Nitrogen, carbon, oxygen and hydrogen concentrations and isotope compositions and major element data from core SE-02b (2017), Surtsey island, Iceland

Nitrogen isotope compositions (δ15Nair = -1.7 ± 4.6‰) are primarily consistent with incorporation of seawater- and meteoric-derived ammonium (NH4+), more likely into secondary K-minerals, palagonitized glass and smectites throughout the core. To a lesser extent, N could be incorporated into Na/Ca-minerals such as analcime in the meteoric water altered zone and sulphates such as anhydrite and gypsum when present. Incorporation of NH4+ into secondary phases occurs with little isotope fractionation consistent with equilibrium fractionation between clay and aqueous NH4+.

We demonstrate that seawater-rock interaction alone can produce the ubiquitous background N enrichment of the oceanic crust without the addition of remobilized N from sedimentary sources. Such magnitude of enrichment is also observed via meteoric water alteration. Localized high enrichments in N in areas of enhanced fluid flow at core SE-02b with exceptionally low δ15N values (as low as -20.1‰), are attributed to abiotic reduction of dissolved N2 under anoxic conditions and/or microbial activity.

We propose that N incorporation into the oceanic crust can occur very efficiently and at very rapid rates in the order of years to a few decades, but becomes inefficient over geological timescales, with major implications for N transfer in subduction zones, N exchange between the oceanic crust, seawater and the atmosphere and for planetary habitability.