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

Abstract

In two contrasting regions of the ocean, the equatorial Pacific and the southern ocean, the δ15N of core top sediments were strongly related to [NO3−] in surface waters. With distance from the equator in the equatorial Pacific, δ15N increased from 7‰ to 16‰ as [NO3−] decreased from 8μM to < 0.1 μM. Going from 60° to 30° S in the SE Indian Ocean, core top δ15N increased from 5‰ to 11‰ as surface [NO3−] decreased from 25μM to < 0.1 μM. These results are strong evidence that sedimentary δ15N in these regions is recording the increasing isotopic enrichment of near‐surface NO3− with its depletion by phytoplankton. In the case of the equatorial Pacific, δ15N 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 [NO3−] consistent with Rayleigh fractionation kinetics for a fractionation factor (ϵu) 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 δ15N 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 δ15N with increasing NO3− depletion is generally transmitted to and preserved in the sediments, an important requirement for further development and application of this important paleoceanographic tool.