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Taphonomy of Fossilized Resins: Determining the Biostratinomy of Amber

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Taphonomy of Fossilized Resins: Determining the Biostratinomy of Amber Viewmetadata, citation and similar papers at core.ac.uk broughtto you by CORE providedby Revistes Catalanes amb Accés Obert ACTA GEOLOGICA HISPANICA, v. 35 (2000), nº 1-2, p. 171-182 Taphonomy of fossilized resins: determining the biostratinomy of amber G.O. POINAR, Jr.(1) and M. MASTALERZ(2) (1) Department of Entomology, Oregon State University, Corvallis,OR 97330. E-mail: [email protected] (2) Indiana Geological Survey, Indiana University, 611 North Walnut Grove, Bloomington, IN 47405-2208. E-mail: [email protected] ABSTRACT Comparing the maturity of fossilized resins with that of their enclosing bedrock can provide information on the maturity, relative age and biostratinomy of amber and copal. A method to determine this is presented here with examples of amber and copal from the Dominican Republic. Maturity of the bedrock was determined by vitrinite reflection and that of the fossilized resin by FTIR analys i s . Vitrinite oxidation values showed maturity states corresponding to lignite and sub-bituminous coal ranks. While the samples from some mines demonstrated that the maturities of the rock and fossilized resin were syngenetic, other samples indicated that recycling of the amber may have occurred. Darkening of the amber (from yel l o w to red) was correlated with increased oxi d a t i o n / w eathering. Th i s method can be a useful tool for understanding the biostratinomy of fossilized resins. Keywo rd s : Am b e r . Copal. Tap h o n o m y. Maturity. Relative age. Dominican Republi c . IN T RO D U C T I O N to the 30-45 million year dates obtained earlier by Cepek (Schlee, 1990). One of the problems dating amber de- Amber from the Dominican Republic is wel l - k n ow n posits by analyzing microfossils in the bedrock is know- for its range of biological inclusions (Poi n a r , 1992; Poi n a r ing whether any of the amber underwent recycling or re- and Poi n a r , 1999; Wu, 1996), thus providing a rare view deposition during its geological history. Some amber of past life in a tropical habitat. However there are still a deposits, such as the Canadian Cretaceous amber in Al - number of unknowns concerning the origin and age of be r ta and the Hat Creek deposits in British Columbia Dominican amber. In fact, there now appears to be as (P i k e, 1993; Poinar et al., 1999) occur in coal seams ma n y age estimates as there are inves t i g ators. The last two which indicates “in situ” production and the age of the studies invol v ed researchers from the American Museum amber would be the same as that of the coal. Other amber of Natural History who published papers with diffe r i n g deposits, such as most of the Mexican and Dominican, are age estimates, (Grimaldi, 1995 (23-30 million yea r s ) ; found in sedimentary rocks and have obvi o u s l y under- It u rr a l d e - V incent and MacPhee, 1996 (15-20 million gone some transportation previous to burial, although years). Neither of these studies gave serious consideration ho w ext e n s i ve and how long this process took is diffic u l t 171 Figure 1. A. Relationship between vitrinite reflectance and a color index of amber; B - Vitrinite reflectance and FTIR-derived ratios: -1 -1 Al1-CH2/CH3 in the 2800-3000 cm stretching region and AL2-CH2/CH3 in the 1300-1500 cm aliphatic bending region. Note lack of correlation; C-A positive correlation between color index of amber and Ox1 (C=O1600-1800/CH2+CH3 2800-3000) index. Note Cotui am- ber not following the trend. 172 Table 1. Vitrinite reflectance (Ro, %), vitrinite oxidation index (Vox) of the samples studied and color index of the associated amber, nd- not determined. to say (Poi n a r , 1992). Another method to determine the one mine (La Toca) indicated a range of up to 40 million maturity and relative age of amber is to chemically exa m - years (Lambert et al., 1985). The question that arose was ine the matrix. This was first accomplished in 1964 with whether these results reflected chronological age diffe r - the infra-red analyses (IRS) of resinites (essentially small ences or maturity changes as a result of contact metamor- bits of amber embedded in low grade coal seams) phism. An additional study of exo m e t h ylene resonances (Murchison and Jones, 1964) and amber (Beck et al., in resin, copal and amber from New Zealand and Au s - 1964; Sav kevich and Shakhs, 1964). Although this tralia (Lambert et al., 1993) showed that exo m e t hy l e n e method is suitable for fin g e r printing the fossilized resin resonances could be used to indicate maturity but that a to identify its province, it cannot be used for dating pur- co r relation between maturity and chronological exists on- poses since the maturity level is dependent on the geo- ly if two conditions are satisfie d , namely that the resin th e r mal regime and not the absolute age (Murchison and originate from the same plant genus or species and that it Jones, 1964; Mustoe, 1985). un d e r go similar diagenetic processes. The first point, that of the resin originating from the same plant genus or Nuclear magnetic resonance (NMR) spectrometry species, is not a serious problem since the spectra can be was also successfully used to fin g e r print amber from var - used to identify the source plant or the identity of the ious deposits (Lambert et al, 1985, 1989). The fin g e r - plants have already been established. It is the second con- prints obtained with NMR spectroscopy as well as with dition that causes concern since in many areas of amber IRS essentially were a characterization of the plant gro u p burial, both regional and contact metamorphism result in that produced the amber. An initial attempt to chronolog- the amber being subjected to differing amounts of heat ic a l l y date Dominican amber using NMR spectroscopy and pressure, which would affect the exo m e t h ylene reso- was based on differences between the exo m e t h ylene res- nances. A likel y example is provided when the amber of onances obtained from amber originating from diffe r e n t Me xico is compared with that of the Dominican Repub- mines. An age range of from 15 to 30 million years was lic. Both amber originates from members of the genus suggested for most of the ambers, however amber from Hy m e n a e a (the Dominican amber tree has been identifie d 173 Figure 2. FTIR spectra of La Toca samples. Note var ying intensitiy of bands assigned to exo c yclic methylene groups (3076,888 cm-1 ). 174 -1 Table 2. FTIR-derived ratios of the amber: Al1-CH2/CH3 in the 2800-3000 cm stretching region; Al2 - Al1-CH2/CH3 in the 1300-1500 -1 cm aliphatic bending region; Ox1 - C=O 1600-1800 /CH2+CH3 2800-3000; Ox2 - C=O 1600-1800/CH2,3 1450; nd: not determined. as Hymenaea prot e r a Poi n a r , 1991b). However it is ap- turity of organic matter in the rock matrix containing the parent by the nature of Mexican amber (hardness, specif- am b e r . It was hoped that such analysis would provide in- ic gra vity) and by the fact that many insect inclusions ac- fo r mation on the maturation of the amber in relation to tu a l l y show evidence of compression and shear pressure the rock and indicate whether the fossilized resins were in that much of the Mexican amber has been subjected to a primary or subsequent depositional site (evidence of re- high levels of heat and pressure. Indeed there is still much cyc l i n g ) . volcanic activity today in the areas of the Mexican mines (P oinar and Poi n a r , 1994). Such contact metamorph i s m increases the rate of maturation, which probably exp l a i n s MA TERIALS AND METHODS why the exo m e t h ylene resonances of Mexican amber are less than those of Dominican amber (Lambert et al., 1985, Rock samples containing amber from several loca- 1989), assuming that the ages are roughly equival e n t . tions in the Dominican Republic (Tab le 1) were collected by the senior author and others during a University of Studies on the taphonomy of fossilized resins are in- Ca l i f o r nia Research Expeditions Program in 1987 (Poi n a r frequent, yet this is an important aspect of amber studies. and Poi n a r , 1994). The samples had been stored at room The sedimentary history or biostratinomy of fossilized temperature since that period. With the exception of the resins can invol v e transport, mixing and sorting by var i - samples from Cotui and El Valle, all of the mines were lo- ous agencies. Amber transported by rivers into the sea or cated in the Cordillera Septentrional roughly betwee n lo w lying deltas, which are later inundated by salt wat e r , Santiago and Puerto Plata in the North e r n Por tion of the often end up in uplifted sedimentary sequences. During co u n t r y.
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