Iron porphyrins in the Permian Kupferschiefer of the lower Rhine Basin, N.W. Germany

Org. Geochem.Vol. 14, No. 6, pp. 659-666. 1989 Printed in Great Britain. All rights reserved

0146-6380/89$3.00+ 0.00 Copyright© 1989PergamonPress plc

Iron porphyrins in the Permian Kupferschiefer of the Lower Rhine Basin, N.W. Germany C. B. ECKARDT1'2, M. WOLF1 and J. R. MAXWELL2 ~Lehrstuhl f~ir Geologie, Geochemie und Lagerst/itten des Erd61s und der Kohle, Aachen University of Technology, Lochnerstrasse 4-20, D-5100 Aachen, F.R.G. 2Organic Geochemistry Unit, University of Bristol, School of Chemistry, Cantock's Close, Bristol BS8 ITS, U.K. (Received 12 August 1988; accepted 20 January 1989) Abstract--Immature samples of the Permian Kupferschiefer from the Lower Rhine Basin in N.W. Germany were analysed for tetrapyrrole pigment type and abundance. The sediment, thought to have been deposited in a marine regime with enhanced salinity, was found to contain high concentrations of metalloporphyrins. The porphyrins are complexed to nickel (Ni) and oxovanadium (V:=O), but high abundances of iron (Fe) porphyrins were also detected using UV/visible spectroscopy and mass spectrometry. The presence in the latter of series of aetioporphyrins, cycloalkanoporphyrins, di-cycloalkanoporphyrins and benz-cycloalkanoporphyrins was confirmed by accurate mass measurements; HPLC co-injection of deoxophylloerythroetioporphyrin (C32 DPEP) with the demetallated iron porphyrins indicated its presence in the sediment as an iron complex. The study provides the first evidence for the occurrence of Fe porphyrins in geological samples other than coals and lignites, and reports the highest concentrations in sedimentary organic matter to date. Key words--Kupferschiefer, metalloporphyrins, iron porphyrins, Fe-deoxophylloerythroetioporphyrin

INTRODUCTION Although the widespread occurrence of nickel and vanadyl porphyrins in sediments from different depositional environments has been reported, there are few reports of porphyrins complexed to other metals. Baker and Palmer (1978) and Baker and Louda (1986) have reported Cu(II) porphyrins in certain immature sediments recovered by the Deep Sea Drilling Project (DSDP), and low concentrations of Ga(III) and Mn(III) porphyrins have been found in immature coals (Bonnett and Czechowski, 1980, 1981, 1984; Bonnett et al., 1987a). Also, there are reports on the occurrence of iron porphyrins (Treibs, 1935; Dunning, 1963; Kroepelin, 1963; Heller, 1967; Rho et al., 1973), although little evidence was given. Recently, the occurrence of iron complexed to deoxophylloerythroetioporphyrin (DPEP) in a crude oil from Venezuela was reported (Franceskin et al., 1986). However, according to Filby and Van Berkel (1987) the Mfssbauer chemical shifts and the quadrupole splitting results are consistent with Fe(llI) inorganic salts and the electronic absorption spectra are similar to those of Ni porphyrins. The occurrence of Fe(III) porphyrins is perhaps best authenticated by Bonnett et aL (1983, 1987b) and Bonnett and Burke (1985), who isolated such species in very low concentrations from a number of lignites and coals, giving evidence based on accurate mass measurements. In the present study we report the wide occurrence of high concentrations of iron porphyrins (up to c.

400/~g/g TOC) in immature samples (R r c. 0.3%) of the Permian Kupferschiefer of the Lower Rhine Basin (N.W. Germany). EXPERIMENTAL

Samples The samples derive from two wells from the Lower Rhine Basin in N.W. Germany (Fig. 1). Sample 19/9 was recovered from a depth of c. 338 m (Table 1), samples 35/P1-PI0 represent the entire sedimentation range of the Kupferschiefer (c. 2 m) from a second well with an average depth of c. 700 m (Table 1). A general characterisation of the sediment has been given by Wedepohl (1964, 1971), Pfittmann and Eckardt (1989) and Eckardt (1989). Extraction and separation The finely ground (<200/~m) core samples were Soxhlet-extracted (72 h), using dichloromethane as solvent. The crude extracts were separated by flashcolumn liquid chromatography (cf. Still et al., 1978) with a constant flow of 5.6 ml/min. The stationary phase was silica gel (Kieselgel 60, 230-400mesh ASTM, Merck). Different metalloporphyrin species were separated, applying a solvent mixture of hexane/ dichloromethane (75/25, v/v) to elute the nickel porphyrins (LC fraction II), dichloromethane to elute the vanadyl porphyrins (LC fraction III) and propanol to elute the iron porphyrins (LC fraction IV). 659

f)6(/

( . I~. E(K ~RI)'I' t'!

at

as their free bases, i.e after demetallatior, v~ith methanesulfonic acid (9OC, 4h~, measuring the absorption at band lV (498 nm) and using an extinc tion coefficient of 15,600 (Chicarelli, 1985). Absorption spectra of the original iron porphyrms were recorded in a mixture of acidic propanol (1 m H('! m propanol) and toluene (I: . v/v~

El-MS Mass spectra were acquired on a Finnigan MAT 8200 mass spectrometer, linked to an INCOS data system. Liquid chromatography fractions containing the metalloporphyrins were introduced by direct insertion probe and heated from ambient to 175~C at 5cC/s, and from 175 to 400°C at 20°C/s. Source conditions were: 70 eV electron energy, 1.0 mA emission current and 225°C source temperature. Spectra were obtained by summing the scans over the entire volatilisation range of the porphyrins (c. 175--350 C). Low resolution spectra (R =- 1000) were obtained by scanning m/z 50 to 700 in 1.1 s. Accurate mass measurements were performed at 5000 resolution, scanning from m/z 200 to 700 in 3 s and calibrating against perfluorkerosene as internal standard.

High performance liquid chromatography (HPLC) Fig. 1. Geographical position of the Lower Rhine Basin.

Blank extraction and separation A core sample (50 g) was homogenised and split into two equal aliquots of 25 g each. The samples were suspended in dichloromethane, and commercially available octaethylporphyrin (Porphyrin Products Ltd, Utah, U.S.A.) added to one of them. After evaporation of the solvent the samples were extracted and separated by column chromatography as detailed above.

HPLC analysis of the demetallated iron porphyrin fraction was performed according to the method of Barwise et al, (1986), adapted, however, to a binary solvent system (Eckardt, 1989). Analyses were performed using an LDC/Milton Roy system, and a Rheodyne 7125 injector valve. Detection was carried out with an LDC/Milton Roy variable wavelength detector set at 400 nm.

Standards"

A standard of chloroferric aetioporphyrin I was synthesised according to the procedure of Buchler (1975), using aetioporphyrin I (courtesy of Professor UV /visible spectroscopy K. M. Smith, University of California, Davis) and Porphyrin concentrations were measured using a FeCI9 as starting materials. The structure was conPerkin Elmer 550 S UV/vis Spectrophotometer. firmed by E l - M S measurements. Accurate mass UV/vis spectra for nickel and vanadyl porphyrins measurements revealed a molecular weight of 567.199 were recorded in dichloromethane solution, quantita- (C~2N4H363sC156Fe requires 567.198), and a mass of tion being performed on the e-absorption maxima at 532.229 for the base peak without the ligand 550 and 570nm, respectively, using extinction (C~2N4H3656Fe requires 532.229). coefficients of 22,000 (Ni) and 17,600 (V~-----O) The sample of DPEP, used for co-injection was (Chicarelli, 1985). Iron porphyrins were quantitated obtained from Serpiano oil shale (Chicarelli, 1985). Table 1. Porphyrin concentrations in samples investigated Porphyrin Yield (/~g/g TOC) Sample

Depth (m)

Ni

V~---O

Fe

19/9 35/P1 35/P2 35/P3 35/P4 35/P5 35/P6 35/P7 35/P8 35/P9 35/P10

337.6 698.8 699.0 699.2 699.4 699.6 699.8 700,0 700.2 700.4 700.6

273 505 481 2040 1120 1452 1213 1586

I34 151 205 663 273 341 537 503 605 475 250

223 80 82 397 235 323 223 169 213 267 104

3074

2411 1411

RESULTS

The Kupferschiefer contains high concentrations of porphyrins. Table 1 shows the yields, up to c. 3900 #g/g TOC, i.e.c. 65,000 ppm of the extractable organic matter (sample 35/P8). The porphyrins are not only complexed to Ni(II) and V~------O,as commonly observed in many sediments, High concentrations of iron porphyrins were also isolated [up to c. 400 #g/g TOC, i.e.c. 5000 ppm of the extractable organic matter (sample 35/P3)]. To our knowledge,

Iron porphyrins in the Permian Kupferschiefer 375

530

.~

6~'9

J

I 3oo

I

B

7o0 nm

Fig. 2. Electronic absorption spectra of chloroferric aetioporphyrin I (A), and liquid chromatography fraction IV from sample 19/9 (B), in 1 m HCI in propanol and toluene (l: 1, v/v). these are the highest concentrations of Fe porphyrins isolated to date from geological samples. Figure 2 shows UV/visible spectra of the liquid chromatography fraction of sample 19/9, containing Fe porphyrins (trace B), and of chioroferric aetioporphyrin I (trace A). Both spectra show a distinct absorption maximum at 629nm, and a diffuse doublet at 530 and 503 nm. The relative intensities within the doublet vary between the standard and the sample, presumably as a result of the contribution of structural types other than aetio porphyrins to the spectrum of the sediment fraction (Fig. 3). Also, both traces are coincident in the Soret band region, showing an absorption maximum at 375 nm. The spectra are consistent with those reported for chioroferric mesoporphyrin IX dimethyl ester in dioxane (Erdman and Corwin, 1947). In order to avoid dimerisation of the compounds (e.g. Fleischer and Srivastava, 1969), the spectra were recorded in an acidic solvent. A mixture of 1 M HCI in propanol and toluene (1:1, v/v) was found to give satisfactory results. Also, the Soret band was sharper than that obtained in the dioxane solution used by Erdman and Corwin (1947). Since no extinction coefficient data for the solvent mixture used were obtained the Fe porphyrins were quantitated as their free bases (see "Experimental"). The low resolution mass spectrum of chloroferric aetioporphyrin I [Fig. 3(a)] shows the expected molecular ions at m/z 567 and m/z 569, consistent

661

with C32N4H3635CI56Fe and C32NaI-I3637CI56Fe. The base peak occurs at m/z 532 (C32N4H3656Fe), due to the loss of the ligand under E1 conditions. The masses observed in the liquid chromatography fraction from sample 19/9 [Fig. 3(b)] are consistent with the presence of porphyrins of different structural types (i.e. aetio, cycloalkano [CAP], di-cycloaikano, benz-aetio and benz-cycloalkano), complexed to 56Fe, extending over a range from c. C27--C34. Ions (m/z 537, 539, 551, 553, 565, 567) which might represent molecular ions of the most abundant CAPs and aetioporphyrins, with chlorine as the ligand, are present, A visually similar carbon number distribution is observed [Fig. 3(c)] after demetallation of the fraction, the molecular ions occurring at 54 daltons lower and corresponding to loss of 56Fe and addition of two hydrogens. Accurate mass measurement of the base peak in the metallated fraction [Fig. 3(b)], i.e. the ion arising from the C32 cycloalkano components, revealed a mass of 530.216 (C32N4H3456Fe requires 530.213). In order to confirm the presence of iron as the complexed metal, accurate mass measurements were also carried out for a second sample, i.e. 35/P3. The accurate masses of the most abundant ions are given in Table 2. These data for selected ions provide further evidence for the presence of Fe aetio porphyrins and Fe CAPs, as well as the presence of compounds of the di-CAP type and the benz-CAP type. Mass spectra of the three liquid chromatography fractions from sample 35/P3 (Fig. 4), containing the three different types of metailoporphyrin species, show distributions covering a carbon number range from c. C~7-C35 but differ with respect to the relative abundance of aetio (A) and cycloalkano ( 0 ) components. The cycloalkano species are most abundant in the V~------Oporphyrin fraction [Fig. 4(b)], as indicated by the CAP/aetio ratio (D/D + E], from Table 2. Accurate mass determination of selected ions in mass spectra of iron porphyrin fraction from sample 35/P3 Observed mass 462.1516 490.1776 504.1992 518.2151 532.2239 560,2651 460.1323 488.1650 502.1850 516.1969 530.2163 500.1683 514.1858 528.1981 542.2162 570.2490 468.1061 496.1381 524.1675

Calculated mass° A mmu ~ 462.1507 490.1820 504.1976 518.2133 532.2289 560.2602 460.1350 488.1663 502.1820 516.1976 530.2133 500.1663 514.1820 528.1976 542.2133 570.2446 468.1037 496.1350 524.1663

0.9 4.4 1.6 1.8 5.0 4.9 2.7 1.3 3.0 1.6 3.0 0.2 3.8 0.5 2.9 4.4 2.4 3.1 1.2

Compound c C27 aetio C29 aetio C30 aetio C3~ aetio C3: aetio C~ aetio C27 CAP C29 CAP Cr0 CAP C31 CAP C32 CAP C30 di-CAP C31 di-CAP C32 di-CAP C33 di-CAP C35 di-CAP C2s benz-CAP C3o benz-CAP C32 benz-CAP

aFor an iron.containing alkyl porphyrin. ~Difference between observed mass and calculated mass in millimass units. "Isomeric structures, with respect to alkyl substituents, not excluded,

f 100 -

(M'35CI)T

5:32

(0

Fe

\ M.+(35CI)

50,

I

567

J~ M+(37CI)

517 502

I

, J - - - i

m/z

400

550

500

....

, ....

! ....

, ....

60q

530

(b)

'°°1!

504

490

476 544

mlz

500

400 100-

476

(C)

50

450 462

+li ,

,,,,~ ~Li

,,°

+

504

mlz

400

565

500

519

600

Fig. 3. Mass spectra of chloroferric aetioporphyrin I (a) and liquid chromatography fraction IV from sample 19/9 before (b), and after (c) demetallation. 662

C30 100t (0)DIE:0.56

•

t

"

ir

oo]

m/z

•

I,

•

_J

.'l._

4~)o""Jt 1'-,--'.-:---,---4.~,~--,

......... , ......

,~

5~)o ....... ' ......... '5~o--'

(b)

I~/tL .ht. ,ILII . J ............ ;bo

C30 •

D/E: 0.64

50-

m/z 400

450

500

550

600

ooo

600

C30

1001 (C) DIE: 0.36 &

50'

mlz

400

40o

500

Fig. 4. Mass spectra of three liquid chromatography fractions from sample 35/P3, containing nickel porphyrins (a), vanadyl porphyrins (b) and iron porphyrins (c). D/E = CAP/(CAP+ aetio) ratio, calculated from molecular ion intensities of CAPs and aetioporphyrins over the range C2s-C3z. • -- aetio porphyrins, • = cycloalkano porphyrins. o.o. 14/6---F

663

C, B. ECKARD] ('l ~/.

664

molecular ion intensities over the range C 2 8 - C 3 2 ) , Higher relative abundances of the aetio species occur in the Ni porphyrin fraction [Fig. 4(a)], although the CAPs still dominate (D/[D + E] = 0.58). In the case of the Fe porphyrins [Fig. 4(c)] the aetio compounds dominate (D/[D + E] = 0.36). It is noteworthy that the Fe porphyrin spectrum of this sample does not show masses corresponding to molecular ions containing a chlorine ligand, unlike 19/9. Figure 5 shows the HPLC distribution of the demetallated Fe porphyrins in sample 19/9. It provides further evidence that Fe is not only complexed to aetio compounds. The most abundant component in this fraction co-eluted with DPEP obtained from Serpiano shale (Chicarelli, 1985). There remained the possibility that the iron complexes were formed from a reaction between porphyrin free-bases and iron during the extraction and work up. To investigate this possibility octaethylporphyrin (OEP) was added to a sample prior to extraction (see "Experimental"). Comparison of the iron porphyrin liquid chromatography fractions by EI-MS with those obtained from the blank, showed that no iron OEP had formed. These results are in agreement with the findings of Bonnett et al. (1983, 1987b), who treated OEP and iron(Ill) salts in the medium used for extraction. However, significant amounts of copper OEP were detected, which were not present in the blank, showing the possibility of a reaction between porphyrin free bases and metal salts during extraction and work up.

DISCUSSION

The Kupferschiefer (c. 230 Myr) of the Lower Rhine Basin has been described as a bituminous, calcareous siltstone of low maturity, containing on average c. 4% organic carbon (Piittmann and Eckardt, 1989: Eckardt, 1989). The geological position of the horizon at the beginning of the Zechstein evaporation sequence in northern Europe (Richter-Bernburg, 1955: Lotze, 1957; Teichmiiller, 1957), and the investigation of its biological marker properties suggest a marine depositional environment with enhanced salinity (Eckardt, 1989). Although there have been unproven reports on the occurrence of Fe porphyrins in sediments and crude oils (see above) there is good evidence for their presence in tow concentrations in lignites and coals. The finding of Fe porphyrins in samples from a marine depositional palaeoenvironment shows that their occurrence is not restricted to coals. Also, their stability under the reducing conditions of a submerged anoxic environment appears to be considerably higher than indicated by Bonnett et al. (1987b), who suppose the compounds to be susceptible to demetallation under these conditions. In one sample the presence of compounds tentatively assigned as chloroferric species was observed (cf. Bonnett et al., 1983. 1987b). At present we are unable to prove if the iron porphyrins exist in the sediment itself as the Fe(ll) or the Fe(III) species. Fe(II) porphyrins are reported to auto-oxidise readily in solution (Buchler, 1975). For example, in the presence of solutions containing acetate ion they

DPEP

,td eP

(mtn) 25

Retention Time

75

Fig. 5. HPLC chromatogram of the demetallatvd Fe porphyrin fraction of sample 19/9.

Iron porphyrins in the Permian Kupferschiefer are converted instantaneously to Fe(III) species with OAc as the ligand (Buchler, 1975). Thus, the suggested chloroferric porphyrins may have been obtained as a result of the extraction with dichloromethane, especially if the solvent contained traces of HCI. Clearly, further studies are required to investigate this possibility. From the similarity between Fe and Ga porphyrin distributions in coal samples, Bonnett et al. (1987a) suggested that the iron was not originally present in the precursor biological tetrapyrroles (i.e. was not incorporated during their biosynthesis), the Ga being of non-biological origin. The present study provides further evidence for this hypothesis, since compounds other than aetio porphyrins are found to be complexed to Fe, particularly DPEP, which is generally thought to derive from chlorophylls. Irrespective of the origin of the aetio compounds, namely from cytochromes (cf. Bonnett et al., 1983, 1987b) or chlorophylls altered by early stage oxidation effects (Barwise and Roberts, 1984), by implication the Fe in the aetio porphyrins of the Kupferschiefer is also of secondary origin. The high proportion of aetio compounds in the Fe porphyrin fractions is noteworthy. The occurrence of Cu porphyrins containing high abundances of aetio species in some DSDP sediments was suggested as indicating an origin from terrestrially derived organic matter, which had undergone oxidation before or during deposition (Baker and Palmer, 1978). Certainly, from organic petrographic analysis there is some evidence of a terrestrial input to the organic matter of the Kupferschiefer of the Lower Rhine Basin (Wolf et al., 1988); also, the relative abundance of Fe porphyrins remains fairly constant throughout the core analysed (c. 10% of total porphyrins, Table 1). Nevertheless, given the high concentrations of Fe porphyrins (up to c. 400 #g/g TOC), it seems more likely that the Fe porphyrins are indigenous, although it is difficult to explain the higher abundances of aetio compounds in these species relative to the Ni and V~----Ospecies. Given the low concentrations of Fe porphyrins reported in coals and lignites in the absence of both Ni and V~----O porphyrins, and their occurrence in a sediment from a shallow marine depositional palaeoenvironment in significant concentrations with both Ni and V = O porphyrins present, further studies of the occurrence of Fe porphyrins in samples from a variety of depositional environments are required in order to investigate the factors controlling the relative abundances of the different metallo complexes (cf. Lewan and Maynard, 1982; Lewan, 1984).

Acknowledgements--We thank the Deutsche Forschungsgemeinsehaft for financial support. Samples were kindly

provided by the Institut fiir Mineralogie und Lagerst~ittenlehre, Aachen University of Technology, and the RAG Niederrhein. We are also grateful to N. Nettekoven for providing facilities.

665 REFERENCES

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