Methanogenesis produces strong ¹³C enrichment in stromatolites of Lagoa Salgada, Brazil: a modern analogue for Palaeo‐/Neoproterozoic stromatolites?

Abstract

Holocene stromatolites characterized by unusually positive inorganic δ(13) CPDB values (i.e. up to +16‰) are present in Lagoa Salgada, a seasonally brackish to hypersaline lagoon near Rio de Janeiro (Brazil). Such positive values cannot be explained by phototrophic fixation of CO2 alone, and they suggest that methanogenesis was a dominating process during the growth of the stromatolites. Indeed, up to 5 mm methane was measured in the porewater. The archaeal membrane lipid archaeol showing δ(13) C values between -15 and 0‰ suggests that archaea are present and producing methane in the modern lagoon sediment. Moreover, (13) C-depleted hopanoids diplopterol and 3β-methylated C32 17β(H),21β(H)-hopanoic acid (both -40‰) are preserved in lagoon sediments and are most likely derived from aerobic methanotrophic bacteria thriving in the methane-enriched water column. Loss of isotopically light methane through the water column would explain the residual (13) C-enriched pool of dissolved inorganic carbon from where the carbonate constituting the stromatolites precipitated. The predominance of methanogenic archaea in the lagoon is most likely a result of sulphate limitation, suppressing the activity of sulphate-reducing bacteria under brackish conditions in a seasonally humid tropical environment. Indeed, sulphate-reduction activity is very low in the modern sediments. In absence of an efficient carbonate-inducing metabolic process, we propose that stromatolite formation in Lagoa Salgada was abiotically induced, while the (13) C-enriched organic and inorganic carbon pools are due to methanogenesis. Unusually, (13) C-enriched stromatolitic deposits also appear in the geological record of prolonged periods in the Palaeo- and Neoproterozoic. Lagoa Salgada represents a possible modern analogue to conditions that may have been widespread in the Proterozoic, at times when low sulphate concentrations in sea water allowed methanogens to prevail over sulphate-reducing bacteria.

Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation

“…The reduction of sulfate produces sulfide, which can be oxidized by oxygen via canonical sulfide oxidation (CSO) (Meysman et al, 2003;van de Velde and Meysman, 2016). Methanogenesis produces methane, which can be oxidized by sulfate in a process called anaerobic oxidation of methane (AOM) or by oxygen (ArOM) (Boetius et al, 2000). The mineralization reactions are expressed via standard limitation-inhibition formulations (Soetaert et al, 1996).…”

“…Specific metabolically produced DIC δ 13 C values are assigned for organoclastic microbial sulfate reduction, AOM and organoclastic methanogenesis (see Table 2). These metabolic pathways have been considered to induce calcite nucleation by changing the surrounding carbonate chemistry, forming an organic matrix or destroying calcification-inhibiting organic mucus (Berner et al, 1970;Visscher et al, 2000;Arp et al, 2001;Ries et al, 2008;Heindel et al, 2013;Birgel et al, 2015). The peak activity of these metabolic pathways coincides with the most reactive upper layers of the sediment column, i.e., highly porous and high metastable calcite polymorph content (Irwin et al, 1977;Marshall, 1992); this is mathematically expressed by Eq.…”

“…This approach has attracted criticism due to the complex multicomponent nature and high potential for diagenetic alteration, which may result in a mixture of primary and secondary diagenetic signals. Major sources of contamination that are known to affect the primary marine carbon isotope signal of bulk-carbonate rock are degradation of organic matter and methane oxidation (Marshall, 1992;Reitner et al, 2005;Wacey et al, 2007;Birgel et al, 2015).…”

“…Foremost, the reactive and biologically active upper section of the sediment column can be envisioned as leaving an imprint on carbonate chemistry, including the carbon isotope composition. This carbon isotope signal will be retained in postdepositional carbonate cements and is controlled by organic carbon (OC) fluxes, pH, alkalinity and the nature of the (sub-)sea-floor microbial communities (Melim et al, 2002;Reitner et al, 2005;Wacey et al, 2007;Birgel et al, 2015;Schobben et al, 2016;Zhao et al, 2016).…”

Microbially-driven formation of Cenozoic siderite and calcite concretions from eastern Austria

“…In contrast, hydrogenotrophic methanogenic archaea are highly selective in terms of their carbon source and preferentially take up 12 CO 2 , which results in a residual dissolved carbonate pool that is strongly enriched in 13 C (δ 13 C values of +24‰; Boehme et al, 1996). Carbonate minerals precipitated in the zone of methanogenesis have therefore positive δ 13 C values, again depending on the extent of mixing of different carbon sources (Irwin et al, 1977;Siegel et al, 1987;Kiriakoulakis et al, 2000;Lash and Blood, 2004;Raiswell and Fisher, 2004;Pearson and Nelson, 2005;Ziegenbalg et al, 2010;Hoffmann-Sell et al, 2011;Meister et al, 2011;Kuechler et al, 2012;Natalicchio et al, 2012;Birgel et al, 2015).…”