Holocene peat initiation in the Ligurian Apennines, northern Italy

Review o1"Palaeobotany and Palynology, 63 (1990): 173-182 Elsevier Science Publishers B.V., Amsterdam

173

Printed in The Netherlands

Holocene peat initiation in the Ligurian Apennines, northern Italy G.M. Cruise Sir John Cass Quaternary Research Unit, Geography Department, Ci O, c~["London Polytechnic, Old Castle Street, London E1 7NT (U.K.)

(Received July 11, 1989: revised and accepted March 27. 1990)

ABSTRACT Cruise, G.M., 1990. Holocene peat initiation in the Ligurian Apennines, northern Italy. Rev. Palaeobot. Palynol., 63:173

182.

The radiocarbon dates obtained from the basal organic sediments of six sites in the Ligurian Apennines are presented and it is shown that most Holocene peat deposits only began to accumulate within the last 5000 years. This paper seeks to examine the significance of climatic, geological and anthropogenic factors on peat growth in eastern Liguria. The absence of known early Holocene peat or lake sediments throughout much of northwest Italy suggests widespread climatic conditions inimical to peat development during that period. Locally, however, there are strong indications that peat formation only occurred after major disruption of local soils which was most likely to have been initiated by human (Chalcolithic) forest disturbances.

Introduction

The complex relationship between initiation of peat growth during the Holocene and the role of climatic change, soil deterioration, mire hydrology and anthropogenic forest clearances has been discussed extensively for sites in northern Europe (Moore, 1975: Pennington, 1975; Moore et al., 1984; Succow and Lange, 1984). As discussed by Moore et al., (1984) blanket peat formation in England and Wales may have been triggered by the effects of Neolithic and later clearances although in parts of Scotland and Norway peat initiation probably commenced before any extensive human activity. In a summary of mire types throughout East Germany, Succow and Lange (1984) have drawn attention to a cessation of sedimentation between approximately 8000 B.P. and 2000 B.P. in many kettle hole and swamp mires. The authors have ascribed this hiatus to the combined effects of a dry climate and water retention by a closed vegetation cover although recommencement of peat growth during the late Holocene may have been due to both climatic deterioration and a

reduction in evapotranspiration rates caused b} forest clearances during the Iron Ag~ and Medieval periods. Continuous Holocene records have been recovered from lake basins throughout the Mediterranean (Bottema, 1974; Jalut et al., 1982; Watts, 1985; Pons and Reille, 1988; Lamb et al., 1989) and from high altitude peats located in some of the mountainous regions bordering the sea (de Beaulieu, 1977; Jalut, 1977). Nevertheless frequent reports of lower-lying basin and valley peats have repeatedly demonstrated an absence of early Holocene sediments. A brief summary of the published data based on available radiocarbon dates and pollen stratigraphy suggests that there may have been three principle phases of peat formation in the Mediterranean region as follows: (1) Accumulation of organic deposits from the "Atlantic" period has been reported from the lower Rh6ne valley (Triat-Laval, 1978) and the Neretva lowlands, Dalmatia (Von Brande, 1973). TriatLaval reported that eight out of the seventeen sites studied in the lower Rh6ne valley only

174

dated from this period and supported this contention with five basal radiocarbon dates ranging from 7610 B.P. to 6450 B.P. Only one basal date of 6915 B.P. was reported by Brande. Although these low-lying coastal sites could have been influenced by changes in sea-level (Aloisi et al., 1978) Triat-Laval considered that Neolithic forest clearance and soil disturbance was a more likely explanation. (2) Several dates from upland basin peats in the Ligurian Apennines indicate the onset of peat accumulation from between 4000 B.P. to 5000 B.P. (Macphail, 1988; see below), a basal date of 4350 B.P. at Litochoro, northeastern Greece was reported by Athanasiadis (1975) and peat formation at around 4500 B.P. of a valley mire at E1 Acebran, southern Spain has been described by Stevenson and Moore (1988). Clay, silt and charcoal in the basal sediments at E1 Acebran were interpreted by the authors as representing forest clearance in the catchment area which led to changes in mire hydrology, although the possibility" that dune migration may have blocked the valley drainage system was also considered. (3) Finally Reille (1977) examined a series of peats in the Rif, Morocco and reported that most of the sequences studied only accumulated from the "Subatlantic" period. Four basal radiocarbon dates varying between 1670 B.P. to 1010 B.P. supported this contention. Similarly two peats in northern Portugal examined by Coud6-Gaussen and Den6fle, 1980) were shown to have begun to accumulate during the period 1170 B.P. to 990 B.P. Here peat formation was also ascribed to destruction of surrounding forests and soil erosion. With the exception of the papers cited above the lack of early Holocene peat sediments and the causes of peat accumulation in the Mediterranean has received little attention in the published literature. The present author has examined the pollen stratigraphy of six Holocene basin peat sites in eastern Liguria, northern Italy (Fig. 1; Macphail, 1988) two of which have been published in detail (Cruise, 1990). It has already been noted that few early Holocene peat or lake sediments have been recovered from the Ligurian Apennines and that most peat deposits only began to accumulate

GM.

CRUISE

J / /

~_~

.

Lago Nerc~

Q

Casaqova

J / delle Lame Lago•

II Agora~e

/ !

•

Prato Mollo /

/

•

0 I

MalorTown

km

5 i

Fig.1. Location

Sestri

Levaill g°ne O . ~ ~

~

10 N I I

N

~-~

L_

o f p e a t sites.

during the last 5000 years. This paper aims to examine the effect that climatic, geological and anthropogenic factors could have had on peat growth in a small part of southern Europe. Peat accumulation in eastern Liguria

The salient characteristics of the sites examined in eastern Liguria are outlined in Table I and summary pollen diagrams showing the four principal taxa together with the available radiocarbon

70

3025±50 GrN-14672 3510±35 GrN 14673

500

4300_+60 BIN-3132 4130+60 BIN-3131

1 small ephemeral stream 100

4855+_40 GrN 14430 4610+40 GrN 1443

Major and minor

100

4180+60 GrN- 14669 351OM50 GrN-14670

None

75

8450±80 GrN-14433 7330_+70 GrN-14434

Negligiable

75

5O4 Bedrock

None available Possibly Lateglacial

5040±100 GrN-14432

DRAINAGE

DIAMETER(m)

SEDIMENTS Depth of peat (cm) Other

14(~ Qp Basal organic sediments

Others

381 Bedrock

2050+60 GrN 14671

145 105 cm silt Bedrock

1 small stream 2 small seasonal streams

Shallow depression bounded by rock ridge

Basin bounded by drift mounds

Basinbounded by rock ridge

Basin bounded by possible arcuate moraine

GEOMORPH.

91 Silt & bedrock

Depression bounded by drift mound & landslide debris ridge Shallow depression bounded by rock ridge

Serpentinite

Quaternary drift over serpentinite

Serpentlnite

Serpentlnite

GEOLOGY

101 Silt & bedrock

Serpentinite Serpentinite

682 Bedrock & gravels

NW

S

NW

NW

S

W

ASPECT

1029

1492

1479

1328

831

1056

ALTITUDE(m)

9o25'E

9o31'E

9o30'E

9o28'E

9o31'E

9o24'E

LONG.

44°18'N

44o28'N

44o30'N

44°29'N

44°18'N

44o33'N

LAT.

I-AGO DELLE LAME

PRATO MOLLO

I.AGO NERO

BARGONE

CASANOVE

SITE

AGORAIE

Peat sites in eastern Liguria.

TABLE I

z

Z Z

> Z >

176

GM

SITE ALTITUDE(M)

LAGO NERO

AGORAIE

1475

1328

NW

NW

ASPECT

J

~

L~

CRUISE

~J

12 L ~

L? L t #

1000-

2000"

/'~

.........

LN4

3000 3510±60BP

LN3

4000

4180t 60 BP

%;

,~ELN2

5000

LN1

6000 t 7000

8000 0

2O

40 60 80 0 20 L ~ ~ L ~

1029 NW

20

I

0

L

0

20

J

20

40

0 100

20

0

20

CASANOVA

BARGONE

1492

1056

831

S

W

S

3

3

~J

L

PRATO MOLLO

LAGO DELLE LAME

U

0

j

d~

I_!. W !2 k ~

J

L5~

L~~

£

L£ L ~

E

L~ L°~

L ~-~

L%

4y_ Ic,°q

____,

4130±60BP 4300± 60BP

.........

;

i

(,

i

7330t 70 F

(P

)

'Q

0

20

40 60

0 100

20

0 10

0

2,0 40

0[~10 010 0L~I0

Fig.2. Holocene vegetation developments in eastern Liguria.

0

2,042

0

2~

0L~20 0[~10

8450 80 BF0 ~ 2 0 • L

40

~ L j00L 110 i

C?

J

HOLOCENE PEAT INITIATION IN LIGURIAN APPENNINES

dates are supplied in Fig.2. All sites are located on serpentinite bedrock at a range of altitudes from 831 m to 1492 m in a variety of geomorphological situations and with contrasting drainage characteristics. The pollen sequences obtained from sites located at altitudes of above 1000m (Fig.2) indicate extensive mixed Abies-Fagus forests in the montane zone of the Ligurian Apennines. Fagus is rarely recorded in pollen diagrams from northern Italy and southern France before approximately 5000 B.P. (de Beaulieu, 1977; Schneider, 1978) and the pollen stratigraphical data are commensurate with a later middle Holocene age for the sediments. Only one of these basins, Casanova, may contain Lateglacial sediments but it has already been argued on the basis of pollen stratigraphy (Macphail, 1988; Cruise, 1990) that there could be a hiatus in the sedimentary record lasting from the Lateglacial until about 5040 B.P. as indicated by the only available radiocarbon assay for this site. The sediments at Agoraie accumulated above bedrock and gravels from shortly before about 4180 B.P. At the shallow sites of Lago Nero and Prato Mollo peat initiation appears to have occurred from 4855 B.P. and 4300 B.P. respectively although some "ageing" of these dates is possible due to recycling of older carbon (see below - - Geological factors). Those dates obtained from Lago delle Lame are inverted, the cause of which is unknown and it is only possible to establish an approximate age of peat inception of 3250 B.P. (an average of the two radiocarbon dates). The oldest dated Holocene sediments (8450 B.P.) were recovered from Bargone, the lowest-lying site, and this has provided records of vegetation developments contrasting with those obtained from higher altitudes (Fig.2). Although Abies frequencies are high in the basal assemblages, these decline and thereafter Quercus and Fagus are more frequent. Due to the lack of a detailed chronology for this site, it is not possible to determine whether discontinuities exist in the sedimentary record. In general however, the pollen stratigraphical data and available radiocarbon dates indicate that at five of the sites examined, Holocene peats only began to accumulate during the last 5000 years.

17"7

Discussion: the history of peat accumulation in eastern Liguria Peat formation is caused by waterlogging leading to a cessation of the breakdown of organic matter. The absence of waterlogging in a depression followed by a later development of peat can be related to changes in one or more of the following variables: (a) Climatic factors - - for example, an increase in effective precipitation could have encouraged waterlogging and peat formation. (b) Geological factors - - an initially freelydraining substrate can become sealed either by natural weathering of clays and silts or by the inwash of eroded soils and superficial deposits from the surrounding area. (c) Anthropogenic factors - - human disturbance of the vegetation could have influenced the development of peat through soil disturbance and increased surface run-off. The possible influence of these factors on the history of peat accumulation in eastern Liguria will now be considered.

(a) Climaticfactors In contrast to neighbouring countries the data base for northwestern Italy is relatively impoverished (see Fig.3), only three dated full Holocene pollen diagrams having been published (Horowitz, 1975; Schneider, 1978). In the northern Apennines there appear to be good pollen-stratigraphical and lithostratigraphical reasons for supposing that continuous sequences occur at the sites of Lagdei (Bertoldi, 1980) and Prato Spilla (J.J. Lowe, pers. commun.). It is not possible however, to determine the extent to which continuity of sedimentation is typical of sites in the northern Apennines as a whole due to the lack of radiocarbon dates and stratigraphical information provided in the published literature (Chiarugi, 1936, 1950; Ferrarini, 1962; Braggio Morucchio and Guido, 1975; Braggio Morucchio et al., 1978, 1980). As shown in Fig.3 twelve sites in the western Italian Alps and Ligurian Apennines have been radiocarbon-dated but there is a dearth of known early Holocene peat or lake sediments in those areas. For instance, Selle

178

G.M. CRUISE

_/

International boundaries Land over 1 0 0 0 m

Lakes z~

Radiocarbon-dated full Holocene sequences Probable full

Holocene sequences 19

Radiocarbon-dated sites with no early Holocene peat or lake sediments Undated sites with probable lack of early Holocene sediments SITE

® 1

Agoraie

DATE QF PEAT INITIATION(BP) 4 1 8 0 + 60

O 2

Lag® Delle Lame

O 3

PratoMollo

4300+60

® 4

Lag® Nero

4855 + 40 5 0 4 0 + 100

3 0 2 5 ± 50 ?

® 5

Casanova

® 6

Bargone

8 4 5 0 + 80

® 7

Selledi

7 4 2 0 ± 120

O 9

Carnino Laghidella 3160±60 ~'erla LagolaManica 6290+60

010

Sestriere

O 8

REF

de Beaulieu(1977) de Beaulieu(1977) Scaife(1987)

3370-+50

Charrier(1970)

® 11 San Sicurio

4200

Cbarrier + Perreti(1977)

o12

RutorGlacier

6485±115

Armando

•

Lagdei

~8ertoldi(1980)

Prato Spilla

J . j Lowe unpublished

1

• 2

et a1(1975)

Fig.3. Peat initiation in N.W. Italy and surrounding areas.

di Carnino (situated at 1905 m in the limestone Marguareis area of Piemonte) was investigated by de Beaulieu (1977) who reported that a cessation of sedimentation had occurred between the end of the Lateglacial and 7420 B.P. At Lag® la Manica (a glacial cirque at 2365 m in the schistose region of the middle Chisone valley, western Italian Alps) peat failed to develop before 6290 B.P. (Scaife, 1987). Of the remainder, eight (over 60% of the total) contain sediments which accumulated only during the last 5000 years. While the available data base is unquestionably inadequate, this lack of known early Holocene organic sediments in a large part of northwest Italy does suggest an interruption of sedimentation on a regional scale which could have been due to widespread climatic conditions inimical to peat development. Palaeoclimatic reconstructions based on general

circulation models supported by pollen, lake-level and marine data at the global scale (COHMAP Members, 1988), multivariate pollen time series analysis of a number of records from southern France (Guiot, 1987) and studies of Alpine glaciers (Porter and Orombelli, 1985) have all strongly indicated a thermal maximum for the period 9000 to 6000 years B.P. with July temperatures being between 2 ° to 4°C higher than at present. Major forest changes during the period 4000 to 5000 B.P. have been widely reported in pollen sequences from southwestern Europe in particular the southward extension of Pinus cembra into the Alpes Maritimes (de Beaulieu, 1977) and the expansion of Abies and Fagus into the Massif Central (de Beaulieu et ai., 1985; Reille et al., 1985; Pons et al., 1988). These significant changes in forest composition at high altitudes together with a concurrent

179

HOLOCENE PEAT INITIATION IN LIGURIAN APPENNINES

development of peat deposits in the study area suggest that peat initiation during the midHolocene is most likely to have occurred as a result of widespread cooler and/or moister conditions. It is probable that high summer temperatures combined with high evapotranspiration rates and possibly a lack of surface run-off due to a closed forest canopy combined to produce rapid decomposition rates that inhibited the development of peat deposits. These conditions would have been most pronounced in small sites with limited drainage, which are characteristics typical of many of the sites in the Ligurian Apennines (Table I). Cooler and/or more humid conditions in the midHolocene would have stimulated the development of peat during that period, although locally mire hydrology would have also been influenced by geological factors and human modification of the landscape.

with a local soil profile, he argues that the silt could not have been derived from weathering of local in situ serpentinite rocks but instead, would have originated from an upper (loessial) silty soil which had developed in drift deposits lying above the bedrock. It was only after erosion and redeposition of these superficial deposits that impermeable conditions were created at Prato Mollo and Lago Nero. At Lago delle Lame peat formation was preceded by the deposition of 1 m of silty deposits (Table I) suggesting that a freely-draining substrate was also sealed by silt that could have been derived from local soils developed in drift. Thus at these three sites water-logging, subsequent rooting by peat-forming plants and a reduction in decomposition rates occurred only after considerable disturbance of local soils. The mechanism by which such soil disturbance could have occurred is discussed below.

(b ) Geological factors

(c) Anthropogenic factors

Studies of the soil and vegetation found on serpentinites have emphasised the shallow, stoney and freely-draining nature of soils derived from the bedrock (Pichi-Serrnolli, 1948; Spence, 1957; Martini and Orsino, 1969; Proctor and Woodell, 1971, Carter et al., 1987) and it has been noted that peat development is very rare on this type of lithology (S. Carter, pers. commun.). Investigations into the weathering and chemical characteristics of Apennine serpentinites (Malquori and Cecconi, 1956; Veniale and van der Marel, 1963) have demonstrated that long-term weathering of these rocks gives rise to clay minerals of the montmorillonite group. It could be argued then, that long periods of weathering of the underlying substrate was a prerequisite for peat formation in eastern Liguria. However detailed micromorphological analyses of the basal organic and minerogenic sediments at Prato Mollo and Lago Nero (Fig.2, Table I) have been undertaken by R.I. Macphail (Courty et al., 1989; Macphail, R.I., 1988; Maggi et al., 1989). In brief he found that the bedrock of both sites had been freely-draining but had been sealed by the deposition of silt, soil fragments and charcoal. On the basis of the mineralogical and grain size characteristics of the serpentinite and comparison

Recent archaeological excavations in the Ligurian Apennines have provided evidence of widespread human activity during the Chalcolith'ic and Bronze Ages at altitudes of between 400m to 800 m (Maggi and Formicola, 1978; Maggi, 1983, 1984, in prep.; Maggi et al., 1985~. Numerous radiocarbon dates from archaeological contexts, charred cereal grains and evidence of substantial mining suggest a major presence of human groups in eastern Liguria during the period 4540 to 3900 B.P. (R. Maggi, pers. commun.). However the peat sites reported here are located at higher altitudes (see Table I) where there is limited archaeological evidence of human activity. Flint assemblages in the higher mountains are most commonly found after modern deforestation and soil erosion have concentrated the artefacts into surface horizons so that the archaeological record is frequently fragmentary and liable to differential preservation. The colonisation and exploitation of high altitude resources in the Apennines and the western Italian Alps in prehistoric times is still not well understood (Nisbet and Biagi, 1987; R. Maggi, pers. commun.) although there is mounting evidence in northern Italy of the spread of upland pastoralism during the Chalcolithic and Bronze

180

G.M. CRUISk

Ages (Barfield et al., 1981; Nisbet, 1983; Greig, 1985; Nisbet and Biagi, 1987). Evidence of mixed stock-rearing and arable farming has been found at the lowest altitudes while in the highest areas there was probably limited summer transhumance between the valleys and high pastures (Nisbet and Biagi, 1987). On base-rich rocks in Liguria there is considerable faunal evidence of domestic stock rearing of sheep/goats and cattle from the Neolithic (Rowly-Conwy, 1989) as well as continued hunting of Red deer until the late Bronze Age (R. Maggi, pers. commun.). Extensive scatters of late Chalcolithic flint arrowheads have been found close to the site at Prato Mollo. These together with evidence of substantial soil disturbance indicate significant human impact upon the local landscape which in turn, led to changes in mire hydrology and the onset of peat formation. R.I. Macphail has also identified both in situ burned peat and coarse charcoal inclusions in the basal peat layers at Prato Mollo. The association of charcoal, soil disturbance and Chalcolithic artefacts strongly suggests deliberate forest burning by man. Burning of a forest can lead to improvements in both the quantity and nutritional quality of the food supplies available to herbivorous animals (Mellars, 1976). The Chalcolithic groups at Prato Mollo presumably used fire in association with hunting and possibly pastoral activities.

which was associated with Chalcolithic burning and forest disturbance phases. Nevertheless this type of evidence is rare in this still largely understudied region and the extent to which human activity influenced the development of peat deposits over a wider geographical area, is not yet possible to determine. A large scale reduction in forest cover during the Chalcolithic and Bronze Ages would have increased overall surface run-off but it is likely that any exploitation of the upland Apennine forests would have taken place against a background of climatic cooling. It is recognised that much of the circumMediterranean area has undergone a long history of anthropogenic interference. What is not understood however, is the relationship between such human activities and the effects of climatic change, geomorphological and geological processes during the last 10,000 years. The timing and processes leading to peat initiation are of significance. An increase in the number of sites for which basal radiocarbon dates are available, would enable a more conclusive evaluation of the extent to which peat formation throughout the Mediterranean region occurred during specific periods in the past and whether such periods coincided with phases of major human activity or climatic conditions conducive to peat growth.

Conclusions

This research formed part of a Ph.D. project funded principally by the DES/British Academy with additional funding supplied by the Sopritendenza Archaeologica della Liguria, the radiocarbon dating laboratory at Groningen, The Netherlands and City of London Polytechnic. In addition the author would like to thank Dr. R.I. Macphail, Prof. J.J. Lowe, Roberto Maggi, Don Shewan and the staff of the Cartographical Unit, City of London Polytechnic. Finally the Corpo Forestale dello Stato is gratefully acknowledged for allowing access to the site at Agoraie.

It is probable that a lack of effective precipitation was the most significant factor in determining the limited development of early Holocene peat deposits in the Ligurian Apennines. In conditions of high summer temperatures and high evapotranspiration rates small sites with limited drainage would have been prone to drying out leading to a cessation of peat formation. Such a conclusion is in accord with recent reconstructions of an early Holocene dry phase for much of Europe and the northern hemisphere (COHMAP Members, 1988). Cooler and moister conditions from the later middle Holocene would have promoted peat growth. Locally, however, peat formation only occurred after substantial disturbance of local soils

Acknowledgements

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