Journal
GEOBIOLOGY
Volume 13, Issue 3, Pages 245-266Publisher
WILEY
DOI: 10.1111/gbi.12130
Keywords
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Funding
- Marie-Curie Research Training Network 'Greenhouse-Gas Removal Apprenticeship and Student Program'
- (GRASP) project [MRTN-CT-2006-035868]
- Swiss National Science Foundation (SNF) [PA00P2-126221]
- Swiss National Science Foundation (SNF) [PA00P2_126221] Funding Source: Swiss National Science Foundation (SNF)
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Holocene stromatolites characterized by unusually positive inorganic C-13(PDB) values (i.e. up to +16 parts per thousand) 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 5mm methane was measured in the porewater. The archaeal membrane lipid archaeol showing C-13 values between -15 and 0 parts per thousand suggests that archaea are present and producing methane in the modern lagoon sediment. Moreover, C-13-depleted hopanoids diplopterol and 3-methylated C-32 17(H),21(H)-hopanoic acid (both -40 parts per thousand) 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 C-13-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 C-13-enriched organic and inorganic carbon pools are due to methanogenesis. Unusually, C-13-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.
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