Microbial nitrification and acidification of lacustrine sediments deduced from the nature of a sedimentary kaolin deposit in central Japan

Tetsuichi Takagi,Katsuhiro Tsukimura,Mihoko Hoshino,Ki-Cheol Shin,Mayumi Jige

doi:10.1038/s41598-021-81627-4

Abstract

Kaolin deposits in the Seto-Tono district, central Japan, were formed by intense kaolinization of lacustrine arkose sediments deposited in small and shallow inland lakes in the late Miocene. Based on mineralogical and stable isotopic (Fe, C, N) studies of Motoyama kaolin deposit in the Seto area, we concluded that it was formed by microbial nitrification and acidification of lacustrine sediments underneath an inland lake. Small amounts of Fe–Ti oxides and Fe-hydroxide in the kaolin clay indicated that iron was oxidized and leached during the kaolinization. The field occurrences indicate that leached ferric iron precipitated on the bottom of the kaolin deposit as limonite crusts, and their significantly fractionated Fe isotope compositions suggest the involvement of microbial activity. The C/N ratios of most of the kaolin clay are distinctly higher than those of modern lacustrine sediment. Although, the possibility of a low-temperature hydrothermal origin of the kaolin deposit cannot be completely ruled out, it is more likely that acidification by dilute nitric acid formed from plant-derived ammonia could have caused the kaolinization, Fe oxidation and leaching. The nitrate-dependent microbial Fe oxidation is consistent with dilute nitric acid being the predominant oxidant.

Highlights

Kaolin is one of the commonest minerals on the Earth’s surface
The X-ray diffraction analysis (XRD) patterns showed that the gaerome and kibushi clays consist mostly of kaolinite, quartz with subordinate feldspars, mica, and minor smectite (Fig. 5)
Dilute nitric acid produced by the nitrification of ammonia could have caused decomposition of feldspars and micas in arkose sediments, resulting in Fe-leaching and intense kaolinization

Summary

Introduction

Kaolin (sensu lato) is one of the commonest minerals on the Earth’s surface. It is the final weathering product of crustal rocks in temperate regions. One possible process is mineral degradation by acid solutions produced by microbial activity The importance of this has been highlighted by studies of kaolin deposits in coastal sediments in the Southeastern United States[4,5]. The kaolin deposits are embedded in the Lower Tokai Group, Seto porcelain clay formation (PCF) and the equivalent Toki PCF. They were deposited 9–11 Ma, based on geochronology of tuff intercalated in the stratigraphy (Ref.[9]). During Pliocene, the Seto and Toki PCF were overlain by Yadagawa formation of the Upper Tokai Group, and the kaolin deposits survived erosion by the uplift after 3 Ma. This study focused on a Motoyama kaolin deposit which is exploited by Akatsuki, Inzo, and Kasen mines in the Seto area (Fig. 2). The climate would have been tropical to subtropical during the deposition[11]

Methods

Results

Conclusion