Nitrogen geochemistry of subducting sediments: New results from the Izu‐Bonin‐Mariana margin and insights regarding global nitrogen subduction

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Abstract
Toward understanding of the subduction mass balance in the Izu‐Bonin‐Mariana (IBM) convergent margin, we present an inventory of N and C concentrations and isotopic compositions in sediments obtained on Ocean Drilling Program (ODP) Legs 129 and 185. Samples from Sites 1149, 800, 801, and 802 contain 5 to 661 ppm total N (organic, inorganic combined) with δ15NAir of −0.2 to +8.2‰ (all δ15N values <+2.5‰ from Site 800). At Site 1149, N content is higher in clay‐rich layers and lower in chert and carbonate layers, and δ15N shows a distinct down‐section decrease from 0 to 120 mbsf (near +8.0 at shallow levels to near +4.0‰). Reduced‐C concentration ranges from 0.02 to 0.5 wt.%, with δ13CVPDB of −28.1 to −21.7‰. The down‐section decreases in δ15N and N concentration (and variations in concentrations and δ13C of reduced C, and Creduced/N) at Site 1149 could help reconcile differences between δ15N values of modern deep‐sea sediments from near the sediment‐water interface and values for forearc metasedimentary rocks. At Site 1149, negative shifts in δ15N, from marine organic values (up to ∼+8‰) toward lower values approaching those for the metasedimentary rocks (+1 to +3‰), are most likely caused by complex diagenetic processes, conceivably with minor effects of changes in productivity and differing proportions of marine and terrestrial organic matter. However, the forearc metamorphic suites (e.g., Franciscan Complex) are known to have been deposited nearer continents, and their lower δ15N at least partly reflects larger proportions of lower‐δ15N terrestrial organic matter. Subduction at the Izu‐Bonin (IB) margin, of a sediment section like that at Site 1149, would deliver an approximate annual subduction flux of 2.5 × 106 g of N and 1.4 × 107 g of reduced C per linear kilometer of trench, with average δ15N of +5.0‰ and δ13C of −24‰. Incorporating the larger C flux of 9.2 × 108 g/yr/linear‐km in carbonate‐rich layers of 1149B (average δ13C = +2.3‰) provides a total C flux of 9.3 × 108 g/yr/linear‐km (δ13C = +1.9‰). Once subducted, sediments are shifted to higher δ15N by N loss during devolatilization, with magnitudes of the shifts depending on the thermal evolution of the margin.

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