- Email: [email protected]
Sulfurization as a preservation mechanism for the δ13C of biomarkers
Accepted Manuscript Note Sulfurization as a preservation mechanism for the δ 13C of biomarkers Yoav O. Rosenberg, Ilya Kutuzov, Alon Amrani PII: DOI: Reference:
S0146-6380(18)30190-6 https://doi.org/10.1016/j.orggeochem.2018.08.010 OG 3773
To appear in:
Organic Geochemistry
Received Date: Revised Date: Accepted Date:
22 May 2018 1 August 2018 22 August 2018
Please cite this article as: Rosenberg, Y.O., Kutuzov, I., Amrani, A., Sulfurization as a preservation mechanism for the δ 13C of biomarkers, Organic Geochemistry (2018), doi: https://doi.org/10.1016/j.orggeochem.2018.08.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sulfurization as a preservation mechanism for the 13C of biomarkers Yoav O. Rosenberg*, Ilya Kutuzov, Alon Amrani Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. *Corresponding author, email: [email protected], phone: +97254 6235905
Abstract We compiled data from studies that examined pairs of compound specific 13C in both the free- and S-bound fractions. The studies compiled span a wide range of biomarker type, rock age (~235Ma to 3Kyr) and paleo-environments in which the sedimentary organic matter was deposited. Despite this variability, majority of data shows that the S-bound fraction is heavier on average by 2‰ compared with the free HC. The hypotheses of different biological sources and fractionation during sulfurization or maturation of the organic matter are reexamined in light of this result. We conclude that the
fractionation between these two fractions should be a result of a more
generalizing mechanism, operating at an early diagenetic stage. Fractionation during biodegradation of the free biomarker fraction can offer such a mechanism. Thus, the Sbound fraction may better represent the original
values of biomarkers used in
paleoenvironmental studies. Future studies should confirm this suggested mechanism. 1
Introduction Through early diagenesis, the preservation of organic compounds (OC) is enhanced
by a sulfurization process, in which inorganic reduced sulfur species react with functionalized OC (Sinninghe Damsté and De Leeuw, 1990; Amrani, 2014). Two principle pathways of sulfurization, namely intramolecular and intermolecular, generate organic sulfur compounds (OSC). The intramolecular pathway generates individual OSC such as thiolanes, while the intermolecular pathway binds different compounds into a
macromolecule structure, thus making it less prone for biologic degradation (Sinninghe Damsté and De Leeuw, 1990). Biomarkers can operationally be divided into three pools: free and intramolecular sulfurized biomarkers in the apolar fraction, and S-bound biomarker (i.e., intermolecular sulfurization) that must be released from the polar and asphaltene fractions (or the kerogen) prior to their GC analysis. The analysis of biomarkers of a given sample usually reveals compounds in these three pools that have the same C-skeleton (e.g., phytane), but differ in their 13C by a few permils and up to 11‰ (e.g., Kohnen et al., 1992a; Forster et al., 2008). Since compound-specific 13C is an important and commonly used tool in paleoenvironmental studies (Hayes et al., 1990; Grice et al., 2005), this apparent fractionation may have important and unexplored implications. Several processes were suggested to explain this difference, where the most common one is a different source input for the biomarkers (e.g., Kohnen et al., 1992b; Schouten et al., 2001). Here we compile data from previous studies which examined pairs of compound specific 13C in both the free- and S-bound fractions. The studies compiled below span a wide range of biomarker type, rock age (~235Ma to 3Kyr) and paleo-oceanic environments in which the sedimentary organic matter was deposited. Based on the intense work done in this field, we show that the S-bound fraction was mostly constant 13
C enriched regardless of sample variability. In light of this result, we review the
different hypotheses suggested for compounds having the same C-skeleton, but different 13C and suggest a new possible alternative. 2
Compilation of literature data Nearly 300 pairs of 13C values of free and S-bound biomarkers from 17 individual
studies were compiled in the present study and presented as Supplementary Material
(SM). Table S1 is sorted by the compounds, and includes pairs of 13C measurements done on the free- and S-bound fractions for phytane, pristane, isoprenoid C16, isoprenoid C18, C25 highly branched isoprenoid (HBI), C35 hopanoid, fatty acids in the C13-C29 range and n-alkanes in the C17-C33 range. General information of each reference is given, including geological period, age, rock formation, geographical site, and method used for desulfurization of the polar phase. Thereafter, the isotopic data is given for the free- and S-bound fractions, along with the delta between them: 1. 13Cbound−free = 13Cs-bound - 13Cfree Few studies also derived 13C for intramolecular S compounds, and Table S1 indicates if the values resemble that of the free- or S-bound fraction. Some studies reported the concentrations of the specific compounds along with their isotopic values. This is given in Table S1 for the calculation of the relative free fraction, defined here as: 2. (Free-biomarker)fraction = [Free- biomarker]/ [Free + S-bound biomarker] Some studies reported their data only in a graphical format and not in a tabular one, and the data points were digitized (see comments, Table S1). The relative error of this process was usually better than 4% (SM 2) and we conclude that the values of the digitized data given here resembles very closely that of the original data. 3
Results Excluding n-alkanes, Fig. 1a shows 13Cs-bound vs. 13Cfree for all compounds in Table
S1 along with the 1:1 agreement line. Most data points in Fig. 1a show that a given Cskeleton is
13
C enriched in the S-bound fraction compared with the free HC fraction.
13Cbound−free narrowly varies between -5 to 6‰ and averaging +2±1‰ (dashed and dotted lines in Fig 1a) regardless the type of the biomarkers, the age of the rock (~235Ma to 3Kyr) and the prevailing paleo-environment of each study. Figure 1b
presents the 13C data for n-alkanes, while the linear regression fit and one standard deviation envelope are from Fig. 1a. Generally, n-alkanes have more variability in 13Cbound−free, though most of the data shows that the S-bound biomarkers are
13
C
enriched. When distinguishing n-alkanes of even and odd C-number, the former show less variability in 13Cbound−free. For some specific biomarkers, a few previous studies found that 13Cbound−free < 0. This was shown for the carotenoid isorenieratane (Hartgers et al., 1994; Putschew et al., 1998), for steroids (Filley et al., 1996; Schouten et al., 2001) and for squalane (Kohnen et al., 1992b). 4
Discussion The mechanisms suggested to account for 13Cbound−free are: Different sources of the biomarkers in the free and S-bound fractions. The
preservation pathway of a biomarker (i.e., intermolecular, or intramolecular sulfurization or no sulfurization) is dictated by its functional sites. Precursors from different organisms with the same C-skeleton (e.g., phytane) can differ in their functional sites (e.g. double bonds) and 13C values, which latter characterize the different fraction of the sample (Kohnen et al., 1992b). For example, the “different precursor” hypothesis was suggested to account for 13Cbound−free in the case of the ubiquitous phytane biomarker (e.g., Kohnen et al., 1992b; Schouten et al., 2001). Indeed, a phytane skeleton (and its
) can be derived from phytol (-23 to -33‰, Table S1) or from archaeol,
produced by archaea (~ -15‰, Kuypers et al., 2001). If different sources of the precursors were responsible for the 13Cbound−free, it should be expected that the magnitude of 13Cbound−free will vary from one case study to the other depending with their local source input and relative sulfurization extent. However, despite the wide sample variability,13Cbound−free is rather uniform among the different compounds (Fig.
1). Thus, for most cases a more generalizing mechanism is probably needed, as was also noted by Forster et al. (2008). Fractionation during sulfurization of organic compounds. Schouten et al. (1995) have tested the possible kinetic C-isotope effect (KIE) using laboratory sulfurization of 1-decene with HSx-. The remaining 1-decene became heavier with increasing sulfurization, and a fractionation factor of -20‰ for the reacting carbon atoms (i.e., not the whole molecule) was calculated. However, Schouten et al. (1995) concluded that such effect is usually not observed in natural samples where the remaining free HC fraction is depleted and not enriched in 13C as also seen in Fig. 1. Since 13C has a more thermodynamically stable bond than
12
C, an equilibrium isotope effect (EIE) during
sulfurization, enriching the S-bound biomarkers by
13
C, can also be speculated.
However, as with the KIE, such fractionation operates on the C atoms of the C-S bond, and should be diluted with the non-reacting C atoms of the molecule. Fig. 1 shows that 13Cbound−free is independent of molecule size. Moreover, 13C of intramolecular S compounds usually resembles the free and not the S-bound fraction (3 out of 5 cases, comment column, Table S1). This suggests that the mechanism leading to 13Cbound−free differentiate between the low MW (free and intramolecular S fractions) and the macromolecular fractions (S-bound fraction). Therefore, we conclude that any isotope effect as a result of sulfurization, if exists, cannot be solely responsible for the rather uniform 13Cbound−free observed. Fractionation through thermal maturation. Working on the Cenomanian/Turonian boundary, Sinninghe Damsté et al. (2008) and Forster et al. (2008) noted the constant ~2‰ difference in 13Cbound−free for phytane, regardless of the 13C excursion in the profiles studied. Although Sinninghe Damsté et al. (2008) regarded their samples as
thermally immature, they suggested that this is a KIE occurring during “mild” maturation, leading to depletion in
13
C of phytane liberated to the free-HC fraction.
However, artificial maturation experiments of type I, II and III kerogens show no change in
of n-alkanes and isoprenoids at lower degree of maturation, and an enrichment of
1-3‰ at a maturity level equivalent to the oil window (Bjorøy et al., 1992). Moreover, the studies compiled here represent either young sediments or source rocks considered immature, and their similar 13Cbound−free may not be ascribed to maturation. Sulfurization as a preservation mechanism of the 13C As discussed above, sulfurization of organic matter serves as a preservation mechanism as it creates cross-linked macromolecules. We suggest that this preservation mechanism preserves better the 13C signature of the S-bound biomarker, while the free fraction becomes
13
C depleted with degradation during diagenesis. Indeed, laboratory
experiments and field data showed that upon anoxic degradation the remaining particulate organic matter (POC) becomes
13
C depleted up to 3‰ (Lehmann et al.,
2002). On the molecular level, laboratory experiments of anoxic degradation showed different trends: remaining fatty acid became
13
C enriched (up to 7‰), alkenones
depleted (up to 6‰) and sterols showed no change (Sun et al., 2004). On this basis, Sun et al. (2004) postulated that the direction of the isotopic shift during degradation depends on the functional moiety being degraded; this moiety might have a different 13C than the average molecule. In a study of biodegradation of oils (i.e., a post maturation process), Sun et al. (2005) found a
13
C enrichment of up to 4‰ for C15-C18 n-alkanes
explained by a KIE. However, in the same set of samples, both phytane and pristane showed depletion in
(up to 2‰). The preferential loss of a
heavier carboxyl
carbon from OM during catagenesis is used to explain the generation of lighter oil
fractions (Galimov, 2006 and references therein). On the atomic level in a given molecule, carbon atoms having alkene and aldehyde functionalities are also
heavier
compared to carbon atoms of alkane bonds (Galimov, 2006). This can further support the view that degradation of molecules through their reactive sites (i.e. by loosing functional groups) can lead to a decrease in the overall
of the molecule. Moreover, since
sulfurization is acting specifically on functionalized groups (e.g. carbonyls) to form C-S bonds it removes this active sites and prevent their further reaction/degradation (Amrani , 2014). If this degradation-preservation hypothesis hold some premise, then 13Cbound−free should increase with the decrease in (Free-biomarker)fraction, accounting for the narrow variation of this variable (~0-6‰). Assuming that Eq. 2 reflects to some extent the degradation-preservation balance during diagenesis, the data for which concentrations were reported as well (Table S1) is depicted in Fig. 2, which shows the expected trend. Linear regression through the origin of the data in Fig. 2 suggests the fractionation factor of a Rayleigh distillation model to be ~1.5‰ for phytane and C25 HBI. The pristane data, taken from Schouten et al., (2001), is somewhat more scattered (R2 = 0.2) and the enrichment factor is higher (-4.5‰). Note that the phytane data is taken from two studies on the Monterey Formation (Schouten et al., 1997; Schouten et al., 2001) while that of the C25 HBI is from an Holocene young sediments (<3Kyr, Sinninghe Damsté et al., 2007). A higher
fractionation of pristane compared to phytane was also observed by
Sun et al. (2005) for the biodegradation of natural oil. The data from laboratory degradation experiments of POC and alkenones discussed above (Lehmann et al., 2002; Sun et al., 2004) is plotted in a similar fashion in Fig. 3; in this case (Free-biomarker)fraction = [Free-biomarker]remaining/[Free-biomarker]initial. Despite
the different C-skeleton and functionalities of alkenones and isoprenoids, the enrichment factor is comparable, and similar to that of the POC. It is also noted that like the steroids field data that showed 13Cbound−free ≈ 0 (Filley et al., 1996; Schouten et al., 2001), Sun et al. (2004) did not observe changes in 13C with steroids degradation. However, the positive 13Cbound−free observed by Hefter (1995) for fatty acids, is not comparable with the enrichment found by Sun et al. (2004) for this biomarker upon degradation. 5
Summary Compilation of almost 300 paired measurements of compound specific
from the
free- and S-bound fractions shows a consistent enrichment in the latter of 2±1‰ on the average. Since the data is of varying sources and ages, it is concluded that different precursors or fractionation during OM sulfurization or maturation cannot account for the observed enrichment of
in the S-bound fraction. Although the molecular
mechanism is unknown yet, biodegradation of the free biomarker fraction at early diagenesis stage can offer a more robust pathway as conceptually depicted in Fig. 4. This in turn suggests that the S-bound fraction may better represent the original
values of
biomarkers used in paleoenvironmental studies. Regardless if our suggested explanation is valid, we have highlighted this intriguing phenomenon and the need to explore it. Future studies, both field and laboratory experiments will have to confirm this (and/or other) hypothesis and mechanism. Acknowledgment. We acknowledge Paul Greenwood for reviewing and improving an earlier version of this manuscript. Y. O. R. and A.A. acknowledges grant No. 15/16 from the Ministry of National Infrastructures Energy and Water Resources of Israel. A.A thanks the Israeli Science Foundation grant No. 1738/16 for partial support of this study. 6
Figure captions
Fig. 1. 13C of isoprenoid based skeletons and fatty acids (a) and n-alkanes (b) in the S-bound vs. free-HC fractions from 17 studies. The bold line is the 1:1 agreement line, while the dashed and dotted lines in both panels are the average±stdev of (a). A complete reference list is given in Table S1. Fig. 2. Changes in 13C vs. Ln(Free-HC)fraction of naturally sulfurized isoprenoids. Fig. 3. Changes in 13C vs. Ln(Free-HC)fraction of degradation experiments of POC (Lehmann et al., 2002) and alkenones (Sun et al., 2004). Fig. 4. Conceptual illustration for the degradation-preservation hypothesis affecting the 7
of free biomarkers through its diagenesis. References
Amrani, A., 2014. Organosulfur Compounds: Molecular and Isotopic Evolution from Biota to Oil and Gas. Annual Review of Earth and Planetary Sciences 42, 733-768. Bjorøy, M., Hall, P.B., Hustad, E., Williams, J.A., 1992. Variation in stable carbon isotope ratios of individual hydrocarbons as a function of artificial maturity. Organic Geochemistry 19, 89-105. Filley, T.R., Freeman, K.H., Hatcher, P.G., 1996. Carbon isotope relationships between sulfide-bound steroids and proposed functionalized lipid precursors in sediments from the Santa Barbara Basin, California. Organic Geochemistry 25, 367-377. Forster, A., Kuypers, M.M.M., Turgeon, S.C., Brumsack, H.-J., Petrizzo, M.R., Sinninghe Damsté, J.S., 2008. The Cenomanian/Turonian oceanic anoxic event in the South Atlantic: New insights from a geochemical study of DSDP Site 530A. Palaeogeography, Palaeoclimatology, Palaeoecology 267, 256-283. Galimov, E.M., 2006. Isotope organic geochemistry. Organic Geochemistry 37, 12001262. Grice, K., Cao, C., Love, G.D., Böttcher, M.E., Twitchett, R.J., Grosjean, E., Summons, R.E., Turgeon, S.C., Dunning, W., Jin, Y., 2005. Photic Zone Euxinia During the Permian-Triassic Superanoxic Event. Science 307, 706-709. Hartgers, W.A., Sinninghe Damsté, J.S., Requejo, A.G., Allan, J., Hayes, J.M., Ling, Y., Xie, T.-M., Primack, J., de Leeuw, J.W., 1994. A molecular and carbon isotopic study towards the origin and diagenetic fate of diaromatic carotenoids. Organic Geochemistry 22, 703-725. Hayes, J.M., Freeman, K.H., Popp, B.N., Hoham, C.H., 1990. Compound-specific isotopic analyses: A novel tool for reconstruction of ancient biogeochemical processes. Organic Geochemistry 16, 1115-1128. Hefter, J., Hauke, V., Richnow, H.H., Michaelis, W., 1995. Alkanoic Subunits in SulfurRich Geomacromolecules, Geochemical Transformations of Sedimentary Sulfur. American Chemical Society, pp. 93-109.
Kohnen, M.E., Schouten, S., Damsté, J.S.S., de Leeuw, J.W., Merrit, D., Hayes, J., 1992a. The combined application of organic sulphur and isotope geochemistry to asses multiple sources of palaeobiochemicals with identical carbon skeletons. Organic Geochemistry 19, 403-419. Kohnen, M.E.L., Schouten, S., Sinninghe Damsté, J.S., de Leeuw, J.W., Merritt, D.A., Hayes, J.M., 1992b. Recognition of paleobiochemicals by a combined molecular sulfur and isotope geochemical approach. Science 256, 358-362. Kuypers, M.M., Blokker, P., Erbacher, J., Kinkel, H., Pancost, R.D., Schouten, S., Damsté, J.S.S., 2001. Massive expansion of marine archaea during a mid-Cretaceous oceanic anoxic event. Science 293, 92-95. Lehmann, M.F., Bernasconi, S.M., Barbieri, A., McKenzie, J.A., 2002. Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis. Geochimica et Cosmochimica Acta 66, 3573-3584. Putschew, A., Schaeffer-Reiss, C., Schaeffer, P., Koopmans, M.P., de Leeuw, J.W., Lewan, M.D., Sinninghe Damsté, J.S., Maxwell, J.R., 1998. Release of sulfur- and oxygen-bound components from a sulfur-rich kerogen during simulated maturation by hydrous pyrolysis. Organic Geochemistry 29, 1875-1890. Schouten, S., Schoell, M., Rijpstra, W.I.C., Sinninghe Damsté, J.S., de Leeuw, J.W., 1997. A molecular stable carbon isotope study of organic matter in immature Miocene Monterey sediments, Pismo basin. Geochimica et Cosmochimica Acta 61, 2065-2082. Schouten, S., Schoell, M., Sinninghe Damsté, J., Summons, R.E., De Leeuw, J., 2001. Molecular biogeochemistry of Monterey sediments, Naples Beach, California: II. Stable carbon isotopic compositions of free and sulphur bound carbon skeletons, in: Isaacs, C., Rullkötter, J. (Eds.), The Monterey Formation: From Rocks to Molecules. Columbia University Press, New York, pp. 175-188. Schouten, S., Sinninghe Damsté, J.S., Kohnen, M.E.L., De Leeuw, J.W., 1995. The effect of hydrosulphurization on stable carbon isotopic compositions of free and sulphur-bound lipids. Geochimica et Cosmochimica Acta 59, 1605-1609. Sinninghe Damsté, J.S., De Leeuw, J.W., 1990. Analysis, structure and geochemical significance of organically-bound sulphur in the geosphere: state of the art and future research. Organic Geochemistry 16, 1077-1101. Sinninghe Damsté, J.S., Kuypers, M.M.M., Pancost, R.D., Schouten, S., 2008. The carbon isotopic response of algae, (cyano)bacteria, archaea and higher plants to the late Cenomanian perturbation of the global carbon cycle: Insights from biomarkers in black shales from the Cape Verde Basin (DSDP Site 367). Organic Geochemistry 39, 1703-1718. Sinninghe Damsté, J.S., Rijpstra, W.I.C., Coolen, M.J.L., Schouten, S., Volkman, J.K., 2007. Rapid sulfurisation of highly branched isoprenoid (HBI) alkenes in sulfidic Holocene sediments from Ellis Fjord, Antarctica. Organic Geochemistry 38, 128-139. Sun, M.-Y., Zou, L., Dai, J., Ding, H., Culp, R.A., Scranton, M.I., 2004. Molecular carbon isotopic fractionation of algal lipids during decomposition in natural oxic and anoxic seawaters. Organic Geochemistry 35, 895-908. Sun, Y., Chen, Z., Xu, S., Cai, P., 2005. Stable carbon and hydrogen isotopic fractionation of individual n-alkanes accompanying biodegradation: evidence from a group of progressively biodegraded oils. Organic Geochemistry 36, 225-238. 8 Figures
S0146-6380(18)30190-6 https://doi.org/10.1016/j.orggeochem.2018.08.010 OG 3773
To appear in:
Organic Geochemistry
Received Date: Revised Date: Accepted Date:
22 May 2018 1 August 2018 22 August 2018
Please cite this article as: Rosenberg, Y.O., Kutuzov, I., Amrani, A., Sulfurization as a preservation mechanism for the δ 13C of biomarkers, Organic Geochemistry (2018), doi: https://doi.org/10.1016/j.orggeochem.2018.08.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sulfurization as a preservation mechanism for the 13C of biomarkers Yoav O. Rosenberg*, Ilya Kutuzov, Alon Amrani Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. *Corresponding author, email: [email protected], phone: +97254 6235905
Abstract We compiled data from studies that examined pairs of compound specific 13C in both the free- and S-bound fractions. The studies compiled span a wide range of biomarker type, rock age (~235Ma to 3Kyr) and paleo-environments in which the sedimentary organic matter was deposited. Despite this variability, majority of data shows that the S-bound fraction is heavier on average by 2‰ compared with the free HC. The hypotheses of different biological sources and fractionation during sulfurization or maturation of the organic matter are reexamined in light of this result. We conclude that the
fractionation between these two fractions should be a result of a more
generalizing mechanism, operating at an early diagenetic stage. Fractionation during biodegradation of the free biomarker fraction can offer such a mechanism. Thus, the Sbound fraction may better represent the original
values of biomarkers used in
paleoenvironmental studies. Future studies should confirm this suggested mechanism. 1
Introduction Through early diagenesis, the preservation of organic compounds (OC) is enhanced
by a sulfurization process, in which inorganic reduced sulfur species react with functionalized OC (Sinninghe Damsté and De Leeuw, 1990; Amrani, 2014). Two principle pathways of sulfurization, namely intramolecular and intermolecular, generate organic sulfur compounds (OSC). The intramolecular pathway generates individual OSC such as thiolanes, while the intermolecular pathway binds different compounds into a
macromolecule structure, thus making it less prone for biologic degradation (Sinninghe Damsté and De Leeuw, 1990). Biomarkers can operationally be divided into three pools: free and intramolecular sulfurized biomarkers in the apolar fraction, and S-bound biomarker (i.e., intermolecular sulfurization) that must be released from the polar and asphaltene fractions (or the kerogen) prior to their GC analysis. The analysis of biomarkers of a given sample usually reveals compounds in these three pools that have the same C-skeleton (e.g., phytane), but differ in their 13C by a few permils and up to 11‰ (e.g., Kohnen et al., 1992a; Forster et al., 2008). Since compound-specific 13C is an important and commonly used tool in paleoenvironmental studies (Hayes et al., 1990; Grice et al., 2005), this apparent fractionation may have important and unexplored implications. Several processes were suggested to explain this difference, where the most common one is a different source input for the biomarkers (e.g., Kohnen et al., 1992b; Schouten et al., 2001). Here we compile data from previous studies which examined pairs of compound specific 13C in both the free- and S-bound fractions. The studies compiled below span a wide range of biomarker type, rock age (~235Ma to 3Kyr) and paleo-oceanic environments in which the sedimentary organic matter was deposited. Based on the intense work done in this field, we show that the S-bound fraction was mostly constant 13
C enriched regardless of sample variability. In light of this result, we review the
different hypotheses suggested for compounds having the same C-skeleton, but different 13C and suggest a new possible alternative. 2
Compilation of literature data Nearly 300 pairs of 13C values of free and S-bound biomarkers from 17 individual
studies were compiled in the present study and presented as Supplementary Material
(SM). Table S1 is sorted by the compounds, and includes pairs of 13C measurements done on the free- and S-bound fractions for phytane, pristane, isoprenoid C16, isoprenoid C18, C25 highly branched isoprenoid (HBI), C35 hopanoid, fatty acids in the C13-C29 range and n-alkanes in the C17-C33 range. General information of each reference is given, including geological period, age, rock formation, geographical site, and method used for desulfurization of the polar phase. Thereafter, the isotopic data is given for the free- and S-bound fractions, along with the delta between them: 1. 13Cbound−free = 13Cs-bound - 13Cfree Few studies also derived 13C for intramolecular S compounds, and Table S1 indicates if the values resemble that of the free- or S-bound fraction. Some studies reported the concentrations of the specific compounds along with their isotopic values. This is given in Table S1 for the calculation of the relative free fraction, defined here as: 2. (Free-biomarker)fraction = [Free- biomarker]/ [Free + S-bound biomarker] Some studies reported their data only in a graphical format and not in a tabular one, and the data points were digitized (see comments, Table S1). The relative error of this process was usually better than 4% (SM 2) and we conclude that the values of the digitized data given here resembles very closely that of the original data. 3
Results Excluding n-alkanes, Fig. 1a shows 13Cs-bound vs. 13Cfree for all compounds in Table
S1 along with the 1:1 agreement line. Most data points in Fig. 1a show that a given Cskeleton is
13
C enriched in the S-bound fraction compared with the free HC fraction.
13Cbound−free narrowly varies between -5 to 6‰ and averaging +2±1‰ (dashed and dotted lines in Fig 1a) regardless the type of the biomarkers, the age of the rock (~235Ma to 3Kyr) and the prevailing paleo-environment of each study. Figure 1b
presents the 13C data for n-alkanes, while the linear regression fit and one standard deviation envelope are from Fig. 1a. Generally, n-alkanes have more variability in 13Cbound−free, though most of the data shows that the S-bound biomarkers are
13
C
enriched. When distinguishing n-alkanes of even and odd C-number, the former show less variability in 13Cbound−free. For some specific biomarkers, a few previous studies found that 13Cbound−free < 0. This was shown for the carotenoid isorenieratane (Hartgers et al., 1994; Putschew et al., 1998), for steroids (Filley et al., 1996; Schouten et al., 2001) and for squalane (Kohnen et al., 1992b). 4
Discussion The mechanisms suggested to account for 13Cbound−free are: Different sources of the biomarkers in the free and S-bound fractions. The
preservation pathway of a biomarker (i.e., intermolecular, or intramolecular sulfurization or no sulfurization) is dictated by its functional sites. Precursors from different organisms with the same C-skeleton (e.g., phytane) can differ in their functional sites (e.g. double bonds) and 13C values, which latter characterize the different fraction of the sample (Kohnen et al., 1992b). For example, the “different precursor” hypothesis was suggested to account for 13Cbound−free in the case of the ubiquitous phytane biomarker (e.g., Kohnen et al., 1992b; Schouten et al., 2001). Indeed, a phytane skeleton (and its
) can be derived from phytol (-23 to -33‰, Table S1) or from archaeol,
produced by archaea (~ -15‰, Kuypers et al., 2001). If different sources of the precursors were responsible for the 13Cbound−free, it should be expected that the magnitude of 13Cbound−free will vary from one case study to the other depending with their local source input and relative sulfurization extent. However, despite the wide sample variability,13Cbound−free is rather uniform among the different compounds (Fig.
1). Thus, for most cases a more generalizing mechanism is probably needed, as was also noted by Forster et al. (2008). Fractionation during sulfurization of organic compounds. Schouten et al. (1995) have tested the possible kinetic C-isotope effect (KIE) using laboratory sulfurization of 1-decene with HSx-. The remaining 1-decene became heavier with increasing sulfurization, and a fractionation factor of -20‰ for the reacting carbon atoms (i.e., not the whole molecule) was calculated. However, Schouten et al. (1995) concluded that such effect is usually not observed in natural samples where the remaining free HC fraction is depleted and not enriched in 13C as also seen in Fig. 1. Since 13C has a more thermodynamically stable bond than
12
C, an equilibrium isotope effect (EIE) during
sulfurization, enriching the S-bound biomarkers by
13
C, can also be speculated.
However, as with the KIE, such fractionation operates on the C atoms of the C-S bond, and should be diluted with the non-reacting C atoms of the molecule. Fig. 1 shows that 13Cbound−free is independent of molecule size. Moreover, 13C of intramolecular S compounds usually resembles the free and not the S-bound fraction (3 out of 5 cases, comment column, Table S1). This suggests that the mechanism leading to 13Cbound−free differentiate between the low MW (free and intramolecular S fractions) and the macromolecular fractions (S-bound fraction). Therefore, we conclude that any isotope effect as a result of sulfurization, if exists, cannot be solely responsible for the rather uniform 13Cbound−free observed. Fractionation through thermal maturation. Working on the Cenomanian/Turonian boundary, Sinninghe Damsté et al. (2008) and Forster et al. (2008) noted the constant ~2‰ difference in 13Cbound−free for phytane, regardless of the 13C excursion in the profiles studied. Although Sinninghe Damsté et al. (2008) regarded their samples as
thermally immature, they suggested that this is a KIE occurring during “mild” maturation, leading to depletion in
13
C of phytane liberated to the free-HC fraction.
However, artificial maturation experiments of type I, II and III kerogens show no change in
of n-alkanes and isoprenoids at lower degree of maturation, and an enrichment of
1-3‰ at a maturity level equivalent to the oil window (Bjorøy et al., 1992). Moreover, the studies compiled here represent either young sediments or source rocks considered immature, and their similar 13Cbound−free may not be ascribed to maturation. Sulfurization as a preservation mechanism of the 13C As discussed above, sulfurization of organic matter serves as a preservation mechanism as it creates cross-linked macromolecules. We suggest that this preservation mechanism preserves better the 13C signature of the S-bound biomarker, while the free fraction becomes
13
C depleted with degradation during diagenesis. Indeed, laboratory
experiments and field data showed that upon anoxic degradation the remaining particulate organic matter (POC) becomes
13
C depleted up to 3‰ (Lehmann et al.,
2002). On the molecular level, laboratory experiments of anoxic degradation showed different trends: remaining fatty acid became
13
C enriched (up to 7‰), alkenones
depleted (up to 6‰) and sterols showed no change (Sun et al., 2004). On this basis, Sun et al. (2004) postulated that the direction of the isotopic shift during degradation depends on the functional moiety being degraded; this moiety might have a different 13C than the average molecule. In a study of biodegradation of oils (i.e., a post maturation process), Sun et al. (2005) found a
13
C enrichment of up to 4‰ for C15-C18 n-alkanes
explained by a KIE. However, in the same set of samples, both phytane and pristane showed depletion in
(up to 2‰). The preferential loss of a
heavier carboxyl
carbon from OM during catagenesis is used to explain the generation of lighter oil
fractions (Galimov, 2006 and references therein). On the atomic level in a given molecule, carbon atoms having alkene and aldehyde functionalities are also
heavier
compared to carbon atoms of alkane bonds (Galimov, 2006). This can further support the view that degradation of molecules through their reactive sites (i.e. by loosing functional groups) can lead to a decrease in the overall
of the molecule. Moreover, since
sulfurization is acting specifically on functionalized groups (e.g. carbonyls) to form C-S bonds it removes this active sites and prevent their further reaction/degradation (Amrani , 2014). If this degradation-preservation hypothesis hold some premise, then 13Cbound−free should increase with the decrease in (Free-biomarker)fraction, accounting for the narrow variation of this variable (~0-6‰). Assuming that Eq. 2 reflects to some extent the degradation-preservation balance during diagenesis, the data for which concentrations were reported as well (Table S1) is depicted in Fig. 2, which shows the expected trend. Linear regression through the origin of the data in Fig. 2 suggests the fractionation factor of a Rayleigh distillation model to be ~1.5‰ for phytane and C25 HBI. The pristane data, taken from Schouten et al., (2001), is somewhat more scattered (R2 = 0.2) and the enrichment factor is higher (-4.5‰). Note that the phytane data is taken from two studies on the Monterey Formation (Schouten et al., 1997; Schouten et al., 2001) while that of the C25 HBI is from an Holocene young sediments (<3Kyr, Sinninghe Damsté et al., 2007). A higher
fractionation of pristane compared to phytane was also observed by
Sun et al. (2005) for the biodegradation of natural oil. The data from laboratory degradation experiments of POC and alkenones discussed above (Lehmann et al., 2002; Sun et al., 2004) is plotted in a similar fashion in Fig. 3; in this case (Free-biomarker)fraction = [Free-biomarker]remaining/[Free-biomarker]initial. Despite
the different C-skeleton and functionalities of alkenones and isoprenoids, the enrichment factor is comparable, and similar to that of the POC. It is also noted that like the steroids field data that showed 13Cbound−free ≈ 0 (Filley et al., 1996; Schouten et al., 2001), Sun et al. (2004) did not observe changes in 13C with steroids degradation. However, the positive 13Cbound−free observed by Hefter (1995) for fatty acids, is not comparable with the enrichment found by Sun et al. (2004) for this biomarker upon degradation. 5
Summary Compilation of almost 300 paired measurements of compound specific
from the
free- and S-bound fractions shows a consistent enrichment in the latter of 2±1‰ on the average. Since the data is of varying sources and ages, it is concluded that different precursors or fractionation during OM sulfurization or maturation cannot account for the observed enrichment of
in the S-bound fraction. Although the molecular
mechanism is unknown yet, biodegradation of the free biomarker fraction at early diagenesis stage can offer a more robust pathway as conceptually depicted in Fig. 4. This in turn suggests that the S-bound fraction may better represent the original
values of
biomarkers used in paleoenvironmental studies. Regardless if our suggested explanation is valid, we have highlighted this intriguing phenomenon and the need to explore it. Future studies, both field and laboratory experiments will have to confirm this (and/or other) hypothesis and mechanism. Acknowledgment. We acknowledge Paul Greenwood for reviewing and improving an earlier version of this manuscript. Y. O. R. and A.A. acknowledges grant No. 15/16 from the Ministry of National Infrastructures Energy and Water Resources of Israel. A.A thanks the Israeli Science Foundation grant No. 1738/16 for partial support of this study. 6
Figure captions
Fig. 1. 13C of isoprenoid based skeletons and fatty acids (a) and n-alkanes (b) in the S-bound vs. free-HC fractions from 17 studies. The bold line is the 1:1 agreement line, while the dashed and dotted lines in both panels are the average±stdev of (a). A complete reference list is given in Table S1. Fig. 2. Changes in 13C vs. Ln(Free-HC)fraction of naturally sulfurized isoprenoids. Fig. 3. Changes in 13C vs. Ln(Free-HC)fraction of degradation experiments of POC (Lehmann et al., 2002) and alkenones (Sun et al., 2004). Fig. 4. Conceptual illustration for the degradation-preservation hypothesis affecting the 7
of free biomarkers through its diagenesis. References
Amrani, A., 2014. Organosulfur Compounds: Molecular and Isotopic Evolution from Biota to Oil and Gas. Annual Review of Earth and Planetary Sciences 42, 733-768. Bjorøy, M., Hall, P.B., Hustad, E., Williams, J.A., 1992. Variation in stable carbon isotope ratios of individual hydrocarbons as a function of artificial maturity. Organic Geochemistry 19, 89-105. Filley, T.R., Freeman, K.H., Hatcher, P.G., 1996. Carbon isotope relationships between sulfide-bound steroids and proposed functionalized lipid precursors in sediments from the Santa Barbara Basin, California. Organic Geochemistry 25, 367-377. Forster, A., Kuypers, M.M.M., Turgeon, S.C., Brumsack, H.-J., Petrizzo, M.R., Sinninghe Damsté, J.S., 2008. The Cenomanian/Turonian oceanic anoxic event in the South Atlantic: New insights from a geochemical study of DSDP Site 530A. Palaeogeography, Palaeoclimatology, Palaeoecology 267, 256-283. Galimov, E.M., 2006. Isotope organic geochemistry. Organic Geochemistry 37, 12001262. Grice, K., Cao, C., Love, G.D., Böttcher, M.E., Twitchett, R.J., Grosjean, E., Summons, R.E., Turgeon, S.C., Dunning, W., Jin, Y., 2005. Photic Zone Euxinia During the Permian-Triassic Superanoxic Event. Science 307, 706-709. Hartgers, W.A., Sinninghe Damsté, J.S., Requejo, A.G., Allan, J., Hayes, J.M., Ling, Y., Xie, T.-M., Primack, J., de Leeuw, J.W., 1994. A molecular and carbon isotopic study towards the origin and diagenetic fate of diaromatic carotenoids. Organic Geochemistry 22, 703-725. Hayes, J.M., Freeman, K.H., Popp, B.N., Hoham, C.H., 1990. Compound-specific isotopic analyses: A novel tool for reconstruction of ancient biogeochemical processes. Organic Geochemistry 16, 1115-1128. Hefter, J., Hauke, V., Richnow, H.H., Michaelis, W., 1995. Alkanoic Subunits in SulfurRich Geomacromolecules, Geochemical Transformations of Sedimentary Sulfur. American Chemical Society, pp. 93-109.
Kohnen, M.E., Schouten, S., Damsté, J.S.S., de Leeuw, J.W., Merrit, D., Hayes, J., 1992a. The combined application of organic sulphur and isotope geochemistry to asses multiple sources of palaeobiochemicals with identical carbon skeletons. Organic Geochemistry 19, 403-419. Kohnen, M.E.L., Schouten, S., Sinninghe Damsté, J.S., de Leeuw, J.W., Merritt, D.A., Hayes, J.M., 1992b. Recognition of paleobiochemicals by a combined molecular sulfur and isotope geochemical approach. Science 256, 358-362. Kuypers, M.M., Blokker, P., Erbacher, J., Kinkel, H., Pancost, R.D., Schouten, S., Damsté, J.S.S., 2001. Massive expansion of marine archaea during a mid-Cretaceous oceanic anoxic event. Science 293, 92-95. Lehmann, M.F., Bernasconi, S.M., Barbieri, A., McKenzie, J.A., 2002. Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis. Geochimica et Cosmochimica Acta 66, 3573-3584. Putschew, A., Schaeffer-Reiss, C., Schaeffer, P., Koopmans, M.P., de Leeuw, J.W., Lewan, M.D., Sinninghe Damsté, J.S., Maxwell, J.R., 1998. Release of sulfur- and oxygen-bound components from a sulfur-rich kerogen during simulated maturation by hydrous pyrolysis. Organic Geochemistry 29, 1875-1890. Schouten, S., Schoell, M., Rijpstra, W.I.C., Sinninghe Damsté, J.S., de Leeuw, J.W., 1997. A molecular stable carbon isotope study of organic matter in immature Miocene Monterey sediments, Pismo basin. Geochimica et Cosmochimica Acta 61, 2065-2082. Schouten, S., Schoell, M., Sinninghe Damsté, J., Summons, R.E., De Leeuw, J., 2001. Molecular biogeochemistry of Monterey sediments, Naples Beach, California: II. Stable carbon isotopic compositions of free and sulphur bound carbon skeletons, in: Isaacs, C., Rullkötter, J. (Eds.), The Monterey Formation: From Rocks to Molecules. Columbia University Press, New York, pp. 175-188. Schouten, S., Sinninghe Damsté, J.S., Kohnen, M.E.L., De Leeuw, J.W., 1995. The effect of hydrosulphurization on stable carbon isotopic compositions of free and sulphur-bound lipids. Geochimica et Cosmochimica Acta 59, 1605-1609. Sinninghe Damsté, J.S., De Leeuw, J.W., 1990. Analysis, structure and geochemical significance of organically-bound sulphur in the geosphere: state of the art and future research. Organic Geochemistry 16, 1077-1101. Sinninghe Damsté, J.S., Kuypers, M.M.M., Pancost, R.D., Schouten, S., 2008. The carbon isotopic response of algae, (cyano)bacteria, archaea and higher plants to the late Cenomanian perturbation of the global carbon cycle: Insights from biomarkers in black shales from the Cape Verde Basin (DSDP Site 367). Organic Geochemistry 39, 1703-1718. Sinninghe Damsté, J.S., Rijpstra, W.I.C., Coolen, M.J.L., Schouten, S., Volkman, J.K., 2007. Rapid sulfurisation of highly branched isoprenoid (HBI) alkenes in sulfidic Holocene sediments from Ellis Fjord, Antarctica. Organic Geochemistry 38, 128-139. Sun, M.-Y., Zou, L., Dai, J., Ding, H., Culp, R.A., Scranton, M.I., 2004. Molecular carbon isotopic fractionation of algal lipids during decomposition in natural oxic and anoxic seawaters. Organic Geochemistry 35, 895-908. Sun, Y., Chen, Z., Xu, S., Cai, P., 2005. Stable carbon and hydrogen isotopic fractionation of individual n-alkanes accompanying biodegradation: evidence from a group of progressively biodegraded oils. Organic Geochemistry 36, 225-238. 8 Figures