Can CO2 outgassing and carbonate precipitation explain the Lomagundi Excursion?
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Keywords: Lomagundi Excursion, Carbon isotope, outgassing
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Abstract
Figures
 Figure 1 Schematic of a conceptual model of carbonate deposition in lagoons during periods of eustatic sea level (a) fall and (b) rise. δ13Ccarb means δ13Ccarbonate. |  Figure 2 δ13CDIC obtained for varying proportions of outgassing (Xog) and carbonate precipitation (Xcp) from Equation 2 at different DIC loss proportions (f). |
| Figure 1 | Figure 2 |
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Introduction
The carbon isotopic composition of carbonates (δ13Ccarbonate) reflects the isotopic composition of the dissolved inorganic carbon (δ13CDIC) pool from which they precipitate with minimal fractionation (ɛcarbonate-bicarbonate ∼0.9 ‰ to 2.7 ‰; Rubinson and Clayton, 1969
Rubinson, M., Clayton, R.N. (1969) Carbon-13 fractionation between aragonite and calcite. Geochimica et Cosmochimica Acta 33, 997–1002. https://doi.org/https://doi.org/10.1016/0016-7037(69)90109-4
). Certain time intervals in geological history record highly positive δ13Ccarbonate (>4 ‰), which are known as carbon isotope excursions (Kump and Arthur, 1999Kump, L.R., Arthur, M.A. (1999) Interpreting carbon-isotope excursions: Carbonates and organic matter. Chemical Geology 161, 181–198. https://doi.org/https://doi.org/10.1016/S0009-2541(99)00086-8
; Hodgskiss et al., 2023Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
). The most prominent among such excursions, in terms of both magnitude and duration, is the Lomagundi-Jatuli Event (LJE), which occurred at ∼2.2–2.0 Ga with δ13Ccarbonate as high as 30 ‰ (Schidlowski et al., 1975Schidlowski, M., Eichmann, R., Junge, C.E. (1975) Precambrian sedimentary carbonates: Carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget. Precambrian Research 2, 1–69. https://doi.org/https://doi.org/10.1016/0301-9268(75)90018-2
; Hodgskiss et al., 2023Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
).Conventionally, global enhancement in organic carbon burial has the most traction among the researchers as the cause of LJE (e.g., Schidlowski et al., 1975
Schidlowski, M., Eichmann, R., Junge, C.E. (1975) Precambrian sedimentary carbonates: Carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget. Precambrian Research 2, 1–69. https://doi.org/https://doi.org/10.1016/0301-9268(75)90018-2
; Bekker and Holland, 2012Bekker, A., Holland, H.D. (2012) Oxygen overshoot and recovery during the early Paleoproterozoic. Earth and Planetary Science Letters 317–318, 295–304. https://doi.org/10.1016/j.epsl.2011.12.012
). According to this hypothesis, a significant increase in global marine productivity and associated organic carbon burial (∼60 %; Schidlowski et al., 1975Schidlowski, M., Eichmann, R., Junge, C.E. (1975) Precambrian sedimentary carbonates: Carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget. Precambrian Research 2, 1–69. https://doi.org/https://doi.org/10.1016/0301-9268(75)90018-2
) during the LJE led to the preferential removal of 12C from DIC pool, thereby enriching the residual DIC and carbonates precipitating from them in 13C. Such a huge organic carbon burial should manifest a significant accumulation of organic-rich sedimentary rocks synchronous with the excursion (Melezhik et al., 1999Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
). But the lack of such organic-rich strata in the sedimentary rock records during the LJE (Melezhik et al., 1999Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
; Hodgskiss et al., 2023Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
) led to the proposition of alternative explanatory models, which are reviewed in Hodgskiss et al. (2023)Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
. However, two processes that have been least explored in explaining the LJE are carbon dioxide (CO2) outgassing (Stiller et al., 1985Stiller, M., Rounick, J.S., Shasha, S. (1985) Extreme carbon-isotope enrichments in evaporating brines. Nature 316, 434–435. https://doi.org/https://doi.org/10.1038/316434a0
) and carbonate precipitation along with their potential interactions (Beeler et al., 2020Beeler, S.R., Gomez, F.J., Bradley, A.S. (2020) Controls of extreme isotopic enrichment in modern microbialites and associated abiogenic carbonates. Geochimica et Cosmochimica Acta 269, 136–149. https://doi.org/10.1016/j.gca.2019.10.022
), which are the focus of this study.top
Why CO2 Outgassing?
Dissolved CO2 is an important component of DIC pool, and variations in its proportion have the potential to modulate δ13CDIC, and in turn δ13Ccarbonate. CO2 outgassing to the atmosphere has been shown to cause significant increase in δ13C in the residual DIC pool in modern day aquatic systems with fractionation factors ranging from −15 to −23 ‰ (Stiller et al., 1985
Stiller, M., Rounick, J.S., Shasha, S. (1985) Extreme carbon-isotope enrichments in evaporating brines. Nature 316, 434–435. https://doi.org/https://doi.org/10.1038/316434a0
), which makes it a plausible mechanism to explain the LJE. The air-water flux of CO2 (FCO2) can be expressed as:Eq. 1
where ΔpCO2 is the pCO2 gradient (pCO2-surface water − pCO2-atmosphere), which determines the direction of the CO2 flux; Kw is the gas transfer velocity (a quadratic function of wind speed); and Ks is the CO2 solubility (a function of temperature and salinity) (Nicholson et al., 2022
Nicholson, S.-A., Whitt, D.B., Fer, I., du Plessis, M.D., Lebéhot, A.D., Swart, S., Sutton, A.J., Monteiro, P.M. (2022) Storms drive outgassing of CO2 in the subpolar Southern Ocean. Nature Communications 13, 158. https://doi.org/https://doi.org/10.1038/s41467-021-27780-w
). The pCO2-atmosphere during the LJE has been calculated to be 20 to 620 times higher than the present day atmospheric level (Kanzaki and Murakami, 2015Kanzaki, Y., Murakami, T. (2015) Estimates of atmospheric CO2 in the Neoarchean–Paleoproterozoic from paleosols. Geochimica et Cosmochimica Acta 159, 190–219. https://doi.org/10.1016/j.gca.2015.03.011
), which may appear to limit CO2 outgassing from the water column. However, air-sea gas exchange under such high pCO2-atmosphere drives the surface ocean rapidly towards equilibrium with the atmosphere, resulting in high pCO2 in surface waters (Galbraith et al., 2015Galbraith, E.D., Kwon, E.Y., Bianchi, D., Hain, M.P., Sarmiento, J.L. (2015) The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere and ocean. Global Biogeochemical Cycles 29, 307–324. https://doi.org/10.1002/2014GB004929
). Under equilibrium condition, the partial pressure of CO2 in surface water equals that of the atmosphere (pCO2-surface-water = pCO2-atmosphere; ΔpCO2 = 0) resulting in no net flux of CO2, i.e. neither net outgassing nor in-gassing. In cases where surface water pCO2 increases due to enhanced organic matter degradation and/or carbonate precipitation (pCO2-surface-water > pCO2-atmosphere; positive ΔpCO2), outgassing of CO2 to the atmosphere is favoured. Outgassing of CO2 is further enhanced potentially in cases of increased surface water temperature due to reduced CO2 solubility (low Ks; Dai et al., 2022Dai, Y., Yu, J., Ren, H., Ji, X. (2022) Deglacial Subantarctic CO2 outgassing driven by a weakened solubility pump. Nature Communications 13, 5193. https://doi.org/10.1038/s41467-022-32895-9
) and stronger winds leading to high gas transfer velocity (high Kw; Nicholson et al., 2022Nicholson, S.-A., Whitt, D.B., Fer, I., du Plessis, M.D., Lebéhot, A.D., Swart, S., Sutton, A.J., Monteiro, P.M. (2022) Storms drive outgassing of CO2 in the subpolar Southern Ocean. Nature Communications 13, 158. https://doi.org/https://doi.org/10.1038/s41467-021-27780-w
). Sedimentological observations in Lomagundi successions suggest higher temperatures associated with greenhouse climate (potentially low Ks ; Shang et al., 2024Shang, G., Zhai, M., Peng, P., Miao, P., Li, Q. (2024) A climate change from icehouse to greenhouse following Huronian glaciation: Evidence from long-term storm deposits of the Paleoproterozoic Hutuo Group in the North China Craton. Journal of Asian Earth Sciences 274, 106289. https://doi.org/10.1016/j.jseaes.2024.106289
) and stronger winds (high Kw) due to prevalence of storm events (Pambo et al., 2006Pambo, F., Guiraud, M., Quesne, D., Gauthier-Lafaye, F., Azzibrouck, G., Lang, J. (2006) The Proterozoic Franceville Basin (SE Gabon): An example of interaction between marine sedimentation and extensional faulting. Africa Geoscience Review 13, 77–106.
; Hill-Svehla and Corcoran, 2023Hill-Svehla, C.M., Corcoran, P.L. (2023) Microbial, tidal, and storm activity in a macrotidal to shallow marine shelf environment during the Paleoproterozoic era. Canadian Journal of Earth Sciences 60, 713–738. https://doi.org/10.1139/cjes-2022-0042
; Shang et al., 2024Shang, G., Zhai, M., Peng, P., Miao, P., Li, Q. (2024) A climate change from icehouse to greenhouse following Huronian glaciation: Evidence from long-term storm deposits of the Paleoproterozoic Hutuo Group in the North China Craton. Journal of Asian Earth Sciences 274, 106289. https://doi.org/10.1016/j.jseaes.2024.106289
). Therefore, we argue that, positive ΔpCO2 along with higher temperature and high wind speed during the LJE could have likely favoured CO2 outgassing from the aquatic systems, leading to high δ13CDIC and eventually high δ13Ccarbonate; this needs to be explored for its role in the LJE.top
Facies Dependency of the Lomagundi Excursion
The lithology of Lomagundi successions worldwide constitutes evaporitic deposits (with calcites and dolomites), dolostones, stromatolitic dolostones, and siliciclastic rocks (Melezhik et al., 1999
Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
; Brasier et al., 2011Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
; Melezhik et al., 2013Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
) inferred to be deposited across a range of sedimentological facies ranging from coastal sabkhas and near shore to open ocean environments (Melezhik et al., 1999Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
). Accordingly, a facies based screening of the global Lomagundi carbonates showed that anomalously high δ13Ccarbonate were confined only to coastal evaporites (+8.1 ± 3.8 ‰) and near shore carbonates (+6.2 ± 2.0 ‰), whereas the open ocean carbonates (+1.5 ± 2.4 ‰) remained unperturbed (Prave et al., 2022Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
). This facies based compilation of Lomagundi carbonates revealed that the LJE was a global near shore carbon anomaly and not a global carbon anomaly (Melezhik et al., 1999Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
; Prave et al., 2022Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
; Hodgskiss et al., 2023Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
). Therefore, any model proposed to explain the LJE should account for both the lack of organic-rich strata as well as the facies dependency of the excursion. Here, we propose and explain a facies dependent, water column scale biogeochemical explanation for the LJE, rather than invoking global scale mechanisms.top
A Depositional Framework for Lomagundi Deposits
Eustatic sea level fluctuations are common in greenhouse climates (Warren, 2010
Warren, J.K. (2010) Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits. Earth-Science Reviews 98, 217–268. https://doi.org/https://doi.org/10.1016/j.earscirev.2009.11.004
). Studies on evaporite deposits globally suggest that greenhouse climate results in the formation of sub-sea level seepage lagoons in which eustatic sea level fluctuations cause the deposition of platform evaporites (during low sea level) alternating with marine shelf carbonates (during high sea level) (Warren, 2010Warren, J.K. (2010) Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits. Earth-Science Reviews 98, 217–268. https://doi.org/https://doi.org/10.1016/j.earscirev.2009.11.004
). The presence of such alternating evaporite and carbonate depositional cycles during Lomagundi successions (Brasier et al., 2011Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
; Melezhik et al., 2013Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
) is consistent with the influence of greenhouse climate inferred during the LJE (Shang et al., 2024Shang, G., Zhai, M., Peng, P., Miao, P., Li, Q. (2024) A climate change from icehouse to greenhouse following Huronian glaciation: Evidence from long-term storm deposits of the Paleoproterozoic Hutuo Group in the North China Craton. Journal of Asian Earth Sciences 274, 106289. https://doi.org/10.1016/j.jseaes.2024.106289
). Additionally, these evaporites and carbonates are inferred to be deposited in restricted to partially restricted lagoonal environments (Brasier et al., 2011Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
; Melezhik et al., 2013Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
). Therefore, we argue that the evaporites with a mean δ13C of +8.1 ± 3.8 ‰ (Prave et al., 2022Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
) might have likely deposited during periods of low sea level, where the lagoons become hydrographically isolated leading to hypersaline conditions (evaporitic lagoon; Fig. 1a). On the other hand, the near shore dolostones and stromatolitic dolostones of Lomagundi with a mean δ13C of +6.2 ± 2.0 ‰ (Prave et al., 2022Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
) might have deposited during high sea level in partially restricted marine shelf lagoons (Fig. 1b). These enclosed lagoons are separated from the open ocean by elevated barriers and hence would be characterised by minimal or intermittent input of sea water from the open ocean (Fig. 1). This proposed depositional framework accounts for the observed irregular appearance of evaporitic signatures and hence anomalous values in the Lomagundi successions (Melezhik et al., 1999Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
, 2013Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
).Figure 1 Schematic of a conceptual model of carbonate deposition in lagoons during periods of eustatic sea level (a) fall and (b) rise. δ13Ccarb means δ13Ccarbonate.
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Facies Dependent Biogeochemical Processes
The sources of DIC in any aquatic system include external inputs (riverine, volcanic, and atmospheric), organic matter degradation, and carbonate dissolution; whereas the sinks include primary productivity and associated organic burial, CO2 outgassing, and carbonate precipitation. Productivity and outgassing leads to a decrease in DIC concentration and increase in δ13C of the remaining DIC pool. Organic matter degradation, on the other hand, increases the DIC pool with a decrease in δ13CDIC. Carbonate dissolution and precipitation act opposite to each other where dissolution increases both concentration and isotopic composition of DIC pool, while precipitation decreases both (Alling et al., 2012
Alling, V., Porcelli, D., Mörth, C.-M., Anderson, L.G., Sanchez-Garcia, L., Gustafsson, Ö., Andersson, P.S., Humborg, C. (2012) Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from δ13C values of DIC. Geochimica et Cosmochimica Acta 95, 143–159. https://doi.org/10.1016/j.gca.2012.07.028
). Therefore, δ13CDIC in our hypothesised lagoon system during the LJE could also be the resultant of these biogeochemical processes and their interactions. Carbonate dissolution appears to be the least relevant process in the context of LJE due to a predominantly carbonate precipitating environment in the water column. Hence, the LJE might have resulted due to the interplay of four processes (productivity, organic matter degradation, CO2 outgassing, and carbonate precipitation) in the lagoonal setting. Although productivity and associated organic burial have the potential to increase the δ13CDIC, low organic carbon concentration in lagoonal LJE carbonates (Karhu, 1993Karhu, J. (1993) Paleoproterozoic evolution of the carbon isotope ratios of sedimentary carbonates in the Fennoscandian Shield. Bulletin of the Geological Survey of Finland, 371, pp. 87.
) and absence of organic-rich strata simultaneous or prior to the LJE (Melezhik et al., 1999Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
), suggests that the organic matter produced in the lagoons might have largely undergone remineralisation. Such extensive remineralisation would have resulted in minimal net organic burial, thereby having negligible effects on both size and isotopic composition of DIC pool during the LJE. Therefore, primary productivity and mineralisation (organic matter degradation) may be assumed to have a neutralising effect on the δ13CDIC during the LJE with CO2 outgassing and carbonate precipitation being the dominant processes controlling the δ13CDIC of the lagoons.Assuming negligible DIC input to alter the pool size and isotopic composition through seepage or restricted exchange in evaporitic and marine shelf lagoons, the range of possible δ13CDIC values that can be achieved in the lagoons from an initial open ocean seawater composition (δ¹3C ∼0 ± 4 ‰; Prave et al., 2022
Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
) through CO2 outgassing and carbonate precipitation can be approximated by treating the lagoons as a closed system and applying a Rayleigh fractionation model. The δ13CDIC of the lagoons at a given time will then be governed by the relative contributions of outgassing and carbonate precipitation.Eq. 2
where, δ13Cinitial is the initial δ13CDIC. Xog is the fraction of DIC being outgassed whereas Xcp is the fraction of DIC precipitated as carbonates (Xog + Xcp = 1). ɛog and ɛcp are the respective fractionation factors (ɛ = δ13Cproduct − δ13Creactant). The term f denotes the fraction of DIC remaining. Equation 2 suggests that varying proportions of CO2 outgassing (Xog) and carbonate precipitation (Xcp) could have resulted in DIC loss from a Lomagundi lagoonal setting, where the former might have increased the δ13CDIC of water column while the latter could have decreased it. Solving Equation 2 for δ13CDIC shows that varying contributions in DIC loss through outgassing and carbonate precipitation can result in different δ13CDIC (see Fig. 2 and Supplementary Information for values used in calculation).
Figure 2 δ13CDIC obtained for varying proportions of outgassing (Xog) and carbonate precipitation (Xcp) from Equation 2 at different DIC loss proportions (f).
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Lomagundi Excursions in an Evaporitic Environment
The carbonates in the Lomagundi evaporites show anomalously high δ13Ccarbonate (+8.1 ± 3.8 ‰; Prave et al., 2022
Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
), which we infer to be deposited in the evaporitic lagoons during eustatic sea level fall (Fig. 1a). The restricted nature of the lagoon and warmer temperatures during the LJE should have preferably favoured DIC loss through evaporative outgassing of CO2 to the atmosphere (Xog > 50 %; the outgassing dominant regime in Fig. 2). Such non-equilibrium CO2 transfer to the atmosphere is observed in the present day Dead Sea brine, which causes a significant increase in δ13C of the residual DIC pool with fractionation factors ranging between −15 and −23 ‰ (Stiller et al., 1985Stiller, M., Rounick, J.S., Shasha, S. (1985) Extreme carbon-isotope enrichments in evaporating brines. Nature 316, 434–435. https://doi.org/https://doi.org/10.1038/316434a0
; Barkan et al., 2001Barkan, E., Luz, B., Lazar, B. (2001) Dynamics of the carbon dioxide system in the Dead Sea. Geochimica et Cosmochimica Acta 65, 355–368. https://doi.org/10.1016/S0016-7037(00)00540-8
). Hence, we infer that intense evaporative loss of CO2 through outgassing might have resulted in significant loss of DIC, thereby enriching remaining DIC pool in 13C in the evaporitic lagoon’s water column during the LJE. Such intense evaporation promotes the precipitation of calcium sulphates (CaSO4) including gypsum and anhydrite. A large proportion of calcites in these settings are interpreted to have formed through early diagenetic calcitisation of the deposited CaSO4 (Brasier et al., 2011Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
; Melezhik et al., 2013Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
). Furthermore, selective removal of Ca2+ from the water column during CaSO4 precipitation increases the Mg/Ca ratio (Brasier et al., 2011Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
). Such Mg-rich conditions would have also favoured dolomite formation, either through direct precipitation or through dolomitisation of pre-existing calcites (Brasier et al., 2011Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
). In this context, we argue that the 13C enriched DIC of the lagoonal waters likely served as the fluid source for such calcitisation and dolomite precipitation (or dolomitisation of calcite) processes, thereby resulting in the formation of anomalously high δ13C evaporitic carbonates. This is evidenced through carbonates in the evaporitic environments of Tulomozero formations from Onega basin where syn-sedimentary calcitisation and dolomitisation have been observed (Brasier et al., 2011Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
; Melezhik et al., 2013Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
).top
Lomagundi Excursion in Marine Shelf Lagoon Environment
It has been observed that carbonates not associated with evaporitic mineral phases also record Lomagundi Excursion, suggesting deposition in a non-hypersaline basin (Goswami et al., 2023
Goswami, A., Sarangi, S., Mohanty, S.P., Patil, D.J., Sarkar, A., Ray, J.S., Das, S., Mohanty, D., Ahmad, S.M., Pradhan, R.M. (2023) Negative δ13Ccarb excursions within early part of the Lomagundi event recorded in the Paleoproterozoic sedimentary carbonates, Aravalli Supergroup, Rajasthan India: Chemostratigraphy and basin evolution. Precambrian Research 399, 107240. https://doi.org/https://doi.org/10.1016/j.precamres.2023.107240
). Moreover, these carbonates show relatively lower δ13Ccarbonate (+6.2 ± 2.0 ‰) compared to carbonates of evaporite deposits (Prave et al., 2022Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
). We infer these carbonates to be marine shelf carbonates deposited in the lagoon during eustatic sea level rise (Fig. 1b). These marine shelf lagoonal carbonates constitute both dolostones (which are inorganically precipitated) and stromatolitic dolostones, which are microbialites formed through microbially induced precipitation of carbonates, where microbial mats influence their mineralisation (Dupraz et al., 2009Dupraz, C., Reid, R P., Braissant, O., Decho, A.W., Norman, R.S., Visscher, P.T. (2009) Processes of carbonate precipitation in modern microbial mats. Earth-Science Reviews 96, 141–162. https://doi.org/10.1016/j.earscirev.2008.10.005
; Melezhik et al., 2013Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
). The precipitation of carbonate from the water column produces CO2; therefore, stromatolitic and inorganic carbonate precipitation in the marine shelf lagoonal setting should eventually produce CO2. This CO2 could either be consumed by phytoplankton or outgassed to the atmosphere (Dupraz et al., 2009Dupraz, C., Reid, R P., Braissant, O., Decho, A.W., Norman, R.S., Visscher, P.T. (2009) Processes of carbonate precipitation in modern microbial mats. Earth-Science Reviews 96, 141–162. https://doi.org/10.1016/j.earscirev.2008.10.005
). Moreover, microbialites should have also produced CO2 due to their characteristically rapid decomposition of biomass, which should have resulted in negligible organic matter accumulation (Jørgensen et al., 1992Jørgensen, B.B., Nelson, D.C., Ward, D.M. (1992) Chemotrophy and decomposition in modern microbial mats. In: Schopf, J.W., Klein, C. (Eds.) The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press, Cambridge, 287–293.
). Evidently, the low organic matter preservation in the LJE carbonates (Karhu, 1993Karhu, J. (1993) Paleoproterozoic evolution of the carbon isotope ratios of sedimentary carbonates in the Fennoscandian Shield. Bulletin of the Geological Survey of Finland, 371, pp. 87.
) likely indicates that most of the microbialite produced organic matter was mineralised to CO2. The CO2 sourced from carbonate precipitation and biomass remineralisation should have likely led to build up of CO2 in the lagoon. Given the warmer temperature (high Ks) due to greenhouse climate, higher wind speeds (high Kw) due to storm events and partially restricted nature of the lagoon, the air-sea flux of CO2 should likely be higher. For the lagoon to outgas CO2 into the atmosphere, the pCO2surface should exceed the pCO2atmosphere. In restricted water bodies like lakes and lagoons, stratification in the water column is common, which can be disrupted by strong winds, promoting vertical redistribution of CO2 from bottom layers to the top layers and resulting in an increase in pCO2 in the surface water (Jennings et al., 2012Jennings, E., Jones, S., Arvola, L., Staehr, P.A., Gaiser, E., Jones, I.D., Weathers, K.C., Weyhenmeyer, G.A., Chiu, C., De Eyto, E. (2012) Effects of weather‐related episodic events in lakes: An analysis based on high‐frequency data. Freshwater Biology 57, 589–601. https://doi.org/10.1111/j.1365-2427.2011.02729.x
). Therefore, we hypothesise that the CO2 produced due to carbonate precipitation and organic matter remineralisation in the Lomagundi marine shelf lagoon should have increased the surface water pCO2 (high ΔpCO2) due to disturbance of stratification through strong winds, which might have favoured outgassing. However, the presence of stromatolitic carbonates and dolostones indicates that even though CO2 outgassing occurred, carbonate precipitation was likely dominant in the shelf lagoons. Therefore, in the marine shelf lagoonal facies, carbonate precipitation (Xcp) might have likely dominated, i.e. the carbonate precipitation dominant regime in Figure 2. As is evident from the fractionation factor of carbonate precipitation, the process over time tends to lower the δ13C of water column DIC (Rubinson and Clayton, 1969Rubinson, M., Clayton, R.N. (1969) Carbon-13 fractionation between aragonite and calcite. Geochimica et Cosmochimica Acta 33, 997–1002. https://doi.org/https://doi.org/10.1016/0016-7037(69)90109-4
), which could be the reason for comparatively lower δ13Ccarbonate in the marine shelf lagoonal carbonates compared to evaporitic carbonates, where outgassing dominated. Our Rayleigh fractionation calculation using Equation 2 for the δ13CDIC and residual DIC (f) data of Lake Laguna Negra (Beeler et al., 2020Beeler, S.R., Gomez, F.J., Bradley, A.S. (2020) Controls of extreme isotopic enrichment in modern microbialites and associated abiogenic carbonates. Geochimica et Cosmochimica Acta 269, 136–149. https://doi.org/10.1016/j.gca.2019.10.022
), a modern day analogy for the lagoonal environment discussed here, suggests that ∼80 % DIC loss through 30–40 % CO2 outgassing can explain the observed δ13CDIC as high as ∼15 ‰ in the lake. We refrain from flux estimates of these processes during LJE due to uncertainty in quantification of several parameters, particularly pCO2surface-water.top
Unperturbed Open Ocean and Coeval δ13Corg
The open ocean carbonates of the LJE show δ13Ccarbonate ∼+1.5 ± 2.4 ‰, which lies within the range of normal sea water isotopic composition (0 ± 4 ‰; Prave et al., 2022
Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
) observed throughout the geologic past. This reflects the conservative nature of open ocean δ13CDIC. Previous studies suggested that the Proterozoic ocean likely had a larger DIC inventory than that of the present day ocean (Bartley and Kah, 2004Bartley, J.K., Kah, L.C. (2004) Marine carbon reservoir, Corg-Ccarb coupling, and the evolution of the Proterozoic carbon cycle. Geology 32, 129–132. https://doi.org/10.1130/G19939.1
; Cantine et al., 2020Cantine, M.D., Knoll, A.H., Bergmann, K.D. (2020) Carbonates before skeletons: A database approach. Earth-Science Reviews 201, 103065. https://doi.org/10.1016/j.earscirev.2019.103065
) and hence could likely be the reason for the unperturbed nature of open ocean δ13CDIC and δ13Ccarbonate. With unperturbed open ocean δ13CDIC and elevated lagoonal δ13CDIC, open ocean organic matter would be expected to have a lower δ¹3Corg than lagoonal organic matter during the LJE. Supporting the above argument, the open ocean deposited Sengoma shales of the Silverton Formation showed δ¹3Corg ranging from −33.7 to −20.8 ‰ with an average of −27.0 ± 3.0 ‰ (Bekker et al., 2008Bekker, A., Holmden, C., Beukes, N.J., Kenig, F., Eglinton, B., Patterson, W.P. (2008) Fractionation between inorganic and organic carbon during the Lomagundi (2.22–2.1 Ga) carbon isotope excursion. Earth and Planetary Science Letters 271, 278–291. https://doi.org/10.1016/j.epsl.2008.04.021
), while the overlying Lomagundi carbonates (δ¹3Ccarbonate ranging from +8.3 to +11.2 ‰) showed relatively higher δ¹3Corg, ranging from −24.8 to −13.9 ‰ (Bekker et al., 2008Bekker, A., Holmden, C., Beukes, N.J., Kenig, F., Eglinton, B., Patterson, W.P. (2008) Fractionation between inorganic and organic carbon during the Lomagundi (2.22–2.1 Ga) carbon isotope excursion. Earth and Planetary Science Letters 271, 278–291. https://doi.org/10.1016/j.epsl.2008.04.021
). Available data indicate a wide range of δ¹3Corg values during the LJE, from as low as −48.2 ‰ to as high as −11 ‰ (Farrell et al., 2021Farrell, Ú.C., Samawi, R., Anjanappa, S., Klykov, R., Adeboye, O.O., et al. (2021) The Sedimentary Geochemistry and Paleoenvironments Project. Geobiology 19, 545–556. https://doi.org/10.1111/gbi.12462
). We attribute the deposits with relatively high δ¹3Corg during the LJE to be associated with lagoons, whereas those with comparatively lower δ¹3Corg to likely represent open ocean depositional settings.top
Uniqueness of Anomalous δ13C to the LJE
The LJE coincided with the subaerial emergence of continents which should have resulted in widespread creation of (partially) restricted intracontinental shallow basins, favouring evaporite deposition and abundant stromatolite reef accretion (Melezhik et al., 1999
Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
; Hodgskiss et al., 2023Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
). Such restricted basins/lagoons are suitable for CO2 outgassing and carbonate precipitation to dominate the carbon cycling (Fig. 1). Specifically, the sea surface temperature was very high (∼60 oC) during the Paleoproterozoic compared to later time intervals, as inferred from resurrected proteins (Gaucher et al., 2008Gaucher, E.A., Govindarajan, S., Ganesh, O.K. (2008) Palaeotemperature trend for Precambrian life inferred from resurrected proteins. Nature 451, 704–707. https://doi.org/10.1038/nature06510
) and oxygen isotope studies (Robert and Chaussidon, 2006Robert, F., Chaussidon, M. (2006) A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts. Nature 443, 969–972. https://doi.org/10.1038/nature05239
). Therefore, we argue that such high temperatures during the LJE could have resulted in increased evaporation and reduced CO2 solubility (Ks in Equation 1), leading to enhanced CO2 outgassing in the lagoons. The observed decline in sea surface temperatures post-LJE (Robert and Chaussidon, 2006Robert, F., Chaussidon, M. (2006) A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts. Nature 443, 969–972. https://doi.org/10.1038/nature05239
; Gaucher et al., 2008Gaucher, E.A., Govindarajan, S., Ganesh, O.K. (2008) Palaeotemperature trend for Precambrian life inferred from resurrected proteins. Nature 451, 704–707. https://doi.org/10.1038/nature06510
) could likely explain the absence of anomalous values in evaporitic carbonates of later geological intervals. Moreover, the Lomagundi carbonates in many regions are overlain by organic-rich shales, which are attributed to the cessation of shallow marine restricted, carbonate precipitating environments post-LJE (Bekker et al., 2003Bekker, A., Karhu, J.A., Eriksson, K.A., Kaufman, A.J. (2003) Chemostratigraphy of Paleoproterozoic carbonate successions of the Wyoming Craton: Tectonic forcing of biogeochemical change? Precambrian Research 120, 279–325. https://doi.org/https://doi.org/10.1016/S0301-9268(02)00164-X
; Hodgskiss et al., 2023Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
). Termination of such environments along with the observed decline in sea surface temperature should have resulted in reduced dominance of CO2 outgassing and carbonate precipitation, which likely explains the abrupt termination of the LJE.top
Acknowledgement
Authors are thankful to the Department of Space, Govt. of India for supporting this work.
Editor: Claudine Stirling
top
References
Alling, V., Porcelli, D., Mörth, C.-M., Anderson, L.G., Sanchez-Garcia, L., Gustafsson, Ö., Andersson, P.S., Humborg, C. (2012) Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from δ13C values of DIC. Geochimica et Cosmochimica Acta 95, 143–159. https://doi.org/10.1016/j.gca.2012.07.028
Show in context Carbonate dissolution and precipitation act opposite to each other where dissolution increases both concentration and isotopic composition of DIC pool, while precipitation decreases both (Alling et al., 2012).
View in article
Barkan, E., Luz, B., Lazar, B. (2001) Dynamics of the carbon dioxide system in the Dead Sea. Geochimica et Cosmochimica Acta 65, 355–368. https://doi.org/10.1016/S0016-7037(00)00540-8
Show in context Such non-equilibrium CO2 transfer to the atmosphere is observed in the present day Dead Sea brine, which causes a significant increase in δ13C of the residual DIC pool with fractionation factors ranging between −15 and −23 ‰ (Stiller et al., 1985; Barkan et al., 2001).
View in article
Bartley, J.K., Kah, L.C. (2004) Marine carbon reservoir, Corg-Ccarb coupling, and the evolution of the Proterozoic carbon cycle. Geology 32, 129–132. https://doi.org/10.1130/G19939.1
Show in context Previous studies suggested that the Proterozoic ocean likely had a larger DIC inventory than that of the present day ocean (Bartley and Kah, 2004; Cantine et al., 2020) and hence could likely be the reason for the unperturbed nature of open ocean δ13CDIC and δ13Ccarbonate
View in article
Beeler, S.R., Gomez, F.J., Bradley, A.S. (2020) Controls of extreme isotopic enrichment in modern microbialites and associated abiogenic carbonates. Geochimica et Cosmochimica Acta 269, 136–149. https://doi.org/10.1016/j.gca.2019.10.022
Show in context However, two processes that have been least explored in explaining the LJE are carbon dioxide (CO2) outgassing (Stiller et al., 1985) and carbonate precipitation along with their potential interactions (Beeler et al., 2020), which are the focus of this study.
View in article
Our Rayleigh fractionation calculation using Equation 2 for the δ13CDIC and residual DIC (f) data of Lake Laguna Negra (Beeler et al., 2020), a modern day analogy for the lagoonal environment discussed here, suggests that ∼80 % DIC loss through 30–40 % CO2 outgassing can explain the observed δ13CDIC as high as ∼15 ‰ in the lake.
View in article
Bekker, A., Karhu, J.A., Eriksson, K.A., Kaufman, A.J. (2003) Chemostratigraphy of Paleoproterozoic carbonate successions of the Wyoming Craton: Tectonic forcing of biogeochemical change? Precambrian Research 120, 279–325. https://doi.org/https://doi.org/10.1016/S0301-9268(02)00164-X
Show in context Moreover, the Lomagundi carbonates in many regions are overlain by organic-rich shales, which are attributed to the cessation of shallow marine restricted, carbonate precipitating environments post-LJE (Bekker et al., 2003; Hodgskiss et al., 2023).
View in article
Bekker, A., Holmden, C., Beukes, N.J., Kenig, F., Eglinton, B., Patterson, W.P. (2008) Fractionation between inorganic and organic carbon during the Lomagundi (2.22–2.1 Ga) carbon isotope excursion. Earth and Planetary Science Letters 271, 278–291. https://doi.org/10.1016/j.epsl.2008.04.021
Show in context Supporting the above argument, the open ocean deposited Sengoma shales of the Silverton Formation showed δ¹3Corg ranging from −33.7 to −20.8 ‰ with an average of −27.0 ± 3.0 ‰ (Bekker et al., 2008), while the overlying Lomagundi carbonates (δ¹3Ccarbonate ranging from +8.3 to +11.2 ‰) showed relatively higher δ¹3Corg, ranging from −24.8 to −13.9 ‰ (Bekker et al., 2008).
View in article
Bekker, A., Holland, H.D. (2012) Oxygen overshoot and recovery during the early Paleoproterozoic. Earth and Planetary Science Letters 317–318, 295–304. https://doi.org/10.1016/j.epsl.2011.12.012
Show in context Conventionally, global enhancement in organic carbon burial has the most traction among the researchers as the cause of LJE (e.g., Schidlowski et al., 1975; Bekker and Holland, 2012).
View in article
Brasier, A.T., Fallick, A.E., Prave, A.R., Melezhik, V.A., Lepland, A. (2011) Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal. Precambrian Research 189, 193–211. https://doi.org/10.1016/j.precamres.2011.05.011
Show in context The lithology of Lomagundi successions worldwide constitutes evaporitic deposits (with calcites and dolomites), dolostones, stromatolitic dolostones, and siliciclastic rocks (Melezhik et al., 1999; Brasier et al., 2011; Melezhik et al., 2013) inferred to be deposited across a range of sedimentological facies ranging from coastal sabkhas and near shore to open ocean environments (Melezhik et al., 1999).
View in article
The presence of such alternating evaporite and carbonate depositional cycles during Lomagundi successions (Brasier et al., 2011; Melezhik et al., 2013) is consistent with the influence of greenhouse climate inferred during the LJE (Shang et al., 2024).
View in article
Additionally, these evaporites and carbonates are inferred to be deposited in restricted to partially restricted lagoonal environments (Brasier et al., 2011; Melezhik et al., 2013).
View in article
Such intense evaporation promotes the precipitation of calcium sulphates (CaSO4) including gypsum and anhydrite. A large proportion of calcites in these settings are interpreted to have formed through early diagenetic calcitisation of the deposited CaSO4 (Brasier et al., 2011; Melezhik et al., 2013).
View in article
Furthermore, selective removal of Ca2+ from the water column during CaSO4 precipitation increases the Mg/Ca ratio (Brasier et al., 2011).
View in article
Such Mg-rich conditions would have also favoured dolomite formation, either through direct precipitation or through dolomitisation of pre-existing calcites (Brasier et al., 2011).
View in article
This is evidenced through carbonates in the evaporitic environments of Tulomozero formations from Onega basin where syn-sedimentary calcitisation and dolomitisation have been observed (Brasier et al., 2011; Melezhik et al., 2013).
View in article
Cantine, M.D., Knoll, A.H., Bergmann, K.D. (2020) Carbonates before skeletons: A database approach. Earth-Science Reviews 201, 103065. https://doi.org/10.1016/j.earscirev.2019.103065
Show in context Previous studies suggested that the Proterozoic ocean likely had a larger DIC inventory than that of the present day ocean (Bartley and Kah, 2004; Cantine et al., 2020) and hence could likely be the reason for the unperturbed nature of open ocean δ13CDIC and δ13Ccarbonate
View in article
Dai, Y., Yu, J., Ren, H., Ji, X. (2022) Deglacial Subantarctic CO2 outgassing driven by a weakened solubility pump. Nature Communications 13, 5193. https://doi.org/10.1038/s41467-022-32895-9
Show in context Outgassing of CO2 is further enhanced potentially in cases of increased surface water temperature due to reduced CO2 solubility (low Ks; Dai et al., 2022) and stronger winds leading to high gas transfer velocity (high Kw; Nicholson et al., 2022).
View in article
Dupraz, C., Reid, R P., Braissant, O., Decho, A.W., Norman, R.S., Visscher, P.T. (2009) Processes of carbonate precipitation in modern microbial mats. Earth-Science Reviews 96, 141–162. https://doi.org/10.1016/j.earscirev.2008.10.005
Show in context These marine shelf lagoonal carbonates constitute both dolostones (which are inorganically precipitated) and stromatolitic dolostones, which are microbialites formed through microbially induced precipitation of carbonates, where microbial mats influence their mineralisation (Dupraz et al., 2009; Melezhik et al., 2013).
View in article
This CO2 could either be consumed by phytoplankton or outgassed to the atmosphere (Dupraz et al., 2009).
View in article
Farrell, Ú.C., Samawi, R., Anjanappa, S., Klykov, R., Adeboye, O.O., et al. (2021) The Sedimentary Geochemistry and Paleoenvironments Project. Geobiology 19, 545–556. https://doi.org/10.1111/gbi.12462
Show in context Available data indicate a wide range of δ¹3Corg values during the LJE, from as low as −48.2 ‰ to as high as −11 ‰ (Farrell et al., 2021).
View in article
Galbraith, E.D., Kwon, E.Y., Bianchi, D., Hain, M.P., Sarmiento, J.L. (2015) The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere and ocean. Global Biogeochemical Cycles 29, 307–324. https://doi.org/10.1002/2014GB004929
Show in context However, air-sea gas exchange under such high pCO2-atmosphere drives the surface ocean rapidly towards equilibrium with the atmosphere, resulting in high pCO2 in surface waters (Galbraith et al., 2015).
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Gaucher, E.A., Govindarajan, S., Ganesh, O.K. (2008) Palaeotemperature trend for Precambrian life inferred from resurrected proteins. Nature 451, 704–707. https://doi.org/10.1038/nature06510
Show in context Specifically, the sea surface temperature was very high (∼60 oC) during the Paleoproterozoic compared to later time intervals, as inferred from resurrected proteins (Gaucher et al., 2008) and oxygen isotope studies (Robert and Chaussidon, 2006).
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The observed decline in sea surface temperatures post-LJE (Robert and Chaussidon, 2006; Gaucher et al., 2008) could likely explain the absence of anomalous values in evaporitic carbonates of later geological intervals.
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Goswami, A., Sarangi, S., Mohanty, S.P., Patil, D.J., Sarkar, A., Ray, J.S., Das, S., Mohanty, D., Ahmad, S.M., Pradhan, R.M. (2023) Negative δ13Ccarb excursions within early part of the Lomagundi event recorded in the Paleoproterozoic sedimentary carbonates, Aravalli Supergroup, Rajasthan India: Chemostratigraphy and basin evolution. Precambrian Research 399, 107240. https://doi.org/https://doi.org/10.1016/j.precamres.2023.107240
Show in context It has been observed that carbonates not associated with evaporitic mineral phases also record Lomagundi Excursion, suggesting deposition in a non-hypersaline basin (Goswami et al., 2023).
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Hill-Svehla, C.M., Corcoran, P.L. (2023) Microbial, tidal, and storm activity in a macrotidal to shallow marine shelf environment during the Paleoproterozoic era. Canadian Journal of Earth Sciences 60, 713–738. https://doi.org/10.1139/cjes-2022-0042
Show in context Sedimentological observations in Lomagundi successions suggest higher temperatures associated with greenhouse climate (potentially low Ks ; Shang et al., 2024) and stronger winds (high Kw) due to prevalence of storm events (Pambo et al., 2006; Hill-Svehla and Corcoran, 2023; Shang et al., 2024).
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Hodgskiss, M.S.W., Crockford, P.W., Turchyn, A.V. (2023) Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion. Annual Review of Earth and Planetary Sciences 51, 301–330. https://doi.org/10.1146/annurev-earth-031621-071250
Show in context Certain time intervals in geological history record highly positive δ13Ccarbonate (>4 ‰), which are known as carbon isotope excursions (Kump and Arthur, 1999; Hodgskiss et al., 2023).
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The most prominent among such excursions, in terms of both magnitude and duration, is the Lomagundi-Jatuli Event (LJE), which occurred at ∼2.2–2.0 Ga with δ13Ccarbonate as high as 30 ‰ (Schidlowski et al., 1975; Hodgskiss et al., 2023).
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But the lack of such organic-rich strata in the sedimentary rock records during the LJE (Melezhik et al., 1999; Hodgskiss et al., 2023) led to the proposition of alternative explanatory models, which are reviewed in Hodgskiss et al. (2023).
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This facies based compilation of Lomagundi carbonates revealed that the LJE was a global near shore carbon anomaly and not a global carbon anomaly (Melezhik et al., 1999; Prave et al., 2022; Hodgskiss et al., 2023).
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The LJE coincided with the subaerial emergence of continents which should have resulted in widespread creation of (partially) restricted intracontinental shallow basins, favouring evaporite deposition and abundant stromatolite reef accretion (Melezhik et al., 1999; Hodgskiss et al., 2023).
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Moreover, the Lomagundi carbonates in many regions are overlain by organic-rich shales, which are attributed to the cessation of shallow marine restricted, carbonate precipitating environments post-LJE (Bekker et al., 2003; Hodgskiss et al., 2023).
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Jennings, E., Jones, S., Arvola, L., Staehr, P.A., Gaiser, E., Jones, I.D., Weathers, K.C., Weyhenmeyer, G.A., Chiu, C., De Eyto, E. (2012) Effects of weather‐related episodic events in lakes: An analysis based on high‐frequency data. Freshwater Biology 57, 589–601. https://doi.org/10.1111/j.1365-2427.2011.02729.x
Show in context In restricted water bodies like lakes and lagoons, stratification in the water column is common, which can be disrupted by strong winds, promoting vertical redistribution of CO2 from bottom layers to the top layers and resulting in an increase in pCO2 in the surface water (Jennings et al., 2012).
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Jørgensen, B.B., Nelson, D.C., Ward, D.M. (1992) Chemotrophy and decomposition in modern microbial mats. In: Schopf, J.W., Klein, C. (Eds.) The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press, Cambridge, 287–293.
Show in context Moreover, microbialites should have also produced CO2 due to their characteristically rapid decomposition of biomass, which should have resulted in negligible organic matter accumulation (Jørgensen et al., 1992).
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Kanzaki, Y., Murakami, T. (2015) Estimates of atmospheric CO2 in the Neoarchean–Paleoproterozoic from paleosols. Geochimica et Cosmochimica Acta 159, 190–219. https://doi.org/10.1016/j.gca.2015.03.011
Show in context The pCO2-atmosphere during the LJE has been calculated to be 20 to 620 times higher than the present day atmospheric level (Kanzaki and Murakami, 2015), which may appear to limit CO2 outgassing from the water column.
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Karhu, J. (1993) Paleoproterozoic evolution of the carbon isotope ratios of sedimentary carbonates in the Fennoscandian Shield. Bulletin of the Geological Survey of Finland, 371, pp. 87.
Show in context Although productivity and associated organic burial have the potential to increase the δ13CDIC, low organic carbon concentration in lagoonal LJE carbonates (Karhu, 1993) and absence of organic-rich strata simultaneous or prior to the LJE (Melezhik et al., 1999), suggests that the organic matter produced in the lagoons might have largely undergone remineralisation.
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Evidently, the low organic matter preservation in the LJE carbonates (Karhu, 1993) likely indicates that most of the microbialite produced organic matter was mineralised to CO2
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Kump, L.R., Arthur, M.A. (1999) Interpreting carbon-isotope excursions: Carbonates and organic matter. Chemical Geology 161, 181–198. https://doi.org/https://doi.org/10.1016/S0009-2541(99)00086-8
Show in context Certain time intervals in geological history record highly positive δ13Ccarbonate (>4 ‰), which are known as carbon isotope excursions (Kump and Arthur, 1999; Hodgskiss et al., 2023).
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Melezhik, V.A., Fallick, A.E., Medvedev, P.V., Makarikhin, V.V. (1999) Extreme 13Ccarb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–red beds’ association in a global context: A case for the world-wide signal enhanced by a local environment. Earth-Science Reviews 48, 71–120. https://doi.org/https://doi.org/10.1016/S0012-8252(99)00044-6
Show in context Such a huge organic carbon burial should manifest a significant accumulation of organic-rich sedimentary rocks synchronous with the excursion (Melezhik et al., 1999).
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But the lack of such organic-rich strata in the sedimentary rock records during the LJE (Melezhik et al., 1999; Hodgskiss et al., 2023) led to the proposition of alternative explanatory models, which are reviewed in Hodgskiss et al. (2023).
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The lithology of Lomagundi successions worldwide constitutes evaporitic deposits (with calcites and dolomites), dolostones, stromatolitic dolostones, and siliciclastic rocks (Melezhik et al., 1999; Brasier et al., 2011; Melezhik et al., 2013) inferred to be deposited across a range of sedimentological facies ranging from coastal sabkhas and near shore to open ocean environments (Melezhik et al., 1999).
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This facies based compilation of Lomagundi carbonates revealed that the LJE was a global near shore carbon anomaly and not a global carbon anomaly (Melezhik et al., 1999; Prave et al., 2022; Hodgskiss et al., 2023).
View in article
This proposed depositional framework accounts for the observed irregular appearance of evaporitic signatures and hence anomalous values in the Lomagundi successions (Melezhik et al., 1999, 2013).
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Although productivity and associated organic burial have the potential to increase the δ13CDIC, low organic carbon concentration in lagoonal LJE carbonates (Karhu, 1993) and absence of organic-rich strata simultaneous or prior to the LJE (Melezhik et al., 1999), suggests that the organic matter produced in the lagoons might have largely undergone remineralisation.
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The LJE coincided with the subaerial emergence of continents which should have resulted in widespread creation of (partially) restricted intracontinental shallow basins, favouring evaporite deposition and abundant stromatolite reef accretion (Melezhik et al., 1999; Hodgskiss et al., 2023).
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Melezhik, V.A., Prave, A.R., Brasier, A.T., Lepland, A., Romashkin, A.E., Rychanchik, D.V., Hanski, E.J., Fallick, A.E., Medvedev, P.V. (2013) 6.3.1 Tulomozero formation: FAR-DEEP holes 10A and 10B. In Melezhik, V.A. et al. (Eds.) Reading the Archive of Earth’s Oxygenation: The Core Archive of the Fennoscandian Arctic Russia-Drilling Early Earth Project (pp. 773–888). Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-29659-8_4
Show in context The lithology of Lomagundi successions worldwide constitutes evaporitic deposits (with calcites and dolomites), dolostones, stromatolitic dolostones, and siliciclastic rocks (Melezhik et al., 1999; Brasier et al., 2011; Melezhik et al., 2013) inferred to be deposited across a range of sedimentological facies ranging from coastal sabkhas and near shore to open ocean environments (Melezhik et al., 1999).
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The presence of such alternating evaporite and carbonate depositional cycles during Lomagundi successions (Brasier et al., 2011; Melezhik et al., 2013) is consistent with the influence of greenhouse climate inferred during the LJE (Shang et al., 2024).
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Additionally, these evaporites and carbonates are inferred to be deposited in restricted to partially restricted lagoonal environments (Brasier et al., 2011; Melezhik et al., 2013).
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This proposed depositional framework accounts for the observed irregular appearance of evaporitic signatures and hence anomalous values in the Lomagundi successions (Melezhik et al., 1999, 2013).
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Such intense evaporation promotes the precipitation of calcium sulphates (CaSO4) including gypsum and anhydrite. A large proportion of calcites in these settings are interpreted to have formed through early diagenetic calcitisation of the deposited CaSO4 (Brasier et al., 2011; Melezhik et al., 2013).
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This is evidenced through carbonates in the evaporitic environments of Tulomozero formations from Onega basin where syn-sedimentary calcitisation and dolomitisation have been observed (Brasier et al., 2011; Melezhik et al., 2013).
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These marine shelf lagoonal carbonates constitute both dolostones (which are inorganically precipitated) and stromatolitic dolostones, which are microbialites formed through microbially induced precipitation of carbonates, where microbial mats influence their mineralisation (Dupraz et al., 2009; Melezhik et al., 2013).
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Nicholson, S.-A., Whitt, D.B., Fer, I., du Plessis, M.D., Lebéhot, A.D., Swart, S., Sutton, A.J., Monteiro, P.M. (2022) Storms drive outgassing of CO2 in the subpolar Southern Ocean. Nature Communications 13, 158. https://doi.org/https://doi.org/10.1038/s41467-021-27780-w
Show in context CO2 outgassing to the atmosphere has been shown to cause significant increase in δ13C in the residual DIC pool in modern day aquatic systems with fractionation factors ranging from −15 to −23 ‰ (Stiller et al., 1985), which makes it a plausible mechanism to explain the LJE. The air-water flux of CO2 (FCO2) can be expressed as:
Eq. 1
where ΔpCO2 is the pCO2 gradient (pCO2-surface water − pCO2-atmosphere), which determines the direction of the CO2 flux; Kw is the gas transfer velocity (a quadratic function of wind speed); and Ks is the CO2 solubility (a function of temperature and salinity) (Nicholson et al., 2022).
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Outgassing of CO2 is further enhanced potentially in cases of increased surface water temperature due to reduced CO2 solubility (low Ks; Dai et al., 2022) and stronger winds leading to high gas transfer velocity (high Kw; Nicholson et al., 2022).
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Pambo, F., Guiraud, M., Quesne, D., Gauthier-Lafaye, F., Azzibrouck, G., Lang, J. (2006) The Proterozoic Franceville Basin (SE Gabon): An example of interaction between marine sedimentation and extensional faulting. Africa Geoscience Review 13, 77–106.
Show in context Sedimentological observations in Lomagundi successions suggest higher temperatures associated with greenhouse climate (potentially low Ks ; Shang et al., 2024) and stronger winds (high Kw) due to prevalence of storm events (Pambo et al., 2006; Hill-Svehla and Corcoran, 2023; Shang et al., 2024).
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Prave, A.R., Kirsimäe, K., Lepland, A., Fallick, A.E., Kreitsmann, T., Deines, Yu. E., Romashkin, A.E., Rychanchik, D.V., Medvedev, P.V., Moussavou, M., Bakakas, K., Hodgskiss, M.S.W. (2022) The grandest of them all: The Lomagundi–Jatuli Event and Earth’s oxygenation. Journal of the Geological Society 179, jgs2021-036. https://doi.org/10.1144/jgs2021-036
Show in context Accordingly, a facies based screening of the global Lomagundi carbonates showed that anomalously high δ13Ccarbonate were confined only to coastal evaporites (+8.1 ± 3.8 ‰) and near shore carbonates (+6.2 ± 2.0 ‰), whereas the open ocean carbonates (+1.5 ± 2.4 ‰) remained unperturbed (Prave et al., 2022).
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This facies based compilation of Lomagundi carbonates revealed that the LJE was a global near shore carbon anomaly and not a global carbon anomaly (Melezhik et al., 1999; Prave et al., 2022; Hodgskiss et al., 2023).
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Therefore, we argue that the evaporites with a mean δ13C of +8.1 ± 3.8 ‰ (Prave et al., 2022) might have likely deposited during periods of low sea level, where the lagoons become hydrographically isolated leading to hypersaline conditions (evaporitic lagoon; Fig. 1a).
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On the other hand, the near shore dolostones and stromatolitic dolostones of Lomagundi with a mean δ13C of +6.2 ± 2.0 ‰ (Prave et al., 2022) might have deposited during high sea level in partially restricted marine shelf lagoons (Fig. 1b).
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Assuming negligible DIC input to alter the pool size and isotopic composition through seepage or restricted exchange in evaporitic and marine shelf lagoons, the range of possible δ13CDIC values that can be achieved in the lagoons from an initial open ocean seawater composition (δ¹3C ∼0 ± 4 ‰; Prave et al., 2022) through CO2 outgassing and carbonate precipitation can be approximated by treating the lagoons as a closed system and applying a Rayleigh fractionation model.
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The carbonates in the Lomagundi evaporites show anomalously high δ13Ccarbonate (+8.1 ± 3.8 ‰; Prave et al., 2022), which we infer to be deposited in the evaporitic lagoons during eustatic sea level fall (Fig. 1a).
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Moreover, these carbonates show relatively lower δ13Ccarbonate (+6.2 ± 2.0 ‰) compared to carbonates of evaporite deposits (Prave et al., 2022).
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The open ocean carbonates of the LJE show δ13Ccarbonate ∼+1.5 ± 2.4 ‰, which lies within the range of normal sea water isotopic composition (0 ± 4 ‰; Prave et al., 2022) observed throughout the geologic past. This reflects the conservative nature of open ocean δ13CDIC
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Robert, F., Chaussidon, M. (2006) A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts. Nature 443, 969–972. https://doi.org/10.1038/nature05239
Show in context Specifically, the sea surface temperature was very high (∼60 oC) during the Paleoproterozoic compared to later time intervals, as inferred from resurrected proteins (Gaucher et al., 2008) and oxygen isotope studies (Robert and Chaussidon, 2006).
View in article
The observed decline in sea surface temperatures post-LJE (Robert and Chaussidon, 2006; Gaucher et al., 2008) could likely explain the absence of anomalous values in evaporitic carbonates of later geological intervals.
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Rubinson, M., Clayton, R.N. (1969) Carbon-13 fractionation between aragonite and calcite. Geochimica et Cosmochimica Acta 33, 997–1002. https://doi.org/https://doi.org/10.1016/0016-7037(69)90109-4
Show in context The carbon isotopic composition of carbonates (δ13Ccarbonate) reflects the isotopic composition of the dissolved inorganic carbon (δ13CDIC) pool from which they precipitate with minimal fractionation (ɛcarbonate-bicarbonate ∼0.9 ‰ to 2.7 ‰; Rubinson and Clayton, 1969).
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As is evident from the fractionation factor of carbonate precipitation, the process over time tends to lower the δ13C of water column DIC (Rubinson and Clayton, 1969), which could be the reason for comparatively lower δ13Ccarbonate in the marine shelf lagoonal carbonates compared to evaporitic carbonates, where outgassing dominated.
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Schidlowski, M., Eichmann, R., Junge, C.E. (1975) Precambrian sedimentary carbonates: Carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget. Precambrian Research 2, 1–69. https://doi.org/https://doi.org/10.1016/0301-9268(75)90018-2
Show in context The most prominent among such excursions, in terms of both magnitude and duration, is the Lomagundi-Jatuli Event (LJE), which occurred at ∼2.2–2.0 Ga with δ13Ccarbonate as high as 30 ‰ (Schidlowski et al., 1975; Hodgskiss et al., 2023).
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Conventionally, global enhancement in organic carbon burial has the most traction among the researchers as the cause of LJE (e.g., Schidlowski et al., 1975; Bekker and Holland, 2012).
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According to this hypothesis, a significant increase in global marine productivity and associated organic carbon burial (∼60 %; Schidlowski et al., 1975) during the LJE led to the preferential removal of 12C from DIC pool, thereby enriching the residual DIC and carbonates precipitating from them in 13C.
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Shang, G., Zhai, M., Peng, P., Miao, P., Li, Q. (2024) A climate change from icehouse to greenhouse following Huronian glaciation: Evidence from long-term storm deposits of the Paleoproterozoic Hutuo Group in the North China Craton. Journal of Asian Earth Sciences 274, 106289. https://doi.org/10.1016/j.jseaes.2024.106289
Show in context Sedimentological observations in Lomagundi successions suggest higher temperatures associated with greenhouse climate (potentially low Ks ; Shang et al., 2024) and stronger winds (high Kw) due to prevalence of storm events (Pambo et al., 2006; Hill-Svehla and Corcoran, 2023; Shang et al., 2024).
View in article
The presence of such alternating evaporite and carbonate depositional cycles during Lomagundi successions (Brasier et al., 2011; Melezhik et al., 2013) is consistent with the influence of greenhouse climate inferred during the LJE (Shang et al., 2024).
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Stiller, M., Rounick, J.S., Shasha, S. (1985) Extreme carbon-isotope enrichments in evaporating brines. Nature 316, 434–435. https://doi.org/https://doi.org/10.1038/316434a0
Show in context However, two processes that have been least explored in explaining the LJE are carbon dioxide (CO2) outgassing (Stiller et al., 1985) and carbonate precipitation along with their potential interactions (Beeler et al., 2020), which are the focus of this study.
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CO2 outgassing to the atmosphere has been shown to cause significant increase in δ13C in the residual DIC pool in modern day aquatic systems with fractionation factors ranging from −15 to −23 ‰ (Stiller et al., 1985), which makes it a plausible mechanism to explain the LJE. The air-water flux of CO2 (FCO2) can be expressed as:
Eq. 1
where ΔpCO2 is the pCO2 gradient (pCO2-surface water − pCO2-atmosphere), which determines the direction of the CO2 flux; Kw is the gas transfer velocity (a quadratic function of wind speed); and Ks is the CO2 solubility (a function of temperature and salinity) (Nicholson et al., 2022).
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Such non-equilibrium CO2 transfer to the atmosphere is observed in the present day Dead Sea brine, which causes a significant increase in δ13C of the residual DIC pool with fractionation factors ranging between −15 and −23 ‰ (Stiller et al., 1985; Barkan et al., 2001).
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Warren, J.K. (2010) Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits. Earth-Science Reviews 98, 217–268. https://doi.org/https://doi.org/10.1016/j.earscirev.2009.11.004
Show in context Eustatic sea level fluctuations are common in greenhouse climates (Warren, 2010).
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Studies on evaporite deposits globally suggest that greenhouse climate results in the formation of sub-sea level seepage lagoons in which eustatic sea level fluctuations cause the deposition of platform evaporites (during low sea level) alternating with marine shelf carbonates (during high sea level) (Warren, 2010).
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Supplementary Information
The Supplementary Information includes:
- Calculations for Figure 2
- Supplementary Information References
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Figures
Figure 1 Schematic of a conceptual model of carbonate deposition in lagoons during periods of eustatic sea level (a) fall and (b) rise. δ13Ccarb means δ13Ccarbonate.
Figure 2 δ13CDIC obtained for varying proportions of outgassing (Xog) and carbonate precipitation (Xcp) from Equation 2 at different DIC loss proportions (f).