Resin from Africa and South America: Criteria for Distinguishing

Chapter 10

Resin from Africa and South America: Criteria for Distinguishing Between Fossilized and Recent Resin Based on NMR Spectroscopy 1

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Joseph B. Lambert , Suzanne C. Johnson , and George O. Poinar, Jr.

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Department of Chemistry, Northwestern University, Evanston, IL 60208 Department of Entomology and Parasitology, College of Natural Resources, University of California, Berkeley, CA 94729

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The carbon-13 nuclear magnetic resonance spectra of resins from Colombia (South America) and Kenya (Africa) are essentially identical to that of modern Hymenaea, when spectra are recorded with either normal or interrupted decoupling. The spectrum of a sample of copal from the Congo in Africa also is nearly identical to that of Hymenaea. In contrast, a resin from Tanzania, Africa, exhibits spectral properties associated with fossilization, such as reduced exomethylene resonances. These observations are consistent with a recent date and little decomposition, i.e., immaturity of the Colombian, Kenyan, and Congolese samples.

Resinous material exuded by a variety of plants may become fossilized over geological time under appropriate conditions of temperature and pressure (/). The starting organic molecules are thought generally to be terpenes, and the fossilized material, which may have been polymerized, cross-linked, and oxidized, is often referred to as amber or resinite, although numerous mineralogical names (succinite, rumanite, burmite) have been applied to specific examples that have unique geographical or paleobotanical origins. The chemical characterization of fossilized resin has been carried out by mass spectrometry (2), infrared spectroscopy (J), and high resolution solid state carbon-13 nuclear magnetic resonance (NMR) spectroscopy (4). The NMR method has been applied on a worldwide basis and now includes characterization of fossil resins from the Caribbean (5), Mexico (C=CH , of which >C= resonates at about δ 148 and =CH at about δ 108. The middle two resonances usually derive from double bond carbons attached only to one other carbon (RCH=), as might be found for a double bond within a ring (endocyclic). Certain fossil resins exhibit neither of the exomethylene resonances. The absence may imply that the original terpenes lacked 2

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Anderson and Crelling; Amber, Resinite, and Fossil Resins ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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L A M B E R T E T AL.

ResinfromAfrica and South America

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this functionality. Thus, whereas exomethylene groups are commonly found in labdane structures, they are lacking in abietane structures (4). Alternatively, the exomethylene group may be removed chemically by diagenetic processes during burial. We have ob served such decreases with time for resins from the Dominican Republic (5) and from New Zealand (8). Thus modern or relatively young resins exhibit strong exomethylene resonances, normally exceeding in intensity those of the endocyclic and related car bons. Over time, the exomethylene resonances degrade in intensity. For Baltic amber (4) they are roughly half the intensity of the endocyclic resonances. For Mexican am ber (6) the exomethylene peaks are quite small, indicative of considerable age or con ditions conducive to degradation. The exomethylene resonances exhibited by the Colombian samples (Figure 1, top and bottom) are quite large. Such observations are indicative of very immature resins and contrast with the spectra of clearly fossilized or mature resins from Mexico or the Baltic Sea. For comparison, Figure 2 (top) gives the spectrum of an authentic sample of modern Hymenaea coubaril with well developed exomethylene resonances, and Figure 3 (top) gives the spectrum of a highly fossilized Mexican sample from Totolapa with quite small exomethylene resonances. The exomethylene peaks of the Kenyan sample (Figure 4, top) are somewhat smaller than those of the Columbian sample, indicating slightly higher maturity. The Tanzanian sample (Figure 4, bottom) is normal for fossilized resin (much reduced exomethylene resonances) and is thought to be of Pliocene origin. Although the African samples resemble Hymenaea, it is also possible they derive from Copaifera sources, for which we presently do not have spec tra of modern samples. The spectra with interrupted decoupling give an alternative diagnostic of these observations. As seen in Figure 1 (middle) for Colombian resin, the only remaining alkene resonances come from carbons without hydrogens. Thus the =CH resonance at δ 108 is gone but the >C= resonance at δ 148 remains (the resonance at δ 140 must come from a double bond that carries one substituent). The large size of the δ 148 peak in the Colombian sample contrasts with the quite small size in Mexican resin (Figure 3, bottom), and is indicative of a very immature sample. The size of the resonance for the Colombian sample is quite comparable with that from authentic Hymenaea (Figure 2, bottom). The Kenyan sample closely follows the pattern of the Colombian samples (Figure 4, middle). It is useful to look for patterns in the saturated portion of the spectra as well (δ 0-90). These resonances for authentic Hymenaea map almost perfectly onto the spectrum for the Colombian resins (compare Figures 1, top, and 2, top), from the little peak at δ 70 and the large peak at δ 40 to four peaks in the region δ 15-30 and the sharp peak with a shoulder at δ 48. With interrupted decoupling the spectra (Figure 1, middle, and Figure 2, bottom) practically can be laid on top of each other (including the carbonyl resonances at δ 180-190). This is extremely strong evidence that the Colombia resins are essentially identical to modern Hymenaea. The Kenya sample (Figure 4, top and middle) follows this same pattern, except that it lacks the small, sharp resonance at δ 34 (just to the right of the largest peak in the spectrum in Figure 1), so that the saturated portion of the spectrum with interrupted decoupling has three rather than four peaks. The spectrum of the Mexican resin with interrupted decoupling (Figure 3, bottom) is quite different from those of the modern resins. Can the Colombian samples be termed copals? This term, originally derived from the Aztec "copalli" ("tree resin") (11), normally refers to semi-fossilized or young 2


AMBER, RESINITE, AND FOSSIL RESINS


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Figure 1. Carbon-13 NMR spectra of resin from Colombia, South America, (top and bottom) with normal decoupling, (middle) same sample as at top with interrupted decoupling.


L A M B E R T ET AL.

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Resin from Africa and South America


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200

~

100

0 ppm

Figure 2. Carbon-13 NMR spectra of Hymenaea coubaril (top) with normal decoupling, (bottom) with interrupted decoupling.


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200

100

0 ppm

Figure 3. Carbon-13 NMR spectra of amber from Totolapa, Mexico, (top) with normal decoupling, (bottom) with interrupted decoupling.


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Resin from Africa and South America

200

λ

•

AM/

A

100

200

100

\ 0

0

ppm

ppm

Figure 4. Carbon-13 NMR spectra of resin (top) from Kenya with normal decoupling, (middle) from Kenya (same sample) with inter rupted decoupling, and (bottom) from Tanzania with normal decoupling.


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amber. We have obtained the spectra of two such materials. The spectrum of the sample from the Congo (Figure 5, top) resembles in every peak both the spectrum of Hymenaea (Figure 2) and those of the Colombian samples (Figure 1). The Manila sample (Figure 5, bottom) exhibits the expected enhanced exomethylene resonances of a recent material but has a quite distinct saturated region, indicative of a different botanical source, presumably Agathis.

1

1

1

1

ι—•—•—•— —ι—•—•—'—·—ι— —•— —•—ι— —'—•—*— 200

100

0 ppm

Figure 5. Carbon-13 NMR spectra from the Congo (top) and from the Philippines (bottom) with normal decoupling.


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ResinfromAfrica and South America

201


:i

Figure 6. Carbon-13 NMR spectra of sample 8-23 from Manitoba, Canada, (top) with normal decoupling, (bottom) with interrupted decoupling. Poinar (12) defined copal as "a recently deposited resin that cannot be molded by hand, has a melting point under 150°C, a surface that partially dissolves (becomes sticky) when acetone is applied and is relatively soft (can be scratched with a fingernail)." Thus copal and amber differ primarily in the extent of fossilization, which depends on age and burial conditions. The Colombian and Kenyan materials were soluble to surface application of acetone, had melting points under 150°C, and could be scratched with a fingernail. Thus the physical observations are in agreement with the NMR spectra that these materials are relatively immature and may be classified as copals. The period of transition from copal to amber has not been fully established, and it probably varies with the type of resin and the conditions of burial (heat, pressure) that control the processes of polymerization, crosslinking, and so on. We present the spectrum of sample 8-23 (Figure 6) as another example of a supposedly ancient resin (Cretaceous Canadian amber in this case) that in fact is recent. The spectra with normal and interrupted decoupling are quite similar to those of the Kenyan sample (Figure 4), suggesting that 8-23 in fact is a modern or immature resin or copal derived from a Hymenaea source. It emphasizes that materials thought with all good intention to be fossilized resin may not be, so that the application of physical or spectroscopic tests are necessary for any case of particular significance or novelty.


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Literature Cited

(1) Anderson, Κ. B.; Winans, R. E.; Botto, R. E. Org. Geochem. 1992, 18, 829-841. (2) Mills, J. S.; White, R.; Gough, L. J. Chem. Geol. 1984-85, 47, 15-19. (3) Beck, C. W. Appl. Spectrosc. Rev. 1986, 22, 57-110. (4) Lambert, J. B.; Beck, C. W.; Frye, J. S. Archaeometry 1988, 30, 248-263. (5) Lambert, J. B.; Frye, J. S.; Poinar, G. O., Jr. Archaeometry 1985, 27, 43-51. (6) Lambert, J. B.; Frye, J. S.; Lee, Τ. Α., Jr.; Welch, C. J.; Poinar, G. O., Jr. In Archaeological Chemistry IV; Allen, R. O., Ed.; American Chemical Society: Washing ton, DC, 1989, pp 381-388. (7) Lambert, J. B.; Frye, J. S.; Poinar, G. O., Jr. Geoarchaeology 1990, 5, 43-52. (8) Lambert. J. B.; Johnson, S.C.;Poinar, G. O., Jr.; Frye, J. S. Geoarchaeology 1993, 8, 141-155. (9) Beck, C. W.; Greenlie, J; Diamond, M. P.; Macciarulo, A. M.; Hannenburg, Α. Α.; Hauck, M. S., J. Archaeol. Sci. 1978, 5, 343-354. (10) Lambert, J. B.; Graham, E.; Smith, M. T.; Frye, J. S. Ancient Mesoamerica 1994, 5, 55-60. (11) Santamaria, F. J. Diccionario de Mejicanismos; Editorial Porrua: Mexico, 1978. (12) Poinar, G. O., Jr. Life in Amber; Stanford University Press: Stanford, CA, 1992. RECEIVED August 14, 1995


Resin from Africa and South America: Criteria for Distinguishing

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