Mangroves store significant amounts of organic carbon in sediments. During carbon burial, methane (CH₄) is produced in anoxic, organic-rich sediments and released to the surface waters via porewater exchange and ebullition. Yet, highly variable CH₄ emissions have been reported in mangroves because of high uncertainty of methane production and oxidation rates. Combining the stable isotopic composition of methane (δ¹³C-CH₄) in porewater and surface water can reveal the fraction of CH₄ oxidized or emitted to the atmosphere. Here, we report high-temporal resolution CH₄ concentrations from creek waters and porewater at two mangrove creeks in Brazil along with measurements of δ¹³C of CH₄. Enriched δ¹³C-CH₄ in top surface layer sediments indicates CH₄ oxidation in the surface sediment before porewaters reach the water column. Tidal pumping in mangrove facilitates CH₄ oxidation in the water column. Surface waters at low tide exhibited a lighter δ¹³C-CH₄ than at high tide. A similar δ¹³C-CH₄ signature between low tide surface water (-70 ± 2 ‰) and porewater (-74 ± 4 ‰) imply that sediment is the source of CH₄. A stable isotope mass balance showed that 30 – 66% of was oxidized within water column, with the rate of 19-121 µmol m^-2 d^-1. Air-sea CH₄ emissions were estimated at both mangroves (68 ± 65 in a pristine and 179 ± 205 µmol m^-2 d^-1 in an urbanized mangrove), on the same order of magnitude as the CH₄ oxidation rate. Overall, our results suggested that CH₄ oxidation in mangrove surface water and sediments partially reduce CH₄ emissions to the atmosphere.

Primary Presenter: Yvonne Yau, Gothenburg (yvonne.yau@gu.se)

Authors:

Yvonne Yau, Department of Marine Sciences, University of Gothenburg   (yvonne.yau@gu.se)

Luiz Cotovicz Jr, Instituto de Ciências do Mar (LABOMAR), Universidade Federal do Ceara, Fortaleza, Brazil, Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research, Warnemunde, Germany  (lcc.junior@gmail.com)

Gwenaël Abril, Laboratoire de Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), UMR 8067, Muséum National d'Histoire Naturelle, CNRS, IRD, SU, UA, Paris, France  (gwenael.abril@mnhn.fr)

Gloria Reithmaier, Department of Marine Sciences, University of Gothenburg, 41319 Gothenburg, Sweden  (gloria.reithmaier@gu.se)

João Barreira, Department of Geochemistry, Fluminense Federal University, Brazil  (jpgbarreira@gmail.com)

Alberto Borges, University of Liège, Chemical Oceanography Unit, Liège, Belgium  (alberto.borges@uliege.be)

Stefano Bonaglia, Department of Marine Sciences, University of Gothenburg, 41319 Gothenburg, Sweden  (stefano.bonaglia@gu.se)

Cédric Morana, University of Liège, Chemical Oceanography Unit, Liège, Belgium  (Cedric.Morana@uliege.be)

Alessandra Fonseca, Department of Oceanography, Federal University of Santa Catarina, Brazil  (alarissa.fonseca@gmail.com)

Isaac Santos, Department of Marine Sciences, University of Gothenburg, 41319 Gothenburg, Sweden  (isaac.santos@gu.se)