e level o f p e a t f o r m a t i o n in a b e a c h - p l a i n , samples h a v e b e e n c o l l e c t e d a l o n g t h e slope o f a d u n e r i d g e 5 k m t o t h e s o u t h - w e s t o f the n o r ­ t h e r n t i p o f the e a s t e r n m o s t b e a c h p l a i n (fig. 6 a, b ) . T h e u p p e r m o s t s a m p l e , t h e p o s i t i o n o f w h i c h is i n d i c a t e d in figure 6 b , a p p e a r e d t o be h e a v i l y p e n e t r a t e d b y r o o t s f r o m t h e o v e r l y i n g few decimeters o f humified p e a t . A l l r o o t s a n d r o o t l e t s , r e c o g n i z a b l e u n d e r t h e b i n o c u l a r m i c r o s c o p e , h a v e been r e m o v e d f r o m t h e s a m p l e . N o l i v i n g r o o t l e t s w e r e o b s e r v e d . I n o r d e r t o d e t e r m i n e t h e r e j u v e n a t i n g effect o f this r o o t c o n t a m i n a t i o n , a s e c o n d s a m p l e , t a k e n at t h e s a m e depth a n d 1 0 c m t o t h e side o f t h e o t h e r s a m p l e , has been d a t e d w i t h o u t h a v i n g been p r e t r e a t e d m a n u a l l y . A check on t h e p r e s e n c e o f l i v i n g r o o t l e t s also was n e g a t i v e . T h e results o f t h e d a t i n g s a r e presented in figure 6 c, t o g e t h e r w i t h a n e s t i m a t e ( b a s e d o n d a t a t o be p u b l i s h e d ) o f t h e level o f p e a t g r o w t h at a r o u n d 3 8 5 0 a n d 3 3 5 0 B P in t h e n o r t h e r n tip o f t h e b e a c h - p l a i n . F r o m t h e t i m e / d e p t h p o s i t i o n o f t h e d a t a - p o i n t s it is a p p a r e n t t h a t , at least b e t w e e n 3 9 0 0 a n d 3 8 0 0 B P , t h e surface o f t h e p e a t in the b e a c h - p l a i n must h a v e possessed a g e n t l e slope t o w a r d s t h e e s t u a r y . T h i s p h e n o m e n o n , which c a n b e e x p l a i n e d as t h e result o f a h a m p e r e d d r a i n a g e in t h e c e n t r a l p a r t s o f the b e a c h - p l a i n ( f r i c t i o n caused b y t h e p e a t ) , w o u l d h a v e b e e n left u n n o t i c e d h a d t h e s a m p l e ( G r N 8 8 6 5 ) n o t been c a r e f u l l y c l e a r e d f r o m all r e c o g n i z a b l e r o o t s a n d rootlets.

A s i m i l a r slope o f t h e s u r f a c e o f t h e p e a t w i l l h a v e e x i s t e d t o w a r d s t h e e s t u a r y o f t h e rivers R h i n e a n d M e u s e in t h e south. O n t h e ( i n f e r r e d ) d r a i n a g e d i v i d e o f the beachp l a i n p e a t , a b o u t 2 k m s o u t h - w e s t o f the s a m p l i n g site (fig. 6 a ) , r e m a i n s o f Sphagnum a n d Eriophorum h a v e b e e n e n c o u t e r e d in t h e u p p e r p a r t o f the p e a t . T h i s agrees w i t h t h e d e c r e a s e in n u t r i e n t s u p p l y t o be e x p e c t e d t h e r e .

2.4.

C o n t a m i n a t i o n

by

older

material

I n q u i t e a n u m b e r o f b o r i n g s t h e L o w e r P e a t a p p e a r e d t o be s e p e r a t e d f r o m the ( p o d z o l i s e d ) sand b e l o w b y a t h i n l a y e r ( 1 — 4 c m ) o f b l a c k , g r e a s y - l i k e , o r g a n i c m a t e r i a l .

As t h e origin o f this m a t e r i a l predates the o n s e t o f c o n t i n u o u s p e a t a c c u m u l a t i o n — it has p r o b a b l y been f o r m e d in an e n v i r o n m e n t o f a l t e r n a t i n g wet a n d d r y c o n d i t i o n s — it w o u l d b e interesting t o k n o w t h e age difference b e t w e e n a s a m p l e o f this b l a c k , structur­ eless, o r g a n i c deposit a n d a s a m p l e from t h e b a s e o f t h e o v e r l y i n g p e a t .

12

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Fig. 6 : 6 a : Geological map of the coastal barrier complex immediately north and south o f T h e Hague; black: coastal barriers covered by Older Dunes; white: beach-plain peat; vertical hatching: clay of Dunkirk deposits (after JELGERSMA et al. 1970), O : Locality where remains o f oligotrophic plants have been encountered in the upper part of the peat; arrows: indicate the direction of the palaeo slope o f the beach-plain peat surface.

6b and 6 c : Position and time/depth plot o f a sample pair of beach-plain peat (fractionized versus non-fractionized). Crosses in the time/depth diagram represent estimated levels of peat growth in the northern tip of the easternmost beach-plain.

Compaction and Other Sources of Error in Obtaining Sea-Level D a t a

66 _L_

64 I

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Fig. 7: Time/depth plot o f a sample pair, taken in the vicinity of Linschoten, from the base o f the Lower Peat and a thin layer o f black, structureless, organic material, seperating the Lower Peat from the podzolised sand below.

F i g u r e 7 shows t h e position o f such a sample p a i r in a c o r e f r o m t h e v i c i n i t y o f L i n ­ schoten ( f o r l o c a t i o n see L in fig. 1 ) . T h e sample o f t h e b l a c k o r g a n i c l a y e r first h a d t o be c a r e f u l l y cleared f r o m t h e m a n y r o o t s a n d r o o t l e t s t h a t decended f r o m t h e p e a t a b o v e .

A c o m p l e t e e l i m i n a t i o n o f the s m a l l e s t r o o t l e t s c o u l d , h o w e v e r , n o t b e achieved.

F r o m t h e significant age difference b e t w e e n t h e t w o samples, a d i f f e r e n c e t h a t w o u l d e v e n h a v e been l a r g e r i f all y o u n g e r r o o t l e t s c o u l d h a v e been r e m o v e d , it is a p p a r e n t t h a t t h e a g e o f a sample f r o m t h e base o f t h e L o w e r P e a t w i l l b e t o o o l d i f t h e s a m p l e c o n t a i n s ( p a r t o f ) t h e black o r g a n i c m a t e r i a l .

2.5.

C o m p a c t i o n

of

deeper

lying

sediments

I n t h e coastal b a r r i e r sampling a r e a , the p r e s e n c e o f l a g o o n a l c l a y a n d p e a t b e l o w t h e ( t w o ) easternmost b a r r i e r ( s ) , a n d o f c l a y ( e y ) beds i n t e r c a l a t e d a t v a r y i n g depths in t h e s h a l l o w m a r i n e sands ( V A N S T R A A T E N 1 9 6 5 ; pers. o b s e r v a t i o n s ) , r e q u i r e s t h a t borings, t o d e t e r m i n e the p r e s e n c e o r a b s e n c e o f c o m p a c t a b l e l a y e r s at p o t e n t i a l s a m p l i n g sites, reach t o sufficient d e p t h . ( I n this r e s p e c t t h e s u i t a b i l i t y o f t h e h a n d - b o r i n g e q u i p m e n t t o p e n e t r a t e sandy d e p o s i t s and t h e a v a i l a b l e m a n p o w e r a r e i m p o r t a n t l i m i t i n g f a c t o r s . ) T h e f o l l o w i n g e x a m p l e m a y serve t o i l l u s t r a t e t h e p o i n t .

A 4 m deep b o r i n g t h r o u g h t h e t o p o f a small, p e a t - c o v e r e d , d u n e in t h e c e n t r e o f t h e ( n o r t h e r n p a r t o f t h e ) easternmost b e a c h - p l a i n , s h o w e d t h e sediments t o consist o n l y o f ( s h e l l - b e a r i n g ) sand. T h e site was a c c e p t e d for s a m p l i n g , until f u r t h e r , less deep borings in t h e surrounding r e v e a l e d the p r e s e n c e o f a thin b a n d o f c l a y , t h e v a r i a t i o n in heigh o f w h i c h s t r o n g l y suggested t h e dune t o h a v e been l o w e r e d (fig. 8 ) , a p p a r e n t l y b y c o m p a c ­ t i o n o f deeper o c c u r r i n g sediments. A b o r i n g t o a d e p t h o f 6 m p r o v e d t h a t f r o m 3 . 7 5 m o n w a r d t h e sediment is m a d e up o f c l a y e y sand, v e r y s a n d y c l a y , a n d m a n y em's thick c l a y beds. T h e a m o u n t o f l o w e r i n g o f t h e dune t o p has b e e n e s t i m a t e d t o b e at least 0 . 6 m .

T h i s l o c a l i t y has b e e n r e j e c t e d for t a k i n g samples.

3.

Acknowledgements

I a m grateful t o M r . R . H o o g e n d o o r n a n d his staff o f the B o o r Technische D i e n s t ( G e o l o g i c a l S u r v e y N e t h e r l a n d s , H a a r l e m ) for p r o v i d i n g t h e e x c e l l e n t c o r e s ; D r . W . G .

M o o k o f t h e G r o n i n g e n r a d i o c a r b o n l a b o r a t o r y f o r d a t i n g the s a m p l e s ; D r . H . C o l l e y f o r c o r r e c t i n g the E n g l i s h t e x t ; M r . H . S i o n for d r a w i n g t h e figures a n d M r . C . v a n der B l i e k f o r his p h o t o g r a p h i c assistence. T h e d a t a p r e s e n t e d in this p a p e r f o r m p a r t o f a n investigation made possible by the Netherlands O r g a n i z a t i o n for the Advancement o f P u r e Scientific R e s e a r c h (full g r a n t 0 7 5 - 9 5 ) .

12

Orson van de Plassche

180

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Fig. 8: Section across the northern slope of a small dune in the centre o f the (northern part of the) easternmost beach-plain. The dune and its surroundings have been lowered by compaction of deeper lying sediments.

4. R e f e r e n c e s J . ( 1 9 5 4 ) : Bodem- en zeespiegelbewegingen in het Nederlandse kustgebied. — Diss.: 85 p., 17 fig.; Wageningen.

H A G E M A N , B . P . ( 1 9 6 9 ) : The Western part of the Netherlands during the Holocene. — Geologie en Mijnbouw, 48 ( 4 ) : 3 7 3 — 3 8 8 , 9 fig.; Rotterdam.

BENNEMA,

S . ( 1 9 6 1 ) : Holocene sea-level changes in the Netherlands. — Meded. Geol. Sticht., Serie C - I V , 7 : 100 p., 50 fig., 3 tab.; Maastricht.

(1966): Sea-level changes during the last 10.000 years. — I n : S A W Y E R , J . S . , ed., World Climate 8000 to 0 B . C . ; Proc. Intern. Symp. 1 8 — 1 9 April, 1 9 6 6 ; London: 5 4 — 7 1 , 10 fig.; London.

JELGERSMA,

—

Compaction and Other Sources o f Error in Obtaining Sea-Level D a t a

—

, D E J O N G , J . , Z A G W I J N , W . H. & VAN R E G T E R E N A L T E N A , J . F . ( 1 9 7 0 ) : The

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coastal dunnes of

the western Netherlands; geology, vegetational history, and archaeology. — Meded. Rijks Geol. Dienst, Nw. Serie, 2 1 : 94—167, 3 9 fig., 10 pt.; Maastricht.

LOUWE KOOIJMANS, L . P . ( 1 9 7 4 ) : The Rhine/Meuse D e l t a ; four studies on its prehistoric occupa­ tion and Holocene geology. — Diss.: 421 p., 124 fig., 27 tab., 16 pt.; Leiden.

— ( 1 9 7 6 ) : Prähistorische Besiedlung im Rhein-Maas Deltagebiet und Bestimmung ehemaliger Wasserhöhen. — Probleme der Küstenforschung im südlichen Nordseegebiet, 1 1 : 1 1 9 — 1 4 3 , 11 fig.; Hildesheim.

PLASSCHE, O. VAN DE ( 1 9 7 9 a ) : Reducing the effect o f root contamination o f peat samples. — Oceanis, Fasc. Hors-Series, 5: 2 1 5 — 2 1 8 , 3 fig.; Paris.

— ( 1 9 7 9 b ) : Sea-level research in the Province o f South-Holland, Netherlands. — In SUGUIO, K., FAIRCHILD, T. R . M A R T I N , L . , & F L E X O R , J . - M . , Ed's., Proc. 1978 Intern. Symp. on coastal evolution in the Quaternary: 5 3 4 — 5 5 1 , 10 fig.; Sao Paolo.

PONS, L. J . & BENNEMA, J . ( 1 9 5 8 ) : De morfologie van het Pleistocene oppervlak in westelijk midden-Nederland, voor zover gelegen beneden gemiddeld zeeniveau (N.A.P.). — Tijds.

K . N . A . G . , L X X V ( 2 ) : 120—139, 7 fig., 1 map.; Leiden.

R O E L E V E L D , W. (1974): T h e Groningen coastal district. A study in holocene geology and lowland physical geography. — Diss.: 252 p., 66 fig., 26 tab.; Amsterdam.

STRAATEN, L . M. J . U . VAN (1954): Radiocarbon datings and changes o f sea-level at Velzen (Netherlands). — Geologie en Mijnbouw, N w . Serie, 16: 2 4 7 — 2 5 3 , 4 fig.; 's-Gravenhage.

— ( 1 9 6 5 ) : Coastal barrier deposits in South- and North-Holland, in particular in the areas around Scheveningen and IJmuiden. — Meded. Geol. Sticht., N w . Serie, 1 7 : 4 1 — 7 5 , 26 fig., 4 tab., 14 pt.; Maastricht.

S T R E I F , H . (1971): Stratigraphie und Faziesentwicklung im Küstengebiet von Woltzeten in Ost­ friesland. — Beihefte Geol. Jahrb., 1 1 9 : 59 S., 10 Abb., 6 T a b . ; Hannover.

V E R B R A E C K , A. in coop, with K O K , H. & VAN M E E R K E R K , M . ( 1 9 7 4 ) : The genesis and age of the riverdunes (donken) in the Alblasserwaard. — Meded. Rijks Geol. Dienst, N w . Serie, 25 (1): 1—8, 12 flg., 2 pt.; Maastricht.

Manuscript received on 18. 12. 1979.

182