Quaternary Science Journal - Radiocarbon measurements of Würm-interstadial samples from Jutland

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Radiocarbon measurements of Würm-interstadial samples from Jutland B y H . T A U B E R , C o p e n h a g e n , a n d H L . DE V R I E S , Groningen Z u s a m m e n f a s s u n g . Der Inhalt von radioaktivem Kohlenstoff in würminterstadialen Proben aus Brörup, Jutland, ist in den Datierungslaboratorien in Kopenhagen und Groningen gemessen worden, Die Proben waren durch verhältnismäßig große Mengen von infiltriertem, jün­ geren Material verunreinigt; nach einer Extraktion von Humussäuren zeigte jedoch keine der Proben eine signifikante Aktivität. Dies läßt darauf schließen, daß das Interstadial von Brörup und die vorausgegangene kalte Periode älter als 50 000 Jahre v. Chr. sind. Wenn die Ansicht von ANDERSEN (1957) richtig ist, daß in Brörup wirklich das Göttweiger Interstadial vorliegt, so würde daraus folgen, daß auch A 1 1 w ü r m und wenigstens der tiefere Teil des G ö t t w e i g e r I n t e r s t a d i a l s älter als 50 000 Jahre v. Chr. sein müßten.

S u m m a r y . The radiocarbon content of Würm-interstadial samples from Brörup, Jutland, has been measured in the dating laboratories in Copenhagen and Groningen. The samples were contaminated with comparatively large amounts of infiltrated younger material, but after an extraction of humic acids none of the samples showed a significant activity. This means that the interstadial at Brörup and the preceding cold period are older than 50.000 B.C. If ANDERSEN (1957) is right in correlating the Brörup interstadial with the „Göttweig Interstadial" this implies that O l d W ü r m and at least the lower part of the G ö t t w e i g i n t e r s t a d i a l also should be older than 50.000 B.C. In „Eiszeitalter und G e g e n w a r t " SVEND T H . A N D E R S E N has given a p r e l i m i n a r y r e p o r t on new p o l l e n analytical investigations of interglacial a n d interstadial fresh w a t e r d e p o ­ sits a t B r ö r u p in J u t l a n d (ANDERSEN 1957). Samples from these deposits a t „ B r ö r u p H o t e l B o g " h a v e been m e a s u r e d for c a r b o n - 1 4 content in the dating laboratories in C o p e n h a g e n a n d Groningen. According to A N D E R S E N (1957), there can be little d o u b t t h a t the samples submitted represent the O l d W ü r m a n d t h e interstadial ( G ö t t w e i g e r interstadial, G R O S S 1956) which separates O l d W ü r m a n d M a i n W ü r m . A cooling of the climate is i n d i c a t e d a t the t o p of the deposits; b u t it is n o t possible to say, w h e t h e r the whole interstadial is represented a t Brörup.

T h e glacial p e a t and m u d layers, from which the samples originate, are p r o t e c t e d by a solifluction deposit of s t o n y s a n d 2 meters thick a n d p r o b a b l y formed d u r i n g a later cold p e r i o d of the last glaciation. T h e layers a r e situated b e l o w the ground w a t e r level, a n d p e n e t r a t i o n of modern rootlets into the l a y e r s is therefore improbable. T h e samples are absolutely non-calcareous, a n d change of activity due t o either older o r younger c a r b o n a t e c a n be excluded. T h e samples w e r e d r i e d a n d stored in closed tins after the boring a n d thus protected against the attack of microorganisms. F o r these reasons the samples w e r e considered especially suitable for r a d i o c a r b o n investigations in this age range, w h e r e o n l y a minute a d m i x t u r e of y o u n g e r m a t e r i a l (less t h a n 1 per cent) will produce a d r a s t i c change in t h e measured age.

T h e s t r a t i g r a p h y a t the locality is the following (cp. A N D E R S E N 1957): 0—0.70 m Made ground 0.70—2.65 — Stony sand 2.65—2.80 — Humous substance ("Dy") K-512 Inter­ 2.80—3.12 — Fine sandy mud K-528 GRO-1264 stadial 3.12—3.74 — Herbaceous peat K-510 GRO-1263 3.74—4.35 — Sandy mud K-526 Old Würm

4.35—5.89 — 5.89—6.00 — 6.00—7.41 —

Mud with sand and silt Clay Mud

Inter­ glacial

7.41—7.46 •— Herbaceous peat 7.46—• — Peat with wood of spruce and alder.

K-511

GRO-1256

H . Tauber und Hl. de Vries

7C

T h e p l a c e of the measured samples in the s t r a t i g r a p h y is shown. T h e samples h a v e been t a k e n only from layers, w h e r e influx of rebedded interglacial m a t e r i a l is either absent o r negligible a n d will h a v e no effect o n t h e d a t i n g results.

T h e samples K - 5 1 2 , 510, a n d 511 were measured in C o p e n h a g e n in a 2-liter CC»2-gas c o u n t e r o p e r a t e d a t 2 atms. (net c o u n t for m o d e r n c a r b o n 24.8 counts p e r minute, back­ g r o u n d 4.8 c p m , a n d b a r o m e t r i c effect 0.12 c p m p e r cm of m e r c u r y ) . Since the a m o u n t of m a t e r i a l w a s limited, a n d t h e samples w e r e considered free of infiltrations, the m a t e r i a l w a s combusted directly w i t h o u t a n y chemical p r e t r e a t m e n t . Each sample w a s m e a s u r e d t w o or three times, a l t e r n a t i n g w i t h b a c k g r o u n d m e a s u r e m e n t s . T h e following net a c t i ­ vities ( a c t i v i t y a b o v e background) w e r e o b t a i n e d : K-512 K-510 K-511

peat peat mud

(2.70—2.80 m) ( 3 . 6 1 — 3 . 6 7 m) (7.07—7.18 m)

0.66 ± 0.06 c p m 0.43 ± 0.08 c p m 0.12 ± 0.04 c p m

These activities correspond to ages of 2 7 . 0 0 0 — 4 0 . 0 0 0 years B. C .

T h e samples G R O - 1 2 6 4 , 1263, a n d 1256 were C 0 2 - g a s counter o p e r a t e d at 3 a t m s . (net c o u n t for 2.4 c p m , a n d b a r o m e t r i c effect 0.05 c p m per cm samples were p r e t r e a t e d w i t h dilute hydrochloric o r d e r to e x t r a c t h u m i c acids (cp. D E V R I E S 1958). net activities w e r e f o u n d : GRO-1264 GRO-1263 GRO-1256

mud peat mud

(2.85—2.90 m) (3.37—3.48 m) (6.81—6.91 m)

m e a s u r e d in G r o n i n g e n w i t h a 2-liter m o d e r n carbon 36.9 cpm, b a c k g r o u n d of m e r c u r y ) . Before combustion t h e a c i d a n d dilute sodium h y d r o x i d e in W i t h this p r e t r e a t m e n t t h e following 0.00 ± 0.03 c p m — 0 . 0 2 ± 0.05 c p m 0.02 ± 0.03 c p m

T h e s e measurements show t h a t n o significant activities were left. T h e samples thus a r e older t h a n the r a n g e of the G r o n i n g e n l a b o r a t o r y , i. e. older t h a n 50.000 B.C. A n age of 50.000 B.C. corresponds to a n a c t i v i t y of 0.06 c p m in this counter.

I n o r d e r to investigate the discrepancies b e t w e e n the measurements in C o p e n h a g e n a n d in G r o n i n g e n , sample G R O - 1 2 6 4 was sent as C O a - g a s in a steel c o n t a i n e r to C o p e n ­ hagen, where it w a s measured as sample K - 5 2 8 . F u r t h e r , a s a m p l e K-526 f r o m 3.96—4.08 m w a s given the same p r e t r e a t m e n t in C o p e n h a g e n a n d then combusted a n d measured. T h e results were the f o l l o w i n g : K-528 K-526

mud mud

(2.85—2.90 m) (3.96—4.08 m)

0.04 ± 0.04 c p m — 0 . 0 1 ± 0.07 cpm

T h e s e activities correspond to ages older t h a n the r a n g e of the C o p e n h a g e n l a b o r a t o r y , i. e. older t h a n 40.000 years B.C. (the difference in r a n g e between the C o p e n h a g e n a n d the G r o n i n g e n stations is due m a i n l y to the different pressures, a n d therefore the different a m o u n t s of sample used a t the t w o stations).

T h e activities initially m e a s u r e d for samples K - 5 1 2 , 510, a n d 511 thus were d u e t o y o u n g e r m a t e r i a l (perhaps h u m i c acids), which m a y be extracted from the samples b y a p r e t r e a t m e n t w i t h hydrochloric acid a n d sodium h y d r o x i d e . T h e a m o u n t of infiltrated m a t e r i a l is surprisingly high. F o r sample K - 5 1 2 it is 2.7 p e r cent or m o r e , depending u p o n t h e age of the m a t e r i a l , a n d even d o w n to 7 meters b e l o w g r o u n d level the effect of this infiltration is noticeable. T h i s w a s considered highly i m p r o b a b l e w h e n t h e m e a s u r e m e n t s w e r e u n d e r t a k e n . I t should be m e n t i o n e d t h a t the „ B r ö r u p H o t e l B o g " is a small b a s i n ( d i a m e t e r a b o u t 60 m ) , so there is a possibility t h a t p a r t of the infiltrations comes n o t from above b u t from the sides. F u r t h e r investigations as to the n a t u r e of the infiltrations a r e p l a n n e d . M o r e o v e r , if it t u r n s o u t t h a t the infiltrated material can be r e m o v e d c o m ­ pletely, it will be tried to date the u p p e r p e a t layer w i t h a n i m p r o v e d p r o c e d u r e , w h i c h extends the r a n g e to 70.000 years (see also DE V R I E S 1958). T h e large difference in age

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before a n d after t h e extraction of t h e infiltrated m a t e r i a l demonstrates t h e influence of an a d m i x t u r e of m o r e recent material into glacial samples. T h e difficulties encountered in securing u n c o n t a m i n a t e d samples older t h a n 2 5 — 3 0 . 0 0 0 years in m a n y cases makes it necessary t o consider r a d i o c a r b o n results for these samples as minimum ages r a t h e r t h a n t r u e d a t i n g s ( c p . D E V R I E S E T AL. 1 9 5 8 , a n d D E V R I E S 1 9 5 8 ) .

T h e m e a s u r e m e n t s thus performed in C o p e n h a g e n a n d in Groningen p r o v e t h a t t h e i n t e r s t a d i a l a t B r ö r u p a n d t h e preceding cold p e r i o d a r e older than 5 0 . 0 0 0 B.C. H e n c e , if t h e correlations b y ANDERSEN ( 1 9 5 7 ) a r e right, O l d W ü r m a n d a t least t h e lower p a r t of t h e i n t e r s t a d i a l which separates O l d W ü r m a n d M a i n W ü r m should also b e older t h a n 5 0 . 0 0 0 B . C . A further discussion of dates for W ü r m - i n t e r s t a d i a l samples a n d t h e c o r r e ­ lation of t h e G ö t t w e i g fossile soil w i t h these dates has been given b y DE V R I E S ( 1 9 5 8 ) .

A c k n o w l e d g e m e n t s . T h e a p p a r a t u s f o r gas counting in t h e C o p e n h a g e n l a b o r a t o r y w a s built w i t h a g r a n t from the W e n n e r - G r e n F o u n d a t i o n for A n t h r o p o l o g i c a l Research a n d a g r a n t from the D a n i s h State Research F o u n d a t i o n , w h o s e s u p p o r t is gratefully acknowledged.

Literature ANDERSEN, SV. Th.: New investigations of interglacial fresh-water deposits in Juriand. A preli­ minary report. — Eisz. u. Geg. 8, S. 181—185, 1957.

GROSS, H . : Das Göttweiger Interstadial, ein zweiter Leithorizont der letzten Vereisung. — Eisz. u. Geg. 7, S. 87-101, 1956.

DE VRIES, HL, BARENDSEN, G. W. & WATERBOLK, H . T j . : Groningen radiocarbon dates I I . Science 127, 129, 1958.

DE VRIES, H L : Radiocarbon dates for Upper Eem and Würm-interstadial samples. - Eisz. u.

Geg. 9 , S. 10-17, 1958.

Manuskr. eingeg. 24. 1. 1958.

Anschriften der Verf.: Henrik Tauber, Kulstof-14 Dateringslaboratoriet, Nationalmuseets Naturvidenskabelige Afdeling, Juliane Maries vej 32, Kopenhagen, Dänemark.

Prof. Dr. H l . de Vries, Natuurkundig Laboratorium, Westersingel 34, Groningen, Netherlands.