Methanogenesis produces strong ¹³C enrichment in stromatolites of Lagoa Salgada, Brazil

Author(s)

D. Birgel, P. Meister, R. Lundberg, T. D. Horath, T. R. R. Bontognali, A. M. Bahniuk, C. E. de Rezende, C. Vasconcelos, J. A. McKenzie

Abstract

Holocene stromatolites characterized by unusually positive inorganic δ

¹³C

_PDB 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 CO

₂ 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

¹³C values between -15 and 0‰ suggests that archaea are present and producing methane in the modern lagoon sediment. Moreover,

¹³C-depleted hopanoids diplopterol and 3β-methylated C

₃₂ 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

¹³C-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

¹³C-enriched organic and inorganic carbon pools are due to methanogenesis. Unusually,

¹³C-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.

Organisation(s)

Department of Geology

External organisation(s)

Eidgenössische Technische Hochschule Zürich, Universität Zürich (UZH), Federal University of Parana, Universidade Estadual do Norte Fluminense Darcy Ribeiro, Max-Planck-Institut für marine Mikrobiologie

Journal

Geobiology

Volume

13

Pages

245-266

No. of pages

22

ISSN

1472-4677

DOI

https://doi.org/10.1111/gbi.12130

Publication date

05-2015

Peer reviewed

Yes

Austrian Fields of Science 2012

105121 Sedimentology

Keywords

ASJC Scopus subject areas

Environmental Science(all), Ecology, Evolution, Behavior and Systematics, Earth and Planetary Sciences(all)

Sustainable Development Goals

SDG 14 - Life Below Water

Portal url

https://ucrisportal.univie.ac.at/en/publications/methanogenesis-produces-strong-13c-enrichment-in-stromatolites-of-lagoa-salgada-brazil(d1c3882a-0f96-4109-bd14-a51e5763ee49).html