[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-P01] Nitrogen isotope geochemical behavior of ammonium between pore water and solids in deep marine sediment

*Toshiro Yamanaka¹, Arisa Sakamoto¹, Yuji Onishi⁷, Ji-Hoon Kim², Lucie C. Pastor³, Andreas P. Teske⁴, Daniel Lizarralde⁵, Tobias W. Höfig⁶, Scientific Party IODP Exp. 385 (1.Tokyo University of Marine Science and Technology, 2.Korea Institute of Geoscience and Mineral Resources, 3.French Research Institute for Exploitation of the Sea (Ifremer), 4.University of North Carolina at Chapel Hill, 5.Woods Hole Oceanographic Institution, 6.Texas A&M University, 7. Research Institute for Humanity and Nature)

Keywords:ammonium, nitrogen isotope, pore water, clay mineral

Nitrogen is supplied from within the deep mantle to the Earth’s surface, hydrosphere, and atmosphere, mostly through volcanic activities. The nitrogen from deep sources is considered to migrate as dinitrogen or ammonia gases depending on the physico-chemical condition. On the other hand, nitrogen recharge from the Earth’s surface into the mantle has been accepted through subduction of ammonium-bearing sediments because of the similarity of charge and ionic radius between ammonium ion (1.43 Å) and potassium ion (1.33 Å) that often substitutes in the interlayers of the potassium-bearing minerals. Nevertheless, the detailed mechanisms as related to the interaction between nitrogen compounds in the ascending hydrothermal fluid and ambient minerals during the path through the lithosphere are not fully understood. The sediment-filled Guaymas Basin is expected to provide suitable samples to reveal behavior of ammonium because abundant sedimentary organic matter supplies sufficient ammonium for stable isotope geochemical study.
During the IODP expedition 385, high ammonium concentrations (up to 39 mM) in the interstitial water (IW) were observed, reflecting mineralization of abundant organic matter in the sediments. Ammonium concentrations generally increase with burial depth, and after reaching the maximum at a certain depth, the concentration is decreasing. Such decrease of ammonium concentrations in deep sediments has been considered to result from scavenging by clay minerals related to illitization. In fact, corresponding phase transition of minerals was observed at this depth in the core sediments. For post cruise analysis, the authors measured nitrogen isotopic ratios (δ¹⁵N values) of IW ammonium for understanding behavior of nitrogen isotopes during the early diagenetic stage. The first results show that the δ¹⁵N values of ammonium reflect those of ambient organic nitrogen (main component of total nitrogen; TN) at the surface layer where the ammonium concentration was increasing with depth, but significantly differ below the maximum of ammonium concentrations. The changing points of δ¹⁵N trends for ammonium possibly corresponds with phase transitions of minerals during diagenesis (opal A to CT, precipitation of carbonate mineral, illitization, etc.).
In the presentation, I will introduce the details of nitrogen behavior in the core samples and the geochemical characteristics of IW obtained from this magmatically active basin.