Land, sea and climate in the northern adriatic region during late pleistocene and holocene

Palaeogeography, Palaeoclimatology, Palaeoecology, 21 ( 1977 ): 1 3 9 - - 1 5 6 © Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s

LAND, SEA AND CLIMATE IN THE NORTHERN DURING LATE PLEISTOCENE AND HOLOCENE

ADRIATIC

REGION

G. C. B O R T O L A M I , 1 J. Ch. F O N T E S , 2 V. M A R K G R A F 3 a n d J. F. S A L I E G E 2

I Istituto di Geologia dell 'UniversitY, Palazzo Carignano, Turin (Italy) 2 Laboratoire de G~ologie Dynamique, Universit~ Pierre et Marie Curie, 4 place Jussieu, 75230 Paris Cedex 05 (France) 3Centre des Faibles Radioactivit~s, Laboratoire mixte CEA--CNRS, B.P. 01, 91190 Gif-sur-Yvette (France) ( R e c e i v e d N o v e m b e r 12, 1 9 7 5 ; revised version a c c e p t e d J u n e 14, 1 9 7 6 )

ABSTRACT B o r t o l a m i , G. C., F o n t e s , J. Ch., Markgraf, V. a n d Sali~ge, J. F., 1977. L a n d , sea a n d c l i m a t e in t h e n o r t h e r n A d r i a t i c region d u r i n g late P l e i s t o c e n e a n d H o l o c e n e . Palaeogeogr., P a l a e o c l i m a t o l . , Palaeoecol., 21 : 1 3 9 - - 1 5 6 . P l e i s t o c e n e a n d H o l o c e n e samples (peats, organic m a t e r i a l s a n d shells) have b e e n obt a i n e d f r o m d i f f e r e n t cores in t h e plain of V e n i c e a n d t h e l o w e r Po Valley. This m a t e r i a l was r a d i o c a r b o n d a t e d in t h e range 4 0 , 0 0 0 B.P. t o present. P l e i s t o c e n e peats p r o v i d e d s u i t a b l e m a t e r i a l for p o l l e n analyses. (1) All t h e P l e i s t o c e n e peats give t h e s a m e r e l a t i o n s h i p b e t w e e n age a n d d e p t h vs soil surface. T h e f r e s h - w a t e r level was t h e same a n d t h e a c c u m u l a t i o n rate of peats r e f l e c t e d t h e s u b s i d e n c e of t h e b o t t o m o f t h e basin. (2) S u b s i d e n c e is n o t c o n s t a n t d u r i n g t h e p e r i o d of t i m e f r o m w h i c h samples were o b t a i n e d . B e t w e e n 4 0 , 0 0 0 a n d 2 2 , 0 0 0 B.P. the rate is 1.3 m m / y e a r , very close to t h e average e s t i m a t e s o b t a i n e d f r o m corings a n d g e o p h y s i c a l i n v e s t i g a t i o n s for t h e w h o l e Q u a t e r n a r y o f t h e region; b e t w e e n 2 2 , 0 0 0 a n d 1 8 , 0 0 0 B.P. t h e s e d i m e n t a t i o n r a t e is 4 to 5 t i m e s h i g h e r t h a n in t h e previous period. This s t r o n g s u b s i d e n c e is c o r r e l a t e d t o t h e ice a c c u m u l a t i o n o n t h e Alps. T h e P l e i s t o c e n e episode e n d s w i t h a s e d i m e n t a t i o n gap, n o P l e i s t o c e n e deposits y o u n g e r t h a n 1 7 , 8 0 0 have b e e n e v i d e n c e d in t h e area. (3) Pollen f r o m peats i n d i c a t e s five climatic stages: (a) 3 9 , 0 0 0 B.P.: cold a n d dry; (b) 3 8 , 0 0 0 - - 3 4 , 0 0 0 : h u m i d a n d w a r m ; (c) 3 3 , 0 0 0 : b r i e f s e t b a c k to a cold a n d dry s t e p p e ; (d) 3 2 , 0 0 0 - - 2 3 , 0 0 0 : dry; (e) 2 2 , 0 0 0 - - 1 8 , 0 0 0 : very dry a n d cold. T h e s e d i f f e r e n t stages can be easily c o r r e l a t e d w i t h t h e climatic f l u c t u a t i o n s already s h o w n in o t h e r regions of Italy, Spain, F r a n c e a n d Greece. (4) T h e episode o f e m e r g e n c e w h i c h follows t h e d e p o s i t i o n of t h e P l e i s t o c e n e peats c o r r e s p o n d s to t h e f o r m a t i o n of an i n d u r a t e d soil rich in c a r b o n a t e s . A n age of approxim a t e l y 1 4 , 0 0 0 B.P. has b e e n o b t a i n e d f r o m this level w h i c h n o longer c o r r e s p o n d s t o a d e p o s i t b u t to a pedogenesis. (5) T h e m a r i n e p a r t of t h e u p p e r P l e i s t o c e n e d e p o s i t s s h o w s a u n i q u e r e l a t i o n s h i p w h e n t h e r a d i o c a r b o n dates are p l o t t e d against t h e p r e s e n t average sea level. The m a i n l a n d samples, h o w e v e r , d o n o t follow this r e l a t i o n s h i p . T h e y have b e e n elevated b y a b o u t 20 m for t h e m o s t n o r t h e r l y l o c a t i o n . This e f f e c t is a t t r i b u t e d to an isostatic r e b o u n d , p o s t e r i o r to t h e m e l t i n g o f t h e Wiirmian ice load o n t h e Alps. 139

140 (6) Because of the eustatic drop of sea level during glaciations, the pre-Holocene landscape was strongly eroded. (7) Continental, coarse and often reworked, deposits are not easily available for a precise stratigraphic reconstruction. (8) Marine deposits do not show a unique correlation between age and depth. The accumulation of sediments at each location is controlled by the morphology over which the Flandrian sea transgressed. At about 3,000 B.P. the transgressive deposits appear more uniform and the sea exceeded its present level.

INTRODUCTION The relationships between age and depth of sediments in the Venice region have been previously investigated by means o f 14C analyses (Fontes and Bortolami, 1972, 1973a). The main object of these investigations was to study the paleosubsidence in the historic city o f Venice and in the surrounding area. Later the study was e x t e n d e d to the Adriatic mainland between Venice and Ravenna (Fontes and Bortolami, 1973b), covering the major part of the large area o f subsidence of the Venice plain and lower Po Valley. The main results can be summarized as follows: (1) uniform sedimentation rate o f a b o u t 1.3 m m / y r in the time interval between 40,000 and 22,000 B.P. ; (2) high sedimentation rate of at least 5 m m / y r between 22,000 and 18,000 B.P.; (3) erosion or lack of sedimentation be t w een 18,000 and 7,000 B.P.; and (4) variable sedimentation rate, depending on locality, during the last 7,000 years. The present paper deals with the paleogeographic and paleoclimatologic aspects of the Venice region as related to its paleosubsidence. A bout 30 new radiocarbon dates are added to the 45 dates previously published and palynologic analyses were p e r f o r m e d on the dated material. Most samples were obtained f r o m several drillings taken f or engineering purposes, but some samples were selected f r om the c ont i nuous cores CNR I and CNR Ibi s, taken for geologic assays (C.N.R., 1971; Favero et al., 1973). Because the layers containing material suitable for 14 C analyses, such as peat, organic remains or shells are discontinuous and infrequent, several locations have been sampled instead o f single vertical profiles. This approach will require m any more dates to permit general conclusions. However, at least two 14C dates are generally available for each hole. UPPER PLEISTOCENE SEDIMENTATION AND SUBSIDENCE In the geographical area of study (Fig.l; Table I), the shallower sediments (60 m) are c o m p o s e d of unconsolidated silts, lime muds and fine detrital deposits including clay minerals. Those deposits are oft en interbedded with layers rich in peat and organic material (C.N.R., 1971). In general, a stratigraphic correlation between ~he different levels from one location to anot her

14l

is difficult. However, when plotting radiocarbon age versus depth with reference to ground surface level, all the points representative of Pleistocene ages show a good correlation (Fig.2). This correlation has only been established within the Venice area itself since south of the Adige River only Holocene deposits are available. k z_

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Fig.1. Location of samples. See Table I for the names of the respective locations. Over the entire region and t h r o u g h o u t the stratigraphie column, Pleistocene peat samples consist of the same t y p e of Cyperaeeae peat. Thus, the growing conditions, i.e. the hydrologic situation, must have been approximately the same during each episode of peat development. For this reason one must reject the c o n c e p t of a gradual filling of pre-existing depressions. Further. more, the good correlation between age and depth of the sediments, independent of facies implies that sedimentation t o o k place within the same basin. It also implies that sedimentation was controlled by the same factor, i.e. the subsidence of the basin floor. As this t ype of sedimentation e x t e n d e d from Venice to Treviso and Padua on the mainland it was probably a general feature for the whole subsidence region as far as the Po Delta. After a continuous subsidence during the interval from 40,000 to 22,000 B.P. with an average sedimentation rate of 1.3 m m / y r , the sedimentation rate

142 TABLE I D e s c r i p t i o n o f the localities s h o w n in Fig.1

No.

Locality

D e p t h vs ground surface

(m) 1

2

3 4 5 6

7 8 9 10 11

12

13 14

15

16 17 18

39.6--40.0 36.0--36.4 19.0--19.4 Treporti 41.3 34.6 27.5 Lido San Nicol5 48.9--49.3 30.5--30.9 L i d o di Pellestrina 4 . 8 - - 5.3 C£ M o n t i r o n 12.3--12.5 Venice T r o n c h e t t o 62.4 CNR--1 and C N R - I bis 5 0 . 7 - - 5 0 . 8 ~:o.'t 37.5--37.6 24.3--24.4 6 . 6 - - 6.7 5.4-- 5.5 Venice civil h o s p i t a l 9.8--10.2 9.8--10.2 V e n i c e civil arsenal 46.9 Alberoni 33.5 Punta Sabbioni 18.0--18.5 12.0--15.0 Alveo Brenta 15.5--15.9 15.4--16.2 15.9--18.3 13.2--13.4 Conca Romea 6 . 4 - - 6.7 36.6--36.9 Candel~ 20.3--20.5 12.9 Lido o f Venice 10.1 7.5 1.9 0.7 22.4 Lido o f Venice ( n o r t h ) 18.6 5.4 5.6-- 6.0 Canale Cenesa 20.0 C o n c a Val Pagliaro 10.5 10.5 Borsea 8.0 Sant ' E r a s m o

D e p t h vs av. sea level (m) 37.8±0.4 34.2±0.4 17.2±0.4 40.4±0.2 33.7±0.2 26.6±0.2 46.1±0.4 27.7±0.4 3.8±0.4 10.4±0.4 60.4±0.2 48.7±0.1 43,7±0.1 35,5±0.1 22,3±0.1 4,6±0.1 3.4±0.1 9.0±0.5 9.0±0.5 45.9±0.3 31.0±0.6 17.8±0.4 13.0±1.7 8.4±0.6 8.3±0.6 9.8±1.3 ii.8±0.3 5.0±0.4 15.7±0.4 +0.3±0.3 12.4±0.2 9.6±0.2 7.0±0.2 1.4±0.2 0.2±0.2 20.4±0.2 16.6±0.2 3.4±0.2 8.1±0.4 14.3±0.3 4.7±0.3 5.5±1.0 3.0±1.0

Age (years B.P.)

* 3 3 . 7 5 0 ± 1000 * 3 0 , 5 3 0 ± 800 " 1 9 . 2 5 0 ± 300 * 3 5 , 6 5 0 ± 1200 29.320 ± 800 * 2 2 , 4 4 0 ± 500 * 3 9 . 4 6 0 ± 1500 * 2 4 , 4 3 0 ± 400 * 2.920 ± 100 * 2 2 . 0 9 0 ± 400 >41,000 > 39.640 36.450 ± 450 34,8O0 ± 5O0 21,750 ± 730 4 , 3 5 0 ± 150 4 , 1 5 0 z 70 20.060 ± 300 26.580 ± 500 * 3 8 . 2 2 0 ± 1200 * 2 8 . 3 6 0 ± 800 * 2 0 . 0 0 0 ± 400 * 5.900 ± 120 " 2 0 , 2 5 0 ± 900 ~'~21,050 ± 400 " 1 8 , 9 0 0 % 400 ~20:95u ± 400 " 1 9 . 4 1 0 ± 800 " 3 1 . 6 3 0 ± 400 " 1 9 . 1 7 0 ± 400 * 5.900 ± 100 * 5.490 ± 100 * 4 . 9 1 0 ± 100 * 3.340 ± 80 * 2.340 ± 90 * 4.930 ± 100 * 4 . 8 4 0 ± 100 * 3.870 ± 90 * 2.020 ± 100 * 7.610 ± 115 * 6.990 ± 120 * 6 . 2 4 0 ± 140 4 . 1 7 0 ± 115

Type

P P P P P P P P OM CM P CM/OM P P OM S 3 OM CM P P P S P P P P P P P S S S S S S S S S OM OM OM OM

143 T A B L E I (continued)

No.

Locality

19

ConcaGrimana

20

Loreo

21

GramigneBando

22 23

Brentelle Villa del B o s c o

24 25 26

Padua-Enpas Arre Sanbruson

27

Bovolenta

28 B

Padua-Gas MottediVolpego

S

Lignano

D e p t h vs ground surface (m)

D e p t h vs av. sea level (m)

Age ( y e a r s B.P.)

16.0--16.5 14.8--15.0 4 . 1 - - 4.6 4 . 1 - - 4.6 3 . 0 - - 3.5

15,7+-0.3 14,3±0.2 3,8!0.2 3,8 ± 0 . 2 4,7 ± 0 . 5

* * * * *

3,460±115 3.300 ±115 2,830±60 3.020±180 4.300-+140

1.5-- 2.0

3.4!0.5

1.500 ± 140

9 . 4 - - 9.7 7.8 3.8

11,0-+0.3 9,6 ± 0.2 5,1 ± 0.2

14,710±400 6.450 ± 110 4 . 2 4 0 ± 120

0.6-- 1.0

2.1±0.3

0 . 6 - - 1.0 9.6 6 . 5 - 7.0 3.6 12.9 4 , 7 - - 5.0 48.5 28.5 21.4--21.6 18,2--18.5

2,1 ± 0 . 3 +3.9 -+ 0.2 4 . 2 5 +- 0.3 1.10±0.2 2 . 1 0 ± 0.2 3.9 ± 0 . 2 46.5 -+0.2 26.5 ± 0 . 2 19.5 ± 0 . 3 16.4 ± 0 . 3

14.0--14.1 9 . 2 - - 9.3 3 . 6 - - 3.8 41.8 23.0

7.0 2.2 +3.3 24.7

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Type

1.970±

OM S OM S OM

OM CM S OM

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1 , 8 5 0 + 210 ~ 170±55 4 , 8 7 5 ± 70 2 . 3 9 5 i 110 17.830 ± 320 3.070±100 37.600±750 25.200±840 23.075±200 22.700±2000 1 9 . 7 1 0 +- 520 1 3 . 8 6 5 _* 3 0 0 5 . 8 0 0 ± 185 4.350-+300 >35.000 **23.450 ±500

S P P OM P P P P P ) f

*'19.620±315 ** 1 . 7 0 0 - + 1 3 2 ** 1.708 ± 8 5 *** 3.840±70

R a d i o c a r b o n d a t e s : P = peat; OM = o r g a n i c m a t e r i a l ; CM = c a r b o n a t e m u d ; S = shell. *Results taken from Fontes and Bortolami' (1973b). **Results taken from Bonatti (1968). ***Result taken from Stefanon (1970).

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Fig.2. R a d i o c a r b o n ages against d e p t h w i t h r e s p e c t t o soil surface. P l e i s t o c e n e samples, all peats growing in t h e s a m e c o n d i t i o n s , i n d i c a t e a r a t h e r c o n s t a n t s e d i m e n t a t i o n rate u n t i l 2 2 , 0 0 0 B.P., t h e last glacial peak, w h e n t h e s e d i m e n t a t i o n rate increases drastically. T h e paleosol f r o m l o c a t i o n 21 does n o t c o r r e s p o n d t o a d e p o s i t . T h e c a r b o n a t e m u d o f l o c a t i o n 7 c o n t a i n s a f r a c t i o n o f " o l d " , i.e. inactive, c a r b o n a n d t h u s is n o use for correlation. H o l o c e n e deposits, b o t h m a r i n e ( m a i n l y shells) a n d c o n t i n e n t a l , do n o t e x h i b i t a u n i q u e p a t t e r n b e c a u s e : (a) t h e a c c u m u l a t i o n does n o t always c o r r e s p o n d t o a biocoenosL, a n d (b) t h e s e d i m e n t a t i o n rate is m o r e c o n t r o l l e d b y t h e b o t t o m m o r p h o l o g y t h a n b y t h e level o f t h e water. F o r details o n H o l o c e n e d e p o s i t s see Fig.5.

145 increases f o u r f o ld at 22,000 B.P. During this interval of fast sedimentation, statistical error on the ages makes it difficult to ascertain the general correlation between age and depth v s ground surface. Two reasons could be responsible for this strong increase in deposition rate: First, a local effect of some small basins to which there could have been a greater a m o u n t of sediment supplied. This, however, seems unlikely for various reasons. No significant variation in the grain size of the sediment occurs. F u r th er mo r e, because of the ext r e m e dryness of climate at that time, no stronger inflow can be assumed. Moreover, the increased rate of peat growth c a n n o t thus be explained. In any case it is difficult to explain local developm e nt of such smaller basins in an area previously flat and swampy at about 22,000 B.P., i.e. before any sig~ificant change in the sea level. Second, the increased sedimentation rate could have been caused by an accelerated downward m o v e m e n t of the basin, i.e. an isostatic effect. As has already been pointed out (Fontes and Bortolami, 1973a, b) the change in the sedimentation rate started with the beginning of the last and strongest phase of the Wilrmian glaciation at about 22,000 B.P. Thus, overloading by ice in the Alps could have led to an immediate increase in the rate of general subsidence of the Venice region. This increased subsidence lasted until ca. 18,000 B.P., after which no record of continental sedimentation has been found. Presumably this i nt e r r upt i on corresponds to the beginning of the deglaciation indicated by the general trend of sea-level fluctuations (Jelgersma, 1961, 1966). As a consequence of the deglaciation, downwarping was arrested. On the mainland there was even an upward movement. This reversal o f crustal m o v e m e n t can be shown by the relationship of age and depth v s present-day sea level (Fig.3). The points of the lagoon localities remain on a general correlation curve, whereas the representative points from mainland localities (Alveo Brenta, Candel/~, Conea Romea, Padua) lie above the curve (e.g., 20 m at Candel/1), proving uplift. Both the increase in subsidence (22,000 to at least 18,000 B.P.) and the subsequent uplift recorded on the mainland are correlated to the successive loading and unloading of ice on the Alps and are thus isostatic. PALYNOLOGIC RESULTS ON PLEISTOCENE LEVELS Previous palynologic analyses of u n d a t e d sections in the region of Venice have shown a pollen sequence from the last glacial period (Bertolani-Marchetti, 1967; Buurman, 1970). Thus, it is of interest to reinvestigate the pollen cont e n t of new sections f r om the Venice lagoon and mainland dated by the radiocarbon m e t h o d (Fontes and Bortolami, 1972, 1973a, b). The time span covered by the analyzed material ranges from 40,000 to 18,000 B.P. thus including the Upper and Inter Wiirm Pleniglaeial of Wiirm III period. We observed within this period five different vegetational phases (see Fig.4) described and i nt er pr et ed in the following. For climatic interpretation one must consider t ha t the vegetational changes in this period range from

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Fig.3. Same relationship b e t w e e n age and depth as in Fig.2. However, here tne depths are related to a general reference level, i.e. the present sea level. This linear correlation, i.e. the observed pattern (Fig.2), for the s e d i m e n t a t i o n rate is preserved even s o m e w h a t better: regular s e d i m e n t a t i o n rate o f a b o u t 1.3 m m / y r b e t w e e n 4 0 , 0 0 0 and 2 2 , 0 0 0 B.P., s u d d e n increase at a b o u t 2 2 , 0 0 0 B.P. and until, at least, 1 8 , 0 0 0 B.P. This pattern applies o n l y for the l o c a t i o n s in the l a g o o n area. Samples on the mainland that previously o b e y e d the general correlation o f age vs ground surface (Fig.2) are n o w shifted upward. This is the case for l o c a t i o n s 11, 24, 26 and especially for l o c a t i o n 13 (Candelh), w h i c h is the northernm o s t sampling p o i n t and the closest to the Alps. The mainland has b e e n tilted up for a b o u t 20 m at Candelh. This e f f e c t involves the P l e i s t o c e n e deposits and m a y be c o n s i d e r e d as an isostatic r e b o u n d f o l l o w i n g the m e l t i n g o f the Wiirmian ice on the Alps, the a c c u m u l a t i o n o f w h i c h had p r o d u c e d the extra s u b s i d e n c e b e t w e e n 2 2 , 0 0 0 and (at least) 1 8 , 0 0 0 B.P. This e f f e c t is n o t n o t i c e a b l e on the e x t e n s i v e cover o f H o l o c e n e deposits.

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148 steppe to open forest and reflect primarily a change in h u m i d i t y much more than in temperature. In the Mediterranean region during the last glaciation, there was a displacement of the lower timberline (or dryness timberline) in response to fluctuations in humidity (see Markgraf, 1974). Phase a. The oldest ~4C date measured for this site is 39,000 B.P. and dates a Gramineae steppe phase with typical steppe shrubs as Juniperus, Arternisia, Ephedra and only scattered pine trees, indicating a cold and dry climate. Phase b. Between 38,000 and 34,000 B.P., the tree pollen percentages increase to 60%, suggesting a climatic change to more humid and warm conditions. At the beginning of this phase (around 38,000 B.P.) a subphase occurred with more than 10% mixed oak forest elements, traces of beech (Fagus) and fir (A bies), followed around 35,000 B.P. by a subphase rich in pine and birch (Betula), up to 5% spruce (Picea) and only traces of mixed oak forest. This vegetational succession may indicate a climatic change from a humid and temperate warm climate to a still warm, but drier climate. The transition to the drier condition is marked by calcareous muds which interrupt the peat sedimentation. Phase c. At 33,000 B.P., a renewed climatic deterioration occurs, with a steppe vegetation rich in Gramineae, Chenopodiaceae, Ephedra and Juniperus and only a few pine stands. Phase d. Between 32,000 and 23,000 B.P. the percentage of pine pollen increases to 50%. The values for Gramineae, Artemisia, Ephedra and Juniperus, however, remain high, indicating an open pine forest in the steppe, reflecting a dry climate. The higher values of pine suggest an increase of temperature. Between 31,000 and 29,000 B.P. the appearance of oak (Quercus), elm (Ulrnus), poplar (Populus) and increased birch (Betula) and alder (Alnus) values suggest a phase of increased humidity which caused a lowering of the lower timberline. Phase e. Between 22,000 and 18,000 B.P. the steppe vegetation is dominating with up to 70% Gramineae, high Arternisia, Juniperus and Chenopodiaceae values and less than 15% tree pollen such as pine and birch. This plant assemblage indicates a very dry and cool climate, especially in its older part where also calcareous muds are frequent at several sites. The Cyperaceae-rich younger part was more humid and contains more abundant riverside vegetation and waterplants. Beneath the pollen sequence described above, i.e. prior to 40,000 B.P., there is a transgressive marine layer consisting of yellowish sands that overlie other peat layers too old to be radiocarbon dated. The pollen composition of these layers shows a vegetational pattern completely different from all the younger phases. Up to 30% mixed oak forest, beech (Fagus), hazel (Corylus), spruce (Picea) and even some Vitis pollen suggest a warm and humid climate, similar to that after glaciation (Postglacial). When correlating the vegetational phases of Venice with those from other areas in the Mediterranean region, striking resemblances appear despite the

149

large distances involved. The comparison sites are the following: Lake of Vico, 50 kin north of Rome (Frank, 1969); Padul, southern Spain (Florschtltz et al., 1971); Cueva Morin, northern Spain (Leroi-Gourhan, 1971); caves of Arcy, France (Leroi-Gourhan and Leroi-Gourhan, 1964), and Tursac, France (Leroi-Gourhan, 1968); Philippi, Macedonia (Wijmstra, 1969) and Ioannina, Macedonia (Bottema, 1967, 1974); and Ghab Valley, Syria (Niklewski and Van Zeist, 1970). The climatic deterioration to cold and dry conditions at 39,000 B.P. {phase a in the Venice area) can be traced in the pollen profiles from Spain, Italy, Greece and Syria by a dominance of open grassland and only scattered pine stands throughout the whole Mediterranean region. The following interstadial between 38,000 and 35,000 B.P. (phase b in Venice) with its traces of thermo-philous vegetation is contemporaneous to the Hengelo interstadial in The Netherlands (Van der Hammen et al., 1967). This interstadial is reported in most Mediterranean profiles, b u t correlation of the two subphases of phase b found in Venice (the older dated 38,000 B.P. and the younger 35,000 B.P.) is only possible with the analyses of Cueva Morin (northern Spain) and Macedonia, where they are called Kalabaki l {dated 36,000 B.P.) and Kalabaki II (dated 37,000 B.P.). The very low pine values in the older Hengelo subphase from Venice are also pronounced in the Cueva Morin profile but are difficult to explain. It can neither be due to climate, since the pine has a very wide ecologic range, nor to the competition of other trees since the total tree pollen did riot exceed 30 to 40% of the total pollen at that time. Between 34,000 and 32,000 B.P. (phase c in Venice) a further dry and cold interval brings back steppe conditions throughout the Mediterranean region, usually strongly marked in the cave analyses, where the rate of sediment deposition is notably increased during cold periods. Another interstadial follows this phase between 32,000 and 23,000 B.P. (phase d in Venice) which culminates in Venice in two climatic optimal subphases around 31,000 and 28,000 B.P. In the analyses of Lake of Vico {Italy) and Padul (Spain), this interstadial is ascribed to the Denekamp interstadial in The Netherlands {Van der H a m m e n et al., 1967). However, an exact correlation of the t w o subphases in Venice is only possible in the profiles from Macedonia and in the cave analyses from Spain and France. In Macedonia those subphases are called Krinides I (dated 32,000 B.P.) and Krinides II {dated 29,000 B.P.) and in France, Arcy (dated 30,000 B.P.) and Kesselt. In all those sites the climatic impact of this interstadial was definitely less strong than during the previous Hengelo interstadial, i.e. the humidity did not increase significantly for this Denekamp interstadial. From 22,000 to 18,000 B.P. {phase e in Venice) extreme dryness and cold must have ruled in the whole Mediterranean region, indicated by an almost treeless steppe vegetation expanding from Spain to Syria {the only exception was Padul, where pine dominates even during this interval. Presumably the climate w a s n o t different from the previous cold spells {phases a and c in Venice), b u t the strong climatic impact is due to the duration of this cold

150 phase without major interruptions. The extreme dryness of this last period of the Wtirmian glaciation might be correlated again with the formation of calcareous muds in the area of Venice. The deepest samples analyzed in our investigation pose the most serious problem of correlation when attempting to relate them to other sites, since these samples cannot be radiocarbon-dated. The vegetational composition strongly resembles the one reported from the early Pleniglacial interstadials called Odderade, Bre~rup and Amersfoort in the Dutch chronology (Van der Hammen et al., 1967). In the Mediterranean region those interstadials occur in the pollen profiles from Padul (northern Spain), Philippi and Ioannina (Macedonia). The vegetational composition of that phase in Venice excludes a correlation to the Moershoofd interstadial, dated between 46,000 and 43,000 B.P. (Van der Hammen et al., 1967), and excludes the Eemian or Riss/Wtirm interglacial. Thus, the marine transgression found in Venice presumably reflects the sea-level rise of those above-mentioned early Pleniglacial interstadials, thus corresponding to the Wfirm III--Wfirm II transgression. Those Odderade, Brc~rup and Amersfoort interstadials are known to have been climatically more favorable than the late Pleniglacial interstadials, thus probably causing important sea-level rises. A high sea-level stage has become known and dated 60,000 B.P. by 23°Th/234U in New Guinea (Chappell, 1974a). Recent efforts to date those interstadials in their type localities in The Netherlands with the m e t h o d of isotope enrichment proposed by Haring et al. (1958) yielded ages between 70,000 and 60,000 B.P. (P. Grootes, pers. comm.). Thus it becomes probable that the deposition of sediment in Venice was interrupted for at least 20,000 years, i.e. between 60,000 and 40,000 B.P. This sedimentation gap seems rather long when compared for example with the gap during the late glacial and Holocene transgression between 18,000 and, at most, 8,000 B.P. Summing up, the pollen analyses from the Venice region fit well into the general picture of the last Pleniglacial vegetation and climate history in the Mediterranean region. Its vegetation pattern during this period especially resembles the analyses from Ioannina (Macedonia) due presumably to the relatively short distance of both localities to mountainous regions. Thus, both sites represent excellent climate indicators, and even a small climate change such as a humidity increase, is immediately reflected by an increase of tree pollen, corresponding to an approach of the forest belt, i.e. a lowering of the dryness timberline. The simultaneous occurrence of certain tree species such as beech (Fagus), fir (A bies) and spruce (Picea) during those relatively short interstadial periods throughout the Mediterranean region is a strong support for the assumption of local refuges for those trees. PLEISTOCENE EMERGENCE

No Pleistocene deposits younger than 17,830 B.P. (locality no. 24) have been recorded in the region. The Pleistocene period ends with a b ~ a k in sedi-

151

m e n t a t i o n evidenced in the lagoon area and probabl y of m ore general extent. The age o f 14,710 _+ 400 B.P. (locality no. 21, Gramigne Bando) given by a carbonate-rich layer does n o t correspond to an episode of sedimentation. This level is located at the t o p of the Pleistocene deposits and slightly indurated and oxidized. The ~3C c o n t e n t {51:~C = + I f',, vs PDB) indicates that the carbonate has crystallized in conditions close to equilibrium with the atmosphere. Thus, the radiometric age is retained as representative of the formation of this level. The palynological association is in agreement with this conclusion. The 1SO c o n t e n t (61sO = -- 5.2":. vs PDB) of the carbonate indicates its precipitation under evaporating conditions in water of continental origin. Those data c o m p a r e d with the facies of the sample suggest that a mechanism of shallow dessication has p r o d u c e d this calcareous crust. In the Venetian area this level would be equivalent to the so-called " C a r a n t o " horizon, the facies of which is very similar. The " C a r a n t o " has previously been interpreted as a paleosol (Matteotti, 1962) considered as an indicator of t he Pleistocene-Holocene b o u n d a r y (Fontes and Bortolami, 1972, 1973a, b). During and after the f o r m a t i o n of the calcareous horizon, erosion of Pleistocene deposits t o o k place, as a consequence of the low level of the sea. As eLresult of this erosion the calcareous crust is largely discontinuous, although its f o r m a t i o n was of general e x t e n t as indicated by its occurrence in the lower Adige plain (Previatello, 1970). In the region of Venice, where the surveys have been m ore detailed, the " C a r a n t o " is recorded near the soil surface in the zone of Marghera some kilometers to war d the mainland. Beneath the lagoon of Venice and in the coastal area (Gatto and Previatello, 1974) it is located at a depth of about 13 m. This slope is greater than that of the t o p o g r a p h y and is very close to that which can be calculated for the Pleistocene deposits between the lagoon and, for example, the locality of CandeltL This shows that the " C a r a n t o " indurated horizon has been d e f o r m e d in the same way as the Pleistocene layers. The tilting effect a t t r i but ed to the melting of the ice would be posterior to f o r m a t i o n of the Caranto horizon. If the Caranto is c o n t e m p o r a n e o u s with the level o f the same facies dated 14,710 B.P. at Gramigne Bando, it would mean that the upward m o v e m e n t of the mainland was active at a b o u t 15,000 B.P. Since this effect of isostatic r e b o u n d (Walcott, 1973; Chappell, 1974b) was highly a t t e n u a t e d when the first horizontal deposits of Holocene age were f o r m e d at ab o u t 7,000 B.P., it means t hat the response of the continental crust to the isostatic reaction was rapid. This conclusion is in agreement with the initiation of an immediate dow nw ar d m o v e m e n t observed at a b o u t 22-23,000 B.P. w i t h o u t any significant time lag at the beginning of the major glaciation. HOLOCENE The oldest Holocene deposits of marine origin recorded in the drillings are dated ab o u t 7,000 B.P.

152

Between approximately 7,000 and 3,000 B.P. there is not a single correlation between the age and the depth of the deposits (Fig.5). The location of the marine deposits is not linked simply to the sea level. The rate of accumulation depends on the coastal currents and principally on the morphology of the transgressed land which previously had been extensively dissected by erosion. Nevertheless, for each given locality the representative points are lying on single characteristic curves. These curves reflect the combined effects of eustatic rise and subsidence. They are contained in the space bounded by the pure eustatic curve of Jelgersma (1961, 1966), and the same curve corrected for a constant subsidence rate of 1.3 mm/year, which was the minimum rate calculated for the late Pleistocene. This leads to two conclusions: (a) the subsidence was still operative during the time interval from 7,000 to 3,000 B.P.; and (b) the rate of the downward movement was less than 1.3 mm/year. At about 3,000 B.P., all the separate curves corresponding to the different sections converge roughly into a single curve. The morphological differences of the territory invaded by the Flandrian transgression have now been levelled. As indicated by the curve, the sea then reached a level higher than the presentday one. Otherwise, it would be difficult to explain why the Upper Holocene deposits are located at about the present~day sea level, especially if the subsidence acted after their deposition. Because of the occurrence of various closed basins, the continental Holocene deposits are difficult to correlate. Furthermore, from the inconsistencies in age obtained, at Bovolenta for instance (locality no. 27), it appears that a large a m o u n t of material must have been reworked on the mainland during the Holocene. The thickness of the reworked deposits at Bovolenta indicates the importance of erosion during the emergence stage of the terminal Pleistocene. PALAEOGEOGRAPHIC RECONSTRUCTION

The main points considered in the present study are schematically summarized in Table II. This reconstruction deals with the zone of the presentday shore in the Venice lagoon where the m a x i m u m of information was available. CONCLUSIONS

(1) The previous evidence on the rate of subsidence and its variation during the late Pleistocene and the Holocene are confirmed (about 1.3 m m / y e a r before the Wfirm pleniglacial, more than 5 m m / y e a r during the pleniglacial, and less than 1.3 m m / y e a r in the Holocene). (2) The isostatic response of the region to the ice loading during the Wi~rm pleniglacial was followed by a rapid rebound the effects of which were highly attenuated before the Holocene. (3) Different stages of subsidence which controlled the sediment accumulation are correlated with climatic episodes evidenced by their pollen spectra.

153

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Fig.5. R e l a t i o n s h i p b e t w e e n r a d i o c a r b o n ages a n d d e p t h vs average sea level for H o l o c e n e san'tples. Circles: m a r i n e or l a g o o n a l samples. Triangles: c o n t i n e n t a l deposits. T h e m a i n p a r t o f t h e samples lie b e t w e e n t h e curve of pure e u s t a t i c rise (Jelgersma, 1 9 6 1 , 1 9 6 6 ) a n d t h e s a m e curve c o r r e c t e d for a c o n s t a n t s u b s i d e n c e rate o f 1.3 m m / y r as d e d u c e d f r o m P l e i s t o c e n e p e a t s e d i m e n t a t i o n a n d c o n s i d e r e d as r e p r e s e n t a t i v e of t h e general geological s u b s i d e n c e . It t h e r e f o r e a p p e a r s t h a t t h e s u b s i d e n c e d u r i n g H o l o c e n e t i m e s was smaller t h a n this regional average. F u r t h e r m o r e , e a c h l o c a t i o n gives its o w n p a t t e r n of ~sedimentation u n t i l a b o u t 3 , 0 0 0 B.P. w h e n all t h e s e p a r a t e curves are converging. Before t h a t t i m e t h e s e d i m e n t a t i o n was especially c o n t r o l l e d b y t h e profile of t h e e x p o s e d l a n d s c a p e w h i c h has b e e n s t r o n g l y e r o d e d d u r i n g t h e late P l e i s t o c e n e e m e r g e n c e .

154 T A B L E II Paleogeographic r e c o n s t r u c t i o n o f t h e V e n i c e area Age (years B°P.)

Vegetal c o v e r

Climate

t

Present - - mixed oak forest- -

I ]

Crustal m o v e m e n t

lagoonal

relative s t a b i l i t y

littoral

<1.3 mm/year

I

I 3,000

Sedimentation

warm; humid ! !

Flandrian transgression

7,000

erosion 15,000

calcareous paleosol

t

I isostatic rebound !

I 18,000

- emergence

÷ > 5 mm/year Gramineae steppe

very dry; cold

subsidence + isostatic e f f e c t

23,000 pine forest

dry

Gramineae steppe

cold; dry

pine a n d b i r c h

warm; dry

pine a n d s o m e mixed oak forest

temperate warm; humid

Gramineae steppe

cold; d r y

32,000 lacustrine fluviatile

34,000

I I ! I I I i !

1.3 m m / y e a r

38,000

39,000

>43,000

>60,000

emergence ?

?

(?) m i x e d oak f o r e s t

warm; humid

marine transgression lacustrine fluviatile

155

(4) The climatic subdivisions based on pollen analyses which were found in this region are in close correlation with those found in sections from Italy, Spain, France and Greece. Five vegetational phases were distinguished. (5) The boundaries of these bioclimatic phases have been established on the basis of numerous radiocarbon ages, which may compensate for the fact that the sampling was scattered because of local conditions of sedimentation. (6) The region appears to lend itself for studies on the Quaternary because: (a) the deposits are well preserved and contain numerous peats rich in pollen; (b) marine transgressions act as indicators of interstadial or interglacial episodes.

ACKNOWLEDGEMENTS

Tihanks are due to the anonymous reviewer, with the initials R.W.F., whose suggestions were most helpful and to A. B. Muller for preparation of the English text.

REFERENCES Bertolani-Marchetti, D., 1967. Vicende climatiche e floristiche d e l l ' u l t i m o glaciale e del postglaciale in sedimenti della laguna veneta. Mere. Biogeogr. Adriat., 1966--67, 7: 193--225. Bonatti, E., 1968. Late-Pleistocene and Postglacial stratigraphy of a s e d i m e n t core from the lagoon of Venice (Italy). Mem. Biogeogr. Adriat., 7 (suppl): 9--26. Bottema, S., 1967. Late Quaternary pollen diagram from Ioannina, n o r t h w e s t e r n Greece. Proc. Prehist. Soc., 33: 26--29. Bottema, S., 1974. Late Quaternary Vegetation History of N o r t h w e s t e r n Greece. Thesis, University of Groningen, 190 pp. Buurman, P., 1970. Pollen analysis of H o l o c e n e and Pleistocene sediments in the neighb o u r h o o d of Portogruaro (Venezia), Italy. Mere. Biogeogr. Adriat., 1969--70, 8 : 4 1 - - 5 1 Chappell, J., 1974a. Relationships b e t w e e n sea-levels, 18 O variations and orbital perturbations, during the past 250,000 years. Nature, 252: 199--202. Chappell, J., 1974b. Late Quaternary glacio- and hydro-isostasy, on a layered earth. Quarternary Res., 4: 429--440. C. N. R. -- L a b o r a t o r i o per lo Studio della Dinamica delle Grandi Masse, 1971. Relazioni sul pozzo Venezia 1. C.N.R. Technical Reports. Favero, V., Alberotanza, L. and Serandrei Barbero, R., 1973. Aspetti paleoecologici, sedimentologici e geochimici dei sedimenti attraversati dal Pozzo Ve I bis C . N . R . C . N . R . -- Lab. per lo Studio della Dinamica delle Grandi Masse, T.R. 6 3 : 5 1 pp. Florschiltz, F., Men~ndez Amor, J. and Wijmstra, T. A., 1971. Palynology of a thick Quarternary succession in S o u t h e r n Spain. Palaeogeogr. Palaeoclimatol. Palaeoecol., 6: 67--85. Fontes, J. Ch, and Bortolami, G., 1972. Subsidence of the Venice area during the past 40,000 years. C.N.R. -- Lab. per 1o Studio della Dinamica delle Grandi Masse, R.T. 5 4 : 1 1 pp. Fontes, J. Ch. ar{d Bortolami, G., 1973a. Subsidence of the Venice area during the past 40,000 years. Nature, 244: 339--341,

156 Fontes, J. Ch. and Bortolami, G., 1973b. Evolution des confins adriatiques septentrionaux au Pleistocene sup~rieur et ~ l'Holoc~ne. In : Les M~th0des quantitatives d'&ude des variations du climat au cours du Pleistocene. Coll. Int. C.N.R.S., No. 219: 155--161. Frank, A. H. E., 1969. Pollen stratigraphy of the Lake of Vico (Central Italy). Palaeogeogr., Palaeoclimatol., Palaeoecol., 6: 67--85. Gatto, P. and Previatello, P., 1974. Significato stratigrafico, comportamento meccanico e distribuzione nella Laguna di Venezia di una argilta sovraconsolidata nota come "Caranto". C.N.R. -- Lab. per lo Studio della Dinamica delle Grandi Massa, T.R. 70: 45 pp. Haring, A., De Vries, A. E. and De Vries, H., 1958. Radiocarbon dating up to 70,000 years by isotopic enrichment. Science, 128: 472--473. Jelgersma, S., 1961. Holocene sea-level changes in the Netherlands. Meded. Geol. Stich., C, 6: 1--101. Jelgersma, S., 1966. Sea-level changes during the last 10,000 years. In: Proc. Int. Symp. World Climate from 8,000 to 0 B.C.R. Meteorol. Soc., pp. 54--71. Leroi-Gourhan, Arl., 1968. L'abri Facteur £ Tursac (Dordogne), Gallia Pr~hist., 11: 123--132. Leroi-Gourhan, Arl., 1971. Cueva Morin. Excavaciones 1966--1968. VII: Analisis polinico de Cueva Morin. Publ. Patronato Cueva Morin. Publ. Patronato Cuevas Prehist., Prov. Santander, Spain. Leroi-Gourhan, Arl. and Leroi-Gourhan, A., 1964. Chronologie des grottes d'Arcy-sur-Cure (Yonne). Gallia Pr~hist., 6: 1--64. Markgraf, V., 1974. Paleoclimatic evidence derived from timberline fluctuations. In: Les M&hodes quantitatives d'~tude des variations du climat au cours du Pleistocene. Coll. Int. C.N.R.S., No. 219: 66--77. Marino, C. M. and Pigorini, B., 1969. Datazione dei sedimenti recenti del Mare Adriatico col metodo del radiocarbonio. Atti Soc. Ital. Sci. Nat., 109: 469--484. Matteotti, G., 1962. Sulle carateristiche dell'argilla prec0mpressa esistente nel sottosuolo di Venezia-Marghera. Not. Ord. Ing. Prov. Padova, 6 : 1 2 pp. Niklewski, J. and Van Zeist, W., 1970. A late Quaternary pollen diagram from Northwestern Syria. Acta Botan. Neerl., 19: 737--754. Previatello, P., 1970. Caratteristiche geotecniche di una argilla sovraconsolidata dell'Adige. Ist. Costr. Maritt. Univ. Padova, Scritti in onore del Prof. Guido Ferro, pp. 147--164. Stefanon, A., 1970. The role of beachrock in the study of the evolution of the North Adriatic Sea. Mem. Biogeogr. Adriat., 1969--70, 8: 79--87. Van der Hammen, T., Maarleveld, G. C., Vogel, J. C. and Zagwijn, N. A., 1967. Stratigraphy, climatic succession and radiocarbon dating of the Netherlands. Geol. Mijnbouw, 46: 79--95. Walcott, R. J., 1973. Structure of the earth from glacio-isostatic rebound. Annu. Rev. Earth Planet. Sci., 1 : 1 5 - - 3 7 . Wijmstra, T. A., 1969. Palynology of the first 30 meters of a 120 m deep section in Northern Greece. Acta Botan. Neerl., 18: 511--527.