Details

Autor(en) / Beteiligte

• Luo, Min
• Torres, Marta E.
• Hong, Wei-Li
• Pape, Thomas
• Fronzek, Julia
• Kutterolf, Steffen
• Mountjoy, Joshu J.
• Orpin, Alan
• Henkel, Susann
• Huhn, Katrin
• Chen, Duofu
• Kasten, Sabine

Titel

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

Ist Teil von

• Earth and planetary science letters, 2020-07, Vol.541, p.116288, Article 116288

Ort / Verlag

Elsevier B.V

Erscheinungsjahr

2020

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• •Release of dissolved Sr2+ with low 87Sr/86Sr, as well as Ca2+ and Ba2+ suggests ongoing volcanic ash alteration.•A concurrent increase in Fe2+ and a depletion of CH4 with a decrease in δ13C of CH4 and DIC suggest Fe-AOM.•We for the first time document the potential linkage between ash alteration and methane oxidation via Fe-AOM.•The rate of Fe-AOM is estimated to be ∼0.4 μmol cm−2 yr−1, equivalent to ∼12% of total CH4 removal. 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 CH4 with a well-defined decrease in δ13C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in δ13C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO42− concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4 and 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 0.4 μmol cm−2 yr−1, which accounts for ∼12% of total CH4 removal 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 CH4 consumption, which at this site is 3.0 μmol cm−2 yr−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.