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

•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.