Biogenically induced bedded chert formation in the alkaline palaeo-lake of the Green River Formation

Ryusei Kuma,Nagayoshi Katsuta,Jessica H. Whiteside,Hidekazu Yoshida,Masayuki Ikeda,Koshi Yamamoto,Hitoshi Hasegawa

doi:10.1038/s41598-019-52862-7

Abstract

Rhythmically bedded cherts are observed in both pelagic marine and lacustrine deposits, but the formation mechanism in the latter remains highly uncertain. Our study of alternating chert–dolomite beds in the Eocene Green River Formation, Utah, USA reveals dense accumulations of organic-matter spheres (30–50 μm diameter) of probable algal cyst origin in the chert layers, and centennial- to millennial-scale periodicities in chert layer deposition. A positive correlation between the degree of degradation of the organic spheres and Si distribution implies decomposition of algal organic matter lead to precipitation of lacustrine chert. As both alkalinity and dissolved silica were likely high in the palaeo-lake waters of the Green River Formation, we hypothesize that decomposition of algal organic matter lowered the pH of sediment pore waters and caused silica precipitation. We propose a formation model in which the initial abundance of algal organic matter in sediment varies with productivity at the lake surface, and the decomposition of this algal matter controls the extent of silica precipitation in sediment. The formation of rhythmically bedded chert–dolomite may be linked to centennial- to millennial-scale climatic/environmental factors that modulate algal productivity, which are possibly tied to solar activity cycles known to have similar periodicities.

Highlights

Bedded cherts are observed in both pelagic marine and lacustrine deposits, but the formation mechanism in the latter remains highly uncertain
Bedded cherts exhibiting periodicity are more rarely found in lacustrine deposits[3,4]; but due to the absence of biogenic siliceous remains in pre-Neogene lacustrine chert, the formation mechanism of such bedded cherts remains far more uncertain
Previous studies have linked the formation of lacustrine chert to the inherently high silica solubility in alkaline waters and the precipitation of silica driven by changes in the pH in lake waters[3] and/or sediment pore waters[4]

Summary

Introduction

Bedded cherts are observed in both pelagic marine and lacustrine deposits, but the formation mechanism in the latter remains highly uncertain. Owen et al.[19] observed spring-vent, pedogenic and shallow-marsh cherts close to fossil springs in the Kenya Rift Many of these studies concluded that lacustrine chert is inorganic in origin, linked to the evaporative concentration of silica and/or subsequent precipitation by changes in the pH of the lake water[3,4]. It remains intriguing, that fossils and biogenic textures are preserved in lacustrine chert[14]. The potential role of the biogenic material and processes in the formation of lacustrine chert was not fully evaluated in previous studies

Methods

Findings

Conclusion