Pyrite-based trace element fingerprints for methane and oil seepage
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Keywords: authigenic pyrite, trace elements, methane seepage, oil seepage
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Abstract
Figures
 Figure 1 Locations of the investigated sediment cores. (a) Gravity core locations from oil seeps at Campeche and Sigsbee Knolls in the southern Gulf of Mexico. (b) Gravity core locations of methane seeps from the South China Sea. |  Figure 2 Mn/Fe ratios and Mo content in methane seepage- and oil seepage-derived authigenic pyrite. Data from Dongsha and Shenhu seepage areas are from Lin et al. (2022). |  Figure 3 Trace element content in methane seepage- and oil seepage-derived authigenic pyrite. (a) Zn and Cu content, (b) Ni and V content; data from Dongsha and Shenhu seepage areas are from Lin et al. (2022). |
| Figure 1 | Figure 2 | Figure 3 |
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Introduction
Marine hydrocarbon seeps support chemosynthesis-based microbial habitats of archaea and sulphate reducing bacteria performing the sulphate driven anaerobic oxidation of methane (SD-AOM). This process triggers the formation of authigenic minerals that record the dynamics and intensity of seepage, redox fluctuations, and the evolution of fluid composition and microbial activity (Feng et al., 2009
Feng, D., Chen, D., Peckmann, J. (2009). Rare earth elements in seep carbonates as tracers of variable redox conditions at ancient hydrocarbon seeps. Terra Nova 21, 49–56. https://doi.org/10.1111/j.1365-3121.2008.00855.x.
; Smrzka et al., 2016Smrzka, D., Zwicker, J., Klügel, A., Monien, P., Bach, W., Bohrmann, G., Peckmann, J. (2016) Establishing criteria to distinguish oil-seep from methane-seep carbonates. Geology 44, 667–670. https://doi.org/10.1130/G38029.1
, 2019aSmrzka, D., Zwicker, J., Misch, D., Walkner, C., Gier, S., Monien, P., Bohrmann, G., Peckmann, J. (2019a) Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico. Sedimentology 66, 2318–2353. https://doi.org/10.1111/sed.12593
). Pyrite is a widespread mineral that forms during sulphate reduction at seeps, which are hotspots of pyrite authigenesis. Detailed studies on pyrite morphology and its stable isotopic and trace element compositions have recently been conducted in seep settings (Lin et al. 2022Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
; Wang et al., 2022Wang, B., Lei, H., Huang, F. (2022) Impacts of sulfate-deriven anaerobic oxidation of methane on the morphology, sulfur isotope, and trace element content of authigenic pyrite in marine sediments of the northern South China Sea. Marine and Petroleum Geology 139, article 105578. https://doi.org/10.1016/j.marpetgeo.2022.105578
).Pyrites from various environments show different trace element patterns depending on morphology, grain size, formation temperatures, and the composition of parent fluids (Gregory et al., 2015
Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
). The trace element inventory of pyrite has been refined into a first order proxy for the deep time evolution of Earth’s biosphere (Large et al., 2014Large, R.R., Halpin, J.A., Danyushevsky, L.V., Maslennikov, V.V., Bull, S.W., Long, J.A., Gregory, D.D., Lounejeva, E., Lyons, T.W., Sack, P.J., McGoldrock, P.J., Calver, C.R. (2014) Trace element content of sedimentary pyrite as a new proxy for deep-time ocean-atmosphere evolution. Earth and Planetary Science Letters 389, 209–220. https://doi.org/10.1016/j.epsl.2013.12.020
). Sedimentary pyrite formation is controlled by the biogeochemical cycles of sulphur, carbon and iron, and constitutes a relevant long term sink for trace elements during early diagenesis (Huerta-Diaz and Morse, 1992Huerta-Diaz, M.A., Morse, T.W. (1992) Pyritization of trace metals in anoxic marine sediments. Geochimica et Cosmochimica Acta 56, 2681–2702. https://doi.org/10.1016/0016-7037(92)90353-K
). Pyrite scavenges and incorporates trace elements from parent fluids, including many redox sensitive and bio-essential trace metals including Mn, Mo, Ni, Cu, Zn, Cr, As and Se (Huerta-Diaz and Morse, 1992Huerta-Diaz, M.A., Morse, T.W. (1992) Pyritization of trace metals in anoxic marine sediments. Geochimica et Cosmochimica Acta 56, 2681–2702. https://doi.org/10.1016/0016-7037(92)90353-K
; Morse and Arakaki, 1993Morse, J.W., Arakaki, T. (1993) Adsorption and coprecipitation of divalent metals with mackinawite (FeS). Geochimica et Cosmochimica Acta 57, 3635–3640. https://doi.org/10.1016/0016-7037(93)90145-M
). Pyrite based proxies for fluid composition, sulphate reduction processes and redox conditions are a trending topic in hydrocarbon seep research (Miao et al., 2022Miao, X., Feng, X., Li, J., Liu, X., Liang, J., Feng, J., Xiao, Q., Dan, X., Wie, J. (2022) Enrichment mechanism of trace elements in pyrite under methane seepage. Geochemical Perspectives Letters 21, 18–22. https://doi.org/10.7185/geochemlet.2211
, Lin et al., 2022Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
; Wang et al., 2022Wang, B., Lei, H., Huang, F. (2022) Impacts of sulfate-deriven anaerobic oxidation of methane on the morphology, sulfur isotope, and trace element content of authigenic pyrite in marine sediments of the northern South China Sea. Marine and Petroleum Geology 139, article 105578. https://doi.org/10.1016/j.marpetgeo.2022.105578
; Domingos et al., 2023Domingos, J.M., Runge, E., Dreher, C., Chiu, T.-H., Shuster, J., Fischer, S., Kappler, A., Duda, J.-P., Xu, J., Mansor, M. (2023) Inferred pyrite growth via the particle attachment pathway in the presence of trace metals. Geochemical Perspectives Letters 26, 14–19. https://doi.org/10.7185/geochemlet.2318
). Hydrocarbon emissions from natural methane and oil seeps represent prominent pathways of carbon transfer from the geosphere to the hydrosphere. Constraining their influence on the marine carbon and sulphur cycles is therefore critical in order to improve the quantification of global methane budgets and to understand the dynamics of – and responses to – natural and anthropogenic oil spills. A pyrite based proxy offers insights into the evolution of fluid composition at seeps, which is critical because fluid composition governs microbial and metazoan ecology (Orcutt et al., 2010Orcutt, B.N., Joye, S.M., Kleindienst, S., Knittel, K., Ramette, A., Reitz, A., Samarkin, V., Treude, T., Boetius, A. (2010) Impact of natural oil and higher hydrocarbons on microbial diversity, distribution, and activity in Gulf of Mexico cold-seep sediments. Deep-Sea Research II 57, 2008–2021. https://doi.org/10.1016/j.dsr2.2010.05.014
). The significance of distinguishing oil- from methane-dominated seep systems has gained traction in recent years in the search for end member system identification (Smrzka et al., 2016Smrzka, D., Zwicker, J., Klügel, A., Monien, P., Bach, W., Bohrmann, G., Peckmann, J. (2016) Establishing criteria to distinguish oil-seep from methane-seep carbonates. Geology 44, 667–670. https://doi.org/10.1130/G38029.1
; Akam et al., 2021Akam, S.A., Lyons, T.W., Coffin, R.B., McGee, D., Naehr, T.H., Bates, S.M., Clarkson, C., Reese, B.K. (2021) Carbon-sulfur signals of methane versus crude oil diagenetic decomposition and U-Th age relationships for authigenic carbonates from asphalt seeps, southern Gulf of Mexico. Chemical Geology 581, 120395. https://doi.org/10.1016/j.chemgeo.2021.120395
; Krake et al., 2022Krake, N, Birgel, D., Smrzka, D., Zwicker, J., Huang, H., Feng, D., Bohrmann, G., Peckmann, J. (2022) Molecular and isotopic signatures of oil-driven bacterial sulfate reduction at seeps in the southern Gulf of Mexico. Chemical Geology 595, 120797. https://doi.org/10.1016/j.chemgeo.2022.120797
), and reliable proxies are continuously being explored and refined.This study presents a first comparison of the trace element compositions of authigenic pyrite derived from methane and oil seeps. Motivated by current efforts to exploit the potential of pyrite based geochemical proxies, we provide new constraints on environmental conditions during seepage and microbial oxidation of heavy hydrocarbons, and establish trace element fingerprints to distinguish methane seeps from oil dominated seeps. These results improve our understanding of trace element liberation during microbial sulphate reduction, and their subsequent incorporation into pyrite during early diagenesis, while emphasising the role of pyrite based trace element geochemistry as a main, or complementary, source of information on redox conditions and fluid compositions in modern, and potentially, ancient seepage environments.
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Study Sites – Endmembers of Hydrocarbon Seepage
Two localities of methane and heavy hydrocarbon seepage were selected for a comparative study of authigenic pyrite forming in different environments. The Campeche and Sigsbee Knolls in the southern Gulf of Mexico (GoM; Fig. 1a) are two salt provinces that exhibit a set of hummocky seafloor structures related to salt tectonism, enabling the seepage of methane and crude oil, as well as the formation of gas hydrates within the sediments and on the seafloor (Sahling et al., 2016
Sahling, H., Borowski, C., Escobar-Briones, E., Gaytán-Caballero, A., Hsu, C.-W., Loher, M., MacDonald, I., Marcon, Y., Pape, T., Römer, M., Rubin-Blum, M., Schubotz, F., Smrzka, D., Wegener, G., Bohrmann, G. (2016) Massive asphalt deposits, oil seepage, and gas venting support abundant chemosynthetic communities at the Campeche Knolls, southern Gulf of Mexico. Biogeosciences 13, 4491–4512. https://doi.org/10.5194/bg-13-4491-2016
). Crude oil and asphalt in the southern GoM fuel microbial sulphate reduction independent of SD-AOM (Joye et al., 2004Joye, S.B., Boetius, A., Orcutt, B.N., Montoya, J.P., Schulz, H.N., Erickson, M.J., Lugo, S.K. (2004) The anaerobic oxidation of methane and sulfate reduction in sediments from Gulf of Mexico cold seeps. Chemical Geology 205, 219–238. https://doi.org/10.1016/j.chemgeo.2003.12.019
), and have shaped a unique environment inhabited by distinct macro- and micro-faunal communities (Orcutt et al., 2010Orcutt, B.N., Joye, S.M., Kleindienst, S., Knittel, K., Ramette, A., Reitz, A., Samarkin, V., Treude, T., Boetius, A. (2010) Impact of natural oil and higher hydrocarbons on microbial diversity, distribution, and activity in Gulf of Mexico cold-seep sediments. Deep-Sea Research II 57, 2008–2021. https://doi.org/10.1016/j.dsr2.2010.05.014
). This study considers the Campeche and Sigsbee Knolls as crude oil dominated end member seepage systems.Figure 1 Locations of the investigated sediment cores. (a) Gravity core locations from oil seeps at Campeche and Sigsbee Knolls in the southern Gulf of Mexico. (b) Gravity core locations of methane seeps from the South China Sea.
The South China Sea (SCS) is a large marginal sea in the western Pacific Ocean located at the transect between the Eurasian, Pacific, and Indian plates (Fig. 1b). The northern SCS is a passive continental margin covered by thick successions of Neogene sediments that promote abundant hydrocarbon generation manifested as mud volcanoes and diapirs, gas chimneys, and seafloor seepage sites. Among the numerous seep sites discovered in the SCS over the past decades, the Dongsha and Shenhu seepage provinces are among the best studied sites, which have been previously studied regarding the genesis of authigenic pyrite (Lin et al., 2022
Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
). To the northeast of these seepage areas, and related to the convergence of the Eurasian and the Philippine Sea plates, lies the south-western Taiwan accretionary prism, which harbours the Yam Seep area located at the northern crest of Four-Way Closure Ridge (FWCR; Tseng et al., 2023Tseng, Y., Römer, M., Lin, S., Pape, T., Berndt, C., Chen, T.-T., Paull, C.K., Caress, D.W., Bohrmann, G. (2023) Yam Seep at Four-Way Closure Ridge: a prominent active gas seep system at the accretionary wedge SW offshore Taiwan. International Journal of Earth Sciences 112, 1043–1061. https://doi.org/10.1007/s00531-022-02280-4
). The Dongsha, Shenhu, and Yam Seep sites are all characterised by seepage of biogenic and thermogenic methane, representing methane dominated end member seepage systems.top
Material and Methods
Authigenic pyrite was obtained from a total of five gravity cores, which were sampled over the course of four cruises between 2015 and 2018 in the southern GoM and the northern SCS (Fig. 1). Between 20 and 30 grams of sediment were sampled from the cores at intervals of 20 cm, freeze dried over 24 hrs, and subsequently powdered by hand using an agate pestle and mortar. Aliquots of unpowdered sediment were then sieved with deionised water through a 0.063 mm sieve, and pyrite aggregates were hand picked under a binocular microscope from the coarse fraction. Pyrite grains were mounted onto epoxy discs, polished to a smooth surface and coated with carbon for scanning electron microscopy and electron probe microanalysis (EMPA). Major element content in pyrite was determined using a Cameca SX-100 electron microprobe. Major and trace element composition of pyrite was determined via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). All element data and additional information are given in the Supplementary Information.
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Results
Manganese, Mo, Cu, and Zn contents in pyrite show different distribution patterns for the two investigated types of hydrocarbon seepage. Pyrite from oil dominated seeps is characterised by higher content of Mn and Mo than in methane seepage-derived pyrite. The Mn enrichment is expressed as Mn/Fe ratios shown in Figure 2, which is two to three orders of magnitude higher in oil seepage-derived pyrite. Molybdenum content is higher by one order of magnitude in oil seep pyrite (Fig. 2). Methane seep pyrite from the four sites shows variable Mn/Fe ratios. The contents of Cu and Zn are higher in oil seep pyrite than in methane seep pyrite, and the distribution of these elements allows for a distinction between the two seepage environments (Fig. 3a). This distinction is less clear for Ni and V (Fig. 3b).
Figure 2 Mn/Fe ratios and Mo content in methane seepage- and oil seepage-derived authigenic pyrite. Data from Dongsha and Shenhu seepage areas are from Lin et al. (2022)
Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
.Figure 3 Trace element content in methane seepage- and oil seepage-derived authigenic pyrite. (a) Zn and Cu content, (b) Ni and V content; data from Dongsha and Shenhu seepage areas are from Lin et al. (2022)
Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
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Discussion
A combined Fe-Mn-Mo fingerprint. Microbial sulphate reduction at seeps is coupled to the oxidation of methane, sedimentary organic matter, and high molecular weight hydrocarbons that make up crude oil (Joye et al., 2004
Joye, S.B., Boetius, A., Orcutt, B.N., Montoya, J.P., Schulz, H.N., Erickson, M.J., Lugo, S.K. (2004) The anaerobic oxidation of methane and sulfate reduction in sediments from Gulf of Mexico cold seeps. Chemical Geology 205, 219–238. https://doi.org/10.1016/j.chemgeo.2003.12.019
; Smrzka et al., 2019aSmrzka, D., Zwicker, J., Misch, D., Walkner, C., Gier, S., Monien, P., Bohrmann, G., Peckmann, J. (2019a) Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico. Sedimentology 66, 2318–2353. https://doi.org/10.1111/sed.12593
). The microbial oxidation rates of these compounds are governed by the microbial consortium capable of using the particular electron donors, which affects sulphate reduction rates and thus the amount of sulphide released to sedimentary pore water (Smrzka et al., 2019aSmrzka, D., Zwicker, J., Misch, D., Walkner, C., Gier, S., Monien, P., Bohrmann, G., Peckmann, J. (2019a) Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico. Sedimentology 66, 2318–2353. https://doi.org/10.1111/sed.12593
). The concentration of dissolved sulphide species in pore fluids in turn influences the solubility and mobility of redox sensitive trace elements, particularly Mo and to a lesser extent Mn. In addition, the oxidation of sedimentary organic matter and crude oil will inevitably lead to the partial or complete breakdown of organic compounds, which are themselves carriers of trace elements (Smrzka et al., 2020Smrzka, D., Feng, D., Himmler, T., Zwicker, J., Hu, Y., Monien, P., Tribovillard, N., Chen, D., Peckmann, J. (2020) Trace elements in methane-seep carbonates: Potentials, limitations, and perspectives. Earth-Science Reviews 208, article 103263. https://doi.org/10.1016/j.earscirev.2020.103263
).Manganese and Mo contents in pyrite allow us to distinguish between the two seepage environments (Fig. 2). The Mo content in oil seep pyrite is up to an order of magnitude higher than in methane seep pyrite and sedimentary pyrite reported from black shales (Gregory et al., 2015
Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
). Molybdenum is incorporated into pyrite under sulphidic conditions, after stabilising as thiomolybdate, and may also be fixed by organic matter (Morse and Luther, 1999Morse, J.W., Luther III, G.W. (1999) Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochimica et Cosmochimica Acta 63, 3373–3378. https://doi.org/10.1016/S0016-7037(99)00258-6
; Gregory et al., 2015Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
). Manganese does not usually reside in pyrite in high concentrations, yet may form sulphides adsorbed to mackinawite; a precursor mineral of pyrite during early diagenesis (Gregory et al., 2015Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
). Manganese is usually incorporated into Ca-rich rhodochrosite or high-Mg calcite under sulphidic conditions during early diagenesis (Suess, 1978Suess, E. (1978) Mineral phases formed in anoxic sediments by microbial decomposition of organic matter. Geochimica et Cosmochimica Acta 43, 339–352. https://doi.org/10.1016/0016-7037(79)90199-6
), but may incorporate into pyrite at high Mn2+ concentrations (Morse and Arakaki, 1993Morse, J.W., Arakaki, T. (1993) Adsorption and coprecipitation of divalent metals with mackinawite (FeS). Geochimica et Cosmochimica Acta 57, 3635–3640. https://doi.org/10.1016/0016-7037(93)90145-M
; Morse and Luther, 1999Morse, J.W., Luther III, G.W. (1999) Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochimica et Cosmochimica Acta 63, 3373–3378. https://doi.org/10.1016/S0016-7037(99)00258-6
). Manganese incorporation into pyrite may also proceed via the uptake of precursor manganese sulphide phases, which are stable at high Mn/Fe ratios and high sulphide levels (Shikazono et al., 1994Shikazono, N., Nakata, M., Tokuyama, E. (1994) Pyrite with high Mn content from the Nankai Trough formed from subduction-induced cold seepage. Marine Geology 118, 303–313. https://doi.org/10.1016/0025-3227(94)90090-6
). A recent study indicates that Mn is distributed randomly within pyrite and held within microcrystals that formed early during diagenesis, suggesting that the partitioning behaviour of dissolved Mn into authigenic minerals is not as straightforward as previously thought (Atienza et al., 2023Atienza, N.M.M., Gregory, D.D., Taylor, S.D., Swing, M., Perea, D.E., Owens, J.D., Lyons, T.W. (2023) Refined views of ancient ocean chemistry: Tracking trace element incorporation in pyrite framboids using atom probe tomography. Geochimica et Cosmochimica Acta 357, 1–12. https://doi.org/10.1016/j.gca.2023.07.013
). Although Mn can be enriched in pyrite formed by SD-AOM (Lin et al., 2022Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
), the Mn/Fe ratios in pyrite from oil seeps are orders of magnitude higher than in pyrite from methane seeps (Fig. 2). High levels of dissolved sulphide favour the incorporation of Mn and Mo into pyrite (Wang et al., 2022Wang, B., Lei, H., Huang, F. (2022) Impacts of sulfate-deriven anaerobic oxidation of methane on the morphology, sulfur isotope, and trace element content of authigenic pyrite in marine sediments of the northern South China Sea. Marine and Petroleum Geology 139, article 105578. https://doi.org/10.1016/j.marpetgeo.2022.105578
), suggesting that the microbial oxidation of crude oil in sediments may enable persistent and highly sulphidic conditions in ambient pore waters controlled by the extent and rate of microbial metabolism. Microbial crude oil oxidation coupled to sulphate reduction enables carbonate precipitation and sulphide production, despite its generally slower microbial oxidation rate compared to SD-AOM (Joye et al., 2004Joye, S.B., Boetius, A., Orcutt, B.N., Montoya, J.P., Schulz, H.N., Erickson, M.J., Lugo, S.K. (2004) The anaerobic oxidation of methane and sulfate reduction in sediments from Gulf of Mexico cold seeps. Chemical Geology 205, 219–238. https://doi.org/10.1016/j.chemgeo.2003.12.019
; Smrzka et al., 2019aSmrzka, D., Zwicker, J., Misch, D., Walkner, C., Gier, S., Monien, P., Bohrmann, G., Peckmann, J. (2019a) Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico. Sedimentology 66, 2318–2353. https://doi.org/10.1111/sed.12593
). Components of crude oils contain variable contents of organic sulphur residing in compounds including thiols, sulphides, and thiophenes (Tissot and Welte, 1984Tissot, B.T., Welte, D.H. (1984) Petroleum formation and occurrences. 2nd edition Springer, Berlin.
). The high sulphur content in oils (>2 wt. %) from the southern GoM and the increase of dissolved hydrogen sulphide gas emitted at oil seeps (Smrzka et al., 2019aSmrzka, D., Zwicker, J., Misch, D., Walkner, C., Gier, S., Monien, P., Bohrmann, G., Peckmann, J. (2019a) Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico. Sedimentology 66, 2318–2353. https://doi.org/10.1111/sed.12593
) suggest that the microbial mineralisation of crude oil may act as an additional source of sulphur to pore waters.The high Mn/Fe ratios of oil seep pyrite may also be due to an effective manganese (oxy)hydroxide shuttle process, transferring adsorbed trace elements from seawater to the sediments (Scholz et al., 2013
Scholz, F., McManus, J., Sommer, S. (2013) The manganese and iron shuttle in a modern euxinic basin and implications for molybdenum cycling at euxinic ocean margins. Chemical Geology 355, 56–68. http://dx.doi.org/10.1016/j.chemgeo.2013.07.006
; Smrzka et al., 2020Smrzka, D., Feng, D., Himmler, T., Zwicker, J., Hu, Y., Monien, P., Tribovillard, N., Chen, D., Peckmann, J. (2020) Trace elements in methane-seep carbonates: Potentials, limitations, and perspectives. Earth-Science Reviews 208, article 103263. https://doi.org/10.1016/j.earscirev.2020.103263
). This shuttle may be intensified by the seepage of crude oil, acting as an additional source of Mn from sedimentary pore waters to bottom waters. Oil seeps at the Campeche Knolls emit oil droplets, oily gas bubbles, and asphalt fragments (Sahling et al., 2016Sahling, H., Borowski, C., Escobar-Briones, E., Gaytán-Caballero, A., Hsu, C.-W., Loher, M., MacDonald, I., Marcon, Y., Pape, T., Römer, M., Rubin-Blum, M., Schubotz, F., Smrzka, D., Wegener, G., Bohrmann, G. (2016) Massive asphalt deposits, oil seepage, and gas venting support abundant chemosynthetic communities at the Campeche Knolls, southern Gulf of Mexico. Biogeosciences 13, 4491–4512. https://doi.org/10.5194/bg-13-4491-2016
), which could act as an additional transport agent for trace elements to bottom waters. The emitted oil components will be oxidised aerobically in the bottom waters around the locus of seepage, releasing adsorbed or incorporated trace elements from organic compounds. This oil enhanced manganese (oxy)hydroxide shuttle would also explain the observed co-enrichment of Mn and Mo (cf. Scholz et al., 2013Scholz, F., McManus, J., Sommer, S. (2013) The manganese and iron shuttle in a modern euxinic basin and implications for molybdenum cycling at euxinic ocean margins. Chemical Geology 355, 56–68. http://dx.doi.org/10.1016/j.chemgeo.2013.07.006
) and to a lesser extent Cu (Fig. 3a) in oil seep pyrite.A combined Cu-Zn-Ni-V fingerprint. Copper and Zn are commonly co-enriched in marine organic matter and seafloor sediments, yet they are taken up into pyrite to different degrees. Copper forms strong complexes with organic matter and precipitates as copper sulphide during sulphate reduction (Morse and Luther, 1999
Morse, J.W., Luther III, G.W. (1999) Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochimica et Cosmochimica Acta 63, 3373–3378. https://doi.org/10.1016/S0016-7037(99)00258-6
). Copper enrichment in oil seep pyrite is likely derived from two sources, either released by reductive dissolution of manganese (oxy)hydroxides leading to a co-enrichment of Mn and Cu (Figs. 2, 3a) and released directly from crude oil during its microbial mineralisation. The affinity for Zn incorporation into pyrite is lower than for Cu due to ZnS precipitation prior to pyrite formation (Morse and Luther, 1999Morse, J.W., Luther III, G.W. (1999) Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochimica et Cosmochimica Acta 63, 3373–3378. https://doi.org/10.1016/S0016-7037(99)00258-6
). Zinc content varies considerably in diagenetic pyrite where it may be present as sphalerite inclusions (Gregory et al., 2015Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
). However, both Zn and Cu are co-enriched in oil seep pyrite, suggesting an overriding effect of microbial crude oil oxidation coupled to sulphate reduction. Nickel and V are micronutrients for phytoplankton growth, and are transported to the seafloor by organic particles and metal (oxy)hydroxides (Smrzka et al., 2019bSmrzka, D., Zwicker, J., Bach, W., Feng, D., Himmler, T., Chen, D., Peckmann, J. (2019b) The behavior of trace elements in seawater, sedimentary pore water, and their incorporation into carbonate minerals: A review. Facies 65, article 41. https://doi.org/10.1007/s10347-019-0581-4
). While Ni is commonly divalent in marine sediments, V is sensitive toward pH, redox conditions and dissolved sulphide concentrations, and V enrichment in reducing sediments relative to average continental crust (Thomson et al., 1998Thomson, J., Jarvis, I, Green, R.H., Green, D.A., Clayton, T. (1998) Mobility and immobility of redox-sensitive elements in deep-sea turbidites during shallow burial. Geochimica et Cosmochimica Acta 62, 643–656. https://doi.org/10.1016/S0016-7037(97)00378-5.
). Whereas Ni is incorporated into pyrite during early diagenesis, V usually resides in the non-sulphide fraction such as organic matter, silicates and carbonates (Huerta-Diaz and Morse, 1992Huerta-Diaz, M.A., Morse, T.W. (1992) Pyritization of trace metals in anoxic marine sediments. Geochimica et Cosmochimica Acta 56, 2681–2702. https://doi.org/10.1016/0016-7037(92)90353-K
; Gregory et al., 2015Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
). In contrast to Cu and Zn, the distribution of Ni and V show large overlap without clear enrichments between methane and oil seep pyrite (Fig. 3b).The release of trace elements during organic matter oxidation represents a source of trace elements during early diagenesis (Smrzka et al., 2020
Smrzka, D., Feng, D., Himmler, T., Zwicker, J., Hu, Y., Monien, P., Tribovillard, N., Chen, D., Peckmann, J. (2020) Trace elements in methane-seep carbonates: Potentials, limitations, and perspectives. Earth-Science Reviews 208, article 103263. https://doi.org/10.1016/j.earscirev.2020.103263
), reflected by the trace element composition of pyrite (Miao et al., 2022Miao, X., Feng, X., Li, J., Liu, X., Liang, J., Feng, J., Xiao, Q., Dan, X., Wie, J. (2022) Enrichment mechanism of trace elements in pyrite under methane seepage. Geochemical Perspectives Letters 21, 18–22. https://doi.org/10.7185/geochemlet.2211
; Chen et al., 2023Chen, C., Wang, J., Algeo, T.J., Zhu, J.-M., Wang, Z., Ma, X., Cen, Y. (2023) Sulfate-driven anaerobic oxidation of methane inferred from trace-element chemistry and nickel isotopes in pyrite. Geochimica et Cosmochimica Acta 349, 81–95. https://doi.org/10.1016/j.gca.2023.04.002
). The present data set expands this distinction regarding the presence of oil compounds and their microbial mineralisation in marine sediments. Crude oils contain trace elements including Ni, V, Cu, and Zn, which are present as metalloporphyrin complexes derived from bacterial and plant pigments, as metal centres of microbial enzymatic cofactors, and in large organic matrices or other metal-binding functional groups (Duyck et al., 2007Duyck, C., Miekeley, N., Porto da Silveira, C.L., Aucélio, R.Q., Campos, R.C., Grinberg, P., Brandao, G.P. (2007) The determination of trace elements in crude oil and its heavy fractions by atomic spectrometry. Spectrochimica Acta Part B 62, 939–951. https://doi.org/10.1016/j.sab.2007.04.013.
). Only Cu and Zn are systematically enriched in oil seep pyrite relative to its methane seep counterpart. These distribution patterns may be controlled by differences in the incorporation behaviour of the respective element into the pyrite structure (Gregory et al., 2015Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
), the trace element composition of the oxidised organic compounds, the varying rates of microbial oil degradation, and the composition of the host sediment (Figure S-2) during pyrite formation.Nickel and V reside in crude oils primarily as petroporphyrin complexes that are primarily part of the asphaltene fraction (Duyck et al., 2007
Duyck, C., Miekeley, N., Porto da Silveira, C.L., Aucélio, R.Q., Campos, R.C., Grinberg, P., Brandao, G.P. (2007) The determination of trace elements in crude oil and its heavy fractions by atomic spectrometry. Spectrochimica Acta Part B 62, 939–951. https://doi.org/10.1016/j.sab.2007.04.013.
). Petroleum hydrocarbons from southern GoM seeps are biodegraded, weathered (Schubotz et al., 2011Schubotz, F., Lipp, J.S., Elvert, M., Kasten, S., Prieto Mollar, X., Zabel, M., Bohrmann, G., Hinrichs, K.-U. (2011) Petroleum degradation and associated microbial signatures at the Chapopote asphalt volcano, Southern Gulf of Mexico. Geochimica et Cosmochimica Acta 75, 4377–4398. https://doi.org/10.1016/j.gca.2011.05.025
) and of low thermal maturity with Zn, Cu and Ni content between 10 and 100 ppm, and V content of up to 400 ppm (Smrzka et al., 2019aSmrzka, D., Zwicker, J., Misch, D., Walkner, C., Gier, S., Monien, P., Bohrmann, G., Peckmann, J. (2019a) Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico. Sedimentology 66, 2318–2353. https://doi.org/10.1111/sed.12593
). The lack of V enrichment in oil seep pyrite is most likely due to its low affinity towards pyrite incorporation, which is lower than for other trace metals (Gregory et al., 2015Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
). Although a slight Ni enrichment in oil seep pyrite is present, it cannot be used as an unambiguous indicator for microbial crude oil mineralisation. Manganese (oxy)hydroxides are also effective transport agents for Ni and V to sediments (Smrzka et al., 2019bSmrzka, D., Zwicker, J., Bach, W., Feng, D., Himmler, T., Chen, D., Peckmann, J. (2019b) The behavior of trace elements in seawater, sedimentary pore water, and their incorporation into carbonate minerals: A review. Facies 65, article 41. https://doi.org/10.1007/s10347-019-0581-4
), yet contrary to Mn and Mo, they are not distinctly enriched in oil seep pyrite. Also, the degree to which Ni and V are released from organic matter during microbial remineralisation may be smaller than for Cu and Zn. For instance, although all four elements reside in metalloporphyrins of organic matter and oils, biodegradation of nickel porphyrins is slower than for copper and vanadyl porphyrins (Sadowski et al., 2007Sadowski, Z., Szubert, A., Maliszewksa, I., Jazdzyk, E. (2007) A view on the organic matter and metalloporphyrins biodegradation as characteristic components of black shale ores. Advanced Materials Research 20-21, 95–98. https://doi.org/10.4028/www.scientific.net/AMR.20-21.95
). Microbial mineralisation of crude oil, which occurs both anaerobically in the sediments and aerobically in bottom waters, may selectively release trace elements from the oils depending on the complexity and size of their respective organic molecules.top
Synthesis
Manganese and Mo are enriched in authigenic pyrite derived from oil seeps compared to pyrite derived from methane seeps. This co-enrichment may be due to a persistent sulphidic environment that is seldom disrupted by pulses of oxygenation due to shifting redox boundaries in the sediment; conditions maintained by the oxidation of crude oil. These conditions would allow for an effective incorporation of Mo into pyrite, while also providing conditions that facilitate Mn uptake by pyrite. The presence of a Mn (oxy)hydroxide shuttle process at oil seeps would also explain the co-enrichment of Mn and Mo in oil seep pyrite. The enrichment of Cu, Zn, and Ni at oil seeps also argues for persistent sulphidic conditions and an additional source of these elements to pore water, both being triggered by the microbial oxidation of crude oil. The presence of an enhanced particulate shuttle process driven by Mn (oxy)hydroxides at oil seeps as invoked above is currently unknown, and represents a yet poorly constrained source for trace elements to bottom waters above seepage environments. Distinguishing the exact sources of trace elements in the two seepage systems that may include crude oil, as well as the dissolution of carbonates and clay minerals in the host sediments, will shed more light on trace element dynamics during early diagenesis. Comparing these results to other seepage sites from more diverse environments will increase the potential of pyrite based trace element proxies.
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Acknowledgements
This research was funded by the “Independent Projects for Postdocs” grant scheme awarded to DS by the Central Research Development Fund of the University of Bremen. We thank Anne Hübner, Stefan Sopke, Janice Malnati, and Andreas Klügel (University of Bremen) for sample preparation and assistance in EMPA analyses.
Editor: Juan Liu
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Data Availability Statement
Data will be made available on the Pangaea database (www.pangaea.de).
top
References
Akam, S.A., Lyons, T.W., Coffin, R.B., McGee, D., Naehr, T.H., Bates, S.M., Clarkson, C., Reese, B.K. (2021) Carbon-sulfur signals of methane versus crude oil diagenetic decomposition and U-Th age relationships for authigenic carbonates from asphalt seeps, southern Gulf of Mexico. Chemical Geology 581, 120395. https://doi.org/10.1016/j.chemgeo.2021.120395
Show in context The significance of distinguishing oil- from methane-dominated seep systems has gained traction in recent years in the search for end member system identification (Smrzka et al., 2016; Akam et al., 2021; Krake et al., 2022), and reliable proxies are continuously being explored and refined.
View in article
Atienza, N.M.M., Gregory, D.D., Taylor, S.D., Swing, M., Perea, D.E., Owens, J.D., Lyons, T.W. (2023) Refined views of ancient ocean chemistry: Tracking trace element incorporation in pyrite framboids using atom probe tomography. Geochimica et Cosmochimica Acta 357, 1–12. https://doi.org/10.1016/j.gca.2023.07.013
Show in context A recent study indicates that Mn is distributed randomly within pyrite and held within microcrystals that formed early during diagenesis, suggesting that the partitioning behaviour of dissolved Mn into authigenic minerals is not as straightforward as previously thought (Atienza et al., 2023).
View in article
Chen, C., Wang, J., Algeo, T.J., Zhu, J.-M., Wang, Z., Ma, X., Cen, Y. (2023) Sulfate-driven anaerobic oxidation of methane inferred from trace-element chemistry and nickel isotopes in pyrite. Geochimica et Cosmochimica Acta 349, 81–95. https://doi.org/10.1016/j.gca.2023.04.002
Show in context The release of trace elements during organic matter oxidation represents a source of trace elements during early diagenesis (Smrzka et al., 2020), reflected by the trace element composition of pyrite (Miao et al., 2022; Chen et al., 2023).
View in article
Domingos, J.M., Runge, E., Dreher, C., Chiu, T.-H., Shuster, J., Fischer, S., Kappler, A., Duda, J.-P., Xu, J., Mansor, M. (2023) Inferred pyrite growth via the particle attachment pathway in the presence of trace metals. Geochemical Perspectives Letters 26, 14–19. https://doi.org/10.7185/geochemlet.2318
Show in context Pyrite based proxies for fluid composition, sulphate reduction processes and redox conditions are a trending topic in hydrocarbon seep research (Miao et al., 2022, Lin et al., 2022; Wang et al., 2022; Domingos et al., 2023).
View in article
Duyck, C., Miekeley, N., Porto da Silveira, C.L., Aucélio, R.Q., Campos, R.C., Grinberg, P., Brandao, G.P. (2007) The determination of trace elements in crude oil and its heavy fractions by atomic spectrometry. Spectrochimica Acta Part B 62, 939–951. https://doi.org/10.1016/j.sab.2007.04.013
Show in context Crude oils contain trace elements including Ni, V, Cu, and Zn, which are present as metalloporphyrin complexes derived from bacterial and plant pigments, as metal centres of microbial enzymatic cofactors, and in large organic matrices or other metal-binding functional groups (Duyck et al., 2007).
View in article
Nickel and V reside in crude oils primarily as petroporphyrin complexes that are primarily part of the asphaltene fraction (Duyck et al., 2007).
View in article
Feng, D., Chen, D., Peckmann, J. (2009). Rare earth elements in seep carbonates as tracers of variable redox conditions at ancient hydrocarbon seeps. Terra Nova 21, 49–56. https://doi.org/10.1111/j.1365-3121.2008.00855.x
Show in context This process triggers the formation of authigenic minerals that record the dynamics and intensity of seepage, redox fluctuations, and the evolution of fluid composition and microbial activity (Feng et al., 2009; Smrzka et al., 2016, 2019a).
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Gregory, D.D., Large, R.R., Halpin, J.A., Baturina, E.L., Lyons, T.W., Wu, S., Danyushevsky, L., Sack, P.J., Chappaz, A., Maslennikov, V.V., Bull, S.W. (2015) Trace element content of sedimentary pyrite in black shales. Economic Geology 110, 1389–1410. https://doi.org/10.2113/econgeo.110.6.1389
Show in context Pyrites from various environments show different trace element patterns depending on morphology, grain size, formation temperatures, and the composition of parent fluids (Gregory et al., 2015).
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The Mo content in oil seep pyrite is up to an order of magnitude higher than in methane seep pyrite and sedimentary pyrite reported from black shales (Gregory et al., 2015).
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Molybdenum is incorporated into pyrite under sulphidic conditions, after stabilising as thiomolybdate, and may also be fixed by organic matter (Morse and Luther, 1999; Gregory et al., 2015).
View in article
Manganese does not usually reside in pyrite in high concentrations, yet may form sulphides adsorbed to mackinawite; a precursor mineral of pyrite during early diagenesis (Gregory et al., 2015).
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Zinc content varies considerably in diagenetic pyrite where it may be present as sphalerite inclusions (Gregory et al., 2015).
View in article
Whereas Ni is incorporated into pyrite during early diagenesis, V usually resides in the non-sulphide fraction such as organic matter, silicates and carbonates (Huerta-Diaz and Morse, 1992; Gregory et al., 2015).
View in article
These distribution patterns may be controlled by differences in the incorporation behaviour of the respective element into the pyrite structure (Gregory et al., 2015), the trace element composition of the oxidised organic compounds, the varying rates of microbial oil degradation, and the composition of the host sediment (Figure S-2) during pyrite formation.
View in article
The lack of V enrichment in oil seep pyrite is most likely due to its low affinity towards pyrite incorporation, which is lower than for other trace metals (Gregory et al., 2015).
View in article
Huerta-Diaz, M.A., Morse, T.W. (1992) Pyritization of trace metals in anoxic marine sediments. Geochimica et Cosmochimica Acta 56, 2681–2702. https://doi.org/10.1016/0016-7037(92)90353-K
Show in context Sedimentary pyrite formation is controlled by the biogeochemical cycles of sulphur, carbon and iron, and constitutes a relevant long term sink for trace elements during early diagenesis (Huerta-Diaz and Morse, 1992).
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Pyrite scavenges and incorporates trace elements from parent fluids, including many redox sensitive and bio-essential trace metals including Mn, Mo, Ni, Cu, Zn, Cr, As and Se (Huerta-Diaz and Morse, 1992; Morse and Arakaki, 1993).
View in article
Whereas Ni is incorporated into pyrite during early diagenesis, V usually resides in the non-sulphide fraction such as organic matter, silicates and carbonates (Huerta-Diaz and Morse, 1992; Gregory et al., 2015).
View in article
Joye, S.B., Boetius, A., Orcutt, B.N., Montoya, J.P., Schulz, H.N., Erickson, M.J., Lugo, S.K. (2004) The anaerobic oxidation of methane and sulfate reduction in sediments from Gulf of Mexico cold seeps. Chemical Geology 205, 219–238. https://doi.org/10.1016/j.chemgeo.2003.12.019
Show in context Crude oil and asphalt in the southern GoM fuel microbial sulphate reduction independent of SD-AOM (Joye et al., 2004), and have shaped a unique environment inhabited by distinct macro- and micro-faunal communities (Orcutt et al., 2010).
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Microbial sulphate reduction at seeps is coupled to the oxidation of methane, sedimentary organic matter, and high molecular weight hydrocarbons that make up crude oil (Joye et al., 2004; Smrzka et al., 2019a).
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Microbial crude oil oxidation coupled to sulphate reduction enables carbonate precipitation and sulphide production, despite its generally slower microbial oxidation rate compared to SD-AOM (Joye et al., 2004; Smrzka et al., 2019a).
View in article
Krake, N, Birgel, D., Smrzka, D., Zwicker, J., Huang, H., Feng, D., Bohrmann, G., Peckmann, J. (2022) Molecular and isotopic signatures of oil-driven bacterial sulfate reduction at seeps in the southern Gulf of Mexico. Chemical Geology 595, 120797. https://doi.org/10.1016/j.chemgeo.2022.120797
Show in context The significance of distinguishing oil- from methane-dominated seep systems has gained traction in recent years in the search for end member system identification (Smrzka et al., 2016; Akam et al., 2021; Krake et al., 2022), and reliable proxies are continuously being explored and refined.
View in article
Large, R.R., Halpin, J.A., Danyushevsky, L.V., Maslennikov, V.V., Bull, S.W., Long, J.A., Gregory, D.D., Lounejeva, E., Lyons, T.W., Sack, P.J., McGoldrock, P.J., Calver, C.R. (2014) Trace element content of sedimentary pyrite as a new proxy for deep-time ocean-atmosphere evolution. Earth and Planetary Science Letters 389, 209–220. https://doi.org/10.1016/j.epsl.2013.12.020
Show in context The trace element inventory of pyrite has been refined into a first order proxy for the deep time evolution of Earth’s biosphere (Large et al., 2014).
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Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
Show in context Detailed studies on pyrite morphology and its stable isotopic and trace element compositions have recently been conducted in seep settings (Lin et al. 2022; Wang et al., 2022).
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Pyrite based proxies for fluid composition, sulphate reduction processes and redox conditions are a trending topic in hydrocarbon seep research (Miao et al., 2022, Lin et al., 2022; Wang et al., 2022; Domingos et al., 2023).
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Among the numerous seep sites discovered in the SCS over the past decades, the Dongsha and Shenhu seepage provinces are among the best studied sites, which have been previously studied regarding the genesis of authigenic pyrite (Lin et al., 2022).
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Mn/Fe ratios and Mo content in methane seepage- and oil seepage-derived authigenic pyrite. Data from Dongsha and Shenhu seepage areas are from Lin et al. (2022).
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Trace element content in methane seepage- and oil seepage-derived authigenic pyrite. (a) Zn and Cu content, (b) Ni and V content; data from Dongsha and Shenhu seepage areas are from Lin et al. (2022).
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Although Mn can be enriched in pyrite formed by SD-AOM (Lin et al., 2022), the Mn/Fe ratios in pyrite from oil seeps are orders of magnitude higher than in pyrite from methane seeps (Fig. 2).
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Miao, X., Feng, X., Li, J., Liu, X., Liang, J., Feng, J., Xiao, Q., Dan, X., Wie, J. (2022) Enrichment mechanism of trace elements in pyrite under methane seepage. Geochemical Perspectives Letters 21, 18–22. https://doi.org/10.7185/geochemlet.2211
Show in context Pyrite based proxies for fluid composition, sulphate reduction processes and redox conditions are a trending topic in hydrocarbon seep research (Miao et al., 2022, Lin et al., 2022; Wang et al., 2022; Domingos et al., 2023).
View in article
The release of trace elements during organic matter oxidation represents a source of trace elements during early diagenesis (Smrzka et al., 2020), reflected by the trace element composition of pyrite (Miao et al., 2022; Chen et al., 2023).
View in article
Morse, J.W., Arakaki, T. (1993) Adsorption and coprecipitation of divalent metals with mackinawite (FeS). Geochimica et Cosmochimica Acta 57, 3635–3640. https://doi.org/10.1016/0016-7037(93)90145-M
Show in context Pyrite scavenges and incorporates trace elements from parent fluids, including many redox sensitive and bio-essential trace metals including Mn, Mo, Ni, Cu, Zn, Cr, As and Se (Huerta-Diaz and Morse, 1992; Morse and Arakaki, 1993).
View in article
Morse, J.W., Luther III, G.W. (1999) Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochimica et Cosmochimica Acta 63, 3373–3378. https://doi.org/10.1016/S0016-7037(99)00258-6
Show in context Molybdenum is incorporated into pyrite under sulphidic conditions, after stabilising as thiomolybdate, and may also be fixed by organic matter (Morse and Luther, 1999; Gregory et al., 2015).
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Manganese is usually incorporated into Ca-rich rhodochrosite or high-Mg calcite under sulphidic conditions during early diagenesis (Suess, 1978), but may incorporate into pyrite at high Mn2+ concentrations (Morse and Arakaki, 1993; Morse and Luther, 1999).
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Copper forms strong complexes with organic matter and precipitates as copper sulphide during sulphate reduction (Morse and Luther, 1999).
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The affinity for Zn incorporation into pyrite is lower than for Cu due to ZnS precipitation prior to pyrite formation (Morse and Luther, 1999).
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Orcutt, B.N., Joye, S.M., Kleindienst, S., Knittel, K., Ramette, A., Reitz, A., Samarkin, V., Treude, T., Boetius, A. (2010) Impact of natural oil and higher hydrocarbons on microbial diversity, distribution, and activity in Gulf of Mexico cold-seep sediments. Deep-Sea Research II 57, 2008–2021. https://doi.org/10.1016/j.dsr2.2010.05.014
Show in context A pyrite based proxy offers insights into the evolution of fluid composition at seeps, which is critical because fluid composition governs microbial and metazoan ecology (Orcutt et al., 2010).
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Crude oil and asphalt in the southern GoM fuel microbial sulphate reduction independent of SD-AOM (Joye et al., 2004), and have shaped a unique environment inhabited by distinct macro- and micro-faunal communities (Orcutt et al., 2010).
View in article
Sadowski, Z., Szubert, A., Maliszewksa, I., Jazdzyk, E. (2007) A view on the organic matter and metalloporphyrins biodegradation as characteristic components of black shale ores. Advanced Materials Research 20-21, 95–98. https://doi.org/10.4028/www.scientific.net/AMR.20-21.95
Show in context For instance, although all four elements reside in metalloporphyrins of organic matter and oils, biodegradation of nickel porphyrins is slower than for copper and vanadyl porphyrins (Sadowski et al., 2007).
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Sahling, H., Borowski, C., Escobar-Briones, E., Gaytán-Caballero, A., Hsu, C.-W., Loher, M., MacDonald, I., Marcon, Y., Pape, T., Römer, M., Rubin-Blum, M., Schubotz, F., Smrzka, D., Wegener, G., Bohrmann, G. (2016) Massive asphalt deposits, oil seepage, and gas venting support abundant chemosynthetic communities at the Campeche Knolls, southern Gulf of Mexico. Biogeosciences 13, 4491–4512. https://doi.org/10.5194/bg-13-4491-2016
Show in context The Campeche and Sigsbee Knolls in the southern Gulf of Mexico (GoM; Fig. 1a) are two salt provinces that exhibit a set of hummocky seafloor structures related to salt tectonism, enabling the seepage of methane and crude oil, as well as the formation of gas hydrates within the sediments and on the seafloor (Sahling et al., 2016).
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This shuttle may be intensified by the seepage of crude oil, acting as an additional source of Mn from sedimentary pore waters to bottom waters. Oil seeps at the Campeche Knolls emit oil droplets, oily gas bubbles, and asphalt fragments (Sahling et al., 2016), which could act as an additional transport agent for trace elements to bottom waters.
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Scholz, F., McManus, J., Sommer, S. (2013) The manganese and iron shuttle in a modern euxinic basin and implications for molybdenum cycling at euxinic ocean margins. Chemical Geology 355, 56–68. https://dx.doi.org/10.1016/j.chemgeo.2013.07.006
Show in context The high Mn/Fe ratios of oil seep pyrite may also be due to an effective manganese (oxy)hydroxide shuttle process, transferring adsorbed trace elements from seawater to the sediments (Scholz et al., 2013; Smrzka et al., 2020).
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This oil enhanced manganese (oxy)hydroxide shuttle would also explain the observed co-enrichment of Mn and Mo (cf. Scholz et al., 2013) and to a lesser extent Cu (Fig. 3a) in oil seep pyrite.
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Schubotz, F., Lipp, J.S., Elvert, M., Kasten, S., Prieto Mollar, X., Zabel, M., Bohrmann, G., Hinrichs, K.-U. (2011) Petroleum degradation and associated microbial signatures at the Chapopote asphalt volcano, Southern Gulf of Mexico. Geochimica et Cosmochimica Acta 75, 4377–4398. https://doi.org/10.1016/j.gca.2011.05.025
Show in context Petroleum hydrocarbons from southern GoM seeps are biodegraded, weathered (Schubotz et al., 2011) and of low thermal maturity with Zn, Cu and Ni content between 10 and 100 ppm, and V content of up to 400 ppm (Smrzka et al., 2019a).
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Shikazono, N., Nakata, M., Tokuyama, E. (1994) Pyrite with high Mn content from the Nankai Trough formed from subduction-induced cold seepage. Marine Geology 118, 303–313. https://doi.org/10.1016/0025-3227(94)90090-6
Show in context Manganese incorporation into pyrite may also proceed via the uptake of precursor manganese sulphide phases, which are stable at high Mn/Fe ratios and high sulphide levels (Shikazono et al., 1994).
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Smrzka, D., Zwicker, J., Klügel, A., Monien, P., Bach, W., Bohrmann, G., Peckmann, J. (2016) Establishing criteria to distinguish oil-seep from methane-seep carbonates. Geology 44, 667–670. https://doi.org/10.1130/G38029.1
Show in context This process triggers the formation of authigenic minerals that record the dynamics and intensity of seepage, redox fluctuations, and the evolution of fluid composition and microbial activity (Feng et al., 2009; Smrzka et al., 2016, 2019a).
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The significance of distinguishing oil- from methane-dominated seep systems has gained traction in recent years in the search for end member system identification (Smrzka et al., 2016; Akam et al., 2021; Krake et al., 2022), and reliable proxies are continuously being explored and refined.
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Smrzka, D., Zwicker, J., Misch, D., Walkner, C., Gier, S., Monien, P., Bohrmann, G., Peckmann, J. (2019a) Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico. Sedimentology 66, 2318–2353. https://doi.org/10.1111/sed.12593
Show in context This process triggers the formation of authigenic minerals that record the dynamics and intensity of seepage, redox fluctuations, and the evolution of fluid composition and microbial activity (Feng et al., 2009; Smrzka et al., 2016, 2019a).
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Microbial sulphate reduction at seeps is coupled to the oxidation of methane, sedimentary organic matter, and high molecular weight hydrocarbons that make up crude oil (Joye et al., 2004; Smrzka et al., 2019a).
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The microbial oxidation rates of these compounds are governed by the microbial consortium capable of using the particular electron donors, which affects sulphate reduction rates and thus the amount of sulphide released to sedimentary pore water (Smrzka et al., 2019a).
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Microbial crude oil oxidation coupled to sulphate reduction enables carbonate precipitation and sulphide production, despite its generally slower microbial oxidation rate compared to SD-AOM (Joye et al., 2004; Smrzka et al., 2019a).
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The high sulphur content in oils (>2 wt. %) from the southern GoM and the increase of dissolved hydrogen sulphide gas emitted at oil seeps (Smrzka et al., 2019a) suggest that the microbial mineralisation of crude oil may act as an additional source of sulphur to pore waters.
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Petroleum hydrocarbons from southern GoM seeps are biodegraded, weathered (Schubotz et al., 2011) and of low thermal maturity with Zn, Cu and Ni content between 10 and 100 ppm, and V content of up to 400 ppm (Smrzka et al., 2019a).
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Smrzka, D., Zwicker, J., Bach, W., Feng, D., Himmler, T., Chen, D., Peckmann, J. (2019b) The behavior of trace elements in seawater, sedimentary pore water, and their incorporation into carbonate minerals: A review. Facies 65, article 41. https://doi.org/10.1007/s10347-019-0581-4
Show in context Nickel and V are micronutrients for phytoplankton growth, and are transported to the seafloor by organic particles and metal (oxy)hydroxides (Smrzka et al., 2019b).
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Manganese (oxy)hydroxides are also effective transport agents for Ni and V to sediments (Smrzka et al., 2019b), yet contrary to Mn and Mo, they are not distinctly enriched in oil seep pyrite.
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Smrzka, D., Feng, D., Himmler, T., Zwicker, J., Hu, Y., Monien, P., Tribovillard, N., Chen, D., Peckmann, J. (2020) Trace elements in methane-seep carbonates: Potentials, limitations, and perspectives. Earth-Science Reviews 208, article 103263. https://doi.org/10.1016/j.earscirev.2020.103263
Show in context In addition, the oxidation of sedimentary organic matter and crude oil will inevitably lead to the partial or complete breakdown of organic compounds, which are themselves carriers of trace elements (Smrzka et al., 2020).
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The high Mn/Fe ratios of oil seep pyrite may also be due to an effective manganese (oxy)hydroxide shuttle process, transferring adsorbed trace elements from seawater to the sediments (Scholz et al., 2013; Smrzka et al., 2020).
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The release of trace elements during organic matter oxidation represents a source of trace elements during early diagenesis (Smrzka et al., 2020), reflected by the trace element composition of pyrite (Miao et al., 2022; Chen et al., 2023).
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Suess, E. (1978) Mineral phases formed in anoxic sediments by microbial decomposition of organic matter. Geochimica et Cosmochimica Acta 43, 339–352. https://doi.org/10.1016/0016-7037(79)90199-6
Show in context Manganese is usually incorporated into Ca-rich rhodochrosite or high-Mg calcite under sulphidic conditions during early diagenesis (Suess, 1978), but may incorporate into pyrite at high Mn2+ concentrations (Morse and Arakaki, 1993; Morse and Luther, 1999).
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Thomson, J., Jarvis, I, Green, R.H., Green, D.A., Clayton, T. (1998) Mobility and immobility of redox-sensitive elements in deep-sea turbidites during shallow burial. Geochimica et Cosmochimica Acta 62, 643–656. https://doi.org/10.1016/S0016-7037(97)00378-5
Show in context While Ni is commonly divalent in marine sediments, V is sensitive toward pH, redox conditions and dissolved sulphide concentrations, and V enrichment in reducing sediments relative to average continental crust (Thomson et al., 1998).
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Tissot, B.T., Welte, D.H. (1984) Petroleum formation and occurrences. 2nd edition Springer, Berlin.
Show in context Components of crude oils contain variable contents of organic sulphur residing in compounds including thiols, sulphides, and thiophenes (Tissot and Welte, 1984).
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Tseng, Y., Römer, M., Lin, S., Pape, T., Berndt, C., Chen, T.-T., Paull, C.K., Caress, D.W., Bohrmann, G. (2023) Yam Seep at Four-Way Closure Ridge: a prominent active gas seep system at the accretionary wedge SW offshore Taiwan. International Journal of Earth Sciences 112, 1043–1061. https://doi.org/10.1007/s00531-022-02280-4
Show in context To the northeast of these seepage areas, and related to the convergence of the Eurasian and the Philippine Sea plates, lies the south-western Taiwan accretionary prism, which harbours the Yam Seep area located at the northern crest of Four-Way Closure Ridge (FWCR; Tseng et al., 2023).
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Wang, B., Lei, H., Huang, F. (2022) Impacts of sulfate-deriven anaerobic oxidation of methane on the morphology, sulfur isotope, and trace element content of authigenic pyrite in marine sediments of the northern South China Sea. Marine and Petroleum Geology 139, article 105578. https://doi.org/10.1016/j.marpetgeo.2022.105578
Show in context Detailed studies on pyrite morphology and its stable isotopic and trace element compositions have recently been conducted in seep settings (Lin et al. 2022; Wang et al., 2022).
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Pyrite based proxies for fluid composition, sulphate reduction processes and redox conditions are a trending topic in hydrocarbon seep research (Miao et al., 2022, Lin et al., 2022; Wang et al., 2022; Domingos et al., 2023).
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High levels of dissolved sulphide favour the incorporation of Mn and Mo into pyrite (Wang et al., 2022), suggesting that the microbial oxidation of crude oil in sediments may enable persistent and highly sulphidic conditions in ambient pore waters controlled by the extent and rate of microbial metabolism.
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Supplementary Information
The Supplementary Information includes:
- Methods
- Results
- Tables S-1 to S-4
- Figures S-1 to S-3
Download Tables S-1 to S-4 (Excel)
Download the Supplementary Information (PDF)
Figures
Figure 1 Locations of the investigated sediment cores. (a) Gravity core locations from oil seeps at Campeche and Sigsbee Knolls in the southern Gulf of Mexico. (b) Gravity core locations of methane seeps from the South China Sea.
Figure 2 Mn/Fe ratios and Mo content in methane seepage- and oil seepage-derived authigenic pyrite. Data from Dongsha and Shenhu seepage areas are from Lin et al. (2022)
Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
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Figure 3 Trace element content in methane seepage- and oil seepage-derived authigenic pyrite. (a) Zn and Cu content, (b) Ni and V content; data from Dongsha and Shenhu seepage areas are from Lin et al. (2022)
Lin, Z., Sun, X., Chen, K., Strauss, H., Klemd, R., Smrzka, D., Chen, T., Lu, Y., Peckmann, J. (2022) Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments. Geochimica et Cosmochimica Acta 333, 75–94. https://doi.org/10.1016/j.gca.2022.06.026
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