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

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 and Kasten, Sabine (2020) Impact of iron release by volcanic ash alteration on carbon cycling in sediments of the northern Hikurangi margin Earth and Planetary Science Letters, 541 (Article number 116288). DOI 10.1016/j.epsl.2020.116288.

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Abstract

Highlights

• 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
C 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.

Abstract

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
C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO 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.

Document Type: Article
Keywords: volcanic ash, diagenesis, Fe-AOM, deep biosphere, Hikurangi margin, RV Sonne, SO247
Research affiliation: MARUM
AWI
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS
Refereed: Yes
DOI etc.: 10.1016/j.epsl.2020.116288
ISSN: 0012-821X
Related URLs:
Date Deposited: 04 May 2020 06:46
Last Modified: 05 May 2020 09:47
URI: http://eprints.uni-kiel.de/id/eprint/49584

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