A dual role of methane seepage intensity on calcium isotopic fractionation
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Keywords: methane seepage, seep carbonates, calcium isotope, sulfate driven anaerobic oxidation of methane, South China Sea
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Abstract
Figures
 Figure 1 (a) World map with the red rectangle showing the study area. (b) Topographic map of the study area and the locations of the seep carbonates (green stars). (c) SY069 seep carbonate. (d) SQW-65 seep carbonate. (e) W08B seep carbonate. The sampling locations of the seep carbonates are illustrated by the red dots. |  Figure 2 Geochemical properties of the seep carbonates (adapted from Bayon et al., 2007). (a) Sr/Ca vs. Mg/Ca. (b) Sr/Ca vs. δ44/40Ca (‰, relative to SRM 915a). (c) Mg/Ca ratios vs. δ44/40Ca (‰ SW). |  Figure 3 Schematic diagram illustrating the calcium isotopic fractionation in open system vs. semi-enclosed system. |
| Figure 1 | Figure 2 | Figure 3 |
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Introduction
Calcium (Ca) is the fifth most abundant element in the Earth’s lithosphere and plays an important role in geological and biological processes (Fantle and Tipper, 2014
Fantle, M.S., Tipper, E.T. (2014) Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy. Earth-Science Reviews 129, 148–177. https://doi.org/10.1016/j.earscirev.2013.10.004
; Griffith and Fantle, 2021Griffith, E., Fantle, M. (2021) Calcium Isotopes (Elements in Geochemical Tracers in Earth System Science). Cambridge University Press, Cambridge. https://doi.org/10.1017/9781108853972
). Ca isotopes have been used as a geochemical proxy in the Ca cycle, carbonate (organic and inorganic) mineralisation, and early diagenesis on both global and regional scales (De La Rocha and DePaolo, 2000De La Rocha, C.L., DePaolo, D.J. (2000) Isotopic Evidence for Variations in the Marine Calcium Cycle Over the Cenozoic. Science 289, 1176–1178. https://doi.org/10.1126/science.289.5482.1176
; Farkaš et al., 2007Farkaš, J., Böhm, F., Wallmann, K., Blenkinsop, J., Eisenhauer, A., van Geldern, R., Munnecke, A., Voigt, S., Veizer, J. (2007) Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms. Geochimica et Cosmochimica Acta 71, 5117–5134. https://doi.org/10.1016/j.gca.2007.09.004
; Kasemann et al., 2008Kasemann, S.A., Schmidt, D.N., Pearson, P.N., Hawkesworth, C.J. (2008) Biological and ecological insights into Ca isotopes in planktic foraminifers as a palaeotemperature proxy. Earth and Planetary Science Letters 271, 292–302. https://doi.org/10.1016/j.epsl.2008.04.007
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). To ensure the reliability and applicability of this proxy, it is necessary to fully understand the factors affecting Ca isotopic fractionation in the seep carbonates (Gussone et al., 2005Gussone, N., Böhm, F., Eisenhauer, A., Dietzel, M., Heuser, A., Teichert, B.M.A., Reitner, J., Wörheide, G., Dullo, W.-C. (2005) Calcium isotope fractionation in calcite and aragonite. Geochimica et Cosmochimica Acta 69, 4485–4494. https://doi.org/10.1016/j.gca.2005.06.003
; Teichert et al., 2009Teichert, B.M.A., Gussone, N., Torres, M.E. (2009) Controls on calcium isotope fractionation in sedimentary porewaters. Earth and Planetary Science Letters 279, 373–382. https://doi.org/10.1016/j.epsl.2009.01.011
; Griffith and Fantle, 2021Griffith, E., Fantle, M. (2021) Calcium Isotopes (Elements in Geochemical Tracers in Earth System Science). Cambridge University Press, Cambridge. https://doi.org/10.1017/9781108853972
). Experimental and theoretical evidence shows that the precipitated mineral is typically enriched in the lighter isotopes of Ca relative to source fluids, and this relationship is strongly dependent on the precipitation rate (Marriott et al., 2004Marriott, C.S., Henderson, G.M., Belshaw, N.S., Tudhope, A.W. (2004) Temperature dependence of δ7Li, δ44Ca and Li/Ca during growth of calcium carbonate. Earth and Planetary Science Letters 222, 615–624. https://doi.org/10.1016/j.epsl.2004.02.031
; Gussone et al., 2005Gussone, N., Böhm, F., Eisenhauer, A., Dietzel, M., Heuser, A., Teichert, B.M.A., Reitner, J., Wörheide, G., Dullo, W.-C. (2005) Calcium isotope fractionation in calcite and aragonite. Geochimica et Cosmochimica Acta 69, 4485–4494. https://doi.org/10.1016/j.gca.2005.06.003
; Blättler et al., 2012Blättler, C.L., Henderson, G.M., Jenkyns, H.C. (2012) Explaining the Phanerozoic Ca isotope history of seawater. Geology 40, 843–846. https://doi.org/10.1130/G33191.1
, 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
; AlKhatib and Eisenhauer, 2017AlKhatib, M., Eisenhauer, A. (2017) Calcium and strontium isotope fractionation in aqueous solutions as a function of temperature and reaction rate; I. Calcite. Geochimica et Cosmochimica Acta 209, 296–319. https://doi.org/10.1016/j.gca.2016.09.035
; Mills et al., 2021Mills, J.V., DePaolo, D.J., Lammers, L.N. (2021) The influence of Ca:CO3 stoichiometry on Ca isotope fractionation: Implications for process-based models of calcite growth. Geochimica et Cosmochimica Acta 298, 87–111. https://doi.org/10.1016/j.gca.2021.01.016
).Cold seep is a widely observed phenomenon on the continental margins (Michaelis et al., 2002
Michaelis, W., Seifert, R., Nauhaus, K., Treude, T., Thiel, V., Blumenberg, M., Knittel, K., Gieseke, A., Peterknecht, K., Pape, T., Boetius, A., Amann, R., Jørgensen, B.B., Widdel, F., Peckmann, J., Pimenov, N.V., Gulin, M.B. (2002) Microbial Reefs in the Black Sea Fueled by Anaerobic Oxidation of Methane. Science 297, 1013–1015. https://doi.org/10.1126/science.1072502
; Teichert et al., 2009Teichert, B.M.A., Gussone, N., Torres, M.E. (2009) Controls on calcium isotope fractionation in sedimentary porewaters. Earth and Planetary Science Letters 279, 373–382. https://doi.org/10.1016/j.epsl.2009.01.011
; Miao et al., 2022Miao, X., Feng, X., Li, J., Liu, X., Liang, J., Feng, J., Xiao, Q., Dan, X., Wei, 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
), which typically develops extensive authigenic carbonates due to a local increase of alkalinity resulted from sulfate driven anaerobic oxidation of methane (SD-AOM) (Michaelis et al., 2002Michaelis, W., Seifert, R., Nauhaus, K., Treude, T., Thiel, V., Blumenberg, M., Knittel, K., Gieseke, A., Peterknecht, K., Pape, T., Boetius, A., Amann, R., Jørgensen, B.B., Widdel, F., Peckmann, J., Pimenov, N.V., Gulin, M.B. (2002) Microbial Reefs in the Black Sea Fueled by Anaerobic Oxidation of Methane. Science 297, 1013–1015. https://doi.org/10.1126/science.1072502
; Teichert et al., 2009Teichert, B.M.A., Gussone, N., Torres, M.E. (2009) Controls on calcium isotope fractionation in sedimentary porewaters. Earth and Planetary Science Letters 279, 373–382. https://doi.org/10.1016/j.epsl.2009.01.011
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). In recent years multiple studies have reported Ca isotope ratios in pore waters and carbonates recovered from methane seeps and discussed the factors controlling the Ca isotopic fractionation in these environments (Teichert et al., 2005Teichert, B.M.A., Gussone, N., Eisenhauer, A., Bohrmann, G. (2005) Clathrites: Archives of near-seafloor pore-fluid evolution (δ44/40Ca, δ13C, δ18O) in gas hydrate environments. Geology 33, 213–216. https://doi.org/10.1130/G21317.1
; Henderson et al., 2006Henderson, G.M., Chu, N.-C., Bayon, G., Benoit, M. (2006) δ44/42Ca in gas hydrates, porewaters and authigenic carbonates from Niger Delta sediments. Geochimica et Cosmochimica Acta 70, A244. https://doi.org/10.1016/j.gca.2006.06.493
; Bradbury and Turchyn, 2018Bradbury, H.J., Turchyn, A.V. (2018) Calcium isotope fractionation in sedimentary pore fluids from ODP Leg 175: Resolving carbonate recrystallization. Geochimica et Cosmochimica Acta 236, 121–139. https://doi.org/10.1016/j.gca.2018.01.040
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). The impact of the carbonate precipitation rate on the Ca isotopic fractionation has also been demonstrated (Teichert et al., 2005Teichert, B.M.A., Gussone, N., Eisenhauer, A., Bohrmann, G. (2005) Clathrites: Archives of near-seafloor pore-fluid evolution (δ44/40Ca, δ13C, δ18O) in gas hydrate environments. Geology 33, 213–216. https://doi.org/10.1130/G21317.1
; Henderson et al., 2006Henderson, G.M., Chu, N.-C., Bayon, G., Benoit, M. (2006) δ44/42Ca in gas hydrates, porewaters and authigenic carbonates from Niger Delta sediments. Geochimica et Cosmochimica Acta 70, A244. https://doi.org/10.1016/j.gca.2006.06.493
; Bradbury and Turchyn, 2018Bradbury, H.J., Turchyn, A.V. (2018) Calcium isotope fractionation in sedimentary pore fluids from ODP Leg 175: Resolving carbonate recrystallization. Geochimica et Cosmochimica Acta 236, 121–139. https://doi.org/10.1016/j.gca.2018.01.040
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). However, since the seep carbonate mineral facies and precipitation rate are mainly determined by the methane seepage intensity (Tang et al., 2008Tang, J., Dietzel, M., Böhm, F., Köhler, S.J., Eisenhauer, A. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: II. Ca isotopes. Geochimica et Cosmochimica Acta 72, 3733–3745. https://doi.org/10.1016/j.gca.2008.05.033
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
; Smrzka et al., 2021Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
), it is reasonable to anticipate methane seepage intensity is one of the key controls on Ca isotopic fractionation. In addition, the depth of the sulfate-methane transition zone (SMTZ) changes when the methane seepage intensity changes (Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). This may affect the extent to which Ca isotope fractionation occurs under shallow open system conditions where dissolved Ca is replenished as carbonate precipitation proceeds, versus deeper closed system conditions where there is no replenishment of dissolved Ca during carbonate formation, allowing for the preferential accumulation of 44Ca, which in turn affects the Ca isotopic composition of the carbonates. Therefore, it is believed that methane seepage intensity plays a dual role on the composition of Ca isotopes in the seep carbonates. In this study, we report the mineralogical composition, Ca isotopic composition, major and trace elements of the seep carbonates recovered from different seep sites on the northern continental slope of the South China Sea (Fig. 1), and further discuss the influence of methane seepage intensity on the Ca isotopic fractionation.Figure 1 (a) World map with the red rectangle showing the study area. (b) Topographic map of the study area and the locations of the seep carbonates (green stars). (c) SY069 seep carbonate. (d) SQW-65 seep carbonate. (e) W08B seep carbonate. The sampling locations of the seep carbonates are illustrated by the red dots.
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Materials and Methods
All the seep carbonates are reported for the first time except W08B (Wei et al., 2020
Wei, J., Wu, T., Zhang, W., Deng, Y., Xie, R., Feng, J., Liang, J., Lai, P., Zhou, J., Cao, J. (2020) Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities. Minerals 10, 1135. https://doi.org/10.3390/min10121135
, 2022Wei, J., Wu, T., Miao, X., Su, P. (2022) Massive Natural Gas Hydrate Dissociation During the Penultimate Deglaciation (∼130 ka) in the South China Sea. Frontiers in Marine Science 9, 875374. https://doi.org/10.3389/fmars.2022.875374
). The sample location, experimental methods, and data processing are described in the Supporting Information.top
Results
The XRD results show that the W08B carbonates are mainly composed of aragonite with a small amount of calcite, while the SY069 and SQW-065 carbonates are dominated by calcite with no aragonite detected. The calcite is primary (Fig. S-2).
The average Sr/Ca and Mg/Ca ratios of W08B are 0.022 and 0.007 (n = 11), respectively. The average Sr/Ca and Mg/Ca ratios of SY069 are 0.005 and 0.047 (n = 9), respectively. The average Sr/Ca and Mg/Ca ratios of SQW-065 are 0.003 and 0.063 (n = 10), respectively (Fig. 2a, Tables S-1, S-2).
Figure 2 Geochemical properties of the seep carbonates (adapted from Bayon et al., 2007
Bayon, G., Pierre, C., Etoubleau, J., Voisset, M., Cauquil, E., Marsset, T., Sultan, N., Le Drezen, E., Fouquet, Y. (2007) Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241, 93–109. https://doi.org/10.1016/j.margeo.2007.03.007
). (a) Sr/Ca vs. Mg/Ca. (b) Sr/Ca vs. δ44/40Ca (‰, relative to SRM 915a). (c) Mg/Ca ratios vs. δ44/40Ca (‰ SW).Calcium isotopic composition (reported as δ44/40Ca) was determined by a Neptune Plus MC-ICP-MS. The average value of W08B δ44/40Ca is −1.17 ‰ SW (n = 11; Fig. 2b,c). The SY069 and SQW-065 δ44/40Ca are relatively high, with average values of −0.92 ‰ SW (n = 9) and −0.68 ‰ SW (n = 10) (Fig. 2b,c), respectively. The δ44/40Ca of all the samples are negatively correlated with the Sr/Ca ratio (R2 = 0.76) and positively correlated with the Mg/Ca ratio (R2 = 0.88).
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Discussion
Precipitation environment of seep carbonates. It is difficult to reconstruct the palaeo carbonate precipitation environment by analysing the pore water because the pore water during carbonate growth cannot be obtained. However, the carbonate mineralogy and geochemistry at methane seeps are mainly influenced by pore water chemistry. Therefore, the seep carbonate precipitation environment can be reconstructed using specific elemental composition and mineral phases (Burton, 1993
Burton, E.A. (1993) Controls on marine carbonate cement mineralogy: review and reassessment. Chemical Geology 105, 163–179. https://doi.org/10.1016/0009-2541(93)90124-2
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
; Smrzka et al., 2021Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
).Strong methane seepage results in a shallow SMTZ (near the seafloor), the presence of dissolved sulfate is thought to inhibit precipitation of high-Mg carbonates and hence favours aragonite formation (Burton, 1993
Burton, E.A. (1993) Controls on marine carbonate cement mineralogy: review and reassessment. Chemical Geology 105, 163–179. https://doi.org/10.1016/0009-2541(93)90124-2
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
). When the SMTZ is located at deeper sediment horizons (weak methane seepage), high-Mg carbonates generally precipitate (Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Smrzka et al., 2021Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
). In this study, W08B is dominated by aragonite, indicating a shallow SMTZ; while SY069 and SQW-065 are mainly calcite, indicating a deeper SMTZ (Fig. 2a, Table S-1). In addition, studies have shown that the carbonate precipitation rate affects the composition of specific trace element ratios (Sr/Ca, Mg/Ca), and this is the basis for us to potentially use the specific trace element ratios to reconstruct the palaeo carbonate precipitation rate (Luff and Wallmann, 2003Luff, R., Wallmann, K. (2003) Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances. Geochimica et Cosmochimica Acta 67, 3403–3421. https://doi.org/10.1016/S0016-7037(03)00127-3
; Tang et al., 2008Tang, J., Dietzel, M., Böhm, F., Köhler, S.J., Eisenhauer, A. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: II. Ca isotopes. Geochimica et Cosmochimica Acta 72, 3733–3745. https://doi.org/10.1016/j.gca.2008.05.033
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Smrzka et al., 2021Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
). Lab experiments and carbonate diagenesis studies have shown that the partitioning of Mg and Sr, and the fractionation of Ca isotopes in inorganic carbonates are related to the carbonate precipitation rates. For the same fluid composition, high Sr/Ca and low Mg/Ca in the carbonates are associated with high carbonate precipitation rates, while low carbonate precipitation rates result in low Sr/Ca and high Mg/Ca (Luff and Wallmann, 2003Luff, R., Wallmann, K. (2003) Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances. Geochimica et Cosmochimica Acta 67, 3403–3421. https://doi.org/10.1016/S0016-7037(03)00127-3
; Tang et al., 2008Tang, J., Dietzel, M., Böhm, F., Köhler, S.J., Eisenhauer, A. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: II. Ca isotopes. Geochimica et Cosmochimica Acta 72, 3733–3745. https://doi.org/10.1016/j.gca.2008.05.033
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). In Figure 2a, W08B is dominated by aragonite with high Sr/Ca and low Mg/Ca, indicating a high precipitation rate, while SY069 and SQW-065 are mainly calcite with low Sr/Ca and high Mg/Ca, indicating a low precipitation rate. This is consistent with previous studies, in which the precipitation rate of aragonite is higher than calcite (Luff and Wallmann, 2003Luff, R., Wallmann, K. (2003) Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances. Geochimica et Cosmochimica Acta 67, 3403–3421. https://doi.org/10.1016/S0016-7037(03)00127-3
; Gussone et al., 2005Gussone, N., Böhm, F., Eisenhauer, A., Dietzel, M., Heuser, A., Teichert, B.M.A., Reitner, J., Wörheide, G., Dullo, W.-C. (2005) Calcium isotope fractionation in calcite and aragonite. Geochimica et Cosmochimica Acta 69, 4485–4494. https://doi.org/10.1016/j.gca.2005.06.003
; Bayon et al., 2007Bayon, G., Pierre, C., Etoubleau, J., Voisset, M., Cauquil, E., Marsset, T., Sultan, N., Le Drezen, E., Fouquet, Y. (2007) Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241, 93–109. https://doi.org/10.1016/j.margeo.2007.03.007
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Blättler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
).Based on the comprehensive analysis of the mineralogy and geochemistry of carbonates, we conclude that W08B carbonate was formed in a shallow SMTZ with high precipitation rate. However, SY069 and SQW-065 are formed in a deep SMTZ with low precipitation rate. Moreover, it is believed that the main controlling factor of this difference is the methane seepage intensity.
The methane seepage intensity affects the Ca isotopic fractionation. In general, carbonate precipitation results in preferential incorporation of light Ca isotopes, which results in pore waters becoming enriched in heavier Ca isotopes (i.e. higher δ44/40Ca values) (Blättler et al., 2012
Blättler, C.L., Henderson, G.M., Jenkyns, H.C. (2012) Explaining the Phanerozoic Ca isotope history of seawater. Geology 40, 843–846. https://doi.org/10.1130/G33191.1
, 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
; AlKhatib and Eisenhauer, 2017AlKhatib, M., Eisenhauer, A. (2017) Calcium and strontium isotope fractionation in aqueous solutions as a function of temperature and reaction rate; I. Calcite. Geochimica et Cosmochimica Acta 209, 296–319. https://doi.org/10.1016/j.gca.2016.09.035
; Mills et al., 2021Mills, J.V., DePaolo, D.J., Lammers, L.N. (2021) The influence of Ca:CO3 stoichiometry on Ca isotope fractionation: Implications for process-based models of calcite growth. Geochimica et Cosmochimica Acta 298, 87–111. https://doi.org/10.1016/j.gca.2021.01.016
). Lab experiments and theoretical models have shown that the Ca isotopic fractionation of the carbonates increases with higher precipitation rates (Tang et al., 2008Tang, J., Dietzel, M., Böhm, F., Köhler, S.J., Eisenhauer, A. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: II. Ca isotopes. Geochimica et Cosmochimica Acta 72, 3733–3745. https://doi.org/10.1016/j.gca.2008.05.033
; DePaolo, 2011DePaolo, D.J. (2011) Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions. Geochimica et Cosmochimica Acta 75, 1039–1056. https://doi.org/10.1016/j.gca.2010.11.020
; Nielsen et al., 2012Nielsen, L.C., DePaolo, D.J., De Yoreo, J.J. (2012) Self-consistent ion-by-ion growth model for kinetic isotopic fractionation during calcite precipitation. Geochimica et Cosmochimica Acta 86, 166–181. https://doi.org/10.1016/j.gca.2012.02.009
). The δ44/40Ca of these samples showed a negative linear correlation with Sr/Ca (R2 = 0.76; Fig. 2b) and positive linear correlation with Mg/Ca (R2 = 0.88; Fig. 2c). The results indicate that high carbonate precipitation rates (aragonite dominated) lead to the accumulation of light Ca isotopes, while low carbonate precipitation rates (calcite dominated) result in heavy Ca isotopes, which is consistent with current understanding (Gussone et al., 2005Gussone, N., Böhm, F., Eisenhauer, A., Dietzel, M., Heuser, A., Teichert, B.M.A., Reitner, J., Wörheide, G., Dullo, W.-C. (2005) Calcium isotope fractionation in calcite and aragonite. Geochimica et Cosmochimica Acta 69, 4485–4494. https://doi.org/10.1016/j.gca.2005.06.003
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Blattler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). In normal marine environments, temperature is considered to be a factor controlling the carbonate precipitation rate (Burton and Walter, 1987Burton, E.A., Walter, L.M. (1987) Relative precipitation rates of aragonite and Mg calcite from seawater: Temperature or carbonate ion control? Geology 15, 111–114. https://doi.org/10.1130/0091-7613(1987)15< 111:RPROAA> 2.0.CO;2
; Gussone et al., 2005Gussone, N., Böhm, F., Eisenhauer, A., Dietzel, M., Heuser, A., Teichert, B.M.A., Reitner, J., Wörheide, G., Dullo, W.-C. (2005) Calcium isotope fractionation in calcite and aragonite. Geochimica et Cosmochimica Acta 69, 4485–4494. https://doi.org/10.1016/j.gca.2005.06.003
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
). However, based on the in situ measurements, the seafloor temperature of W08B (3.5 °C) is similar to that of SY069 (3.1 °C), and slightly lower than that of SQW-065 (8.3 °C) (Supplementary Information). Therefore, temperature difference as the main factor affecting the carbonate precipitation rate can be excluded. In addition, temperature changes in other cold seeps are also not considered as the potential factor in controlling the carbonate precipitation rate (Bradbury and Turchyn, 2018Bradbury, H.J., Turchyn, A.V. (2018) Calcium isotope fractionation in sedimentary pore fluids from ODP Leg 175: Resolving carbonate recrystallization. Geochimica et Cosmochimica Acta 236, 121–139. https://doi.org/10.1016/j.gca.2018.01.040
; Thiagarajan et al., 2020Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
; Blattler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). However, the HCO3− concentration varies significantly because the SD-AOM can generate large amounts of HCO3− (Michaelis et al., 2002Michaelis, W., Seifert, R., Nauhaus, K., Treude, T., Thiel, V., Blumenberg, M., Knittel, K., Gieseke, A., Peterknecht, K., Pape, T., Boetius, A., Amann, R., Jørgensen, B.B., Widdel, F., Peckmann, J., Pimenov, N.V., Gulin, M.B. (2002) Microbial Reefs in the Black Sea Fueled by Anaerobic Oxidation of Methane. Science 297, 1013–1015. https://doi.org/10.1126/science.1072502
). Compared with the weak methane seepage, SD-AOM is stronger in the strong methane seepage, and more HCO3− will be produced, which increases the alkalinity and the carbonate saturation of the environment (Luff and Wallmann, 2003Luff, R., Wallmann, K. (2003) Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances. Geochimica et Cosmochimica Acta 67, 3403–3421. https://doi.org/10.1016/S0016-7037(03)00127-3
). In this case, the precipitation rate of the carbonates will be higher in the strong methane seepage than that in the weak methane seepage. Therefore, it is believed that the carbonate precipitation rate caused by the methane seepage intensity is an important factor affecting the Ca isotopic composition.The seepage intensity not only controls the carbonate precipitation rate, but also controls the depth of the SMTZ (Smrzka et al., 2021
Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
), which in turn controls the magnitude of Ca isotopic fractionation. Strong methane seepage results in a shallow SMTZ, and the pore water has a strong mixing effect with the overlying seawater, resulting in a relatively open system (Smrzka et al., 2021Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
). In this scenario, the Ca2+ diffusion rate is high, and the 40Ca consumed by the carbonate formation can be replenished from seawater in time, leading to no enrichment in 44Ca. Therefore, light Ca isotopes are more enriched in the aragonite (Fig. 3a). In addition, sufficient SO42− supply will also inhibit the formation of calcite by inhibiting Mg entry into the crystal lattice (Burton, 1993Burton, E.A. (1993) Controls on marine carbonate cement mineralogy: review and reassessment. Chemical Geology 105, 163–179. https://doi.org/10.1016/0009-2541(93)90124-2
; Goetschl et al., 2019Goetschl, K.E., Purgstaller, B., Dietzel, M., Mavromatis, V. (2019) Effect of sulfate on magnesium incorporation in low-magnesium calcite. Geochimica et Cosmochimica Acta 265, 505–519. https://doi.org/10.1016/j.gca.2019.07.024
), resulting in the precipitation of aragonite, which is consistent with the light Ca isotopic values observed in the W08B aragonite (Fig. 2). Conversely, weak methane seepage results in a deep SMTZ. The thick overlying sediment prevents the exchange of pore water and seawater, resulting in a relatively closed environment (Blattler et al., 2021Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
). The low Ca2+ diffusion rate is unable to effectively replenish the consumed 40Ca, therefore, the pore fluid will become enriched in 44Ca, which means later carbonate minerals will also be enriched in 44Ca (Fig. 3b). In addition, carbonate formation in this environment is dominated by calcite (Goetschl et al., 2019Goetschl, K.E., Purgstaller, B., Dietzel, M., Mavromatis, V. (2019) Effect of sulfate on magnesium incorporation in low-magnesium calcite. Geochimica et Cosmochimica Acta 265, 505–519. https://doi.org/10.1016/j.gca.2019.07.024
; Smrzka et al., 2021Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
), therefore calcite records heavy Ca isotopic values (Fig. 3b). Therefore, it is suggested that methane seepage intensity also controls the openness of the cold seep environment and further affects the Ca isotopic composition.Figure 3 Schematic diagram illustrating the calcium isotopic fractionation in open system vs. semi-enclosed system.
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Conclusions
In the study, a dual role of methane seepage intensity on the Ca isotopic fractionation of the seep carbonates is demonstrated for the first time. It is found that authigenic carbonates collected from different seep sites in the northern continental slope of the South China Sea show obvious differences in Ca isotopes. In the high methane seepage intensity environment, the SMTZ is located in the shallow sediment or even near the seafloor. In a closed system (i.e. when the SMTZ is located in the deep sediment), seep carbonate precipitation prefers to incorporate light 40Ca, hence resulting in the enrichment of heavy 44Ca in the pore water. In this case, with time, the ongoing carbonate precipitation (mostly high-Mg calcite) should indeed be accompanied by a heavy Ca isotope composition. In contrast, in an open system (i.e. when the SMTZ is located near the seafloor), aragonite is precipitated from a relatively light 40Ca pool in the pore water, hence explaining why the aragonite displays light Ca isotopic composition. These results suggest that the Ca isotope of the seep carbonates is potentially a good proxy for understanding and constraining the palaeo SD-AOM activities in the cold seeps, which is of great significance to study the global calcium cycle.
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Acknowledgements
This research was funded by Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0201), National Key Research and Development Program of China (No. 2016YFC0304905-03), Guangdong Province Marine Economic Development (Six Major Marine Industries) Special Fund Project ([2021] No. 58), China Geological Survey Project (No. DD20190224) and Hainan Institute of Chinese Engineering Development Strategies (21-HN-ZT-01). We also thank all the crew and participants in the TS07-02 and TS07-03 expeditions for their support and constructive discussions. JW highly appreciates Mr. Pinxian Wang and Prof Kang Ding for providing the opportunity to participate in the TS07-02 and TS07-03 research cruises. We also appreciate Editor Claudine Stirling and Postdoctoral researcher Tingting Chen (University of Science and Technology of China) for their thoughtful suggestions on a revised draft of the manuscript.
Editor: Claudine Stirling
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References
AlKhatib, M., Eisenhauer, A. (2017) Calcium and strontium isotope fractionation in aqueous solutions as a function of temperature and reaction rate; I. Calcite. Geochimica et Cosmochimica Acta 209, 296–319. https://doi.org/10.1016/j.gca.2016.09.035
Show in context Experimental and theoretical evidence shows that the precipitated mineral is typically enriched in the lighter isotopes of Ca relative to source fluids, and this relationship is strongly dependent on the precipitation rate (Marriott et al., 2004; Gussone et al., 2005; Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
View in article
In general, carbonate precipitation results in preferential incorporation of light Ca isotopes, which results in pore waters becoming enriched in heavier Ca isotopes (i.e. higher δ44/40Ca values) (Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
View in article
Bayon, G., Pierre, C., Etoubleau, J., Voisset, M., Cauquil, E., Marsset, T., Sultan, N., Le Drezen, E., Fouquet, Y. (2007) Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241, 93–109. https://doi.org/10.1016/j.margeo.2007.03.007
Show in context Geochemical properties of the seep carbonates (adapted from Bayon et al., 2007).
View in article
This is consistent with previous studies, in which the precipitation rate of aragonite is higher than calcite (Luff and Wallmann, 2003; Gussone et al., 2005; Bayon et al., 2007; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
Blättler, C.L., Henderson, G.M., Jenkyns, H.C. (2012) Explaining the Phanerozoic Ca isotope history of seawater. Geology 40, 843–846. https://doi.org/10.1130/G33191.1
Show in context Experimental and theoretical evidence shows that the precipitated mineral is typically enriched in the lighter isotopes of Ca relative to source fluids, and this relationship is strongly dependent on the precipitation rate (Marriott et al., 2004; Gussone et al., 2005; Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
View in article
In general, carbonate precipitation results in preferential incorporation of light Ca isotopes, which results in pore waters becoming enriched in heavier Ca isotopes (i.e. higher δ44/40Ca values) (Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
View in article
Blättler, C.L., Hong, W.-L., Kirsimäe, K., Higgins, J.A., Lepland, A. (2021) Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates. Geochimica et Cosmochimica Acta 298, 227–239. https://doi.org/10.1016/j.gca.2021.01.001
Show in context Ca isotopes have been used as a geochemical proxy in the Ca cycle, carbonate (organic and inorganic) mineralisation, and early diagenesis on both global and regional scales (De La Rocha and DePaolo, 2000; Farkaš et al., 2007; Kasemann et al., 2008; Blättler et al., 2021).
View in article
Experimental and theoretical evidence shows that the precipitated mineral is typically enriched in the lighter isotopes of Ca relative to source fluids, and this relationship is strongly dependent on the precipitation rate (Marriott et al., 2004; Gussone et al., 2005; Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
View in article
In recent years multiple studies have reported Ca isotope ratios in pore waters and carbonates recovered from methane seeps and discussed the factors controlling the Ca isotopic fractionation in these environments (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
Cold seep is a widely observed phenomenon on the continental margins (Michaelis et al., 2002; Teichert et al., 2009; Miao et al., 2022), which typically develops extensive authigenic carbonates due to a local increase of alkalinity resulted from sulfate driven anaerobic oxidation of methane (SD-AOM) (Michaelis et al., 2002; Teichert et al., 2009; Blättler et al., 2021).
View in article
The impact of the carbonate precipitation rate on the Ca isotopic fractionation has also been demonstrated (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
However, since the seep carbonate mineral facies and precipitation rate are mainly determined by the methane seepage intensity (Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021; Smrzka et al., 2021), it is reasonable to anticipate methane seepage intensity is one of the key controls on Ca isotopic fractionation.
View in article
In addition, the depth of the sulfate-methane transition zone (SMTZ) changes when the methane seepage intensity changes (Blättler et al., 2021).
View in article
Therefore, the seep carbonate precipitation environment can be reconstructed using specific elemental composition and mineral phases (Burton, 1993; Blättler et al., 2021; Smrzka et al., 2021).
View in article
For the same fluid composition, high Sr/Ca and low Mg/Ca in the carbonates are associated with high carbonate precipitation rates, while low carbonate precipitation rates result in low Sr/Ca and high Mg/Ca (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
This is consistent with previous studies, in which the precipitation rate of aragonite is higher than calcite (Luff and Wallmann, 2003; Gussone et al., 2005; Bayon et al., 2007; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
In general, carbonate precipitation results in preferential incorporation of light Ca isotopes, which results in pore waters becoming enriched in heavier Ca isotopes (i.e. higher δ44/40Ca values) (Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
View in article
The results indicate that high carbonate precipitation rates (aragonite dominated) lead to the accumulation of light Ca isotopes, while low carbonate precipitation rates (calcite dominated) result in heavy Ca isotopes, which is consistent with current understanding (Gussone et al., 2005; Thiagarajan et al., 2020; Blattler et al., 2021).
View in article
In addition, temperature changes in other cold seeps are also not considered as the potential factor in controlling the carbonate precipitation rate (Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blattler et al., 2021).
View in article
Conversely, weak methane seepage results in a deep SMTZ. The thick overlying sediment prevents the exchange of pore water and seawater, resulting in a relatively closed environment (Blattler et al., 2021).
View in article
Bradbury, H.J., Turchyn, A.V. (2018) Calcium isotope fractionation in sedimentary pore fluids from ODP Leg 175: Resolving carbonate recrystallization. Geochimica et Cosmochimica Acta 236, 121–139. https://doi.org/10.1016/j.gca.2018.01.040
Show in context In recent years multiple studies have reported Ca isotope ratios in pore waters and carbonates recovered from methane seeps and discussed the factors controlling the Ca isotopic fractionation in these environments (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
The impact of the carbonate precipitation rate on the Ca isotopic fractionation has also been demonstrated (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
In addition, temperature changes in other cold seeps are also not considered as the potential factor in controlling the carbonate precipitation rate (Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blattler et al., 2021).
View in article
Burton, E.A. (1993) Controls on marine carbonate cement mineralogy: review and reassessment. Chemical Geology 105, 163–179. https://doi.org/10.1016/0009-2541(93)90124-2
Show in context Therefore, the seep carbonate precipitation environment can be reconstructed using specific elemental composition and mineral phases (Burton, 1993; Blättler et al., 2021; Smrzka et al., 2021).
View in article
Strong methane seepage results in a shallow SMTZ (near the seafloor), the presence of dissolved sulfate is thought to inhibit precipitation of high-Mg carbonates and hence favours aragonite formation (Burton, 1993; Thiagarajan et al., 2020).
View in article
In addition, sufficient SO42− supply will also inhibit the formation of calcite by inhibiting Mg entry into the crystal lattice (Burton, 1993; Goetschl et al., 2019), resulting in the precipitation of aragonite, which is consistent with the light Ca isotopic values observed in the W08B aragonite (Fig. 2).
View in article
Burton, E.A., Walter, L.M. (1987) Relative precipitation rates of aragonite and Mg calcite from seawater: Temperature or carbonate ion control? Geology 15, 111–114. https://doi.org/10.1130/0091-7613(1987)15< 111:RPROAA> 2.0.CO;2
Show in context In normal marine environments, temperature is considered to be a factor controlling the carbonate precipitation rate (Burton and Walter, 1987; Gussone et al., 2005; Thiagarajan et al., 2020).
View in article
De La Rocha, C.L., DePaolo, D.J. (2000) Isotopic Evidence for Variations in the Marine Calcium Cycle Over the Cenozoic. Science 289, 1176–1178. https://doi.org/10.1126/science.289.5482.1176
Show in context Ca isotopes have been used as a geochemical proxy in the Ca cycle, carbonate (organic and inorganic) mineralisation, and early diagenesis on both global and regional scales (De La Rocha and DePaolo, 2000; Farkaš et al., 2007; Kasemann et al., 2008; Blättler et al., 2021).
View in article
DePaolo, D.J. (2011) Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions. Geochimica et Cosmochimica Acta 75, 1039–1056. https://doi.org/10.1016/j.gca.2010.11.020
Show in context Lab experiments and theoretical models have shown that the Ca isotopic fractionation of the carbonates increases with higher precipitation rates (Tang et al., 2008; DePaolo, 2011; Nielsen et al., 2012).
View in article
Fantle, M.S., Tipper, E.T. (2014) Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy. Earth-Science Reviews 129, 148–177. https://doi.org/10.1016/j.earscirev.2013.10.004
Show in context Calcium (Ca) is the fifth most abundant element in the Earth’s lithosphere and plays an important role in geological and biological processes (Fantle and Tipper, 2014; Griffith and Fantle, 2021).
View in article
Farkaš, J., Böhm, F., Wallmann, K., Blenkinsop, J., Eisenhauer, A., van Geldern, R., Munnecke, A., Voigt, S., Veizer, J. (2007) Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms. Geochimica et Cosmochimica Acta 71, 5117–5134. https://doi.org/10.1016/j.gca.2007.09.004
Show in context Ca isotopes have been used as a geochemical proxy in the Ca cycle, carbonate (organic and inorganic) mineralisation, and early diagenesis on both global and regional scales (De La Rocha and DePaolo, 2000; Farkaš et al., 2007; Kasemann et al., 2008; Blättler et al., 2021).
View in article
Goetschl, K.E., Purgstaller, B., Dietzel, M., Mavromatis, V. (2019) Effect of sulfate on magnesium incorporation in low-magnesium calcite. Geochimica et Cosmochimica Acta 265, 505–519. https://doi.org/10.1016/j.gca.2019.07.024
Show in context In addition, sufficient SO42− supply will also inhibit the formation of calcite by inhibiting Mg entry into the crystal lattice (Burton, 1993; Goetschl et al., 2019), resulting in the precipitation of aragonite, which is consistent with the light Ca isotopic values observed in the W08B aragonite (Fig. 2).
View in article
In addition, carbonate formation in this environment is dominated by calcite (Goetschl et al., 2019; Smrzka et al., 2021), therefore calcite records heavy Ca isotopic values (Fig. 3b).
View in article
Griffith, E., Fantle, M. (2021) Calcium Isotopes (Elements in Geochemical Tracers in Earth System Science). Cambridge University Press, Cambridge. https://doi.org/10.1017/9781108853972
Show in context Calcium (Ca) is the fifth most abundant element in the Earth’s lithosphere and plays an important role in geological and biological processes (Fantle and Tipper, 2014; Griffith and Fantle, 2021).
View in article
To ensure the reliability and applicability of this proxy, it is necessary to fully understand the factors affecting Ca isotopic fractionation in the seep carbonates (Gussone et al., 2005; Teichert et al., 2009; Griffith and Fantle, 2021).
View in article
Gussone, N., Böhm, F., Eisenhauer, A., Dietzel, M., Heuser, A., Teichert, B.M.A., Reitner, J., Wörheide, G., Dullo, W.-C. (2005) Calcium isotope fractionation in calcite and aragonite. Geochimica et Cosmochimica Acta 69, 4485–4494. https://doi.org/10.1016/j.gca.2005.06.003
Show in context To ensure the reliability and applicability of this proxy, it is necessary to fully understand the factors affecting Ca isotopic fractionation in the seep carbonates (Gussone et al., 2005; Teichert et al., 2009; Griffith and Fantle, 2021).
View in article
Experimental and theoretical evidence shows that the precipitated mineral is typically enriched in the lighter isotopes of Ca relative to source fluids, and this relationship is strongly dependent on the precipitation rate (Marriott et al., 2004; Gussone et al., 2005; Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
View in article
This is consistent with previous studies, in which the precipitation rate of aragonite is higher than calcite (Luff and Wallmann, 2003; Gussone et al., 2005; Bayon et al., 2007; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
The results indicate that high carbonate precipitation rates (aragonite dominated) lead to the accumulation of light Ca isotopes, while low carbonate precipitation rates (calcite dominated) result in heavy Ca isotopes, which is consistent with current understanding (Gussone et al., 2005; Thiagarajan et al., 2020; Blattler et al., 2021).
View in article
In normal marine environments, temperature is considered to be a factor controlling the carbonate precipitation rate (Burton and Walter, 1987; Gussone et al., 2005; Thiagarajan et al., 2020).
View in article
Henderson, G.M., Chu, N.-C., Bayon, G., Benoit, M. (2006) δ44/42Ca in gas hydrates, porewaters and authigenic carbonates from Niger Delta sediments. Geochimica et Cosmochimica Acta 70, A244. https://doi.org/10.1016/j.gca.2006.06.493
Show in context In recent years multiple studies have reported Ca isotope ratios in pore waters and carbonates recovered from methane seeps and discussed the factors controlling the Ca isotopic fractionation in these environments (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
The impact of the carbonate precipitation rate on the Ca isotopic fractionation has also been demonstrated (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
Kasemann, S.A., Schmidt, D.N., Pearson, P.N., Hawkesworth, C.J. (2008) Biological and ecological insights into Ca isotopes in planktic foraminifers as a palaeotemperature proxy. Earth and Planetary Science Letters 271, 292–302. https://doi.org/10.1016/j.epsl.2008.04.007
Show in context Ca isotopes have been used as a geochemical proxy in the Ca cycle, carbonate (organic and inorganic) mineralisation, and early diagenesis on both global and regional scales (De La Rocha and DePaolo, 2000; Farkaš et al., 2007; Kasemann et al., 2008; Blättler et al., 2021).
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Luff, R., Wallmann, K. (2003) Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances. Geochimica et Cosmochimica Acta 67, 3403–3421. https://doi.org/10.1016/S0016-7037(03)00127-3
Show in context In addition, studies have shown that the carbonate precipitation rate affects the composition of specific trace element ratios (Sr/Ca, Mg/Ca), and this is the basis for us to potentially use the specific trace element ratios to reconstruct the palaeo carbonate precipitation rate (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Smrzka et al., 2021).
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For the same fluid composition, high Sr/Ca and low Mg/Ca in the carbonates are associated with high carbonate precipitation rates, while low carbonate precipitation rates result in low Sr/Ca and high Mg/Ca (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021).
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This is consistent with previous studies, in which the precipitation rate of aragonite is higher than calcite (Luff and Wallmann, 2003; Gussone et al., 2005; Bayon et al., 2007; Thiagarajan et al., 2020; Blättler et al., 2021).
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Compared with the weak methane seepage, SD-AOM is stronger in the strong methane seepage, and more HCO3− will be produced, which increases the alkalinity and the carbonate saturation of the environment (Luff and Wallmann, 2003).
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Marriott, C.S., Henderson, G.M., Belshaw, N.S., Tudhope, A.W. (2004) Temperature dependence of δ7Li, δ44Ca and Li/Ca during growth of calcium carbonate. Earth and Planetary Science Letters 222, 615–624. https://doi.org/10.1016/j.epsl.2004.02.031
Show in context Experimental and theoretical evidence shows that the precipitated mineral is typically enriched in the lighter isotopes of Ca relative to source fluids, and this relationship is strongly dependent on the precipitation rate (Marriott et al., 2004; Gussone et al., 2005; Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
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Miao, X., Feng, X., Li, J., Liu, X., Liang, J., Feng, J., Xiao, Q., Dan, X., Wei, 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 Cold seep is a widely observed phenomenon on the continental margins (Michaelis et al., 2002; Teichert et al., 2009; Miao et al., 2022), which typically develops extensive authigenic carbonates due to a local increase of alkalinity resulted from sulfate driven anaerobic oxidation of methane (SD-AOM) (Michaelis et al., 2002; Teichert et al., 2009; Blättler et al., 2021).
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Michaelis, W., Seifert, R., Nauhaus, K., Treude, T., Thiel, V., Blumenberg, M., Knittel, K., Gieseke, A., Peterknecht, K., Pape, T., Boetius, A., Amann, R., Jørgensen, B.B., Widdel, F., Peckmann, J., Pimenov, N.V., Gulin, M.B. (2002) Microbial Reefs in the Black Sea Fueled by Anaerobic Oxidation of Methane. Science 297, 1013–1015. https://doi.org/10.1126/science.1072502
Show in context Cold seep is a widely observed phenomenon on the continental margins (Michaelis et al., 2002; Teichert et al., 2009; Miao et al., 2022), which typically develops extensive authigenic carbonates due to a local increase of alkalinity resulted from sulfate driven anaerobic oxidation of methane (SD-AOM) (Michaelis et al., 2002; Teichert et al., 2009; Blättler et al., 2021).
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However, the HCO3− concentration varies significantly because the SD-AOM can generate large amounts of HCO3− (Michaelis et al., 2002).
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Mills, J.V., DePaolo, D.J., Lammers, L.N. (2021) The influence of Ca:CO3 stoichiometry on Ca isotope fractionation: Implications for process-based models of calcite growth. Geochimica et Cosmochimica Acta 298, 87–111. https://doi.org/10.1016/j.gca.2021.01.016
Show in context Experimental and theoretical evidence shows that the precipitated mineral is typically enriched in the lighter isotopes of Ca relative to source fluids, and this relationship is strongly dependent on the precipitation rate (Marriott et al., 2004; Gussone et al., 2005; Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
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In general, carbonate precipitation results in preferential incorporation of light Ca isotopes, which results in pore waters becoming enriched in heavier Ca isotopes (i.e. higher δ44/40Ca values) (Blättler et al., 2012, 2021; AlKhatib and Eisenhauer, 2017; Mills et al., 2021).
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Nielsen, L.C., DePaolo, D.J., De Yoreo, J.J. (2012) Self-consistent ion-by-ion growth model for kinetic isotopic fractionation during calcite precipitation. Geochimica et Cosmochimica Acta 86, 166–181. https://doi.org/10.1016/j.gca.2012.02.009
Show in context Lab experiments and theoretical models have shown that the Ca isotopic fractionation of the carbonates increases with higher precipitation rates (Tang et al., 2008; DePaolo, 2011; Nielsen et al., 2012).
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Smrzka, D., Zwicker, J., Lu, Y., Sun, Y., Feng, D., Monien, P., Bohrmann, G., Peckmann, J. (2021) Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide. Chemical Geology 580, 120357. https://doi.org/10.1016/j.chemgeo.2021.120357
Show in context However, since the seep carbonate mineral facies and precipitation rate are mainly determined by the methane seepage intensity (Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021; Smrzka et al., 2021), it is reasonable to anticipate methane seepage intensity is one of the key controls on Ca isotopic fractionation.
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Therefore, the seep carbonate precipitation environment can be reconstructed using specific elemental composition and mineral phases (Burton, 1993; Blättler et al., 2021; Smrzka et al., 2021).
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When the SMTZ is located at deeper sediment horizons (weak methane seepage), high-Mg carbonates generally precipitate (Thiagarajan et al., 2020; Smrzka et al., 2021).
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The seepage intensity not only controls the carbonate precipitation rate, but also controls the depth of the SMTZ (Smrzka et al., 2021), which in turn controls the magnitude of Ca isotopic fractionation.
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In addition, studies have shown that the carbonate precipitation rate affects the composition of specific trace element ratios (Sr/Ca, Mg/Ca), and this is the basis for us to potentially use the specific trace element ratios to reconstruct the palaeo carbonate precipitation rate (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Smrzka et al., 2021).
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Strong methane seepage results in a shallow SMTZ, and the pore water has a strong mixing effect with the overlying seawater, resulting in a relatively open system (Smrzka et al., 2021).
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In addition, carbonate formation in this environment is dominated by calcite (Goetschl et al., 2019; Smrzka et al., 2021), therefore calcite records heavy Ca isotopic values (Fig. 3b).
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Tang, J., Dietzel, M., Böhm, F., Köhler, S.J., Eisenhauer, A. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: II. Ca isotopes. Geochimica et Cosmochimica Acta 72, 3733–3745. https://doi.org/10.1016/j.gca.2008.05.033
Show in context However, since the seep carbonate mineral facies and precipitation rate are mainly determined by the methane seepage intensity (Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021; Smrzka et al., 2021), it is reasonable to anticipate methane seepage intensity is one of the key controls on Ca isotopic fractionation.
View in article
In addition, studies have shown that the carbonate precipitation rate affects the composition of specific trace element ratios (Sr/Ca, Mg/Ca), and this is the basis for us to potentially use the specific trace element ratios to reconstruct the palaeo carbonate precipitation rate (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Smrzka et al., 2021).
View in article
For the same fluid composition, high Sr/Ca and low Mg/Ca in the carbonates are associated with high carbonate precipitation rates, while low carbonate precipitation rates result in low Sr/Ca and high Mg/Ca (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
Lab experiments and theoretical models have shown that the Ca isotopic fractionation of the carbonates increases with higher precipitation rates (Tang et al., 2008; DePaolo, 2011; Nielsen et al., 2012).
View in article
Teichert, B.M.A., Gussone, N., Eisenhauer, A., Bohrmann, G. (2005) Clathrites: Archives of near-seafloor pore-fluid evolution (δ44/40Ca, δ13C, δ18O) in gas hydrate environments. Geology 33, 213–216. https://doi.org/10.1130/G21317.1
Show in context In recent years multiple studies have reported Ca isotope ratios in pore waters and carbonates recovered from methane seeps and discussed the factors controlling the Ca isotopic fractionation in these environments (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
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The impact of the carbonate precipitation rate on the Ca isotopic fractionation has also been demonstrated (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
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Teichert, B.M.A., Gussone, N., Torres, M.E. (2009) Controls on calcium isotope fractionation in sedimentary porewaters. Earth and Planetary Science Letters 279, 373–382. https://doi.org/10.1016/j.epsl.2009.01.011
Show in context To ensure the reliability and applicability of this proxy, it is necessary to fully understand the factors affecting Ca isotopic fractionation in the seep carbonates (Gussone et al., 2005; Teichert et al., 2009; Griffith and Fantle, 2021).
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Cold seep is a widely observed phenomenon on the continental margins (Michaelis et al., 2002; Teichert et al., 2009; Miao et al., 2022), which typically develops extensive authigenic carbonates due to a local increase of alkalinity resulted from sulfate driven anaerobic oxidation of methane (SD-AOM) (Michaelis et al., 2002; Teichert et al., 2009; Blättler et al., 2021).
View in article
Thiagarajan, N., Crémière, A., Blättler, C., Lepland, A., Kirsimäe, K., Higgins, J., Brunstad, H., Eiler, J. (2020) Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments. Geochimica et Cosmochimica Acta 279, 204–219. https://doi.org/10.1016/j.gca.2020.03.015
Show in context In recent years multiple studies have reported Ca isotope ratios in pore waters and carbonates recovered from methane seeps and discussed the factors controlling the Ca isotopic fractionation in these environments (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
The impact of the carbonate precipitation rate on the Ca isotopic fractionation has also been demonstrated (Teichert et al., 2005; Henderson et al., 2006; Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
However, since the seep carbonate mineral facies and precipitation rate are mainly determined by the methane seepage intensity (Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021; Smrzka et al., 2021), it is reasonable to anticipate methane seepage intensity is one of the key controls on Ca isotopic fractionation.
View in article
Strong methane seepage results in a shallow SMTZ (near the seafloor), the presence of dissolved sulfate is thought to inhibit precipitation of high-Mg carbonates and hence favours aragonite formation (Burton, 1993; Thiagarajan et al., 2020).
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When the SMTZ is located at deeper sediment horizons (weak methane seepage), high-Mg carbonates generally precipitate (Thiagarajan et al., 2020; Smrzka et al., 2021).
View in article
In addition, studies have shown that the carbonate precipitation rate affects the composition of specific trace element ratios (Sr/Ca, Mg/Ca), and this is the basis for us to potentially use the specific trace element ratios to reconstruct the palaeo carbonate precipitation rate (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Smrzka et al., 2021).
View in article
For the same fluid composition, high Sr/Ca and low Mg/Ca in the carbonates are associated with high carbonate precipitation rates, while low carbonate precipitation rates result in low Sr/Ca and high Mg/Ca (Luff and Wallmann, 2003; Tang et al., 2008; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
This is consistent with previous studies, in which the precipitation rate of aragonite is higher than calcite (Luff and Wallmann, 2003; Gussone et al., 2005; Bayon et al., 2007; Thiagarajan et al., 2020; Blättler et al., 2021).
View in article
The results indicate that high carbonate precipitation rates (aragonite dominated) lead to the accumulation of light Ca isotopes, while low carbonate precipitation rates (calcite dominated) result in heavy Ca isotopes, which is consistent with current understanding (Gussone et al., 2005; Thiagarajan et al., 2020; Blattler et al., 2021).
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In normal marine environments, temperature is considered to be a factor controlling the carbonate precipitation rate (Burton and Walter, 1987; Gussone et al., 2005; Thiagarajan et al., 2020).
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In addition, temperature changes in other cold seeps are also not considered as the potential factor in controlling the carbonate precipitation rate (Bradbury and Turchyn, 2018; Thiagarajan et al., 2020; Blattler et al., 2021).
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Wei, J., Wu, T., Zhang, W., Deng, Y., Xie, R., Feng, J., Liang, J., Lai, P., Zhou, J., Cao, J. (2020) Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities. Minerals 10, 1135. https://doi.org/10.3390/min10121135
Show in context All the seep carbonates are reported for the first time except W08B (Wei et al., 2020, 2022).
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Wei, J., Wu, T., Miao, X., Su, P. (2022) Massive Natural Gas Hydrate Dissociation During the Penultimate Deglaciation (∼130 ka) in the South China Sea. Frontiers in Marine Science 9, 875374. https://doi.org/10.3389/fmars.2022.875374
Show in context All the seep carbonates are reported for the first time except W08B (Wei et al., 2020, 2022).
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Supplementary Information
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Figures
Figure 1 (a) World map with the red rectangle showing the study area. (b) Topographic map of the study area and the locations of the seep carbonates (green stars). (c) SY069 seep carbonate. (d) SQW-65 seep carbonate. (e) W08B seep carbonate. The sampling locations of the seep carbonates are illustrated by the red dots.
Figure 2 Geochemical properties of the seep carbonates (adapted from Bayon et al., 2007
Bayon, G., Pierre, C., Etoubleau, J., Voisset, M., Cauquil, E., Marsset, T., Sultan, N., Le Drezen, E., Fouquet, Y. (2007) Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241, 93–109. https://doi.org/10.1016/j.margeo.2007.03.007
). (a) Sr/Ca vs. Mg/Ca. (b) Sr/Ca vs. δ44/40Ca (‰, relative to SRM 915a). (c) Mg/Ca ratios vs. δ44/40Ca (‰ SW).
Figure 3 Schematic diagram illustrating the calcium isotopic fractionation in open system vs. semi-enclosed system.