Desert varnish and petroglyphs on sandstone – Geochemical composition and climate changes from Pleistocene to Holocene (Libya)

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Chemie der Erde 68 (2008) 31–43 www.elsevier.de/chemer

Desert varnish and petroglyphs on sandstone – Geochemical composition and climate changes from Pleistocene to Holocene (Libya) Martin Dietzela,, Hans Kolmera, Peter Po¨ltb, Sanja Simicb a

Institute of Applied Geosciences, Graz University of Technology, Rechbauerstrasse 12, A-8010 Graz, Austria Research Institute for Electron Microscopy, Graz University of Technology, Steyrergasse 17/III, A-8010 Graz, Austria

b

Received 4 September 2006; accepted 30 January 2007

Abstract Desert varnish of pristine sandstone and petroglyph surfaces from Takabart Kabort (Naturalistic Bubaline Art School) and Alamas (Tanzina Art School) can be well classified by their (SiO2+Al2O3):MnO2, Al2O3:SiO2, and P2O5:CaO ratios. Specific ratios are due to the occurrence of clay minerals like illite, kaolinite, smectite, and feldspar, quartz, carbonates like calcite and dolomite, manganese oxyhydroxides, and apatite. Their occurrence corresponds to the local origin and composition of the primary aeolian material. In general, the analyzed desert varnish shows lamination patterns characterized by alternating MnO2-rich and -poor layers (p25 wt% MnO2) at rather constant iron oxyhydroxide content (6 wt% Fe2O3). Varnish on non-engraved surfaces exhibits three MnO2-rich layers, whereas varnish-coated petroglyphs reveal minor lamination patterns corresponding to the dating of petroglyphs by rock art. The older Naturalistic Bubaline Art School petroglyphs (about 6–4 ka BP) and the younger Tazina Art School petroglyphs (about 3.8–3 ka BP) contain only two and one MnO2-rich layer, respectively. It is assumed that the occurrence of such microlaminations is caused by climate changes in North Africa. Three humid periods are discerned from the Terminal Pleistocene to Holocene in the literature. Such periods are suitable to induce manganese accumulation by biotic and abiotic processes. Accordingly, the distinct lamination patterns gained from this study verify the dating of petroglyphs by rock art. From another point of view, classification of the above petroglyphs may be provided by analyses of microlaminations independently on cultural historical aspects. r 2007 Elsevier GmbH. All rights reserved. Keywords: Desert varnish; Rock varnish; Petroglyphs; Manganese layers; Palaeoclimate

1. Introduction The traditional term desert varnish comprises dark coatings on rock surfaces, which occur worldwide in a huge variety of environmental settings most frequently Corresponding author. Tel.: +43 316 873 6360; fax: +43 316 873 6876. E-mail address: [email protected] (M. Dietzel).

0009-2819/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2007.03.001

in drylands (e.g. Dorn, 1998, 2006). Such coatings are of variable thickness ranging from a few up to about several hundred microns (e.g. Haberland, 1975; Oberlander, 1994; Liu and Broecker, 2000; this study). Desert varnish comprises a paper-thin solid mixture typically composed of manganese and iron oxyhydroxides, clay minerals, and additional grains of quartz, feldspar, and carbonates (e.g. Potter and Rossman, 1977, 1979). Small quantities of apatite, gypsum, and barite may be also

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M. Dietzel et al. / Chemie der Erde 68 (2008) 31–43

present (Engel and Sharp, 1958; Nagy et al., 1991; Raymond et al., 1991; Reneau et al., 1992). In former studies, analyses of desert varnish are proposed as a tool for dating of petroglyphs (e.g. Hunt, 1961; Oberlander, 1994; Cremaschi, 1996; Dragovich, 1998; Watchman, 2000; Dorn, 2001; Lee and Bland, 2003). However, precaution has to be given on interpretations as thickness and kind of varnish strongly depend on the individual composition of the host rock and the specific natural environments (Dorn and Oberlander, 1981; Dorn, 1998, 2006). It is well established that desert varnish may show microlaminations with respect to the manganese content. A correlation between manganese concentrations and environmental changes was first deciphered by Perry and Adams (1978) (see also Liu et al., 2000). Dorn (1984, 1994) verified such behaviour by K/Ar-dated volcanic host rocks, where the sensitivity of Mn:Fe laminations is related to fluctuations in alkalinity within a minimum time period of 10–25 ka. These results are in accordance with climate records obtained from stable oxygen isotope analyses of marine foraminifera in North Atlantic deep sea sediments (Broecker and Denton, 1989). Moreover, recent blind test studies show that manganiferous microstratigraphy of varnish may be a reliable tool to record changes in past climate (Liu, 2003; Marston, 2003; Phillips, 2003). The formation of MnO2-rich layers in desert varnish can be caused by microbial activity of bacteria and fungi (e.g. Krumbein, 1969; Dorn and Oberlander, 1981; Nagy et al., 1991; Grote and Krumbein, 1992; Ehrlich, 1996; Krinsley, 1998; Dorn, 1998, 2006). Manganese accumulating bacteria preferentially evolve at transitional climate conditions. Moderate humid environment ensure optimum growth conditions for such organisms. Accordingly, Hunt and Mabey (1966) pointed out that at least small amounts of moisture are necessary for a potential desert varnish formation. However, the so-called polygenetic model for the formation of desert varnish is well established, which combines biotic enhancement of manganese with abiotic processes (e.g. Dorn, 1998, 2006; Krinsley, 1998; Thiagarajan and Lee 2004). Following this model, desert varnish is a product of common weathering of clay minerals (e.g. illite, smectite, and chlorite) and manganese accumulating bacteria. Neither silicate weathering nor bacteria activity itself will create desert varnish. Weathering processes and diagenesis lead to feathering of clay minerals into nanometerscaled monolayers along (001) planes. Simultaneously, manganese-accumulating bacteria are able to enhance MnO2 content up to several orders of magnitude above the geochemical background. Manganese is mostly obtained from direct aqueous atmospheric deposition, and is accumulated at the cell walls of the bacteria.

At a second stage, divalent manganese ions can be remobilized by degradation of the cells. Subsequently, manganese is oxidized and precipitated within the feathered fragments of the clay minerals. In principle, redox reactions between MnII and MnIV are largely governed by pH, and transformation to tetravalent manganese is rather slow below pH 8.5. However, microbial oxidation can significantly stimulate the transformation kinetics at near neutral pH conditions most likely for rock varnish. The layered structures of clay minerals serve as templates for a final deposition of manganese oxyhydroxides with e.g. nanometer scale todorokite–birnessite-like structure. Desert varnish is formed in a wide variety of environmental settings. In desert areas such as Central Sahara desert varnish cannot be formed under present hyper-arid conditions. However, any rock type may be completely covered by desert varnish giving evidence for prior ‘‘pluvial’’ periods as at least a moderate humidity is necessary for the remobilization of MnII from the cell walls of the bacteria. The idea of the present study is to examine manganese microlaminations in the desert varnish on sandstone in the Messak Mountains (Fezzan/Libya). Laminations may reflect the short timed palaeoclimatic fluctuations produced by several humid periods within the last 15 ka BP. In North Africa, up to three humid periods are perceived from the Terminal Pleistocene to Holocene (Muzzolini, 1995, 2001). Sensitivity of such microlaminations seems too low to record short living alkalinity fluctuations during the Holocene (Dorn, 1984), whereas MnO2-rich layers may correspond to the relative humid periods and the chronology of the historical petroglyphs. However, requirements for the present investigation are (1) a uniform lithology of the sampling sites to avoid compositional variations of the substrate, (2) a reliable chronology of climatic changes within the sampling area, and (3) a feasible age classification of petroglyphs e.g. by rock arts.

2. Study area and sites of sampling The sandstone plateau of the Messak Settafet (‘‘black plateau’’) extends about 300 km in SW–NE direction (Fig. 1). The lateral extension reaches 60 km with elevations up to 1200 m a.s.l. The boundary to the Northeast (Ubari sand sea) is characterized by steep escarpments with level changes up to several 100 m. In Southeastern direction, the sandstone plateau dips with flat inclination towards the Murzuq sand sea. The rocks of the study area are cross-bedded coarse grained to conglomeratic quartzitic sandstones with thin insertions of shale from Lower Cretaceous (Geological Research and Mining Department, 1985). The sandstone formation

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comprises uniform lithology throughout the whole study area. The Messak plateau itself is strongly dissected by numerous recently inactive canyon-like wadis of Middle Pleistocene age (Cremaschi, 1996). Samples of desert varnish were collected in two wadis, Takabart Kabort for Naturalistic Bubaline style gravures and Alamas for Tazina style gravures, respectively (see Fig. 1 and Table 1). In order to minimize damage to cultural heritage, sampling sites were selected, which already show an advanced decay by bulk sandstone weathering. Sampling of 7–10 cm diameter fragments was carried out at the bottom of the rock walls, where rock engravings had been fallen off by spalling. Fragments of Naturalistic Bubaline style petroglyphs were clearly identified by the remains of their gravures

12°00

Ubari sand sea

Libya

26°30 0

13°00

30 km

N

W

.A

W

la

.T

ak

m

.K

ab

as

Wadi Berjuj

or

t

Murzuq sand sea 25°30

Fig. 1. Location map of the Messak Settafet area (Fezzan, Libya; see van Albada and van Albada, 2000) and the sampling sites of rock varnish in wadi Alamas and Takabart Kabort (m).

33

on the steep rock walls. The smaller Tazina style petroglyphs of wadi Alamas are typically carved in flat laying rock panels with low inclinations (Pichler and Rodrigue, 2003). Engraving fragments of 3–5 cm diameter were sampled, which had been also already fallen off by spalling. Sampling of non-carved surfaces was conducted within a distance of a few metres to the engravings to avoid impact to the petroglyphs (see Table 1).

3. Methods The mineralogical composition of scraped desert varnish was analyzed by X-ray Diffraction using a Philips PW 1800 (CoKa). The chemical composition of desert varnish was measured along microsections. The optimum location for the linescans was selected from scanning electron microscope (SEM) images. For this purpose, the samples, host rock with desert varnish, were embedded in epoxy resin. Subsequently, drypolished microsections were prepared. Finally, the surface of the specimens was coated with a thin carbon layer (around 20 nm) to avoid charging. In the following paragraphs, the term plateau data refers to linescans through microdepressions on varnish-coated surfaces of non-engraved rock samples. The term gravure data refers to linescans through microdepressions at petroglyph engravings. The microanalyses have been performed by energydispersive X-ray spectrometry (EDXS) in a SEM. A ThermoNORAN Voyager EDXS attached to a Zeiss DSM 982 Gemini was used. Elements can be detected with a detection sensitivity of around 0.1 wt%. The size

Table 1.

Sampling sites and characteristics of selected rock samples

No.

Denotation

Location

Animal species

Rock art school

Gravure depth (mm)

Sample type

1

Plateau 1

Takabart Kabort

Non-engraved

—

—

2

Plateau 2

Takabart Kabort

Non-engraved

—

—

3

Engraving 1

Takabart Kabort

Cattle

8–10

4

Engraving 2

Takabart Kabort

Cattle

5

Plateau 1

Alamas

Non-engraved

Naturalistic Bubaline Naturalistic Bubaline —

—

6

Plateau 2

Alamas

Non-engraved

—

—

7

Engraving 1

Alamas

Gazelle

Tazina

3–5

8

Engraving 2

Alamas

Gazelle

Tazina

3–5

Rock wall about 3 m from sample 3 Rock wall about 4 m from sample 4 Uncoupled fragment at the base of a rock wall Uncoupled fragment at the base of a rock wall Rock wall about 5 m from sample 7 Rock wall about 4 m from sample 8 Uncoupled fragment at the base of a cliff Uncoupled fragment at the base of a cliff

7–9

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of the volume, where interactions of the electrons with the specimen take place, depends on the electron energy (E0 ¼ 12 keV). The diameter of the smallest analysis volume (point analysis) is about 1 mm3. To get more precise information, automated quantitative linescans with equally spaced points through the varnish were acquired, with the respective profiles and point spacings recorded at backscatter electron (BSE) images. The Noran Voyager standardless quantification routine, including the PROZA correction procedure, has been used for the calculation of the concentrations. An analytical error o10% is expected for the major elements.

4. Results and discussion Preparation of microsections clearly revealed known problems with desert varnish on sandstones in arid areas. Platy parts of the surface easily disintegrate due to typical sandstone weathering. Desert varnish in measurable dimensions rather occurs in microdepressions, where it is obviously protected against abrasion or washout. Microtopographical elevations are almost free of varnish. The depths of the microdepressions vary between 50 and 500 mm. Desert varnish appears light to middle brown coloured due to admixtures of manganese and iron oxyhydroxides. No distinction can be made by optical microscopy and SEM between microsections of gravures and plateaus (e.g. Figs. 2A and B). However, chemical analyses of desert varnish from the Messak show specific element correlations.

4.1. Silica, aluminium, and manganese In Figs. 3A–D, an inverse correlation of (SiO2+ Al2O3) versus MnO2 content is shown. The scattering at low MnO2 concentrations is mostly related to various amounts of quartz. The respective chemical composition of desert varnish from non-engraved surfaces (Figs. 3A and B), Tazina School engravings (Fig. 3C), and Naturalistic Bubaline School engravings (Fig. 3D) shows an almost similar relationship. Nevertheless, slight differences in correlation between the desert varnish from Alamas and Takabart Kabort are obtained from the regression lines. The sites can be distinguished by the slopes of the regression lines from 1.6 to 1.7 and from 1.3 to 1.4, respectively. In Figs. 4A and B, the ternary plots of SiO2, Al2O3, and 10 MnO2 concentrations for the two wadis show that the desert varnish from Alamas is somewhat enriched in Al2O3:SiO2 ratio (p46%) versus that from the Takabart Kabort (almost p37%). The silicate composition of dusts remained rather constant during the whole time span of varnish formation but depends

Fig. 2. SEM images of rock varnish at microdepressions: (A) rock varnish at non-engraved sandstone surface from Takabart Kabort (plateau 2) and (B) Rock varnish at a Tazina School petroglyph from Alamas (engraving 1). The white lines show the location of the linescans for microsection analyses.

on the local environment. The origin of the materials in dusts may be almost related to vast areas covered by deeply reworked palaeosoils (Cremaschi and Trombino, 1998). The projecting points for albite, kaolinite, smectite, and muscovite (illite) give clear evidence about the varnish bulk composition. The occurrence of the above minerals was verified by XRD analyses of scraped material from the respective desert varnish. High quartz contents are rather due to quartz grains of the primary sandstone. It is

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concluded from XRD that significant amounts of Al2O3 and Fe2O3 are present as amorphous solids.

4.2. Calcium, manganese, and phosphorus

Fig. 3. Plot of (SiO2+Al2O3) versus MnO2 contents of rock varnish from wadi Alamas (Tazina School) and Takabart Kabort (Naturalistic Bubaline School): (A, B) rock varnish at non-engraved rocks and (C, D) rock varnish at engravings. Slopes and ordinate intersections of the regression lines are (A) 1.7 and 88; (B) 1.3 and 85; (C) 1.6 and 86; (D) 1.4 and 85, respectively.

In Figs. 5A–D, the CaO versus MnO2 contents of the analyzed desert varnish are shown. A positive correlation between MnO2 and CaO exists at a comparably low CaO content of desert varnish from Takabart Kabort over the whole concentration range (Figs. 5B and D). XRD analyses of respective scraped varnish material show only minor amounts of calcite (CaCO3) and apatite (simplified formula: Ca5(PO4)3(OH)). However, desert varnish from Alamas (Figs. 5A and C) displays no significant correlation between calcium and manganese. From XRD analyses significant amounts of calcite, apatite, and to a lesser extend of dolomite (CaMg (CO3)2) are identified in scrapped desert varnish. In this case, a potential correlation between calcium and manganese is masked by the above Ca-containing minerals (see dashed lines in Figs. 5A and C). Accordingly, Figs. 6A and C show a good correlation between CaO and P2O5 contents in desert varnish of Alamas confirming the presence of the considerable amounts of apatite in the respective scraped varnish materials. Deviation of the slope of the regression lines (0.67–0.54, respectively) from the ideal correlation for apatite (P2O5:CaO ¼ 0.78) may be caused by additional amounts of calcite and dolomite. Desert varnish from Takabart Kabort contains minor amounts of apatite with P2O5p2 wt% (Figs. 6B and D), and no correlation between CaO and P2O5 is observed. Phosphorus may be primarily of aeolian origin verified (1) by the absence of P2O5 in the primary sandstone and (2) by the considerable P2O5 concentrations (up to 14 wt% of P2O5; Kolmer, unpublished) in shore sediments of fresh water lakes in the Murzuq sand

Fig. 4. Triplot of SiO2, Al2O3, and 10MnO2 concentrations (in wt%) for rock varnish from (A) wadi Takabart Kabort (Naturalistic Bubaline School) and (B) wadi Alamas (Tazina School). Full squares and circles represent linescans at plateaus 1 and 2, respectively. Open symbols denote respective linescans at engravings.

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Fig. 5. Plot of CaO versus MnO2 contents for rock varnish from wadi Alamas (Tazina School) and Takabart Kabort (Naturalistic Bubaline School). (A, B) rock varnish at nonengraved rocks and (C, D) rock varnish at engravings. Slopes of the regression lines are (B) 0.11 and (D) 0.14, respectively. Dashed lines in (A) and (C) represent the mean regression line of (B) and (D): [CaO] ¼ 0.12[MnO2]+1.2.

sea gained by guano deposition. High bio-mass production along shores of perennial lakes can be used for the reconstruction of climate changes (Pachur, 2001). Nevertheless, additional phosphorus may be obtained by in situ guano precipitation.

4.3. Manganese microlaminations In Figs. 7A and B, the characteristic MnO2 distribution through desert varnish of non-engraved sandstones from Takabart Kabort and Alamas are shown. Most conspicuous are significant enrichments of manganese through the varnish transects. Three MnO2-rich layers are distinctly separated by MnO2-poor layers. The outer MnO2 enrichment of the desert varnish is always higher or broader, which may be related to elevated accumulation rates or times.

Fig. 6. Plot of P2O5 versus CaO contents for rock varnish from wadi Alamas (Tazina School) and Takabart Kabort (Naturalistic Bubaline School): (A, B) rock varnish at nonengraved rocks and (C, D) rock varnish at engravings. Slopes of the regression lines are (A) 0.67 and (C) 0.54, respectively. P2O5:CaO ratio for apatite is about 0.78. Note that the dashed lines in (A) and (C) correspond to the respective graduation of the ordinate in (B) and (D).

Desert varnish of Naturalistic Bubaline School engravings (Figs. 7C and D) typically shows two MnO2-rich layers, whereas varnish of the younger Tazina School engravings (Figs. 7E and F) display only one MnO2-rich layer. To draw conclusions about palaeoclimate, usually Mn:Fe ratios are plotted along the linescans of desert varnish. In the present study, this method fails as the Fe2O3 content is rather constant through the desert varnish (Fe2O3 ¼ 673 wt%).

4.4. Climate record The overall climate history of North Africa and the Fezzan from Late Pleistocene to Holocene is well known by studies on lacustrine fresh-water sediments

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Fig. 7. Characteristic manganese microlaminations of rock varnish from Messak area reflecting climatic fluctuations. MnO2-rich layers refer to humid periods: (A) transects of non-engraved rock surfaces from Takabart Kabort (plateau 2; linescan in Fig. 2A); (B) Alamas (plateau 2); (C, D) transects of Naturalistic Bubaline School petroglyph from Takabart Kabort (engraving 1 and engraving 2, respectively); (E, F) transects of Tazina School petroglyph from Alamas (engraving 1, linescan in Fig. 2B, and engraving 2, respectively). The right hand of the diagrams (0 mm) refers to the outer surface of the rock varnish (see linescans of Figs. 2A and B).

from Ethiopia to Mauretania (Gasse, 1977; Gasse et al., 1980; Rognon, 1987), the wadi Howar river system, Nubia (Pachur and Kro¨pelin, 1987; Pachur, 2001; Kuper and Kro¨pelin, 2006), and the Fezzan (e.g.

Cremaschi and Di Lernia, 1995; Cremaschi et al., 1996; Cremaschi, 1998a, b; Cremaschi and Trombino, 1998, 1999). However, the following three humid periods, quite different in intensity, are documented

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Hyperarid Period

Great Humid Period

ar id

20

ka BP

B

19 18 17 16

15 14 13 12 11 10

Rock Art School 7

6

5

6

5

9

4

8

7

6

5

4

3

2

3

Present Arid Period

Third Humid Period

Postaterian

Postneolithic

Return of Rainfall

humid

Neolithic Humid Period

A

Arid Period

M. Dietzel et al. / Chemie der Erde 68 (2008) 31–43

Middle Holocene Arid Period

38

2

1

0

1

0

Naturatistic Bubaline Tazina Horse Camel

C Cultural Epoch

10

9

8

7

4

3

2

1

0

Epipalaeolithic Mesolithic Early pastoral Middle Pastoral Late Pastoral

Fig. 8. (A) Paleoclimate in North Africa with humid and arid periods (adapted from Muzzolini, 1995). (B) Short chronology; Cultural historical evolution of principal art schools in the study area according to Muzzolini (1993). (C) Long chronology (Cremaschi and Di Lernia 1998; Mori, 1978; Lhote, 1976).

from the Terminal Pleistocene to Holocene, tentatively summarized by Muzzolini (2001) (Fig. 8A). The terms ‘‘pluvial’’ or ‘‘humid’’ indicate rather overall climate conditions than actual amounts and annual distribution of precipitations (see also Le Quellec, 2004). 4.4.1. Great humid period Fontes and Gasse (1989) postulate the reestablishment of more humid conditions at about 10 to 9 ka BP after an arid period of debated duration (e.g. Gasse, 1977; Rognon, 1987; Fontes and Gasse, 1989; SmykatzKloss et al., 2000). This is supported by the existence of large freshwater lakes in the ergs of Ubari and Murzuq, and the early to middle Holocene lake archipel Ptolemy, Nubia (Pachur, 2001, Kuper and Kro¨pelin, 2006), the latter being connected to the Nile by wadi Howar. For lacustrine base sediments of this wadi Pachur and Kro¨pelin (1987) estimated an age of 9.4 ka BP. This humid period was also found by Cremaschi’s group using dated strata in caves and shelters. The onset of the wet period is indicated by travertine formation between

the 14 and 9th millennium BP (Carrara et al., 1998) showing substantial increase of precipitation in the Acacus Mountains (Cremaschi, 1998b). During the late 10th millennium, the wet environment declined and epipalaeolithic hunters settled in the area. The shift from Epipalaeolithic to Mesolithic cultures and changes in their socio-economic strategies strongly indicate adaptation towards aridity (Cremaschi and Di Lernia, 1995). Accordingly, from 8th to the beginning of 7th millennium, dry conditions became more and more severe, stratigraphic gaps occur, sediment layers were eroded, and aeolian sand layers are recorded (Cremaschi et al., 1996). The ‘‘Great Humid Period’’ (Muzzolini, 1995, 2001) ends with a temporary arid period of about 1 ka duration, the Middle Holocene Arid Period which is not revealed by Kuper and Kro¨pelin (2006). 4.4.2. Neolithic humid period Doherty et al. (2000) postulated the evidence of a second humid period beginning at about 6 ka BP by palaeobotanic investigations. At this time period, fresh

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water lakes of large extension existed in both, the Ubari and Murzuq sand sea (Petit-Maire, 1991; Pachur, 2001). This second humid period is termed as the Neolithic Humid Period ending at about 4.5 ka BP (Muzzolini, 2001). The evolution of environmental conditions may be obtained by pollen analyses gained from dated dung layers (Trevisan Grandi et al., 1998; Mercuri et al., 1998). Pollen sequences taken from caves and shelters in the Acacus Mountains cover a time span ranging from the 7th to 4th millennium BP, from the onset of the Neolithic Humid Period to its approximate end, when arid conditions returned at least in the lowlands. For the above periods, pollen analyses of trees, shrubs, and herbs show clearly in distinct layers a gradual change from wet savannah climate to a desert climate. The former corresponding to recent Sahel climate with precipitation of 150–300 mm/year, the latter corresponding to recent intra desert wadis, e.g. wadi Hanna in Egypts Gilf Kebir. Human settlement became well established all over the Eastern Sahara, fostering the development of cattle pastoralism (Kuper and Kro¨pelin 2006). Conclusions about annual distribution of precipitation could be drawn from seasonal occupations of different sites (lowlands or mountains) derived from pollen sequences. With the beginning of 5th millennium, climate deteriorates to more arid even hyper-arid conditions, the Postneolithic Arid Period. 4.4.3. Third humid period After a few hundred years, the Postneolithic Arid Period is replaced by humid pulsations of short duration (e.g. Gasse, 1977). The decrease of humid intensity was lasting about 1–1.5 ka as revealed by water-level oscillations observed with fresh-water lakes (Ja¨kel, 1978; Gasse et al., 1980; Kutzbach and Street-Perrott, 1985, Muzzolini, 2001; Pachur, 2001). This 3rd humid period was also verified by numerous studies on palaeosoils, lake sediments, cave or shelter deposits of the Tadrart Acacus, Messak, wadi Uan Kasa, and Murzuq sand sea (Cremaschi et al., 1996; Cremaschi, 1998b; Trombino, 1998). The open lakes or ponds desiccated or turned into sabkhas, but the ground-water level in the wadis should had been high enough to ensure water supply from near surface groundwater supporting conditions of life of local nomad population (Le Quellec, 2004). Accordingly, the Roman province Africa supplied Italy with significant amounts of food. The present climate within the study area is hyperarid with a mean annual precipitation of p10 mm and a mean annual temperature of about 22 1C.

4.5. Dating of petroglyphs The Messak Settafet area is one among several areas throughout the Sahara, which is particularly rich in

39

petroglyphs (Muzzolini, 1995; van Albada and van Albada, 2000). The petroglyphs are classified by stone tools, ceramics, and rock art (Cremaschi and Di Lernia, 1998, and Muzzolini, 1995, 2001). The creation age of these petroglyphs obviously belongs to different cultural epochs (Monod, 1932; Frobenius, 1937; Mori, 1965, 1978; Muzzolini, 1993, 2001). In Fig. 8B the four Rock Art Schools for the Fezzan are shown according to the ‘‘short chronology’’ postulated by Muzzolini (1993). Following Muzzolini’s (1993) concept, the oldest Rock Art School, the Naturalistic Bubaline School, represents large undomesticated mammals such as giant buffalos, elephants, giraffes, lions, and ostrichs. This wild fauna group is related to the time period between 6 and 4 ka BP and corresponds approximately to the Neolithic Humid Period (Fig. 8A). This humid period was followed by the Postneolithic Arid Period from about 4.5 to 3 ka BP. The Third Humid Period began at about the middle of the 4th millennium. At the same time rock art of the Tazina School occurred in the Morroccan and Algerian Atlas and some hundred years later in the Fezzan, which may be due to regional differences in climate development. The respective fauna was similar to the wild fauna group, but can be easily distinguished from the former style by scenic composition, stylistic devices, and artistic techniques (Pichler and Rodrigue, 2003). In comparison to the Neolithic Humid Period giraffes, ostrichs, and gazelles occur at higher frequency. Moreover, oryxantelopes appeared reflecting sub desert steppe climate (Muzzolini, 1995). To place the Tazina-style correctly in a chronological scheme is still difficult because of its expansion from South Morocco, through Algeria far into Libya. All petroglyphs belonging to the above two schools are coated with desert varnish, whereas those of the following Horse School (about 2.6 ka BP) and Camel School period (about 2.2–1.5 ka BP) show no desert varnish coatings. In contrast to the ‘‘short chronology’’, the ‘‘long chronology’’ (Mori, 1965; Lhote, 1976; Cremaschi, 1996; Cremaschi and Di Lernia, 1998) generally relates the wild fauna group depictions to epipalaeolithic hunters, such placing these gravures several millennia and a whole humid period earlier (Fig. 2C). Recent genetic analyses of African cattle breeds indicate that at about 10 ka BP wild cattle may have already undergone an indigenous African domestication (Hanotte et al., 2002). Moreover, the occurrence of domesticated cattle was verified by Smith (1978) in Greece at about 8.2 ka BP. They appeared in Africa together with domesticated sheeps and goats via the Isthmus of Suez as early as 7.8 ka BP (Stokstad, 2002). Accordingly, all younger gravures of Pastoral group mammals, often associated with domesticated cattle and increasing numbers of human figures will have to be placed in earlier periods, which is in accordance with dated rock

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art paintings in a shelter in Central Acacus (Ponti and Sinibaldi, 2005). In Muzzolin’s ‘‘short chronology’’ Tazina style covers the whole 4th millennium, whereas Cremaschi (1996) places a giraffe figure resembling to Tazina style in the late pastoral, which includes also the 5th millennium. Whether the ‘‘long chronology’’ or the ‘‘short chronology’’ is valid has to be confirmed by archaeological studies.

5. Conceptional model for evolution of manganese microlaminations, petroglyphs chronology and palaeoclimate Cremaschi (1992, 1996) was among the first who brought desert varnish formation on sandstone surfaces of the Messak into context of historical age, chronology of petroglyphs, and palaeoclimate. Desert varnish analyses of gravures of Bubaline antiquus (epipalaeolithic, 9.8–8.9 ka BP), elephant (early pastoral, 7.4–6.4 ka BP), giraffe (late pastoral, 5.0–3.5 ka BP), and camel (2–1 ka BP) are compared to varnish analyses on adjacent non-engraved rock surfaces. Main results of Cremaschi’s (1996) observations are (1) that desert varnish formation began after carving of the bubalus and elephant figures contemporaneous or later than early pastoral. (2) The furrows of the bubalus figure were corroded by weathering in a ‘‘wet’’ environment before the onset of varnish formation. These observations raise questions about climatic conditions during the Great Humid Period. This era may be due to an oscillation period with rather humid and rather arid conditions, especially during its final stage from the end of 9th millennium to the onset of the Middle Holocene Arid Period. During the decline of the Great Humid Period palaeoclimate almost certainly passed through conditions similar to the later Neolithic Humid Period, where varnish formation is well established. However, during Holocene three humid periods can be distinguished, which provide a distinct climate for formation of desert varnish by sequences oscillating between rather humid and rather arid conditions. Accordingly, during the initial Great Humid Period desert varnish developed containing one (the first) MnO2-rich layer. The following Middle Holocene Arid Period terminated ongoing Mn enrichment. Following Muzzolini’s (1995) short chronology, the oldest petroglyphs of the Naturalistic Bubaline School engravings coincide with the Neolithic Humid Period (6.5–4.5 ka BP; see Fig. 8A and B). At this time, artists began carving mythical imaginations on rock surfaces. By deeply carving (about 10 mm), the already existing desert varnish was destroyed and a pristine surface was produced. On this new surface, varnish formation continued until the end of the Neolithic Humid Period.

Again this varnish would contain a single (second) Mn-rich layer. At the same time, the varnish also formed and accumulated further on non-engraved surfaces already varnished. Thus, desert varnish on non-engraved surfaces should contain two Mn-rich layers, whereas the Naturalistic Bubaline School engravings show only one Mn-rich layer. The following Postneolithic Arid Period terminated Mn enrichment. At the same time, the varnish of non-engraved rock surfaces contained two MnO2-rich layers, whereas the Naturalistic Bubaline School engravings showed only one MnO2-rich layer. During the Third Humid Period, a new artistic style appears, the Tazina School. Again varnish-covered rock surfaces were carved and along the engravings new pristine surfaces were created. Subsequently, varnish formation continued until the end of the Third Humid Period, where varnish formation irrevocably came to an end till recent time. Consequently, this varnish contains an additional (the third) Mn-rich layer. Environmental conditions favouring the formation of MnO2-rich layers were never repeated since this period. Accordingly, desert varnish covering the Tazina-School engravings typically contains one MnO2-rich layer (Figs. 8E and F). The varnishes covering the Naturalistic Bubaline engravings comprise two MnO2-rich layers (Figs. 8C and D), whereas all non-engraved rock surfaces were covered by desert varnish containing three MnO2-rich layers (Figs. 8A and B).

6. Summary and conclusions Desert varnish composition of the samples collected from the Messak Mountains shows some specific elemental relationships, which provide insight into varnish formation on sandstone in a desert-like environment. The characteristic negative correlation between (SiO2+Al2O3) and MnO2 reflects the coexistence of clay minerals with manganese oxyhydroxide phases. Desert varnish of the sites Alamas and Takabart Kabort may be easily distinguished by specific (SiO2+ Al2O3):MnO2 and SiO2:Al2O3 ratios due to local origin of dust material (e.g. composition and association of silicates) and precipitation environments (see Figs. 3 and 4). Moreover, desert varnish of Alamas (Tazina School) and Takabart Kabort (Naturalistic Bubaline School) can be clearly deciphered by the concentrations of CaO, MnO2, and P2O5. Tazina School engravings and nonengraved varnish of Alamas contain high amounts of calcite and apatite. Thus, CaO and P2O5 contents are positively correlated (see also analytical data for desert varnish from wadi Mathendush in Cremaschi (1992) and (1996)). The slopes of the regression lines in Figs. 6A

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and C correspond to respective calcite to apatite ratios in the varnish. On the other hand, Naturalistic Bubaline School engravings and non-engraved varnish of Takabart Kabort show no correlation between CaO and P2O5. As amounts of apatite and calcite are low, such desert varnish can be identified by an almost constant CaO:MnO2 ratio of about 0.12 (see Figs. 5B and D). The results of the present study indicate that manganese microlamination through the desert varnish from petroglyphs present on the sandstone in the studied area may be a reliable tool to evaluate past climate changes from Pleistocene to Holocene if natural environmental conditions and rock art dating are well established (see also Marston, 2003; Lee and Bland, 2003). Main aspects are that (1) Occurrence of three humid periods in the Holocene, the Great Humid Period, the Neolithic Humid Period, and the Third Humid Period is deduced from climate records (see Section 4.4) recently confirmed by Le Quellec (2004). (2) Sedimentological and pollen analyses indicate that these humid periods represent oscillating sequences of rather humid and arid conditions favouring desert varnish formation. (3) MnO2-rich layers were formed at moderate humid conditions, whereas MnO2-poor layers are referred to rather dry periods (see Section 1). Non-engraved varnish coatings are exposed since the Terminal Pleistocene. Such varnish typically shows three MnO2-layers corresponding to the Great Humid Period, Neolithic Humid Period, and Third Humid Period, respectively (see Figs. 7 and 8). (4) Desert varnish on the Naturalistic Bubaline School petroglyphs was formed in the Neolithic Humid Period. Accordingly, two MnO2-rich layers occur in the varnish on these engravings. (5) Desert varnish on Tazina School petroglyphs is about 2 ka younger than that on Naturalistic Bubaline School petroglyphs. Thus, a single MnO2-rich layer can be observed representing the Third Humid Period. From another point of view, geochemical analyses can provide an indirect method for classification of petroglyphs, which does not depend on cultural historical aspects. However, for any promising application of geochemical signatures to palaeoclimate and petroglyph research accurate knowledge of the local history and environmental settings is required.

Acknowledgements The authors thank M. Hierz for support by performing the mineralogical analyses as well as K. Heide and

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W. Smykatz-Kloss for handling and reviewing the manuscript.

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