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 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 μ...
Published in: | Global Biogeochemical Cycles |
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Main Authors: | , |
Format: | Article in Journal/Newspaper |
Language: | English |
Published: |
AGU (American Geophysical Union)
2012
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Subjects: | |
Online Access: | https://oceanrep.geomar.de/id/eprint/34315/ https://oceanrep.geomar.de/id/eprint/34315/1/nitro.pdf https://doi.org/10.1029/93GB03396 |
Summary: | 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. |
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