Cycling of CO₂ and N₂ Along the Hikurangi Subduction Margin, New Zealand: An Integrated Geological, Theoretical, and Isotopic Approach

We present a quantitative assessment of the input and output of CO₂ and N₂ along the Hikurangi margin based on the chemical and stable isotope composition of sediments and basalts (from IODP 375), previously accreted metasedimentary rocks, and volcanic/hydrothermal gases (together with noble gas data for the latter). We compare these results with 3-D thermo-petrologic models for four lithologic structures, representing different plateau inputs. The model results indicate that 59%–85% of initially subducted C and 5%–12% of N is lost from the slab during metamorphism, with both volatiles being dominantly sourced from altered oceanic crust with some contribution from subducted sediment at the forearc-arc transition (75–90 km depth). The δ¹³C_VPDB and CO₂/³He values for the arc gases range from −8.3 to −1.4‰ and 2 × 10⁹ to 2.7 × 10¹¹, indicating contributions from slab carbonate, organic C, and mantle C of 67%, 30%, and 3%, respectively. The δ¹⁵N_air and N₂/³⁶Ar values of arc gases are −1.0 to +2.3‰ and 1.54 × 10⁴ to 1.9 × 10⁵, indicating slab and mantle contributions of 74% and 26%. The δ¹³C signature of gases requires addition of organic C by tectonic erosion and/or shallow crustal assimilation. These calculations yield whole-margin fluxes of 5.4–7.0 Tg/yr for CO₂ and 0.0022–0.0057 Tg/yr for N_2, corresponding to ∼2.2% and 1%–30% of the global CO₂ and N₂ flux from subaerial volcanoes worldwide (assuming no loss during transit). This unique assessment of volatile cycling could prove useful in refining regional and global estimates of volatile recycling efficiency.