Archaeological Chemistry - American Chemical Society

Chapter 7

Bitumen in Neolithic Iran: Biomolecular and Isotopic Evidence 1

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Michael W. Gregg , Rhea Brettell , and Benjamin Stern

Downloaded by CORNELL UNIV on June 17, 2017 | http://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0968.ch007

1

Department of Anthropology, University of Toronto, Toronto, Ontario M5S 3G3, Canada Department of Archaeological Sciences, University of Bradford, Bradford, West Yorkshire BD7 1DP, United Kingdom

2

This paper presents the results of the chemical analysis of materials recovered from two of the earliest agricultural villages in southwestern Iran and a late Neolithic pastoral encampment in nearby Khuzistan. Gas chromatography - mass spectrometry (GC-MS) revealed biomarker compounds characteristic of bitumen in residues from ceramic vessels supporting the excavators' contention that the interior surfaces of some vessels were coated with a thin layer of such material and confirmed that 'fragments' collected during excavation were indeed bitumen. Biomolecular and isotopic (δD and δ C) analysis of the bitumen indicated that the sources utilized lie in the Susa and Deh Luran regions of southwestern Iran. 13

© 2007 American Chemical Society

Glascock et al.; Archaeological Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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138 Bitumen use is well attested in the later Chalcolithic and Bronze Age periods of southern Mesopotamia (7, 2), with its earliest reported use in pottery vessels coming from the site of Tell Sabi Abyad in northern Syria, dated to 6100 B.C. (3). However, the presence of bitumen has been recorded in both the aceramic and ceramic levels of the early Neolithic sites of A l i Kosh and Chagha Sefid (Figure 1) excavated by Frank Hole, Kent Flannery and James Neely during the early 1960s, the earliest ceramic horizons at both sites dating between 6800 - 7200 B . C . (4, 5). Similarly, evidence of 'bituminous earth' was also recovered from the late Neolithic pastoral encampment at Tepe Tula'i (Figure 1) excavated by Frank Hole in 1973, and provisionally dated between 6200 - 5900 B . C . (6, 7). Bitumen appears to have been utilized as waterproofing for basketry, reed matting, and dwellings, and as a hailing agent for stone axes and flint tools (8, 9). However, Hole et al. also identified residues, presumed to be bitumen, adhering to ceramic fragments. This coating may have functioned as a ceramic sealant and/or repair agent. Alternatively, the primary function of the pottery vessels may have been for the collection and heating of bitumen for use as a waterproofing and adhesive material (70, 77).

Materials and Methods Materials As part of a wider study into the initial development and use of ceramics in the Middle East, organic residues in pottery from 19 of the earliest villages and pastoral encampments in the Zagros mountains and in the Levant are in the process of being analyzed by gas chromatography - mass spectrometry ( G C MS). Most of these materials were acquired from extant collections with 200 pottery fragments having been examined to date. These included samples from A l i Kosh (Figure 2) and Chagha Sefid, two of the earliest agricultural villages in southwestern Iran. The molecular signatures characteristic of bitumen were identified in 4 sherds from these sites (Table I). Since Hole had reported recovering 'bituminous earth' from A l i Kosh, Chagha Sefid, and Tepe Tula'i (6, 10, 77), earthen fragments that appeared to be encrusted with bitumen (Figure 2) were subsequently obtained from the Iranian National Museum in Tehran (Table I). One modern bitumen sample for source identification (Table I, C M ) was also collected from a seep called Chersh Merghir, (spring of tar in Farsi; Ν 32° 4 Γ 26"; Ε 47° 19' 56"), where the foothills of the Zagros mountains meet the Mesopotamian plain.



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Figure 1. Location of sites in southwestern Iran mentioned in the text; the early Neolithic villages of Ali Kosh and Chagha Sefid and the late Neolithic pastoral encampment of Tepe Tula 7.

Figure 2. left, reed-impressed, bitumen-encrusted, mudbrickfragment (AK) from Ali Kosh; right, pottery fragment (AK1) recoveredfrom the Mohammad Jaffar horizon at Ali Kosh, dated to 7100 B. C



Modern bitumen

Archaeological bitumen samples

Archaeological pottery samples

CM

TTD1.02

TTZ

TTI

AK

CS2 INT

CS1 INT RES

AK2 INT

AK1 INT RES

Find State

Sherd Earthy chunks Earthy chunks

Sherd

Sherd

Sherd

Bitumen recovered from Tepe Tula'i Earthy excavations chunks Bitumen recovered from Tepe Tula'i Earthy excavations chunks Bitumen recovered from modern seep Viscous at Chersh Meghir liquid

Description

Pottery fragment with visible residue from Ali Kosh Pottery fragment from Ali Kosh Pottery fragment with visible residue from Chageh Sefid Pottery fragment from Chageh Sefid Bitumen recovered from Ali Kosh excavations Bitumen recovered from Tepe Tula'i excavations

Table I. Details for the Archaeological and Modern Samples.


Color

Black

Orange brown Brown Cream layers Brown Pink/cream layers Yellow-brown Cream inclusions Grey/brown

Dark brown

Orange brown

Dark brown

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Methods The pottery sherds from A l i Kosh and Chagha Sefid were subsampled by grinding portions (0.6 g) into a fine powder using a high-speed modeling drill fitted with an abrasive tungsten bit. Subsamples of the archaeological 'bitumen' fragments were homogenized by crushing with a mortar and pestle, and a portion of the tar collected from Chersh Meghir was selected. The lipid fractions were then obtained by repeated (three times) solvent extraction using dichloromethane:methanol ( D C M / M e O H , 2:1, v/v) with ultrasonication to aid dissolution. Separation of the solid and soluble fractions was attained by centrifiiging at 2000 rpm for five minutes, the solvent soluble fraction being decanted and combined. The solvent was then evaporated on a warm hot-plate under a stream of nitrogen gas to produce the 'total extract'. A subsample of this 'total extract' was then diluted in D C M for analysis by G C - M S . This was conducted using an Hewlett Packard 5890 series II G C , fitted with a 15 m χ 0.25 mm id, 0.1 mm film thickness OV1 phase fused silica column ( M E G A ) connected to a 5972 series mass selective detector. The splitless injector and interface were maintained at 300°C and 340°C respectively. The helium carrier gas was held at a constant inlet pressure of 1 psi. The G C oven was temperature programmed at 50°C for 2 minutes then increased by 10°C per minute to a maximum of 340°C, at which the temperature was held for 10 minutes. The column was directly inserted into the ion source where electron impact (70 eV) spectra were obtained. The resulting chromatograms were examined for the molecular and characteristic fragment ions of various lipid classes including biomarkers of petroleum such as terpanes (m/z 191) and steranes (m/z 217) (3, 12, 13). In order to assist in further characterizing the bitumen samples, bulk isotopic analysis (8 C and 8D) of the asphaltene fraction was undertaken as this fraction provides the most representative isotopic composition of the sample rather than the 'total extract' as used for G C - M S analysis (3). The asphaltene was obtained by repeated (three times) washing of a portion of the 'total extract' in 5 ml of nhexane with ultrasonication to aid dissolution. Separation of the solid asphaltene fraction was assisted by centrifiiging, the solvent being decanted. Any remaining solvent was then removed on a warm hot-plate under a stream of nitrogen gas. Bulk carbon isotopic values were obtained using continuous flow isotope ratio mass spectrometry (IR-MS), the samples being flash combusted in a column containing chromium oxide ( C r 0 ) and silvered cobalt (I) oxide held at a temperature of 1020°C. The resultant gases were then reduced to C 0 in a column of elemental copper at 680°C and passed through a water trap of magnesium perchlorate before being separated in a G C column for introduction to the M S (Finnigan delta plus X L ) . The reference C 0 gas was standardized against the international standard IAEA600 (δ -27.5 ± 0.2) and three methionine 13

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3

2

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142 standards (δ -26.6) were run as quality control check samples during batch analysis. Hydrogen values were obtained by elemental analyzer-isotope ratio mass spectrometry (EA-IR-MS), the samples being released into a furnace set at 1080°C and thermally decomposed to H and C O over glassy carbon. Any traces of water were then removed by a magnesium perchlorate trap and any C 0 formed by a Carbosorb trap before the H was resolved by a packed column gas chromatograph held at 35°C. The resultant chromatographic peak was then passed into the ion source of the IR-MS, ionized and accelerated and gas species of different mass separated in a magnetic field. These were simultaneously measured on a Faraday cup universal collector array with masses 2 and 3 being monitored for deuterium. 2

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Results and Discussion

Molecular Characterization Comparison of the chromatograms obtained from the modern bitumen seep, the archaeological bitumen and the pottery samples demonstrated the essential similarity of the data obtained with the terpane distribution patterns showing parallels in the range and relative abundances of the diagnostic hopanes (examples shown in Figures 3, 4 and Table II). The dominant families present were 17a(H),2ip(H)-hopanes, Ts (18a(H)-22,29,30-trisnorneohopane) and Tm (17a(H)-22,29,30-trisnorhopane) and tricyclopolyprenanes, with subordinate molecular classes being the methyl-ap-hopanes, 17P(H),21a(H)-hopanes and hexahydrobenzohopanes. Thus, the residues in the ceramics and the 'bituminous earth' collected during excavation of these Neolithic sites can clearly be identified as bitumen. Molecular identification of bitumen sources relies upon assessment of the presence/absence of biomarker hopanes such as oleanane, "a unique genuine chronostratigraphic biomarker" relating oil type to specific geological periods (13), and gammacerane, and ratios of their relative abundance (3, 12). The archaeological bitumen samples and the modern sample from Chersh Meghir all have minor peaks in the expected location of oleanane; however, this compound was only tentatively identified as the mass spectra do not show the required molecular ion and fragments and there is no clear indication of oleanane in any of the archaeological ceramic samples. These findings are somewhat unexpected as, although oleanane has never been observed in bitumen from Iraq and Syria, it generally occurs in pronounced abundances in sources from Iran, in particular, from Khuzestan and Fars provinces (75). This lack of clearly identifiable


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es

(N

m

On « « JL 16

18

20

22

24

28

26

Retention Time (mins)

1 ff ÎN S S 1

w 16

18

20

22 24 26 Retention Time (mins)

1

III ί S fûLvwr^ 28

Figure 3. Selected ion chromatogram (m/z 191) for two archaeological ceramic samples: (a) AK2INTRES and (b) CS1INTRES. Key to peak identification is shown in Table II.



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1 6

18

20

22

^24~

26

28

26

28

Retention Time (min)

t/l

C

1 6

18

20

22

24

Retention Time (min)

Figure 4. Selected ion chromatograms (m/z 191) of "bituminous earthsamples: (a) AK and (b) TTI. Key to peak identification is shown in Table II.



23/3 24/3 25/3 24/4 26/3 28/3 29/3 Ts Tm 28αβΗ 29αβΗ 30ΜβαβΗ 29βαΗ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 30αβΗ 31ΜόαβΗ 30βαΗ 3^HS+R Gammacerane 32αβΗε+Η 33αβΗε+Ρ 34αβΗε+Ρ 35αβΗε+Β 36αβΗε+Κ

?

Brief Identification

Peak number 2

1

17a,21 β-homodishopane (22S) + (22R) 17a,2^-trishomohopane (22S) + (22R) 17a,2^-tetrakishomohopane (22S) + (22R) 17a,2^-pentakishomohopane (22S) + (22R) 17a,2^-sextakishomohopane (22S) + (22R)

C30H52

17a,2^-homohopane (22S) + (22R)

17β,21α-Γ)ορ8ηβ (moretane)

2

2

2

2

2

24

C3 tricyclopolyprenane C tricyclopolyprenane C5 tricyclopolyprenane C4 tetracyclic terpane C e tricyclopolyprenane C8 tricyclopolyprenane C9 tricyclopolyprenane 18a-22,29,30-trisnorneohopane 17a-2229,30-trisnorhopane 17α,21 β-29,30-

Archaeological Chemistry - American Chemical Society

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