Nitrogen isotopes in bulk marine sediment: linking seafloor observations with subseafloor records

[摘要] The stable isotopes of nitrogen offer a unique perspective on changes in thenitrogen cycle, past and present. However, the presence of multiple forms ofnitrogen in marine sediments can complicate the interpretation of bulk nitrogenisotope measurements. Although the large-scale global patterns of seafloor δ¹⁵Nhave been shown to match process-based expectations, small-scale heterogeneity onthe seafloor, or alterations of isotopic signals during translation into thesubseafloor record, could obscure the primary signals. Here, a public database of nitrogen isotopemeasurements is described, including both seafloor and subseafloor sediment samplesranging in age from modern to the Pliocene, and used to assess these uncertainties.In general, good agreement is observed between neighbouring seafloor sites withina 100 km radius, with 85% showing differences of < 1‰. There isalso a good correlation between the δ¹⁵N of the shallowest (< 5 ka)subseafloor sediments and neighbouring seafloor sites withina 100 km radius (R² = 0.83), which suggests a reliable translation ofsediments into the buried sediment record. Meanwhile, gradualδ¹⁵N decreases over multiple glacial–interglacial cycles appear toreflect post-depositional alteration in records from the deep sea (below 2000 m).We suggest a simple conceptual model to explain these 100-kyr-timescale changesin well-oxygenated, slowly accumulating sediments, which calls on differentialloss rates for pools of organic N with different δ¹⁵N. We concludethat bulk sedimentary nitrogen isotope records are reliable monitors of pastchanges in the marine nitrogen cycle at most locations, and could be furtherimproved with a better understanding of systematic post-depositional alteration.Furthermore, geochemical or environmental criteria should be developed in order to effectively identify problematic locationsand to account for confounding influences where possible.

[学科分类] 地球化学与岩石