Nitrogen, carbon, oxygen and hydrogen concentrations and isotope compositions and major element data from core SE-02b (2017), Surtsey island, Iceland
Description
Abstract 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), matching the global average for altered oceanic crust (11 μg/g; ranging in ages from 5-170 Ma). 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.
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Institutions
- Lehigh UniversityPennsylvania, Bethlehem