Ammonium Behavior and Nitrogen Isotope Characteristics of 2:1 Clay Minerals from Submarine Hydrothermal System in the Wakamiko Crater of Kagoshima Bay, Southwestern Japan

2:1 clay minerals such as smectite incorporating ammonium were extracted to investigate the ammonium behavior and nitrogen isotope characteristics for two different sediment cores which were collected from shimmering sites on seafloor of the Wakamiko crater, southwestern Japan. Inorganic nitrogen contents in clay fraction were estimated by calibration curve based on consistently decreasing carbon and nitrogen ratio during the treatment to decompose organic materials, after removing inorganic carbon. The results show that the proportions of inorganic nitrogen for total nitrogen in clay fraction of SWS site(Core#1094MR: av. 18.2%) are higher than those in SES site(Core#1093MG: av. 11.5%). Relatively good crystallinity of the former suggests that exchangeable ammonium was transformed to non-exchangeable ammonium during more evolving diagenetic process. Nitrogen isotope variance of clay fraction(SES site: Core#1093MG: -4.4 similar to +0.2 %, av. -2.4 %; SWS site: Core#1094MR: -0.7 similar to +3.0 %, av. +1.5 %) during sequential decomposition of exchangeable ammonium suggests that heat flow derived from deep magma led to nitrogen isotope fractionation between dissolved ammonium and ammonia in the fluids involved in the formation of 2:1 clay mineral incorporating ammonium with local temperature variation.

Automated summary

In this study, clay minerals such as smectite incorporating ammonium were extracted to investigate ammonium behavior and nitrogen isotope characteristics. The results showed differences in inorganic nitrogen content between sediment cores, with a transformation of exchangeable ammonium to non-exchangeable ammonium during evolving diagenetic process. The nitrogen isotope variance in clay fraction suggests heat flow from deep magma led to nitrogen isotope fractionation.