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On the origin of sedimentary aliphatic macromolecules: A comment on recent publications by Gupta et al.
Organic Geochemistry Organic Geochemistry 38 (2007) 1585–1587 www.elsevier.com/locate/orggeochem
Discussion
On the origin of sedimentary aliphatic macromolecules: A comment on recent publications by Gupta et al. Jan W. de Leeuw
*
Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands University Utrecht, Organic Geochemistry, Fac. Geosciences, Budapestlaan 4, 3584 CD Utrecht, The Netherlands University Utrecht, Palaeoecology, Beta Faculty, Budapestlaan 4, 3584 CD Utrecht, The Netherlands Received 21 March 2007; received in revised form 14 May 2007; accepted 17 May 2007 Available online 2 June 2007
In a number of recent papers Gupta et al. (2005, 2006a,b, 2007a,b,c) describe the existence and mechanism of formation of aliphatic geomacromolecules in sedimentary environments as an alternative for the selective preservation mechanism of non-hydrolysable resistant aliphatic biomacromolecules such as cutan and algaenan. In these papers, Gupta et al. amply refer to many of our papers regarding the selective preservation pathway for cutans and algaenans, but for reasons difficult to understand no references are made to our publications which demonstrate that in addition to the selective preservation of resistant aliphatic biomacromolecules, non-biological aliphatic geomacromolecules can also be produced from low-molecular-weight lipids upon (oxidative) polymerization (Blokker, 2000; Kuypers et al., 2002; Versteegh et al., 2004; De Leeuw et al., 2005, 2006). By not making reference to these papers Gupta et al. wrongly suggest that we are of the opinion that resistant aliphatic macromolecules in sediments result exclusively from the preservation of resistant biomacromolecules. Already in 2000 Blokker (2000) and later on in Versteegh et al. (2004) and Ref. cited therein and in De Leeuw et al. (2005), we used a wide variety *
Present address: Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands. Tel.: +31 222 369366; fax: +31 222 319674. E-mail address: [email protected]
of complementary analytical and chemical methods, as well as electron microscopy, to recognize morphologically and chemically the abundant presence of a highly aliphatic geopolymer produced from low-molecular-weight unsaturated lipids, a neoformed geopolymer highly comparable to the biomacromolecules cutan and algaenan. Because this aliphatic geopolymer was present in a very immature and well-preserved Eocene sediment, we excluded the formation of such geopolymers by thermal processes and speculated that oxidative polymerization during (very) early diagenesis might have occurred. This is in contrast with the mechanism suggested by Gupta et al. (2007a). These authors suggest that these types of aliphatic geopolymers are formed gradually during diagenesis by thermal stress based on laboratory experiments involving the heating of low-molecular-weight lipids for certain periods of time. Although such polymers can be formed in this way during heating experiments in the laboratory, this is by no means evidence that such experiments reflect natural processes. Kuypers et al. (2002), studying the insoluble organic matter of an Albian black shale using PyGC/MS and RuO4 oxidation, was forced to conclude that neoformation of an isoprenoid aliphatic geopolymer had occurred during diagenesis, and that this explained the abundant presence of isoprenoid compounds in the pyrolysates and in the
0146-6380/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2007.05.010
1586
J.W. de Leeuw / Organic Geochemistry 38 (2007) 1585–1587
products of RuO4 oxidation. A detailed chemical structure of this diagenetically formed isoprenoid geopolymer – by crosslinking of functionalized tetramethylicosane (TMI) and pentamethylicosane (PMI) derivatives – was presented. Finally, in a recent review paper De Leeuw et al. (2006) discussed the origin of aliphatic polymers in sediments, concluding that at least two separate though parallel pathways explaining the presence of aliphatic polymers in sediments have to be considered, namely: (1) the selective preservation of non-hydrolysable aliphatic biomacromolecules such as cutan and algaenan and their possible further transformation by additional cross-linking and (2) the oxidative polymerization of low-molecularweight functionalized lipids, a process similar to the drying of oil paint made from linseed oil. This neoformation of aliphatic geopolymers is also seen as the solution to a long-standing enigma, i.e., the highly aliphatic nature of morphologically well-preserved fossil insects, Rhabdopleura and shrimps (e.g., Baas et al., 1995; Briggs et al., 1995; Briggs, 1999). We reported that this aliphatic nature is likely the result of a gradual replacement of chitin by an aliphatic geopolymer produced from functionalized lipids upon oxidative polymerization with complete conservation of the morphology (De Lee-
Fig. 1. Interrelationships between extant biomass, bio- and geomacromolecules, kerogen and fossil fuels (modified after Tegelaar et al., 1989).
uw et al., 2006). For reasons mentioned above, we question that this replacement is due to the formation of an aliphatic geomacromolecule by thermal processes (cf. Gupta et al., 2007a). Based on the cited (and other uncited papers), it should be realized that the present state of the art with respect to sedimentary aliphatic macromolecules reflects a more balanced view regarding their origin, i.e., through selective preservation on the one hand and neoformation from low-molecularweight lipids on the other hand, than in the past. This balanced view is shown in Fig. 1 exemplifying the role of aliphatic and other bio- and geomacromolecules in the geological part of the carbon cycle.
References Baas, M., Briggs, D.E.G., Van Heemst, J.D.H., Kear, A.J., De Leeuw, J.W., 1995. Selective preservation of chitin during the decay of shrimps. Geochimica et Cosmochimica Acta 59, 945– 951. Briggs, D.E.G., Kear, A.J., Baas, M., De Leeuw, J.W., Rigby, S., 1995. Decay and composition of the hemichordate Rhabdopleura: Implications for the taphonomy of graptolites. Lethaia 28, 15–23. Briggs, D.E.G., 1999. Molecular taphonomy of animal and plant cuticles: Selective preservation and diagenesis. Philosophical Transactions of the Royal Society of London 354, 7–17. Blokker, P., 2000. Structural analysis of resistant polymers in extant algae and ancient sediments. Ph.D. Thesis, Chapter 9, pp. 107–113. De Leeuw, J.W., Poole, I., Blokker, P., Van Bergen, P.F., 2005. Resistant bio- and geomacromolecules in fossil spores, pollen and wood. In: Gonzalez-Vila, F.J., Gonzalez-Perez, J.A., Almendros, G. (Eds.), Organic Geochemistry: Challenges for the 21st Century, Akron Grafica, Sevilla vol. 1, p. 45. De Leeuw, J.W., Versteegh, G.J.M., Van Bergen, P.F., 2006. Biomacromolecules of plants and algae and their fossil analogues. Plant Ecology 189, 209–233. Gupta, N.S., Michels, R., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Pancost R.D., 2005. Experimental evidence for lipids as a source for aliphatic component of fossils and sedimentary organic matter. In: Gonzalez-Vila, F.J., Gonzalez-Perez, J.A., Almendros, G. (Eds.), Organic Geochemistry: Challenges for the 21st Century, Akron Grafica, Sevilla vol. 1, p. 46–47. Gupta, N.S., Collinson, M.E., Briggs, D.E.G., Evershed, R.P., Pancost, R.D., 2006a. Reinvestigation of the occurrence of cutan in plants: Implications for the leaf fossil record. Paleobiology 32, 432–449. Gupta, N.S., Michels, R., Briggs, D.E.G., Evershed, R.P., Pancost, R.D., 2006b. The organic preservation of fossil arthropods: An experimental study. In: Proceedings of the Royal Society of London, 2777–2783. Gupta, N.S., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Michels, R., Pancost, R.D., 2007a. Molecular preservation of plant and insect cuticles from the Oligocene Enspel Forma-
J.W. de Leeuw / Organic Geochemistry 38 (2007) 1585–1587 tion, Germany: Evidence against derivation of aliphatic polymer from sediment. Organic Geochemistry 38, 404–418. Gupta, N.S., Michels, R., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Pancost, R.D., 2007b. Experimental evidence for the formation of geomacromolecules from plant leaf lipids. Organic Geochemistry 38, 28–36. Gupta, N.S., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Michels, R., Jack, S.K., Pancost, R.D., 2007c. Evidence for the in situ polymerisation of labile aliphatic organic compounds during the preservation of fossil leaves: Implications for organic matter preservation. Organic Geochemistry 38, 499–522.
1587
Kuypers, M.M.M., Blokker, P., Hopmans, E.C., Kinkel, H., Pancost, R.D., Schouten, S., Sinninghe Damste´, J.S., 2002. Archaeal remains dominate marine organic matter from the early Albian oceanic anoxic event 1b. Palaeogeography, Palaeoclimatology, Palaeoecology 185, 211–234. Tegelaar, E.W., De Leeuw, J.W., Derenne, S., Largeau, C., 1989. A reappraisal of kerogen formation. Geochimica et Cosmochimica Acta 53, 3103–3106. Versteegh, G.J.M., Blokker, P., Wood, G.D., Collinson, M.E., Sinninghe Damste´, J.S., De Leeuw, J.W., 2004. Oxidative polymerization of unsaturated fatty acids as a preservation pathway for dinoflagellate organic matter. Organic Geochemistry 35, 1129–1139.
Discussion
On the origin of sedimentary aliphatic macromolecules: A comment on recent publications by Gupta et al. Jan W. de Leeuw
*
Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands University Utrecht, Organic Geochemistry, Fac. Geosciences, Budapestlaan 4, 3584 CD Utrecht, The Netherlands University Utrecht, Palaeoecology, Beta Faculty, Budapestlaan 4, 3584 CD Utrecht, The Netherlands Received 21 March 2007; received in revised form 14 May 2007; accepted 17 May 2007 Available online 2 June 2007
In a number of recent papers Gupta et al. (2005, 2006a,b, 2007a,b,c) describe the existence and mechanism of formation of aliphatic geomacromolecules in sedimentary environments as an alternative for the selective preservation mechanism of non-hydrolysable resistant aliphatic biomacromolecules such as cutan and algaenan. In these papers, Gupta et al. amply refer to many of our papers regarding the selective preservation pathway for cutans and algaenans, but for reasons difficult to understand no references are made to our publications which demonstrate that in addition to the selective preservation of resistant aliphatic biomacromolecules, non-biological aliphatic geomacromolecules can also be produced from low-molecular-weight lipids upon (oxidative) polymerization (Blokker, 2000; Kuypers et al., 2002; Versteegh et al., 2004; De Leeuw et al., 2005, 2006). By not making reference to these papers Gupta et al. wrongly suggest that we are of the opinion that resistant aliphatic macromolecules in sediments result exclusively from the preservation of resistant biomacromolecules. Already in 2000 Blokker (2000) and later on in Versteegh et al. (2004) and Ref. cited therein and in De Leeuw et al. (2005), we used a wide variety *
Present address: Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands. Tel.: +31 222 369366; fax: +31 222 319674. E-mail address: [email protected]
of complementary analytical and chemical methods, as well as electron microscopy, to recognize morphologically and chemically the abundant presence of a highly aliphatic geopolymer produced from low-molecular-weight unsaturated lipids, a neoformed geopolymer highly comparable to the biomacromolecules cutan and algaenan. Because this aliphatic geopolymer was present in a very immature and well-preserved Eocene sediment, we excluded the formation of such geopolymers by thermal processes and speculated that oxidative polymerization during (very) early diagenesis might have occurred. This is in contrast with the mechanism suggested by Gupta et al. (2007a). These authors suggest that these types of aliphatic geopolymers are formed gradually during diagenesis by thermal stress based on laboratory experiments involving the heating of low-molecular-weight lipids for certain periods of time. Although such polymers can be formed in this way during heating experiments in the laboratory, this is by no means evidence that such experiments reflect natural processes. Kuypers et al. (2002), studying the insoluble organic matter of an Albian black shale using PyGC/MS and RuO4 oxidation, was forced to conclude that neoformation of an isoprenoid aliphatic geopolymer had occurred during diagenesis, and that this explained the abundant presence of isoprenoid compounds in the pyrolysates and in the
0146-6380/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2007.05.010
1586
J.W. de Leeuw / Organic Geochemistry 38 (2007) 1585–1587
products of RuO4 oxidation. A detailed chemical structure of this diagenetically formed isoprenoid geopolymer – by crosslinking of functionalized tetramethylicosane (TMI) and pentamethylicosane (PMI) derivatives – was presented. Finally, in a recent review paper De Leeuw et al. (2006) discussed the origin of aliphatic polymers in sediments, concluding that at least two separate though parallel pathways explaining the presence of aliphatic polymers in sediments have to be considered, namely: (1) the selective preservation of non-hydrolysable aliphatic biomacromolecules such as cutan and algaenan and their possible further transformation by additional cross-linking and (2) the oxidative polymerization of low-molecularweight functionalized lipids, a process similar to the drying of oil paint made from linseed oil. This neoformation of aliphatic geopolymers is also seen as the solution to a long-standing enigma, i.e., the highly aliphatic nature of morphologically well-preserved fossil insects, Rhabdopleura and shrimps (e.g., Baas et al., 1995; Briggs et al., 1995; Briggs, 1999). We reported that this aliphatic nature is likely the result of a gradual replacement of chitin by an aliphatic geopolymer produced from functionalized lipids upon oxidative polymerization with complete conservation of the morphology (De Lee-
Fig. 1. Interrelationships between extant biomass, bio- and geomacromolecules, kerogen and fossil fuels (modified after Tegelaar et al., 1989).
uw et al., 2006). For reasons mentioned above, we question that this replacement is due to the formation of an aliphatic geomacromolecule by thermal processes (cf. Gupta et al., 2007a). Based on the cited (and other uncited papers), it should be realized that the present state of the art with respect to sedimentary aliphatic macromolecules reflects a more balanced view regarding their origin, i.e., through selective preservation on the one hand and neoformation from low-molecularweight lipids on the other hand, than in the past. This balanced view is shown in Fig. 1 exemplifying the role of aliphatic and other bio- and geomacromolecules in the geological part of the carbon cycle.
References Baas, M., Briggs, D.E.G., Van Heemst, J.D.H., Kear, A.J., De Leeuw, J.W., 1995. Selective preservation of chitin during the decay of shrimps. Geochimica et Cosmochimica Acta 59, 945– 951. Briggs, D.E.G., Kear, A.J., Baas, M., De Leeuw, J.W., Rigby, S., 1995. Decay and composition of the hemichordate Rhabdopleura: Implications for the taphonomy of graptolites. Lethaia 28, 15–23. Briggs, D.E.G., 1999. Molecular taphonomy of animal and plant cuticles: Selective preservation and diagenesis. Philosophical Transactions of the Royal Society of London 354, 7–17. Blokker, P., 2000. Structural analysis of resistant polymers in extant algae and ancient sediments. Ph.D. Thesis, Chapter 9, pp. 107–113. De Leeuw, J.W., Poole, I., Blokker, P., Van Bergen, P.F., 2005. Resistant bio- and geomacromolecules in fossil spores, pollen and wood. In: Gonzalez-Vila, F.J., Gonzalez-Perez, J.A., Almendros, G. (Eds.), Organic Geochemistry: Challenges for the 21st Century, Akron Grafica, Sevilla vol. 1, p. 45. De Leeuw, J.W., Versteegh, G.J.M., Van Bergen, P.F., 2006. Biomacromolecules of plants and algae and their fossil analogues. Plant Ecology 189, 209–233. Gupta, N.S., Michels, R., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Pancost R.D., 2005. Experimental evidence for lipids as a source for aliphatic component of fossils and sedimentary organic matter. In: Gonzalez-Vila, F.J., Gonzalez-Perez, J.A., Almendros, G. (Eds.), Organic Geochemistry: Challenges for the 21st Century, Akron Grafica, Sevilla vol. 1, p. 46–47. Gupta, N.S., Collinson, M.E., Briggs, D.E.G., Evershed, R.P., Pancost, R.D., 2006a. Reinvestigation of the occurrence of cutan in plants: Implications for the leaf fossil record. Paleobiology 32, 432–449. Gupta, N.S., Michels, R., Briggs, D.E.G., Evershed, R.P., Pancost, R.D., 2006b. The organic preservation of fossil arthropods: An experimental study. In: Proceedings of the Royal Society of London, 2777–2783. Gupta, N.S., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Michels, R., Pancost, R.D., 2007a. Molecular preservation of plant and insect cuticles from the Oligocene Enspel Forma-
J.W. de Leeuw / Organic Geochemistry 38 (2007) 1585–1587 tion, Germany: Evidence against derivation of aliphatic polymer from sediment. Organic Geochemistry 38, 404–418. Gupta, N.S., Michels, R., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Pancost, R.D., 2007b. Experimental evidence for the formation of geomacromolecules from plant leaf lipids. Organic Geochemistry 38, 28–36. Gupta, N.S., Briggs, D.E.G., Collinson, M.E., Evershed, R.P., Michels, R., Jack, S.K., Pancost, R.D., 2007c. Evidence for the in situ polymerisation of labile aliphatic organic compounds during the preservation of fossil leaves: Implications for organic matter preservation. Organic Geochemistry 38, 499–522.
1587
Kuypers, M.M.M., Blokker, P., Hopmans, E.C., Kinkel, H., Pancost, R.D., Schouten, S., Sinninghe Damste´, J.S., 2002. Archaeal remains dominate marine organic matter from the early Albian oceanic anoxic event 1b. Palaeogeography, Palaeoclimatology, Palaeoecology 185, 211–234. Tegelaar, E.W., De Leeuw, J.W., Derenne, S., Largeau, C., 1989. A reappraisal of kerogen formation. Geochimica et Cosmochimica Acta 53, 3103–3106. Versteegh, G.J.M., Blokker, P., Wood, G.D., Collinson, M.E., Sinninghe Damste´, J.S., De Leeuw, J.W., 2004. Oxidative polymerization of unsaturated fatty acids as a preservation pathway for dinoflagellate organic matter. Organic Geochemistry 35, 1129–1139.