Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization

In two contrasting regions of the ocean, the equatorial Pacific and the southern ocean, the δ 15 N of core top sediments were strongly related to [NO 3 − ] in surface waters. With distance from the equator in the equatorial Pacific, δ 15 N increased from 7‰ to 16‰ as [NO 3 − ] decreased from 8μM to < 0.1 μM. Going from 60° to 30° S in the SE Indian Ocean, core top δ 15 N increased from 5‰ to 11‰ as surface [NO 3 − ] decreased from 25μM to < 0.1 μM. These results are strong evidence that sedimentary δ 15 N in these regions is recording the increasing isotopic enrichment of near‐surface NO 3 − with its depletion by phytoplankton. In the case of the equatorial Pacific, δ 15 N values for sinking particles collected at 150 m matched well the core top sediment values, demonstrating little diagenetic alteration of the near‐surface generated isotopic signal. These equatorial Pacific data sets have variations with near‐surface [NO 3 − ] consistent with Rayleigh fractionation kinetics for a fractionation factor (ϵ u ) of 2.5‰. This value is substantially lower than previously found for temperate or polar regions, perhaps as a result of differences in phytoplankton species assemblage or growth condition. In the southern ocean south of the polar front, comparison of δ 15 N values for opal‐rich sediments south and sinking particles indicates an apparent +5‰ diagenetic enrichment relative to the surface‐generated signal that requires further investigation. This exception aside, our observations show that the surface‐water relationship of increasing δ 15 N with increasing NO 3 − depletion is generally transmitted to and preserved in the sediments, an important requirement for further development and application of this important paleoceanographic tool.