Abstract
Abstract. The stable isotopes of nitrogen offer a unique perspective on changes in the nitrogen cycle, past and present. However, the presence of multiple forms of nitrogen in marine sediments can complicate the interpretation of bulk nitrogen isotope measurements. Although the large-scale global patterns of seafloor δ15N have been shown to match process-based expectations, small-scale heterogeneity on the seafloor, or alterations of isotopic signals during translation into the subseafloor record, could obscure the primary signals. Here, a public database of nitrogen isotope measurements is described, including both seafloor and subseafloor sediment samples ranging in age from modern to the Pliocene, and used to assess these uncertainties. In general, good agreement is observed between neighbouring seafloor sites within a 100 km radius, with 85% showing differences of < 1‰. There is also a good correlation between the δ15N of the shallowest (< 5 ka) subseafloor sediments and neighbouring seafloor sites within a 100 km radius (R2 = 0.83), which suggests a reliable translation of sediments into the buried sediment record. Meanwhile, gradual δ15N decreases over multiple glacial–interglacial cycles appear to reflect post-depositional alteration in records from the deep sea (below 2000 m). We suggest a simple conceptual model to explain these 100-kyr-timescale changes in well-oxygenated, slowly accumulating sediments, which calls on differential loss rates for pools of organic N with different δ15N. We conclude that bulk sedimentary nitrogen isotope records are reliable monitors of past changes in the marine nitrogen cycle at most locations, and could be further improved with a better understanding of systematic post-depositional alteration. Furthermore, geochemical or environmental criteria should be developed in order to effectively identify problematic locations and to account for confounding influences where possible.
Highlights
Nitrogen is a critical nutrient element in marine ecosystems, limiting the growth of marine life over most of the oceans (Falkowski, 1997)
We find the maximum number of records between 5 and 20 ka before present (BP) (Fig. 4b)
The clustered spatial distribution is even more prevalent in the collection of subseafloor sediment records, with most sites located on the continental shelves and slopes of the Pacific, Arabian Sea and southeastern Atlantic
Summary
Nitrogen is a critical nutrient element in marine ecosystems, limiting the growth of marine life over most of the oceans (Falkowski, 1997). The goals of this paper are twofold: first, to provide a brief technical description of the NICOPP database for future investigators who will focus on specific aspects of the data therein; second, to use the database in order to evaluate the degree to which downcore sedimentary δ15N records reflect the δ15N of nearby core-top data, through a spatial comparison with neighbouring observations and a temporal comparison with observations throughout the Late Pleistocene. This provides a novel global view on spatial variability in δ15N and diagenesis. The database will be updated as new records become available, and the authors welcome additional contributions
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
