Impact of iron release by volcanic ash alteration on carbon cycling in sediments of the northern Hikurangi margin

Susann.Henkel [ at ]


We present geochemical data collected from volcanic ash-bearing sediments on the upper slope of the northern Hikurangi margin during the RV SONNE SO247 expedition in 2016. Gravity coring and seafloor drilling with the MARUM-MeBo200 allowed for collection of sediments down to 105 meters below seafloor (mbsf). Release of dissolved Sr2+with isotopic composition enriched in 86Sr (87Sr/86Sr minimum = 0.708461 at 83.5 mbsf) is indicative of ash alteration. This reaction releases other cations in the 30-70 mbsf depth interval as reflected by maxima in pore-water Ca2+and Ba2+concentrations. In addition, we posit that Fe(III) in volcanogenic glass serves as an electron acceptor for methane oxidation, a reaction that releases Fe2+measured in the pore fluids to a maximum concentration of 184 μM. Several lines of evidence support our proposed coupling of ash alteration with Fe-mediated anaerobic oxidation of methane (Fe-AOM) beneath the sulfate-methane transition (SMT), which lies at ∼7 mbsf at this site. In the ∼30-70 mbsf interval, we observe a concurrent increase in Fe2+and a depletion of CH4with a well-defined decrease in δ13C-CH4values indicative of microbial fractionation of carbon. The negative excursions in δ13C values of both DIC and CH4are similar to that observed by sulfate-driven AOM at low SO2−4concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4and DIC. Mass balance considerations reveal that the iron cycled through the coupled ash alteration and AOM reactions is consumed as authigenic Fe-bearing minerals. This iron sink term derived from the mass balance is consistent with the amount of iron present as carbonate minerals, as estimated from sequential extraction analyses. Using a numerical modeling approach we estimate the rate of Fe-AOM to be on the order of 0.4μmol cm−2yr−1, which accounts for ∼12% of total CH4removal in the sediments. Although not without uncertainties, the results presented reveal that Fe-AOM in ash-bearing sediments is significantly lower than the sulfate-driven CH4consumption, which at this site is 3.0μmol cm−2yr−1. We highlight that Fe(III) in ash can potentially serve as an electron acceptor for methane oxidation in sulfate-depleted settings. This is relevant to our understanding of C-Fe cycling in the methanic zone that typically underlies the SMT and could be important in supporting the deep biosphere.

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DOI 10.1016/j.epsl.2020.116288

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Luo, M. , Torres, M. E. , Hong, W. L. , Pape, T. , Fronzek, J. , Kutterolf, S. , Mountjoy, J. J. , Orpin, A. , Henkel, S. , Huhn, K. , Chen, D. and Kasten, S. (2020): Impact of iron release by volcanic ash alteration on carbon cycling in sediments of the northern Hikurangi margin , Earth andPlanetaryScienceLetters, 541 . doi: 10.1016/j.epsl.2020.116288

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