title: 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.
authors.affiliation: Department of Geodynamics and Sedimentology, University of Vienna, Vienna, Austria.
authors.forename: D
authors.initials: D
authors.lastname: Birgel
authors.forename: P
authors.initials: P
authors.lastname: Meister
authors.forename: R
authors.initials: R
authors.lastname: Lundberg
authors.forename: T D
authors.initials: TD
authors.lastname: Horath
authors.forename: T R R
authors.initials: TR
authors.lastname: Bontognali
authors.forename: A M
authors.initials: AM
authors.lastname: Bahniuk
authors.forename: C E
authors.initials: CE
authors.lastname: de Rezende
authors.forename: C
authors.initials: C
authors.lastname: Vasconcelos
authors.forename: J A
authors.initials: JA
authors.lastname: McKenzie
mesh_terms: D001419:Bacteria / Q000378:metabolism*; D001938:Brazil; D002244:Carbon / Q000737:chemistry*; D019015:Geologic Sediments / Q000737:chemistry* / Q000382:microbiology; D055593:Geological Phenomena; D008697:Methane / Q000096:biosynthesis*
chemical_list: D002244:Carbon; D008697:Methane