Late Holocene environmental change in the coastal southern Somalia inferred from Achatina and rhizoliths

Journal of African Earth Sciences 49 (2007) 79–89 www.elsevier.com/locate/jafrearsci

Late Holocene environmental change in the coastal southern Somalia inferred from Achatina and rhizoliths R. Matteucci b

a,*

, G. Belluomini b, L. Manfra

a

a Dipartimento di Scienze della Terra, Universita` degli Studi di Roma La Sapienza, P.le Aldo Moro, 5, 00185 Roma, Italy Istituto per le Tecnologie Applicate ai Beni Culturali, CNR, Area della Ricerca di Roma, Via Salaria, Km 29.300, 00016 Roma, Italy

Received 4 May 2006; received in revised form 24 April 2007; accepted 3 July 2007 Available online 10 July 2007

Abstract Surfaces of poorly cemented carbonate dunes of the coast of southern Somalia contain as exhumed bodies treelet rhizoliths and subfossil shells of the giant land snail Achatina. Present-day coastal dunes in southern Somalia are poorly vegetated and do not support living Achatinas. Thus, the presence of these subfossils provides evidence for a more humid period in the past: the subfossil giant land snails and the rhizoliths indicate a palaeoenvironment which was probably similar to the modern environment of the coastal belt of Kenya and of the southernmost corner of coastal Somalia, where living Achatinas are frequent and the forest–savannah mosaic vegetation type [Bonnefille, R., 1985. Evolution of the continental vegetation: the palaeobotanical record from East Africa. South African Journal of Science 81, 267–270] is widespread. According to radiometric ages of Achatina shells and geological observations, aridification process probably took place in the Late Holocene. Somali data are consistent with the known regional climatic trends. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Holocene; Somalia; Achatina; Rhizoliths; Radiometric ages; Aridification

1. Introduction 1.1. Foreword The Holocene climate was globally highly variable, with periods of rapid, dramatic change (Mayewski et al., 2004). Regional climatic trends are influenced by topographical pictures and local circulation pattern; they can be fairly revealed only by a really well distributed records. In East Africa, which shows a great diversity in climates mainly linked to topographical features, the most important evidence for the Holocene climate change is provided by the past lake-level fluctuations in the Ethiopian Rift Valley and pollen diagrams from inner mountainous areas; no paleoclimatic data are available for the coastal belt for the entire Holocene. This paper discusses the presence of subfossil giant land snail Achatina and of rhizoliths in the coastal dunes of *

Corresponding author. Tel.: +39 0 649914789; fax: +39 0 64454729. E-mail address: [email protected] (R. Matteucci).

1464-343X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2007.07.001

southern Somalia; they can be used as a paleoclimatic proxy, confirming the climate deterioration occurring in the East-African region after the Mid Holocene. 1.2. Regional Mid-Late Holocene climate record The entire Afro-Asian northern monsoonal belt underwent an increasing aridification process during the Late Holocene (Hoelzmann et al., 1998; Gasse, 2000; Lu¨ckge et al., 2001; Nicoll, 2004). At Mid to Late Holocene transition, the climatic trend to modern pattern started as a consequence of changes in monsoon intensity and atmospheric circulation patterns (Kutzbach and Street-Perrot, 1985; Kutzbach and Liu, 1997). During the Mid-Holocene the Inter Tropical Convergence Zone (ITCZ) was shifted northward and the summer monsoon humid air masses moved with greater intensity as is observed today (Gasse and Van Campo, 1994; Rodriguez et al., 2000; Lu¨ckge et al., 2001; Gasse, 2002). In most of tropical Africa, rainfall began to decrease after 5.8 kyr BP (deMenocal et al., 2000; Gasse, 2000), in apparent association whit a northern

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deglaciation event (Stager et al., 2003); the pronounced shifting to drier environmental conditions, recorded in many sites between 5 and 3 kyr BP (Bonnefille and Chalie´, 2000), was centered about 4 kyr BP (Marchant and Hooghiemstra, 2004). The aridification process was oscillating and highly unstable, with short-term drying events (Bonnefille and Mohammed, 1994; Gasse, 2000; Schilman et al., 2001). In the northern tropics of Sudan and Egypt the increased aridity after 5–4.5 kyr BP contributed to the demise of the Old Kingdom in the Nile valley (Hassan, 1997; Salzmann and Waller, 1998; Gasse, 2002; Nicoll, 2004); an abrupt drying event at 4 kyr BP is related to the collapse of Mesopotamian civilization (Cullen et al., 2000); a dramatic break was observed at about 3.5 kyr BP in India (Caratini et al., 1994); in coastal Pakistan (Rad von et al., 1999), precipitation decreased after 4.0–3.5 kyr BP; in Madagascar, the first major change in vegetation as a consequence of natural dessication is placed about 3 kyr BP (Burney, 1993; Matsumoto and Burney, 1994). In East Africa, the climatic stepover began relatively abruptly at around 4.5–4 kyr BP (Maitima, 1991; Beuning et al., 1997; Barker et al., 2000; Benvenuti et al., 2002; Dramis et al., 2003; Lamb et al., 2004); in the last 3 kyr BP, pollen diagrams register significant climatic changes in south-eastern Ethiopia, with a cooler climate following the colder and humid phase of Mid-Holocene times, a drier period between 1.8 and 0.95 kyr BP and major climatic variations in the last millennium (Bonnefille and Mohammed, 1994; Machado et al., 1998). 2. Modern environmental setting

the coastal ones and show a yellow-brown colour, mainly due to the persistent lateritic coatings of some quartz grains. Quartz granule content varies from 40% to 60% in weight of the innermost white dunes or of the recent deposits due to deflation of the old ‘‘red dune’’ to 15– 20% of the present-day beach. North of Muqdishu, most of the white dunes are almost completely made of quartz granules of aeolian and fluvial origin (70–80% in weight). Mineralogical composition of the red dune is mainly siliciclastic: 79–85% quartz grains, 16–25% feldspar and accessory minerals, 2–3% carbonate grains (Angelucci et al., 1995 and unpublished data). Only some of the mostly coastal white dunes are stabilized by vadose cementation. Long axes of both red and white dunes run essentially parallel to the coast (Fig. 1). The recent carbonate dunes of the African coast of the Indian Ocean were formed during the post-glacial sea-level rise (Illenberger and Rust, 1988; Carbone et al., 1999), mainly after the sea-level bypassed the edge of the continental shelf, about 8 kyr BP, according to Hopley (1994) and Ramsay and Cooper (2002). Carbonate grains (miliolids and other foraminiferids, small bioclastic fragments) for the Somali white dunes were supplied by the skeletal sandy beach and coral reefs bounding the coast on the seaward side; skeletal grains were blown off the beach by the coastwise directed summer monsoon. A progradation of the coast occurred during the marine regression to the present sea-level, after the sea reached a higher level of about 2 m a.s.l., around 5 kyr BP (Jerardino, 1995; Carbone et al., 1999; Carbone and Accordi, 2000; Teller et al., 2000; Ramsay and Cooper, 2002). The ‘‘red dune’’ and the white dunes of the innermost part of the coastal strip are now subject to deflation and remodelling.

2.1. Physiography and geology 2.2. Climate and vegetation The coast of southern Somalia is characterized by the presence of the ‘‘Merka red dune’’, an ancient dune ridge, running parallel to the coast and separating a narrow coastal strip from the alluvial plain of the Shabelle river (Fig. 1). The river flows perpendicularly from the inner mountains to the coast, turning southwards and running along the coast from Muqdishu to the village of Gelib, where it is incorporated into a marshland, usually without reaching the sea. The ‘‘red dune’’ is a siliciclastic polyphase complex, whose width ranges from 8 to 10 km south of Muqdishu to 100 km north (Carbone et al., 1984; Angelucci et al., 1995). Its maximum height is about 150 m a.s.l., near the town of Merka. On the east side, the dune partly overlies a coral-rich marine sequence which outcrops along the shore; on the western side, the dune underlies the Shabelle alluvial deposits (Fig. 4). Wide mobile quartz dunefields lie on the ancient ‘‘red dune’’. In the narrow coastal strip, there are well developed white mixed quartz–carbonate dunes, lying on the marine carbonate substrate or on the eastern side of the ‘‘red dune’’. The innermost mixed quartz–carbonate dunes are richer in quartz grains than

Climate of southern Somalia is semi-arid or arid, with a bimodal rainfall distribution, influenced by monsoonal winds. The main rainy season is between March and June, centered on April–May (‘‘Gu’’ season, in Somali). The second, less abundant and more variable rain season is between September and November (‘‘Der’’ season; ‘‘Jilal’’ and ‘‘Hagai’’ are the dry seasons, in Somali). However, along the coast, the Jilal season is rainy, so there is a unique rainy season from March to September, entirely regulated by the SW monsoonal wind (Fig. 2); its core is in June and its 300–600 mm/yr abundance (Kismayu station, Fantoli, 1960) decreases from south to north; on the Kenyan border, the annual rainfall even reaches 600–700 mm/yr. If we take a mean monthly rainfall of 50 mm as the limit between dry and humid months (Rizzo, 1977), only in the southern corner along the coast the wet season is longer than three months without interruption (Fig. 2c and d, Kysmayu and Muqdushu stations, after data in Fantoli, 1960). Insolation is very high, with a long dry season from November to May. The annual mean temperature is of 26–27 C°, with low seasonal fluctuations.

R. Matteucci et al. / Journal of African Earth Sciences 49 (2007) 79–89 Fig. 1. A. Geological map of the Somali coast near Muqdishu: a – reef limestones; b – yellow-red sands (‘‘Merka red dune’’); c – quartz–calcareous sands (white dunes); d – gray quartz–calcareous sands (interdune depressions); e – present-day shore sands; f – yellow quartz sands (mobile dunes); g – Jasiira pond; h – long axes of the dunes. B. Map of the coast between Muqdishu and Kysmayu, showing the ‘‘Merka red dune’’ alignment and the distribution of living Achatina. C. Geographic location of the studied area.

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for preliminary dating, in different sites (Figs. 1 and 4): three specimens come from the surface of the ‘‘red dune’’ and six from the surface of the white dunes; five of the latter specimens were collected from the innermost white dunes, south of Muqdishu and one from the siliciclastic white poorly stabilized dune, north of Muqdishu. In addition, one recent specimen was collected on the Shabelle river bank near the Afgoye village, for environmental correlation with the subfossil ones; one fossil specimen, badly preserved, come from the top of the Buur Bithale hill, near the town of Belet Weyne, in the innermost part of the central Somalia. For dating, the shells were pretreated using standard methods, i.e. superficially scraped and leached with hydrogen peroxide and washed by ultrasounds; finally, the selected innermost parts were leached with diluted acid. 4. Field observations 4.1. Rhizoliths

Fig. 2. (a) Schematic of the general pattern of monsoonal winds over Horn of Africa (solid lines, winter monsoon; shades lines, summer monsoon). (b) Mean annual rainfall over Horn of Africa (in mm), from various sources (Fantoli, 1960; Rizzo, 1977; Nicholson, 2000). (c) and (d) Annual distribution of humid months (rainfall more than 50 mm), in Muqdishu (c) and Kysmayu (b) coastal regions.

In the present-day vegetation map of East Africa by Bonnefille (1985), Somalia (Fig. 3) is almost entirely desert; only the western regions are Acacia/Commiphora steppe, a type of vegetation which occurs in semi-desert areas of East Africa up to altitudes of about 1500 m. The coastal southernmost corner (south of the equator) is made up of forest– savannah mosaic, a vegetation type distributed along the whole coastal zone as far as Mozambique. Strips of riverine forest are preserved along the Juba and Shabelle rivers; extent and continuity of the riverine forest were dramatically reduced by agricultural development, mainly in the Shabelle valley. In the Somali coastal zone, the open woodland vegetation (‘‘boscaglia’’ in italian; Pichi-Sermolli, 1957; Pignatti, 1990) is characterized by sparse umbrellalike treelets and thorny shrubs, both principally consisting of species of Acacia. Arboreal Acacia mainly develops on the western side of the ‘‘red dune’’, which retain permeating water longer than carbonate dunes. Herbs grow during the rainy seasons; but eastward, on the red and the white dunes, vegetation is either very scarce or totally absent (Fig. 5a and b). 3. Material and methods Field observations were carried out by R.M. as a part of a project on the coastal marine sedimentary processes in Southern Somalia. Eleven Achatina shells were collected

Rhizoliths are local common organo-sedimentary structures in the white dune sands, outcropping on the top of the dunes and in the interdune flats. According to Plaziat (1971), Klappa (1980) and Pfefferkorn and Fuchs (1991), they are easily recognizable, by their length (some being more than 2 m long), their branching morphology (generally downward, with decreasing diameters) and their location (subaereal conditions). In the poorly stabilized white dunes, which predominate in the innermost part of the coastal strip, rhizoliths are mainly formed by cemented sand grains, with a vague concentric arrangement. Some appear as weakly cemented subcylindrical rinds, almost entirely empty, while others preserve well-developed inner rhizomicritized envelopes. Rhizoliths are almost completely exhumed from the substrate by wind-removal of surrounding poorly or not cemented dune sand. They form tangles of subcylindrical, branched pieces, chaotically arranged (Fig. 6e and f). Few of the pieces show a subvertical position, being still partially included into the sand. Small pieces, blown off by the wind, are scattered around over wide areas. Rhizoliths show variable diameters (max 10 cm) and lengths (max up to 2 m). The longest pieces show numerous laterals and a decreasing diameter from end to end. In the stabilized coastal white dunes, all the rhizoliths are formed by micrite envelopes and micrite-cemented sand grains; the axial tubule is empty or partially filled. Rhizoliths, well anchored into the substrate by the limited wind-removal of cemented sand, mainly show a vertical position and emerge for a maximum of 0.5 m. Taproots show numerous laterals, radially branching, strongly downward inclined; some have a very large diameter (up to 30 cm) and belong to arboreal plants (Fig. 6a–d). In the ‘‘red dune’’, R.M. observed only rare large siliceous subcylindrical bodies, some of which partly infilled in the sand, recognizable as in situ petrified trunks of trees.

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Fig. 3. (a) Simplified vegetation map of present-day eastern Africa, modified from Bonnefille (1985) and Pickford (1995); (b) Inferred vegetation map before the late Holocene aridification in the studied coastal area.

ACH 1 (26980 ± 340)

ACH 7 (6533 ± 62) ACH 6 (6041 ± 87) ACH 8 (5655 ± 49) ACH 2 (5536 ± 43) SOM 3 ACH (3960 ± 90)

SOM 2 AN (6170 ± 65) SOM 3 AN (6100 ± 100) SOM 2 PO (5180 ± 60)

ACH 4 (Recent)

200

ACH 3 (1871 ± 27)

100 m 0

Alluvial plain

Cretaceous limestone

Webi Shabelle

Buur Bithale (Belet Weyne) WNW

Buur Bithale

Red dune

s.l. White dune

Muqdishu

200 100 0 m

Reef limestone

~ 300 km Coast

ESE

Fig. 4. Simplified geological section between Buur Bithale hill and the coast, with the location of Achatina shells used for dating.

Glennie and Evamy (1968) used the Arabic word ‘‘dikaka’’, meaning shrub-covered dune sand, to designate plant-root structures associated with aeolian sand. The somali ‘‘dikaka’’ differs from that studied by Glennie and Evamy, being constituted mainly by rhizoliths with larger

diameters and with a predominantly vertical orientation. According to Cannon (1911), in arid environment, most perennials, such as Acacias, have root systems with taproots and laterals which are both well developed; they can exploit both deeper aquifers and intergranular water

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Fig. 5. (a) Inland white dunes, south of Muqdishu; on the second floor, the ‘‘red dune’’; (b) Achatina and Otopoma shells on the surface of a poorly cemented white dune.

during the rainy season. Thus, Somali ‘‘dikaka’’ derives from a ‘‘boscaglia’’-like vegetation, partly formed by treelets, rather than shrubs. Similar vertical root casts were described by Cohen (1982) from the Pleistocene Koobi Fora Formation, Kenya. Petrographical and geochemical data (Mount and Cohen, 1984) confirmed their interpretation as a vertical root system of phreatophytes, which grew in a semi-arid-climate with an intermittent water supply, exploiting deeper aquifers. Vertical rhizocretions from the Egyptian coast of the Red Sea were described by Freytet et al. (1994), fossil rhizoliths of a variety of plants from Egypt by Bown (1982). 4.2. Subfossil Achatina The giant African land snail is a gastropod endemic to the Afro-tropical region, but it has been widely spread, by man, in tropical countries around the world; it is infamous for the damages caused in the regions in which it was accidentally introduced (Mead, 1979). Achatina is a terrestrial snail which tolerates wide changes of temperature and long dry periods. Its vital activity starts when the relative air humidity exceeds 50% (Aoki, 1978). At present, Achatina is common in the coastal zone of Kenya characterized by the forest–savannah mosaic type of vegetation; it occurs, but not so frequently, in the semi-arid Acacia/ Commiphora woodland zone (‘‘nyike’’ belt). It is known from the Serengeti plains in Tanzania and from the tropical rain forest, but it is absent or very rare in the Kenyan highlands, as at Kukamega, in western Kenya (Pickford, 1995). In Somalia, living Achatina is common in the southernmost coastal corner, south of the Juba river mouth and, less frequent, along the Shabelle river, up to the north of Balad (R.M., personal observations). Connolly (1928) and Bacci (1951) report findings of Achatina from numer-

ous localities within the Shabelle river valley and in the western part of central Somalia (Acacia/Commiphora woodland zone). Along the coast, between Muqdishu and Merka, no living specimens were found; nevertheless, elderly Somalis remember rare giant land snail individuals living in temporary ponds in the Muqdishu town. Achatina originates from East Africa; it is known in Kenya since the lower Miocene and is frequent in terrestrial Pliocene and Quaternary in Kenya and Tanzania (Pickford, 1995). Along the coast near Muqdishu, Achatina tests are frequent in the continental deposits which lie on the raised coral reefs (Carbone et al., 1984). On the surface of red and white coastal dunes, Achatina (Achatina lactea Reeve, in Nardini, 1933), accompanied by the helicid Otopoma (Georgia guillainopsis Bourgouignat, in Nardini, 1933) can be found very often up to 5–10 specimens in 10 m2. The density of tests on the surfaces of the dunes (Fig. 5) is mainly related to the wind-removal of the surrounding dune sand. In fact, Achatina tests, made heavier by sandinfill, can be found more frequently on the surfaces of the poorly cemented white dunes, where the gastropod assemblage is quite enriched by wind-induced exhumation. Generally, these tests show a white, brilliant colour (Fig. 7), due to polishing by sand abrasion over a long period and to wind-induced rolling down along dune slopes; some of them show carbonate encrustations and superficial rootlet traces. 5. Radiometric dating Eleven specimens were collected for radiometric dating. All the Achatina individuals, except the badly preserved ACH 1, are referable to the same species. Their morphology is very close to that of A. fulica Bowdich 1822, the most common of the more than 70 species of the genus known

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Fig. 6. (a–d) Coastal well cemented white sand dunes, South of Muqdishu; (a) Top view of a dune, showing the emergence of vertical rhizoliths, with various diameters; (b) Lateral view of a dune, showing deeply developed vertical rhizoliths; (c) Detail showing radial, downward-inclined laterals, which pass through a deeply inclined dunal forest; (d) Vertically developed rhizolith, showing the micritic envelope around the axial tubule and many stumps of laterals downward inclined. (e) and (f) Inland uncemented white sand dunes, South of Muqdishu; (e) comprehensive view of a rhizolith tangle, showing only a few pieces in a vertical position; (f) detail of recumbent pieces, showing their weakly cemented cortexes.

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Fig. 7. A white, brilliant Achatina shell, weakly polished by sand abrasion. (a) apertural view; (b) opposite view.

from East Africa. However, they slightly differ for their granulate external surface and the white colour. White living specimens (A. fulica var. ? lactea) were observed along the Shabelle river by R.M. The summary of 14C dating results is in Table 1. 6. Discussion and conclusions 6.1. Dating Reliability of radiocarbon ages obtained from terrestrial snails is relatively low (Evin et al., 1980; Goodfriends and Stipp, 1983; Mussi et al., 1995), particularly in the case of

Table 1 Results of

14

C dating on Achatina shells

Lab code (R-)

Sample name and collection site

Conventionala

1632 1633 1634 1881 2403 2404 2405 2406 2680 2681 2682

SOM SOM SOM SOM ACH ACH ACH ACH ACH ACH ACH

6587 ± 65 6100 ± 100 4380 ± 90 5600 ± 60 26,980 ± 340 5536 ± 43 1871 ± 27 Recent 6041 ± 67 6533 ± 62 5655 ± 49

a

snails from arid or semi-arid carbonate areas (Goodfriends, 1987; Head, 1999). This mainly depends on the possible presence of bicarbonate derived from older carbonate substrate incorporated in the snail shell. Nevertheless, the modern specimen collected on the Shabelle river bank and the three specimens from the red dune lived on a substrate made up of siliciclastic sediment, with very poor carbonate content. These ages seems reliable. Five of the six dates obtained from the Achatina shells found on the white carbonate dunes are in agreement with the former; only the date obtained for the shell found on the siliciclastic white dune, north of Muqdishu, indicates a considerably younger age. Also, in spite of the limited number of radiometric data and of their scarce reliability, we observe that the radiometric ages obtained for eight of the nine Achatina shells found on the dunes correspond to periods before the climatic break towards drier conditions occurring in East Africa at around 4.5–4 kyr BP, well documented in pollen diagrams, lakelevel histories and the faunal record (Jolly et al., 1994; Mohammed et al., 1995; Dramis et al., 2003; Lamb et al., 2004). Seven of the dated Achatinas lived at the end of the well-known African humid period, whose abrupt termination is placed, in North Africa, between 5 and 6 kyr BP (deMenocal et al., 2000), one lived before the rapid African dry oscillation of Gasse (2000), occurring at around 4.2– 4 kyr BP; the only younger radiometric age (ACH3 – 1871 ± 21 yr BP), could indicate a progressive or oscillating rarefaction of the giant snail presence in the area, in accordance with the long-term fluctuations of the Late Holocene documented in East Africa. According Lu¨ckge et al. (2001), the period between 2 and 1.5 kyr BP was the wettest over the last 3 kyr BP; a significant wet event at 1.7 kyr BP was described by Barker et al. (2000) in the Lake Massoko region (Tanzania). Moreover, a very rare presence of live Achatina in the modern times was observed by ederly Somalis.

2 3 3 2 1 2 3 4 6 7 8

AN – red dune AN – red dune ACH– white dune PO – red dune – B. Bithale – white dune – white dune – Afgoye – white dune – white dune – white dune

14

C age (y BP ± 1r )

d13Cb (&,VPDB)

Calibrated age rangec (y BP ± 2r )

+0.68 +0.80 +0.80 +0.80 7.50 7.68 7.78 12.13 8.01 8.18 8.35

7566–7427 7159–6764 5210–4846 6445–6305 6398–6288 1868–1735 5207–4735 5617–5363 4600–4359

Conventional ages were obtained using Libby’s (1955) half-life (5568 ± 30 y); BP is Before Present (AD 1950). Determinations of carbon isotope composition (d13C) were performed on pure CO2 by means of a dual inlet triple collector isotope ratio mass spectrometer (SIRA II VG Isotech, Middlewich, UK). Gaseous samples were sequentially introduced into the dual inlet apparatus and balanced against an internal CO2 reference gas calibrated versus the international standard VPDB. Sample and reference isotope ratios were measured and used for Craig corrected d13C calculations. c Calibrated ages were obtained with the INTCAL98 recommended calibration curve (Stuiver et al., 1998) and all the errors are quoted as two sigma uncertainty. b

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6.2. Rhizoliths Rhizoliths are proof of an older vegetation cover established on the pre-existing dunes during a long-term increase in rainfall (Glennie, 1999); the reduced vegetation cover of the present-day indicates a subsequent increasing aridity. Coastal white carbonate dunes probably formed during the post-glacial sea-level rise, mainly after the sea bypassed the edge of the continental shelf, about 8 kyr BP. Development of deep-root vertical systems depends on the exploitation of deeper aquifers. The present water table is very slightly higher than the sea-level, reaching a depth of less than 3 m at 2–3 km from the coast (Dal Pra` et al., 1986); the fresh water aquifer is exploitable by surficial hand-wells only along the coastal interdune depressions. The Somali ‘‘dikaka’’ possibly indicates a dune colonization following the post-glacial rise in the water table, which probably reached its maximum height, along the Indian Ocean coast of Africa about 5 kyr BP, when the sea-level was higher than at present (Ramsay and Cooper, 2002). 6.3. Climate deterioration Radiometric dating of numerous Achatina tests sampled in stratigraphic succession could give good informations on the Upper Pleistocene (?) – Holocene climatic evolution in the region. Nevertheless present evidence allows us to postulate a general climatic deterioration after the Holocene climatic optimum, when the climate was wetter than today along the coast of the Horn of Africa, as reported in the Dahlak Islands (southern Red Sea) by Belluomini et al. (1980). The environmental conditions in the area, during the Mid-Holocene, were probably similar to the modern ones currently found in coastal Kenya and southernmost Somalia (rainfall more than 500–600 mm/year; rainfall season longer than three months). The climatic deterioration must have developed progressively from north to south (Fig. 3b), and consisted, probably, in a moderate reduction in the mean annual rainfall (today, in Muqdishu, 300– 500 mm/year) and in a moderate variation in the rainfall seasonality. Even the siliciclastic mobile dunefields, which are growing on the ‘‘red dune’’, threatening some coastal villages, indicate aridity (Lancaster, 1981) and are consistent with the Late Holocene regression regime and drying climate. The spatial reconstruction of climate in East Africa at 6 kyr BP in Peyron et al. (2000), based on inland sites, states wetter conditions than today north of 3°S and slightly drier, to the south. The site of Muqdishu indicate wetter conditions along the coast south of 2°N. 6.4. Human impact Rare flint artifacts were found by R.M. on the red dune; they are badly preserved and show different preservation characteristics (Mussi, personal communication). However, they clearly testify to the presence of prehistoric

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human groups in the dune coastal zone during the Holocene. Studied Somali industries (sites of Juba valley, Mussi, 1990; Coltorti and Mussi, 1993) indicate that human groups colonized southern Somalia later the end of Pleistocene and earlier than the diffusion of pottery (before 5 kyr B.P. in the Buur region, according to Brandt, 1986). Thus, man could have contributed to the environmental deterioration of the Somali coastal belt mainly favouring fires with their devastating effects (Burchard, 1998; Wooler et al., 2000), killing megaherbivores and interfering with the ecologic balance. Nevertheless, the persistence of the forest/savannah mosaic vegetation and of living Achatina in the coastal Kenya and in the southern coastal corner of Somalia, indicate that the human impact could have been only a secondary cause of the documented deterioration. Acknowledgements Special thank from the authors to A. Angelucci, Universita` ‘‘La Sapienza’’, Rome, for providing some Achatina specimens and to M.Mussi, Universita` ‘‘La Sapienza’’, Rome, for helpful informations about the flint artifacts collected by R.M. on the ‘‘red dune’’. Thanks for drawings to Maurizio Salvati, Earth Sciences Department. The work was supported by a grant from University ‘‘la Sapienza’’, Rome, co-ordinated by R. Matteucci. References Angelucci, A., De Gennaro, M., De Magistris, M.A., Di Girolamo, P., 1995. Mineralogical, geochemical and sedimentological analysis on recent and quaternary sands of the littoral region between Mogadishu and Merka (Southern Somalia) and their economic implication. Geologica Romana 31, 249–263. Aoki, J., 1978. Ecological distribution of the land snail, Achatina fulica, and some possibilities of its ecological control. Edaphologia 18, 21–28. Bacci, G., 1951. Elementi per una malacofauna dell’Abissinia e della Somalia. Annali del Museo civico e Stazione naturale di Genova 65, 1– 44. Barker, P., Telford, R., Merdaci, O., Williamson, D., Taieb, M., Vincens, A., Gibert, E., 2000. The sensitivity of a Tanzanian crater lake to catastrophic tephra input and four millennia of climate change. Holocene 10, 303–310. Belluomini, G., Esu, D., Manfra, L., Matteucci, R., 1980. Gasteropodi dulcicoli e terrestri nell’isola di Dahlak Kebir – testimonianze di una fase umida olocenica nelle Isole Dahlak, Mar Rosso. Bollettino Malacologico 16, 369–390. Benvenuti, M., Carnicelli, S., Belluomini, G., Dainelli, N., Di Grazia, S., Ferrari, G.A., Iasio, C., Sagri, M., Ventra, D., Balemwald, Atnafu, Seifu, Kebede, 2002. The Ziway-Shala lake basin (main Ethiopian rift, Ethiopia): a revision of basin evolution with special reference to the Late Quaternary. Journal of African Earth Sciences 35, 247–269. Beuning, K.R.M., Talbot, M.R., Kelts, K., 1997. A revised 30,000-year paleoclimatic and paleohydrologic history of Lake Albert, East Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 136, 259–279. Bonnefille, R., 1985. Evolution of the continental vegetation: the palaeobotanical record from East Africa. South African Journal of Science 81, 267–270. Bonnefille, R., Chalie´, F., 2000. Pollen-inferred precipitation time-series from equatorial mountains, Africa, the last 40 kyr BP. Global and Planetary Change 26, 25–50.

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