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 δ 13 C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in δ 13 C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO 4 2 − 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.

中文翻译：

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火山灰蚀变释放铁对北 Hikurangi 边缘沉积物中碳循环的影响

摘要 我们展示了 2016 年 RV SONNE SO247 探险期间从 Hikurangi 边缘北部上坡的火山灰沉积物中收集的地球化学数据。使用 MARUM-MeBo200 进行的重力取芯和海底钻探允许收集深达 105 米的沉积物海底（mbsf）。释放出富含 86Sr 的同位素组成的溶解 Sr2+（87Sr/86Sr 最小值 = 0.708461，83.5 mbsf）表明灰分改变。该反应在 30-70 mbsf 深度区间释放其他阳离子，如孔隙水 Ca2+ 和 Ba2+ 浓度的最大值所反映。此外，我们假设火山成因玻璃中的 Fe(III) 作为甲烷氧化的电子受体，该反应释放在孔隙流体中测量到的最大浓度为 184 μM 的 Fe2+。几条证据支持我们提出的在硫酸盐-甲烷转变 (SMT) 下灰分蚀变与 Fe 介导的甲烷厌氧氧化 (Fe-AOM) 的耦合，该转变位于该站点的~7 mbsf。在~30-70 mbsf 间隔中，我们观察到 Fe2+ 的同时增加和 CH4 的消耗，δ 13 C-CH4 值明显下降，表明微生物对碳进行分馏。DIC 和 CH4 的 δ 13 C 值的负偏移与在低 SO 4 2 - 浓度下由硫酸盐驱动的 AOM 观察到的类似，并且只能通过 CH4 和 DIC 之间微生物介导的碳同位素平衡来解释。质量平衡考虑表明，通过耦合灰分蚀变和 AOM 反应循环的铁被作为自生含铁矿物消耗。根据连续提取分析估计，从质量平衡得出的铁汇项与作为碳酸盐矿物存在的铁量一致。使用数值建模方法，我们估计 Fe-AOM 的速率约为 0.4 μmol cm-2 yr-1，占沉积物中 CH4 总去除量的 12%。尽管并非没有不确定性，但所呈现的结果表明，含灰沉积物中的 Fe-AOM 显着低于硫酸盐驱动的 CH4 消耗，在该地点为 3.0 μmol cm-2 yr-1。我们强调，灰分中的 Fe(III) 可能在硫酸盐耗尽环境中作为甲烷氧化的电子受体。这与我们对通常位于 SMT 之下的甲烷区中 C-Fe 循环的理解有关，并且可能对支持深层生物圈很重要。