Pervasive and rapid enrichment of nitrogen in newly erupted oceanic basalt

This is the authors' abstract. We don't add key points for this paper.

The alteration of oceanic crust by seawater is a critical sink in the global nitrogen (N) cycle, yet the mechanisms and timescales of N enrichment remain poorly constrained. We present N concentrations and isotope compositions from a 192-meter drill core (SE-02b) retrieved from the then 50-year-old Surtsey Island in Iceland, a pristine analog for rapid alteration of young oceanic basalt. The altered basaltic tuffs show significant N enrichment (average 11 ± 8 μg/g), relative to unaltered Icelandic basalt glasses (0.028 ± 0.026 μg/g) and MORB (∼1.1 to 1.5 μg/g), matching the global average for altered oceanic crust (11 μg/g; ranging in ages from 5 to 170 Ma). Nitrogen isotope compositions (δ 15 N air = −1.7 ± 4.6 ‰) are primarily consistent with incorporation of seawater- and meteoric-derived ammonium (NH 4 + ), 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 NH 4 + into secondary phases occurs with little isotope fractionation consistent with equilibrium fractionation between clay and aqueous NH 4 + . 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 δ 15 N values (as low as −20.1 ‰), are attributed to abiotic reduction of dissolved N 2 under anoxic conditions and/or microbial activity. We propose that N incorporation into the oceanic crust can occur rapidly in days to decades rather than at a constant rate over geologic time. This has major implications for N transfer in subduction zones, N exchange between the oceanic crust, seawater and the atmosphere and for planetary habitability.