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Published October 20, 2024 | Version Published
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Nitrogen isotope homogenization of dissolved ammonium with depth and ¹⁵N enrichment of ammonium during incorporation into expandable layer silicates in organic-rich marine sediment from Guaymas Basin, Gulf of California

Creators

  • 1. ROR icon California Institute of Technology

Contributors

Abstract

Sedimentary nitrogen isotopic ratios are used as a proxy for ancient biogeochemical cycles on Earth's surface. It is generally accepted that sediment hole tops record primary signatures because organic nitrogen (ON) is predominant in this part of the hole. In contrast to such early to middle diagenetic stages, it is well known that heavier nitrogen isotope 15N tends to enrich in sedimentary rocks during later diagenetic and metamorphic stages. However, there are some critical gaps in our understanding of nitrogen isotopic alteration associated with abiotic processes during early-middle diagenesis. In this study, we examined the isotope ratios of ammonium nitrogen in interstitial water (IW) and total nitrogen (TN), including exchangeable ammonium and mineral nitrogen, in the solid-phase of organic-rich-sediment recovered by International Ocean Discovery Program (IODP) Expedition 385 cores drilled in the Guaymas Basin, Gulf of California, that contained ammonium-rich IW. The isotopic ratios (δ15N value) of TN are the most variable with depth compared to any other type of nitrogen. This variation can be interpreted as reflecting changes in the water mass environment in the basin caused by glacial–interglacial climate changes, modifying the δ15N values of the marine primary producers. Thus, the δ15N value of TN is a proxy for environmental change in the basin, while each component of TN shows different trends. The δ15N values of IW and exchangeable ammonium did not exhibit significant changes with depth, but the latter values are about 3 ‰ enriched in 15N. This may be due to advective transport of solute into adjacent layers followed by the formation of an isotopic equilibrium between IW and exchangeable ammonium in the case of fast sediment accumulation rate. The δ15N value of exchangeable ammonium is the highest among the other types of nitrogen with one exception, where the δ15N value of TN is the highest. The calculated δ15N values of ON based on mass balance are almost the same as those of associated TN in the shallow sediment layers (< 150 m below seafloor), but the difference in the δ15N values of TN and ON are significant in the deeper layers, where proportions of ON contents are <50%. In particular, in the layer where the δ15N value of TN is the highest, that of ON shows an even higher value and the difference reaches 3.5 ‰. The δ15N values of mineral nitrogen are similar to that of IW ammonium except the surface layers. Under such conditions, when δ15N value of TN is intermediate between those of mineral nitrogen and exchangeable ammonium, calculated δ15N value of ON is close to that of TN. On the other hand, if δ15N value of TN is out of the range between mineral nitrogen and exchangeable ammonium, it causes further difference in δ15N value of ON. It means that the fluctuation of δ15N values of TN is reduced relative to those of ON through depth. It has been considered that δ15N value of TN in sediment is similar to that of ON, and changes in the δ15N value of TN due to diagenesis are limited, but in such environment ON fluctuations over depth may be slightly underestimated.

Copyright and License

© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Acknowledgement

We sincerely thank the IODP technical staff and the R/V JOIDES Resolution crew of Expedition 385 for their invaluable assistance. Dr. Chitoshi Mizota helped us with recovery of ammonium using the steam distillation technique, XRD analyses, and clay mineral separation and provided many useful comments to an early version of this manuscript. We appreciate the valuable comments given by Dr. Eva Stüeken, Dr. Benjamin Uveges, Dr. Vasileios Mavromatis and two other anonymous reviewers. T.Y. acknowledges support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan through a Grant-in-Aid for Scientific Research (C) (18K03756) and Scientific Research (B) (21H01590 and 21H01172).

Funding

T.Y. acknowledges support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan through a Grant-in-Aid for Scientific Research (C) (18K03756) and Scientific Research (B) (21H01590 and 21H01172).

Contributions

Toshiro Yamanaka: Conceptualization, Methodology, Writing – original draft. Arisa Sakamoto: Investigation. Kanon Kiyokawa: Investigation. Jaeguk Jo: Investigation, Data curation, Visualization. Yuji Onishi: Data curation, Visualization. Yoshihiro Kuwahara: Investigation, Data curation, Visualization. Ji-Hoon Kim: Investigation. Lucie C. Pastor: Investigation. Andreas Teske: Project administration. Daniel Lizarralde: Project administration. Tobias W. Höfig: Data curation, Writing – review & editing.

Data Availability

Data will be made available on request.

Conflict of Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Toshiro Yamanaka reports financial support was provided by the Ministry of Education, Culture, Sports, Science and Technology of Japan. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Additional details

Funding

Ministry of Education, Culture, Sports, Science and Technology
18K03756
Ministry of Education, Culture, Sports, Science and Technology
21H01590
Ministry of Education, Culture, Sports, Science and Technology
21H01172

Dates

Accepted
2024-05-31
Accepted
Available
2024-06-01
Available Online

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Caltech groups
Division of Geological and Planetary Sciences (GPS), Seismological Laboratory
Publication Status
Published