Geologica Acta, Vol.11, Nº 3, September 2013, 359-370 DOI: 10.1344/105.000001871 Available online at www.geologica-acta.com Physico-chemical analysis of Albian (Lower Cretaceous) amber from San Just (Spain): implications for palaeoenvironmental and palaeoecological studies J. DAL CORSO 1 G. ROGHI 2 E. RAGAZZI 3 I. ANGELINI 1 A. GIARETTA 2 C. SORIANO 4 X. DELCLÒS 5 H.C. JENKYNS 6 1 Dipartimento di Geoscienze, Università degli Studi di Padova Via Gradenigo 6, 35131, Padova, Italy. Dal Corso E-mail: [email protected] Angelini E-mail: [email protected] 2 Istituto di Geoscienze e Georisorse (IGG-CNR) via Gradenigo 6, 35131, Padova, Italy. Roghi E-mail: [email protected] 3 Dipartimento di Scienze del Farmaco, Università degli Studi di Padova L.go Meneghetti 2, 35131, Padova, Italy. E-mail: [email protected] 4 European Synchrotron Radiation Facilities (ESRF) Rue Jules Horowitz, 6, 38000, Grenoble, France E-mail: [email protected] 5 Department d’Estratigrafia, Paleontologia i Geociències Marines, Facultat de Geologia, Universitat de Barcelona Martí i Franquès s/n, 08028 Barcelona, Spain. E-mail: [email protected] 6 Department of Earth Sciences, University of Oxford South Parks Road, Oxford OX1 3AN, UK. E-mail: [email protected] ABSTRACT Amber from a Lower Cretaceous outcrop at San Just, located in the Eastern Iberian Peninsula (Escucha Formation, Maestrat Basin), was investigated to evaluate its physico-chemical properties. Thermogravimetric (TG) and Differential Thermogravimetric (DTG) analyses, infra-red spectroscopy, elemental and C-isotope analyses were performed. Physico-chemical differences between the internal light nuclei and the peripheral darker portions of San Just amber can be attributed to processes of diagenetic alteration that preferentially took place in the external amber border colonized by microorganisms (fungi or bacteria) when the resin was still liquid or slightly polymerized. δ13C values of different pieces of the same sample, from the nucleus to the external part, are amber remarkably homogeneous, as are δ13C values of the darker peripheral portions and lighter inner parts of amber the same samples. Hence, neither invasive microorganisms, nor diagenetic alteration, changed the bulk isotopic composition of the amber. δ13C values of different amber samples range from -21.1‰ to -24‰, as expected for C 3 plant-derived material. C-isotope analysis, coupled to palaeobotanical, TG and DTG data and infra-red spectra, suggests that San Just amber was exuded by only one conifer species, belonging to either the Cheirolepidiaceae or Aracauriaceae, coniferous families probably living under stable palaeoenvironmental and palaeoecological conditions. KEYWORDS Amber. C-isotopes. Thermogravimetric analyses. Infra-red spectroscopy. Cretaceous. 359 J. DAL CORSO et al. Physico-chemical analysis of San Just amber InTRODuCTIOn Amber from an outcrop at San Just (Teruel Province) Barcelona Zaragoza Utrillas in the Iberian Peninsula (Fig. 1) represents a fossilized natural resin that originated in an Early Cretaceous forest Madrid Escucha (Peñalver and Delclòs, 2010). San Just amber is one of the Lisboa Valencia richest known Early Cretaceous deposits for its biological inclusions, yielding fungal hyphae, trichomes of ferns or conifers, pollen grains, and many different arthropods such Valdeconejos 1 Km as arachnids and hexapods (Peñalver et al., 2007; Peñalver FIGURE 1 Map of the study area. Right panel: location of the study and Delclòs, 2010). The horizons yielding amber derive area. San Just amber deposit is represented by an X. from the Escucha Formation, that has been interpreted as deposited in shallow-marine to freshwater environments in the Maestrat Basin (Cervera et al., 1976; Querol and San Just amber typically shows a peripheral portion Salas, 1988; Querol et al., 1992; Peyrot et al., 2007a,b; darker than the nucleus and opaque (Peñalver et al., Delclòs et al., 2007; Peñalver et al., 2007). The Escucha 2007). Peñalver et al. (2007) observed filamentous Formation is composed of three members: a lower, a structures by optical and scanning electron microscope. middle and an upper member (namely, Regachuelo, These structures are abundant in the more external part of Barriada and La Orden Members). San Just Amber the peripheral portion and scarcer towards the boundary has been found in the organic-rich levels in the interval with the inner and lighter part of the amber, and were first extending from the top of the Regachuelo Member to the interpreted as alteration/degradation structures (Peñalver base of the La Orden Member, corresponding to freshwater et al., 2007). However, similar structures are present in swamp facies (Querol et al., 1992; Peñalver et al., 2007; the outer part of many other Cretaceous amber that have Villanueva-Amadoz et al., 2010) and recently dated to been described as colonies of fossil fungi (Ascaso et al., the middle–early late Albian based upon angiosperm 2005) or bacteria (Breton and Tostain, 2005; Schmidt and pollen (Villanueva-Amadoz et al., 2010). The amber is Schäfer, 2005; Breton, 2007). Nevertheless, diagenetic associated with abundant conifer remains belonging to alteration and the effects of microbiological intervention the Cheirolepidiacean conifer Frenelopsis (Peñalver et al., can coexist. 2007; Alcalá et al., 2009). Although many previous studies have suggested that the association Frenelopsis–amber To investigate this issue further we decided to further characterises brackish and estuarine environments (Gomez explore some physico-chemical properties (thermal et al., 2008; Néraudeau et al., 2003, 2005, 2009; Peyrot et behaviour, infra-red spectra, elemental composition al., 2005a), in the Cretaceous of Spain Frenelopsis is also and C-isotope signature) of San Just amber, in order found in freshwater environments (Gomez et al., 2001; to understand the changes from the darker peripheral Gomez et al., 2002; Villalba-Breva et al., 2012). San Just portion of the amber, and whether degradation or amber is also associated with fossils plant macro-remains microbiological inclusions produced the peripheral and sporomorphs of ferns, Ginkgoales, Bennettitales, zones. In addition, the C-isotope signature of San Just Caytoniales, Cycadales, Coniferales and angiosperms, amber was also studied to determine the ecology and the which together suggest warm and humid environmental environmental conditions of the resin-producing plants. conditions (Gomez et al., 1999, 2000; Gomez 2002; It has been pointed out that the δ13C signature of amber Díez et al., 2005; Sender et al., 2005; Peyrot et al., could be a useful tool for palaeobotanical, palaeoclimatic 2005b, 2007a,b; Villanueva-Amadoz et al., 2010). An and palaeoenvironmental purposes (Nissenbaum and Araucariacean palaeobotanical affinity has been suggested Yakir, 1995; Murray et al., 1998; Nissenbaum et al., for San Just amber by Peñalver et al. (2007) by comparison 2005; McKellar et al., 2008, 2011; Dal Corso et al., with other Spanish amber localities (Alonso et al., 2000; 2011). The C-isotope signature of amber is the result Chaler and Grimalt, 2005). However, the presence of of a complex series of carbon-fractionation processes abundant Cheirolepidiacean plant remains in the San Just involving CO taken up by plants from the atmosphere. 2 deposits and comparison with other Lower Cretaceous During photosynthesis the following factors influence the amber-bearing outcrops from western Spain (Menor- C-isotope fractionation of C plants (the only metabolic 3 Salvan et al., 2010; Najarro et al., 2010) could also suggest group assumed to be present during the Cretaceous): a Cheirolepidiacean paleobotanical affinity. Existing data i) The carbon-isotope composition of the atmosphere show that San Just amber can clearly be attributed to (δ13C ); ii) Diffusion of CO from the air into the atm 2 conifers, but the attribution to a specific conifer family, leaves (-4.4‰); iii) Biochemical fractionation during most likely Cheirolepidiaceae or Araucariaceae, is not carboxylation (-27 to -29‰); and iv) Ecological factors, possible yet. such as water stress (+3 to +6‰), nutrient shortage (-4‰), Geologica Acta, 11(3), 359-370 (2013) 360 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber light limitation (-5 to -6‰) and low temperature (-3‰) signal) and derived DTG data from the recorded TG signal. (Arens et al., 2000; Dal Corso et al., 2011 and references The Main Thermal Point (MATHEP) from the DTG data therein). Carbon isotopes are further fractionated during corresponding to the maximal rate of weight loss was the biosynthesis of specific plant compounds: with used as a comparison parameter, as previously described regard to amber, diterpenoids of probable resin origin (Ragazzi et al., 2003, 2009). have higher values compared with associated coals (+1 to +2‰) and with triterpenoids derived from plant Elemental analysis epicuticular waxes (+2 to +4‰) (Schoell et al., 1994). Moreover, insect infestation has been recently linked to A CE-Instruments EA 1110 Automatic Elemental an observed 1‰ enrichment of modern and fossil resin Analyser, equipped with AS 200 autosampler and Mettler δ13C in a similar way to the 13C enrichment in resin of Toledo AT21 Comparator, was used. The instrument plants living under water-stress conditions (McKellar et consists of a simultaneous carbon–hydrogen–nitrogen al., 2011). Thus, the C-isotope signatures of C plants and sulphur analyser based on reliable dynamic flash 3 vary widely (from approximately -20‰ to -35‰; e.g. combustion and GC separation (with helium as carrier Cerling and Harris, 1999), depending on a combination gas), followed by thermal conductivity detectors (TCD). of physical, biochemical and ecological factors. Large A workstation, which permits complete automation from δ13C variability has been also observed in amber (e.g., weight entry to storage of results, consists of Eager 200 McKellar et al., 2008; Dal Corso et al., 2011). Although Software installed on a compatible XT/AT microcomputer the high δ13C variability of amber could make the building with a graphic printer. The Eager 200 Software offers the of reliable δ13C curves for chemostratigraphic studies possibility of using K factors and linear regression for amber difficult, this geochemical variability itself can offer calculation. Weight entry via the RS 232CC link is fully information about the ecology of resin-producing plants automated. The powdered sample (about 2mg) was placed and the environmental conditions under which they grew. into a tin capsule and loaded into the AS 200 autosampler. The calibration standards for carbon–hydrogen–nitrogen and sulphur were prepared from known amounts of MATERIALS AnD METHODS sulfanilamide (CH N O S). 6 8 2 2 Amber analysed in this study derives from an outcrop Infra-red spectroscopic analyses named San Just (acronym SJU), located near Escucha village (Fig. 1) (Peñalver and Delclòs, 2010). Absolute Small amber pieces were cut from selected amber ages are given according to the International Stratigraphic samples after a careful optical microscopy study. The aim Chart (International Commission on Stratigraphy 2012, of this preliminary work was to obtain pieces from each available at: http://www.stratigraphy.org/) and Gradstein et amber sample representative of the areas showing different al. (2012). physical characteristics (i.e. colour, opacity and presence of inclusions). The pieces were analysed with Diffuse- Thermogravimetric (TG) and differential thermogravi- Reflectance Infra-Red Fourier Transform (DRIFT) using metric (DTG) analyses a Nicolet NEXUS 760 FTIR instrument, equipped with a Collector II accessory. Infra-red spectra were recorded in TG and DTG profiles were obtained for each sample the spectral range 4000–400cm-1, with a resolution of 4cm-1, using a prototypal C.N.R. instrument (I.G.G. C.N.R., using 64 scans accumulation. The data were processed with Padova, Italy) named “Le Chatelier”. A type S (Pt-10% the Nicolet Instrument Corporation program: OMNIC, Rh/Pt) thermocouple placed inside an electric furnace version 5.1. The DRIFT analysis was performed on very provided sample and furnace measurements. Amber pieces small amounts of amber (0.2mg), using a micro-cup. The were taken from both the darker peripheral portion and amber pieces were mixed with 20mg of dry KBr, ground in the lighter internal part of fossil resins. These pieces were an agate mortar, and the mixture was placed directly into pulverized in an agate mortar before measurement (mass the sample holder. 500mg, particle size <75µm) and inserted into a platinum crucible, placed on a quartz glass support interfaced with Carbon-isotope analyses a Mettler Toledo AB 104 balance. The heating rate was 10°C/min in air, starting from room temperature (20°C) Separated pieces of San Just amber samples were and reaching a maximum temperature of 700°C. The ash crushed into a fine powder and treated with 3M HCl in residue was 3.45±1.66 wt% (mean±1SD), and the range order to remove possible microscopic residual carbonates was 0.74% to 5.4%. A computer, equipped with custom- deriving from the amber-bearing sediment. A fixed mass made software written in Lab View 5.1 language recorded of this powder (1.5mg) was placed in a tin capsule. δ13C sequential temperature and weight for each sample (TG analyses were performed at the Research Laboratory Geologica Acta, 11(3), 359-370 (2013) 361 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber for Archaeology and the History of Art (University of The thermal behaviour of the fossil resin may depend Oxford) with a Carlo Erba NA 1108 elemental analyser on different factors such as the age of the sample, degree coupled to a SERCON Geo 20/20 IRMS running in of polymerization, chemical composition, diagenetic continuous flow mode with a He carrier gas (flow rate alteration and palaeobotanical origin. Thus, it is not 100ml/min). The instrumental precision was determined possible to obtain a unique model for estimating the as the standard deviation of an alanine in-house standard age of the sample on the basis of the DTG peak data. As (δ13C =-26.94±0.14‰, n=43) routinely checked against already underlined in previous investigations (Ragazzi et alanine international standards IAEA-CH-6 and IAEA-CH-7 and al., 2003, 2009), the thermal method should be seen as a traceable back to the VPDB standard. complementary technique to be used together with other methods of age determination. It is best used as an overall Statistical analysis method for the estimation of resin maturation, including any process that might be involved in taphonomic Data are expressed as mean ±SD. For correlation modification. analysis of thermal data, we measured the Pearson correlation coefficient R in order to find a measure of We did not use the coefficient of determination R2, dependence between the geological age and the value of which represents the proportion of variance explained the Main Thermal Point (MATHEP). It is well known that by the model (R2 can also be taken as a measure of the Pearson correlation coefficient R indicates the strength how good a predictor might be if constructed from the of a linear relationship between two variables; it should be modelled values. Since several variables are involved, taken into account that the value of R cannot completely including physico-chemical and taphonomical ones, it is characterize the relationship between the two considered not possible to expect a direct prediction of the age just variables. by means of this model). A B FIGURE 2 Filamentous structures in San Just amber (SJU 8 sample). A) Peripheral darker portion, B) border between the peripheral darker portion and the inner lighter portion. White bar = 50μm. Geologica Acta, 11(3), 359-370 (2013) 362 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber RESuLTS AnD DISCuSSIOn 0 100 200 300 400 500 600 700 Pieces of San Just amber samples were initially checked 0 under the optical microscope to observe the differences % -20 s bFeiltawmeeennt othues dsatrrkuecrt upreersi phareer alv aenryd tahbeu lnigdhatnetr iinnn etrh ep odrtaiorknesr. ght los --6400 TG peripheral part and become less abundant approaching the wei -80 border with the inner lighter nucleus (Fig. 2A, B), as already -100 described by Peñalver et al. (2007). These structures are 0 very similar to the filaments described by other authors as microbiological inclusions (Ascaso et al., 2005; Breton and -2 Tostain, 2005; Schmidt and Schäfer, 2005; Breton 2007), and in particular to the fungal hyphae found in the Lower n) -4 Cretaceous Álava amber by Ascaso and colleagues (Ascaso mi %/ -6 e(2t 0a0l7., 2an0d0 3r,e 2fe0r0e5n)c.e As st hoebreseinrv) eidn bmyo Sdcehrnm eidxta manpdl eDs itlhcheseer ative ( -8 DTG fungal hyphae or bacteria can grow in liquid resin. However, eriv it has been postulated that filamentous microorganisms can D -10 SJU 6.1 also grow on the surface when resin is already solidified, SJU 6.2 producing a thin dark peripheral crust (Néraudeau et al., -12 SJU 6.3 2011). The fact that we observe a pervasive colonization SJU 6.4 in San Just amber (Fig. 2B) suggests that fungi or bacteria -14 invaded the soft resin while it was sitting in water or on 0 100 200 300 400 500 600 700 wet soil. Temperature °C FIGURE 3 Thermogravimetric (TG) and Differential Thermogravimetric Thermogravimetric (TG) and differential thermogravi- (DTG) profiles of specimens from SJU 6 amber samples. metric analyses (DTG) Characteristics of the specimens are shown in Table I. Data obtained from thermal analyses of the samples to any major component, although an apparent maximum of Albian amber found in the San Just area are shown in is detectable in the range 314–352°C and 551–575°C (see Table I (Electronic Appendix available at www.geologica- secondary peaks in Table I). acta.com), Figure 3 and Figure 4. The TG and DTG curves show unambiguous patterns with an overall three-phase Elemental analysis profile (Figs. 3, 4). Two samples were investigated; one (SJU6) presented a minimal alteration surface, whereas The elemental composition of San Just amber samples the other (SJU8) showed the common two-coloured is presented in Table II. The carbon content is around 78–79 appearance, with a darker outer part and a lighter nucleus. wt%, hydrogen content ranges around 11–12 wt%, oxygen The MATHEP values of the analysed pieces obtained from and trace elements are about 9%, sulphur between 0.45– the sample SJU6 ranged from 412 to 422°C (see Table I), 0.65 wt%, and nitrogen is present in traces (0.03 wt%). and represented the loss of more than 70% of the sample. The remaining mass of the sample was lost above 450°C, Infra-red spectroscopic analysis showing some minor peaks (secondary peaks) between 351 and 616°C. Eleven amber samples from San Just were analysed by DRIFT. Amber pieces from the inner and the peripheral The external characteristics of sample SJU8 are shown portions of studied samples were analysed. The DRIFT in Figure 4A. TG profiles of the peripheral portion (P) and spectra of the inner light amber portions show similar inner part (I) of San Just amber (sample SJU8) are presented features for all samples (see the typical spectra in Fig. 5, in Figure 4B. The TG profile of the inner amber portion is black line). The main peaks are due to the C–H bonds, as similar to the profile of sample SJU6 presented in Figure expected for a fossil resin (Langehneim and Beck, 1965). In 3, showing the same pattern with three main DTG peaks particular, the more intense bands are at: i) 2929±3cm−1 and (Fig. 4, SJU8.2G and SJU8.4G). On the other hand, the TG 2947±3cm−1 (shoulder), due to the asymmetric stretching profile of the darker peripheral portion (Fig. 4, SJU8.1A of the CH and CH bonds; ii) 2868±1cm-1 and 2852±2cm-1 2 3 and SJU8.3A) presents an almost single-phase pattern, (shoulder), due to the symmetric stretching of the CH and 2 from 250 to 650°C, and a DTG profile characterized by CH bonds; and iii) 1456±1cm−1 and 1376±1cm-1, due to the 3 the presence of several peaks, which are difficult to assign bending of the CH and CH bonds. 3 2 Geologica Acta, 11(3), 359-370 (2013) 363 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber SJU 8.1A SJU 8.4G SJU 8.2G SJU 8.3A FIGURE 4 A) SJU 8 amber specimens: position of sub-samples analysed for δ13C and thermogravimetry. B) comparison between TG and DTG profiles of the lighter inner portions (left graph) and the darker peripheral ones (right graph) of the four specimens. A strong absorption band is present at about 1700cm-1 is present in the DRIFT spectra at about 3450cm-1 and is and is assigned to carbonyl stretching. The intensity of this related to the OH stretching of alcohols and carboxylic peak is higher or comparable to the intensity of the C–H acids. The possibility that the absorption at about 3450cm-1 bending peaks. Moreover, this peak is composed of several is influenced by the presence of water in the KBr may be poorly resolved bands: it shows the main absorption at ruled out because of the use of dried material, the record of 1720±3cm-1 and a shoulder (or rarely, a second separable a reference background before collection of each spectrum, peak) at 1701±3cm-1. In a few cases, the absorption at and the relatively short time needed for a DRIFT analysis 1701±3cm-1 shows similar or slightly higher intensity when compared with the classic preparation and analysis than the peak at 1720±3cm-1. A rather intense broad band with KBr pellets. Geologica Acta, 11(3), 359-370 (2013) 364 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber In the range between 1300-900cm-1, three intense The carbonyl absorption band in the DRIFT spectra peaks (with several shoulders) and two weak peaks of San Just amber is complex and not easy to resolve. have been systematically observed, respectively Consequently, in order to enhance the resolution, at 1227±1cm-1, 1160±3cm-1, 1032±1cm-1 (intense), deconvolution and second derivative processes 1095±1cm-1 and 975±1cm-1 (weak). This spectral zone is have been applied to the spectra (Fig. 6A, B). After difficult to interpret, since the absorption bands could be deconvolution, all but two of the spectra show only due to numerous different vibrations, namely: the C–O two main peaks: at 1724±3cm-1 (1726±5cm-1 using the stretching of esters, alcohols and carboxylic acids, and second derivative) and at 1698±4cm-1. These bands are the C–H out-of-plane vibrations of several structures. due, respectively, to ketone and carboxylic acid groups. Comparing the DRIFT spectra obtained with previous work Only in two spectra is a shoulder of the ketone band (Beck, 1970; Derrick et al., 1999; Guiliano et al., 2007) discernible at about 1735–1745cm-1; this absorption is the bands at 1227±1cm-1, 1160±3cm-1, 1032±1cm-1 could due to the presence of esters. be tentatively assigned, respectively, to C–O stretching of carboxylic acids (Guiliano et al., 2006); C–O stretching The main characteristics of the San Just amber spectra of aliphatic esters; and symmetric bending of the C–H are the high intensity of the peak at 1032±1cm-1 and the cyclohexane (Beck, 1970) or C–O stretching (Guiliano et good resolution of the peak at 975±1cm-1 (generally only al., 2006). A few very weak peaks are also present in the a shoulder in the FTIR spectra on numerous other amber range of 850–700cm-1. The dark peripheral portions show varieties). Other characteristic but weak and unassigned DRIFT spectra similar to those obtained from the inner peaks are present at 888±1cm-1 (commonly only a small lighter part (Fig. 5), the main differences being related shoulder), 856±1cm-1 and 814±1cm-1. The band at about to changes in the intensity of the bands at about 1700cm- 888cm-1 in the infra-red spectra of fossil resin is usually 1 (in several samples the peak shows a unique absorption attributable to out-of-plane C–H bending in the exocyclic at 1713±4cm-1); 1032±1cm-1 (more intense in the dark methylene groups. These groups also give bands at peripheral portion) and 975±1cm-1 (weaker in the dark 3050–309cm-1 and 1640cm-1 (Beck, 1970; Derrick et al., peripheral portion). Only two samples illustrate changes 1999; Guiliano et al., 2007). Even though a very small in the range of 1250–1150cm-1; they do not display strong absorption is present at 888cm-1 in the spectra of San Just peaks but only broad absorption. ambers, the other associated peaks are not apparent, so 100 90 e c 80 n a mitt s n a Tr % 70 νO-H νC=O Ketone, acid and esters Main absorptions: 60 νC-O of carboxylic acid C-H νsym of -CH2 and -CH3 C-H ∂sym and ∂sym νC-O of aliphatic esters C-H νasym of -CH2 and -CH3 of -CH2 and -CH3 C-H ∂sym cycloexane 50 3000 2000 1000 Wavenumber (cm-1) FIGURE 5 DRIFT spectra (transmittance mode) of two fossil resin specimens from the SJU8 amber sample: a specimen from the darker peripheral portion (grey line) and a specimen from the lighter inner part of the sample (black line). The assignments of the main absorption peaks are reported. (The black spectrum was translated along the y-axis in order to display both spectra properly). Geologica Acta, 11(3), 359-370 (2013) 365 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber presents MATHEP values that agree with the previously 0.24 A reported age vs MATHEP correlation (Ragazzi et al., 1721 1702 2003, 2009) and confirm that MATHEP values seem to e increase with increasing age (R=0.68, Fig. 7): this pattern c 0.16 n a may be due to the palaeobotanical origin of the resin, b or but it has also been explained by differences in chemical s Ab 0.08 groups and bonds occurring with age, probably depending on the maturation process of resin (Ragazzi et al., 2003). According to this model, MATHEP values observed 0 for San Just amber would represent a higher degree of B maturation than younger ambers (Fig. 7), in agreement 1776 1605 with the results of infra-red spectral analysis (see infra- s unit 1726 1696 red spectroscopic analysis paragraph). By contrast, TG ary analyses of the darker peripheral amber portions yield bitr results that are more difficult to interpret. The TG profile Ar of the peripheral portion of San Just amber sample (SJU 8) shows an almost linear weight loss between 300 to 600°C, that led to a less-defined DTG profile than those 1900 1800 1700 1600 of the nuclei (Fig. 4). SJU 6 amber sample probably Wavenumber (cm-1) experienced a minor degree of alteration, since TG and DTG profiles of the darker peripheral portion are very FIGURE 6 DRIFT spectra (absorbance mode) of sample SJU6 in the similar to the TG and DTG profiles of the inner lighter range of 1900–1550 cm-1. A) DRIFT spectrum after deconvolution; part (Fig. 3). B) DRIFT spectrum (grey line) and second derivative (black line). Infra-red analyses of external weathered crusts and we may exclude the presence of terminal olefin. Previous internal parts of Upper Albian and Lower Cenomanian Raman studies have proved that the amount of exocyclic French amber samples were carried out by Néraudeau methylene groups decrease with increasing maturation of et al. (2011). Although relative spectra and absorption the fossil resin (Edwards and Farwell, 1996; Winkler et al., peaks are not directly discussed, the authors did not find 2001; Brody et al., 2001). The amber from San Just, as any significant variation in the FTIR spectra. Infra-red suggested in a previous work (Peñalver et al., 2007), has a spectral analyses of the peripheral and internal portions high degree of maturation. of San Just amber samples (this study and Peñalver et al., 2007) suggest that there are few differences in terms of Carbon-isotope composition Several pieces obtained from seven amber samples 250 y = -803.03 + 2.1587x R= 0.67719 from the San Just outcrop were analysed for the δ13C. These amber pieces show δ13C values ranging from -21.1‰ to 200 -24‰ (Table III), with a mean ±1SD of -22.5±1.01‰, falling within the range expected for C plant-derived 3 material. C-isotope values of different amber pieces within the same sample are very similar, as shown for SJU5, Ma) 150 SJU6 and SJU8 samples (Table III). Moreover, analyses E ( G performed on the darker peripheral portion and on the inner A 100 lighter portion of single samples (SJU6 and SJU8) did not show any significant differences in the C-isotope signature. 50 Physico-chemical differences between peripheral portions and nuclei of San Just amber 0 Lighter inner portions of amber from San Just show 350 370 390 410 430 450 similar TG profiles and MATHEP values, ranging from MATHEP (°C) 412 to 429°C. Previous studies have shown that the FIGURE 7 MATHEP vs AGE plot. Red squares: San Just amber, black MATHEP values of amber are correlated with their age and squares: data from Ragazzi et al. (2003 and 2009) obtained from maturation (Ragazzi et al., 2003, 2009). San Just amber various samples of amber from different geological contexts. Geologica Acta, 11(3), 359-370 (2013) 366 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber the compounds present. DRIFT spectra of the peripheral within a closed system prevent any significant C-isotope and inner samples are very similar, suggesting a small fractionation. If externally grown fungi had been degree of chemical transformation, differing mainly in the incorporated in the fluid resin, a difference in C-isotope intensity of bands due to C–O and C=O bonds. This result composition of amber in the fungus-containing areas might is consistent with oxidation and degradation processes that have been observed. may act in the outer part of the amber. As a result of these processes, the amounts and the types of functional groups It is also interesting to note that the sulphur content in containing double bonds and O may change. Thus, the San Just amber is approximately 0.5wt% (Table II). This present observation of shoulder loss and flat TG profile in value is much lower than the sulfur content found in coals the darker peripheral portion of the amber (Fig. 4) can be coming from the Alliaga-Utrillas sub-basin (6.15wt%: here interpreted as the effect of oxidation and/or diagenetic Querol et al., 1992), actually the same depocentre that transformation, with possible rearrangement of chemical yielded the San Just amber. It seems likely that the amber bonds originally present among the main components. was not affected by any post-depositional invasion of sulphur, unlike other organic substrates such as wood. As previously stated, the difference in colour between This observation supports the hypothesis that the amber the darker peripheral part and the inner lighter part of many structure remained closed during its geological history San Just amber samples, and the presence of filamentous and little or no transfer of elements from the surrounding structures, have been interpreted as evidence for some sediment has occurred. kind of alteration/degradation of the original fossil resin or related to the presence of poorly defined biological Physico-chemical variability of San Just amber inclusions (Peñalver et al., 2007). Our observations support the presence of microbiological inclusions, most It is known that fossil resin from the same deposit likely fungal hyphae (Fig. 2). However, our physico- can be compositionally different: e.g. Lower Cretaceous chemical data equally indicate that alteration has occurred. ambers from Álava show different infra-red spectra Of course, the two hypotheses are not mutually exclusive. and high variability in chemical composition (Alonso The most likely model to explain the alteration in the et al., 2000; Chaler and Grimalt, 2005). However, all peripheral part is: i) soft resin was originally colonized by analysed San Just amber specimens exhibit similar microorganisms (fungi or bacteria) growing into the resin and peculiar physico-chemical features. TG and DTG mass; ii) upon solidification of the resin, the microorganisms profiles of SJU-6 and SJU-8 samples, excluding data stopped growing; and iii) amber was subsequently from the altered portions, have a very similar shape and buried by sediment, whereupon the portion containing MATHEP (Fig. 3; Fig. 4). These two samples also show the microorganisms was preferentially altered because similar infra-red spectra: both the absorption frequencies the biological structures present facilitated diagenetic and the relative intensity of the bands are comparable in processes such as oxidation (due to several factors such all the samples. as UV rays, salt, etc.). Indeed, fine tubules made by the activity of microorganisms would have increased the resin San Just amber δ13C values vary over a range of 2.9‰ area exposed to weathering agents and would have speeded (from -21.1‰ to -24.0‰). When compared with recent up oxidation. Supporting this assertion is the fact that the gymnosperm resin, this variability is similar to that obtained altered peripheral zones of the amber correspond exactly by Stern et al. (2008) for a comparable number of resin to the portion containing the microbiological filamentous samples produced by Pinus sylvestris trees growing in the structures. same English localities and sampled at the same altitudes. The range of modern resin δ13C values becomes larger if The C-isotope composition of separated peripheral and all the sampled localities are considered together, and/or if inner pieces belonging to the same amber sample is very different Pinus species are examined (Stern et al., 2008). similar (Table III). It appears that the transformation of the Such a pattern could be reasonably linked to the effect main components in San Just amber, as suggested by TG that local environmental conditions have on plant carbon- profiles and DRIFT analyses, did not affect its C-isotope isotope signatures. composition (Table III). Hence, during the alteration processes, the system must have remained closed with A number of ecological factors can influence 13C/12C respect to carbon isotopes. Moreover, the microbiological fractionation during C photosynthesis. If these factors 3 inclusions did not change the bulk C-isotope composition (e.g. water availability, temperature, salinity) are different of the amber. Fungal hyphae and bacteria incorporated in (or change through time), the δ13C of plants that assimilate modern resin use the compounds present as a substrate atmospheric CO with the same C-isotope composition 2 to grow (Schmidt and Dilcher, 2007 and references will be different. Since amber has a strong potential to therein). Consequently, the biological processes acting retain its pristine C-isotope signature, it is possible to Geologica Acta, 11(3), 359-370 (2013) 367 DOI: 10.1344/105.000001871 J. DAL CORSO et al. Physico-chemical analysis of San Just amber make comparisons with the behaviour of modern resin iv) All TG and DTG profiles and infra-red spectra of C-isotope values (this study, Dal Corso et al., 2011, samples SJU-6 and SJU-8 have similar characteristics, McKellar et al., 2011). McKellar et al. (2008) tried to suggesting that San Just amber is chemically and physically explain the highly variable δ13C of Cretaceous Canadian homogeneous within the deposit. Moreover, a comparison amber. They reported a range of 4.8‰ for 13 amber with δ13C variability data of modern gymnosperm resin, samples from the Campanian Cedar Lake deposit, and coupled with palaeobotanical information, suggests that 6.2‰ for 28 amber samples from the coeval Campanian the San Just amber was produced by a single conifer Grassy Lake deposit, and linked such a wide variability species living under stable environmental conditions. to variable forest conditions and stressed plants. The comparison of San Just amber C-isotope variability with modern resin C-isotope ranges suggests that San Just ACKnOWLEDGMEnTS amber was exuded by plants, possibly a conifer species (see introduction for a discussion on San Just amber The authors would like to thank Peter Ditchfield for isotope botanical affinity), growing under stable environmental analyses at the Research Laboratory for Archaeology and the conditions. As previously noted, San Just amber is History of Art (University, of Oxford, UK), Alessandro Pavese concentrated in a sedimentary facies of the Escucha (University of Milan, Italy) for supporting the use of the IR Fm. that has been interpreted as formed in a freshwater instrument and Sandra Boesso (Università degli Studi di Padova, swampy plain where the conifer forest probably grew Italy) for TG analyses. Thanks go to Nereo Preto (Università under stable ecological/climatic conditions and the resin degli Studi di Padova, Italy) for useful suggestions and a pre- experienced limited or no transport. revision of the manuscript. We thank Didier Néraudeau and an anonymous reviewer for their comments that greatly improved the manuscript. This study has been partially funded by the COnCLuSIOnS CGL2011–23948/BTE “The Cretaceous amber from Spain: a pluridisciplinary study II” of the Spanish Ministerio de Economía The integrated data presented here add useful y Competitividad. information to our understanding of fossil resin of the San Just deposit. REFEREnCES i) TG, DTG analyses and infra-red spectra show that differences between the darker periphery and inner part of Alcalá, L., Cobos, A., Delclòs, X., Luque, L., Mampel, L., San Just amber are due to diagenetic alteration/degradation Royo-Torres, R., after Soriano, C. 2009. Mesozoic terrestrial concentrated in those zones where microbiological ecosystems in Teruel. Fundamental, 14, 93-130. inclusions are present. Diagenetic alteration took place Alonso, J., Arillo, A., Barrón, E., Corral, J.C., Grimalt, J., primarily in the external parts of the amber lumps that had López, J.F., López, R., Martínez-Delclòs, X., Ortuño, V., been colonized by microorganisms when the resin was still Peñalver, E., Trincão, P.R., 2000. A new fossil resin with liquid. The presence of microbiological inclusions probably biological inclusions in Lower Cretaceous deposits from made the peripheral amber portion more susceptible to Álava (Northern Spain, Basque-Cantabrian Basin). Journal diagenetic alteration. of Paleontology, 74(1), 158-178. Arens, N.C., Jahren, A.H., Amundson, R., 2000. Can C3 ii) Amber could retain its pristine C-isotope composition, plants faithfully record the carbon isotopic composition even if it had experienced some diagenetic alteration of atmospheric carbon dioxide? Paleobiology, 26(1), during burial in sediments. Moreover, fungal hyphae or 137-164. bacteria, observed in the peripheral darker portion of some Ascaso., C., Wierzchos, J., Corral, J.C., López, R., Alonso, J., San Just amber samples, did not change the bulk δ13C 2003. New applications of light and electron microscopic amber signature, supporting the hypothesis that they used techniques for study of microbiological inclusions in amber. enclosing compounds to grow into the soft resin. Thus, Journal of Palaeontology, 77(6), 1182-1192. amber can be suggested as a tool for palaeoenvironmental, Ascaso, C., Wierzchos, J., Speranza, M., Gutiérrez, J.C., Martín- palaeoecological and palaeoeclimatic reconstructions González, A., De Los Ríos, A., Alonso, J., 2005. Fossil protist based on its δ13C signature, as it seems to represent a and fungi in amber and rock substrates. Micropaleontology, closed system with respect to C-isotopes as soon as it 51(1), 59-72. polymerizes. Beck, C. W., 1970. Amber in Archaeology. Archaeology, 23, 7-11. Breton, G., 2007. La bioaccumulation de microorganismes dans iii) MATHEP data obtained from the light inner nuclei l’ambre , analyse comparée d’un ambre cénomanien et d’un of San Just amber fit with the MATHEP vs age correlation ambre sparnacien, et de leurs tapis algaires et bactériens. of Ragazzi et al., 2008, 2009. Comptes Rendus Palevol, 6(1-2), 125-133. Geologica Acta, 11(3), 359-370 (2013) 368 DOI: 10.1344/105.000001871