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ACTA GEOLOGICA HISPANICA, v. 35 (2000), nº 1-2, p. 171-182

Taphonomy of fossilized resins: determining the of

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 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 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 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 ). 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 , 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. These localities lie in the Altimira facies of the El these processes, amber from different plant sources and Ma m e y For mation (Upper ) which is shale-sand- di f ferent ages can become mixed , thus making dating and stone interspersed with a conglomerate of wel l - r o u n d e d pa l e o - r e c o n s t r uction diffic u l t . pe b b les (Eberle et al, 1980). The El Valle source origi- nates in the eastern portion of the Dominican Republi c The present study was underta k en to determine the while the Cotui site is located in a val l e y between the rate of maturity of amber from various “mines” in the Do- Cordilleras Central and Septentrional. The numbers fol- minican Republic and compare them with the rate of ma- lo wing the La Toca samples indicate the distance in feet

175 Figure 3. FTIR spectra of El Valle, Palo Alto Low and Cotui ambers. Note differences in exocyclic methylene groups (3076, 1645- 1652, and 888 cm -1), with the strongest in Cotui to almost undetectable in El Valle.

176 -1 Table 3. Relative intensity of exocyclic methylene groups (with peaks at 3081, 1644, and 888 cm ) and their ratio to CH 2 and CH3 in the aliphatic stretching region (E-integration area of exocyclic methylene groups). of various amber locations along the slope of a mountain. flectance mode at a resolution of 8 cm-1 with 1,024 scans It appeared that these represented different veins that collected from each specimen. transected the mountside. The IR signal was recorded in the region of 400 to Rock samples were prepared as polished blocks fol- 4000 cm-1 wavelength. Bands were identified by compar- lo wing standard petrogra p h y procedures (Bustin et al., ison with published assignments (Painter et al., 1981; 1985). A Leitz MPV-2 microscope was used to identify Wang and Griffith, 1985; Goodarzi and McFarlane, 1991; or ganic matter types and to measure random vitrinite re- Mc F arlane et al., 1993). The following ratios were calcu- -1 flectance in oil. Vitrinite particles were recorded as either lated: CH2 /C H 3 in the 2800-3000 cm stretching regi o n -1 un ox i d i z e d , slightly oxidized or strongly oxidized and a (A L 1), CH2 /C H 3 in the 1300-1500 cm aliphatic bending vitrinite oxidation index (Vox) was calculated as a ratio of re gion (AL2), C=O16 0 0 - 1 8 0 0 / CH2 +C H 3 28 0 0 - 3 0 0 0 (O x 1), and oxidized to unoxidized vitrinite based on no fewer than 20 C= O 16 0 0 - 1 8 0 0 /C H 2,3 1450 (O x 2). For selected samples, a ra- vitrinite particles per sample. Random vitrinite re- tio of exo c yclic alkenes/ CH2 + CH3 28 0 0 - 3 0 0 0 was also cal- flectance was determined from 10 to 25 measurements, culated. These ratios utilize band integration areas rather depending on the availability of material. than absolute concentrations of functional gro u p s .

Amber separated from the host rock was first given a color value, based on a scale from 1 to 3: 1- light yel l o w, RE S U LT S 1.5- yel l o w, and yel l o w with orange edges, 2- dark yel l o w to light orange, 2.5- dark orange to light brown and 3- Or ganic petro gra p h y of the rock matrix br o wn. This index would be correlated with other para- meters. For analysis of the fossilized resin, crushed amber The amount of organic matter in the rock matrix or copal was subjected to Fourier Tra n s f o r m infrared ranged from traces to several percent. It was dominated an a l ysis (FTIR) and analyzed with a Nicolet 20SXC spec- by vitrinite that occurred as var i a b le size fragments. Th e trometer equipped with a DTGS detector and diffuse re- vitrinite often showed remnants of woody cell struc t u r e , flectance accessory. With this procedure, a small amount although homogenous telocollinite was also present, of- of amber (ca 2 % by weight) was mixed with fin e l y ten in association with framboidal pyrite. In some speci- ground KBr and loaded into the compartment of a diffu s e mens, the vitrinite showed little signs of oxidation whi l e reflectance attachment. Spectra were obtained in re- in others it was oxi d i z e d , with prominent relief and wea t h -

177 Figure 4. Relationship between vitrinite oxidation index (Vox) of amber-bearing rocks and Ox2 (C=O1600-1800/CH2,3 1450) for the am- ber studied. Line shows a linear trend between vitrinite oxidation index of the rock matrix (Vox) and oxidation index of amber (Ox2) for the majority of samples. ering cracks. The Vox index ranged from 0.25 to 17 (Tabl e = 0.5) indicating that the color darkens as these ratios in- 1) , with the least oxidized vitrinite coming from Cibao, crease (fig. 1C). Palo Quemado II, La Toca 128, La Toca 30 and Palo Al t o High and the highest from Cotui, Palo Alto Middle and FTIR analysis of the ambers showed distinct diffe r - Los Cacos. Macerals of the liptinite group represented by ences between them with regard to the distribution of often yel l o w fluorescing liptodetrinite (probably of functional groups. Figure 2 presents FTIR spectra of three and algal origin) were dispersed in the samples. amber samples occurring in close proximity to each oth- er . The spectra are dominated by signals from aliphatic With one exception, the maturity of the organic mat- groups (2800-3000 cm-1 and 1350- 1480 cm-1 ) and oxy - ter in the rock corresponded to a lignitic or sub-bitumi- genated groups (1600-1800 cm-1 ). The closer spectral nous coal rank, as inferred from the vitrinite reflectance an a l ysis shows differing intensities of CH2 and CH3 range of 0.36% to 0.49%. The exception was the sample groups between individual amber specimens. The propor- from Cotui where the vitrinite showed a reflectance rang- tion of CH2 to CH3 (reflecting the length and branching ing from 3.2% to 6%. The organic particles were often of aliphatic chains {Lin and Ritz, 1993}) is highest in El porous and ves i c u l a t e d , highly oxidized and resemble d Valle, Palo Quemado 1 and Los Cacos (Tab le 2), suggest- those found in therma l l y altered zones near volcanic in- ing that the longest chain and least branched aliphatic tru s i o n s . st r uctures occur in these three samples. This proportion is expressed by Al 1 (aliphatic stretching region) and Al 2 (bending mode region). Ratios Ox1 and Ox2 in c l u d e Amber samples aliphatic and oxygenated groups in the equations and usu- al l y reflect the oxidation level of the organic matter: CH2 The color of the amber obtained from the rock matrix, and CH3 are known to be consumed during oxidation with which varied from light yel l o w (1) to dark orange-brown the C=O groups being formed (Kister et al., 1988; Vas a l - (2.5) (Tab le 1) did not show a correlation with the vitri- lo et al., 1991; Pradier, 1992). These and similar ratios nite reflectance of the matrix rock (Fig. 1A). Nor did the ha ve been used as a measure of oxidation (Gethner, 1987; vitrinite reflectance always show a correlation with FTIR- Lynch et al., 1988; Goodarzi and McFarlaine, 1991). As - de r i ved ratios (Fig. 1B). The color index did show a sig- suming that CH2 and CH3 were equally abundant in the 2 ni f icant correlation with oxidation indices (Ox1, Ox2) (r fossilized resin, the least oxidized ambers would be from

178 La Taco 128 and La Toca 30 and the highest oxi d i z e d %, exo c yclic alkenes began to disappear. This stage of would be samples from Cotui, Palo Quemado 1, El Hijo maturity is not surprising since many of the amber de- and La Toca ground. posits (including resinites) found world-wide today occur in similar ranking coal deposits (Poi n a r , 1991a, 1992). In The concentration of exo c yclic alkenes in diterpe n o i d fact there are no reports of recogn i z a b le amber or fo r m also differentiates the various resins. Th e s e resinites associated with anthracite coals since carboniza- compounds are represented by bands at 3076, 1645 and tion associated with the increase in maturity will result in 888 cm-1 (S t r e i b l et al., 1976). Based on the intensity of resinite fluidity and migration which can occur even in bands representing these compounds, the fossilized resins some bituminous coals (Poi n a r , 1991a). In a study of studied can be divided into three groups (Tab le 3) com- resinites and amber associated with coal deposits of prising those of high intensity (A)(characterized by a ra- kn o wn age in New Zealand, it was possible to find amber tio of isocyclic alken e s / C H 2 + CH3 2800-3000 higher in sub-bituminous deposits dating from the , than 0.01), medium intensity (B) and low intensity (C). , Eocene and Cretaceous and in lignite beds The samples with intense exo c yclic alkanes were Cotui, dating from the Miocene and Oligocene (Poi n a r , 1991a; La Toca 128, Las Aquitas and La Toca 30. Those with Sh e r wo o d , 1986). medium intensity were Cibao, Palo Alto high, Palo Al t o middle, Palo Alto low, Palo Quemado II, El Hijo and La The present study also showed that in most samples a Toca ground. Those with low intensity were Los Cacos, similar level of maturity occurred in the amber and the Palo Quemado 1 and El Valle. Figure 3 shows exa m p l e s enclosing rock matrix, suggesting that the two matured of FTIR spectra of ambers representing each of these tog e t h e r . One of several exceptions to this trend was the groups, clearly demonstrating the most distinct bands at sample from Cotui which in contrast to all others, showed 3076 and 888 cm-1 in Cotui and almost undetectible in El a lower degree of maturity than that found in the matrix Valle. Other notable bands belonged to aldehydes (2725 rock. Here the vitrinite in the host rock showed increased cm -1 ) which are usually weak to absent except in La Ca- maturity (as expressed by vitrinite reflectance), while the cos, Cibao and La Toca 30. No aromatic compounds wer e amber showed ver y low maturity as indicated by intense de t e c t a bl e . exo c yclic methylene groups (Fig. 3). This could be ex- plained if the Cotui sample originated from a diffe r e n t Amber specimens of group A were the most heteroge - plant genus or species or if the Cotui material had a dif- neous with significant variations in functional groups ob- ferent biostratinomy. Differences between Cotui material se r ved. The Cotui sample was especially unique and had and amber from other areas of the Dominican Republi c the exo c yclic alkenes much more intense than in the oth- was also noted in an earlier study on the NMR spectra of ers (almost three times higher in the Cotui sample) (Fig . amber (Lambert et al., 1985). The spectra of Cotui mate- 3 and Tab le 3). The Cotui sample also revealed an intense rial obtained in the latter study differed from all others band at 1274 cm-1 that could represent aryl - O - a r yl func- enough for it to be placed in a separate catego r y. Later C14 tions (Osawa and Shih, 1971). This band did not appear in dating studies revelaed that the Cotui material is under an y other samples. Also found in the Cotui sample was a 1000 years old (Schlee, 1990; Poi n a r , unpublished re- dominant band at 1696 cm-1 (F ig. 2) which could be as- sults). However , in the age analysis of Dominican amber signed to stretching vibrations of the -COOH group with based on exo m e t h ylane resonance peaks, it was assessed the carbonyl group conjugated with C=C in the ring (Bel- with the amber samples (Lambert et al., 1985). This re- la m y, 1980). sulted in a biased estimate of the Cotui age (15-17 million years) which was based on the assumption that all of the material originated from the same Hy m e n a e a tree species. DI S C U S S I O N A recent analysis of the Cotui copal indicates that the plant source is the extant Hymenaea courbaril (M a s t a l e r z The present study indicates that the maturity of the and Poi n a r , unpublished observations) while all of the dispersed organic matter in the rock matrix containing amber samples probably originated from the ext i n c t samples of amber from the Dominican Republic corre - species Hymenaea prot e r a Poinar (1991b). This diffe r - sponds to lignitic and subbituminous coal ranks as ex- ence in plant origin may explain why an anomalous date pressed by vitrinite reflectance values ranging from 0.32 of these deposits was obtained in a previous study using % to 0.49 % (see Teichmuller and Tei c h m u l l e r , 1982). In nuclear magnetic resonance (Lambert et al., 1985). Th e his study of resinites and associated coals, Murae et al. Cotui material probably represents deltaic deposits whi c h (1995) showed that at a vitrinite reflectance of about 0.4 were incorporated into sediment typical of that region re-

179 sulting in a high maturity host rock contrasting with the amber produced from the same genus of trees. This is lo w maturity copal. Although Grimaldi (1995) stated that quite clear in comparing Dominican amber with Colom- Burleigh and Wh a l l e y (1983) and Schlee (1984) carbon- bian copal (dated at < 1000 years; Poi n a r , 1996), both dated other deposits of Dominican amber (from Baya g u a - from Hy m e n a e a trees. So there can be a general corre l a - na) Burleigh and Wh a l l e y did not include any samples tion between the maturity of fossilized resin and color, but from the Dominican Republic in their study and Schlee as this study shows, amber of the same age group can var y on l y carbon-dated Cotui deposits. in color as a result of differential .

The importance of diagenetic factors in modifying the The present inves t i g ation demonstrates that it is possi- infra-red spectra of amber and resinites has already been ble to compare the maturation of amber with that of dis- discussed by Mustoe (1985) and Murchison and Jones persed organic matter in the embedding rock matrix. It (1964), respectivel y. Changes in infrared spectra of coal should be emphasized that what is being measured here is as a result of increased oxidation has also been estab- the degree of maturation which has no bearing on the lished (Gray and Lowenhaupt, 1989). ch r o n o l o gical age of the deposits. In fact Given (1964) al- ready stated in regards his chemical study of coal macer- In the present study, aside from the Cotui sample, four als that “ the correlation between the degree of metamor- other amber samples did not show a maturation at the phism and geological age is not close”. This study also same level as the rock matrix (Fig. 4). The amber from El demonstrates the usefulness of comparison of oxi d a t i o n Hijo and Palo Quamado 1 showed a higher degree of ox- indicators for both host rock and amber. Such a compari- idation than would be expected from the analysis of or- son can provide an understanding of the pre-depositional ganic matter in the rock matrix. This suggests that the am- hi s t o r y of not only amber but also other types of organ i c bers had either been exposed to severe weathering before ma t t e r . li t h i f ication or that they had been re-cycled. The sample from El Hijo was obtained from strata in a wet clay de- While the methods employed here are fai r l y standard, posit along a river and may have undergone some exp o - ad v anced analytical techniques could provide further in- sure, but the Palo Quamado 1 sample came from rock fo r mation on this topic. Such studies are useful in under- strata that had not previ o u s l y been exposed and may have standing amber biostratinomy, especially in relation to the been recycled. In addition, amber from La Cacos and Pa- origins of their biological inclusions. lo Alto Middle show relativel y low oxidation in contrast to the high oxidation index of the organic matter in the rock matrix (Fig. 4). This suggests that either some mix- ACK N O WLEDGMENTS ing occurre d , resulting in more recent amber being mixed with carbon remains from earlier deposits or that the or- The senior author thanks the University of California Re- ganic matter from the rock matrix was strongly oxi d i z e d search Expedition Program, Berkel e y, California for fin a n c i a l before deposition. su p p o r t to visit and collect amber and associated bedrock from various mines in the Dominican Republic. Thanks are also ex- This study also shows that the color of the amber sam- tended to Ken Anderson, Patrick Hatcher and Roberta Poinar for ples can have no relation to maturity. Some colors like red, their revi e ws of earlier versions of this manuscript. clear yel l o w and blue are characteristic of certain amber mines and several reasons for this have been proposed, such as differences in hardness, age, and wea t h e r i n g RE F E R E N C E S (Po i n a r , 1992). In fact dealers in the Dominican Republi c told the senior author that in order to make yel l o w amber Beck, C.W., Wil bu r , E., Meret, S., 1964. Infra-red spectra and re d , simply place it in the oven at 500 F for 15 minutes. the origin of amber. Nature, 201, 256-257. While the present study shows that there is no corre l a t i o n Be l l a m y, L.J., 1980. The infrared spectra of complex molecules. be t w een color and most functional groups, there is a sig- In: Ad v ances in Infrared Group Frequencies. Chapman and ni f icant correlation between color and the Ox1 and Ox2 ra - Hall, 299 pp., New Yor k . tios. These ratios, especially Ox1, are considered indices Burleigh, R., Whalley, P., 1983. On the relative geological ages of oxidation or weathering (Goodarzi and McFar l a n e , of amber and copal. Journal of natural History, 17, 919- 1991) and thus strongly indicate that weathering brought 921. about the darker color. Also it is a general phenomenon Bustin, R.M., Cameron, A., Grieve, D., Kalkreuth, W., 1985. that young amber or copal is lighter in color than mature Coal Pet e r o l og y , its principles, Methods and Ap p l i c a t i o n ,

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