Generalized conditions of spherical carbonate concretion formation around decaying organic matter in early diagenesis

Hidekazu Yoshida,Sirono Sin-Ichi,Masayo Minami,Richard Metcalfe,Koshi Yamamoto,Nagayoshi Katsuta

doi:10.1038/s41598-018-24205-5

Hidekazu Yoshida, Sirono Sin-Ichi + Show 4 more

Open Access

https://doi.org/10.1038/s41598-018-24205-5

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Abstract

Isolated spherical carbonate concretions observed in marine sediments are fascinating natural objet trouve because of their rounded shapes and distinct sharp boundaries. They occur in varied matrices and often contain well preserved fossils. The formation process of such concretions has been explained by diffusion and rapid syn-depositional reactions with organic solutes and other pore water constituents. However, the rates, conditions and formation process of syngenetic spherical concretions are still not fully clear. Based on the examination of different kinds of spherical concretions from several locations in Japan, a diffusion based growth diagram was applied to define the generalized growth conditions of spherical concretions formed around decaying organic matter. All analytical data imply that the spherical concretions formed very rapidly, at least three to four orders of magnitude faster than previously estimated timescales. The values indicate that spherical concretions are preferentially grown within clay- to silt-grade marine sediments deposited in relatively deep (a few tens of metres) environments dominated by diffusive solute transport, very early in diagenesis.

Highlights

Spherical, isolated carbonate concretions occur throughout the world in marine argillaceous sedimentary rocks of widely varying geological ages
We extend the theory of the ‘cross-plot diagram’ by applying it to concretions of different ages at several localities to develop a general model for the formation conditions of spherical carbonate concretions in marine sediments
Based on the investigation of several spherical concretions of different ages, the general applicability of a ‘Diffusion-growth rate cross-plot’ has been demonstrated for constraining the formation conditions of isolated spherical concretions that are widely distributed in marine argillaceous rocks deposited in relatively deep seabed environments

Summary

Introduction

Spherical, isolated carbonate concretions occur throughout the world in marine argillaceous sedimentary rocks of widely varying geological ages. Front on the concretion surface (Fig. 1) is developed by reactions between HCO3− and Ca2+ ions as concretions grow outwards, and has a certain characteristic width ‘L’ where CaCO3 has been precipitated This front is developed in any kind of carbonate-rich spherical concretion formed syn-genetically during burial of marine sediments with organic carbon sources in the concretions[3,18,19]. In this case, the relationship between ‘L’, diffusion coefficient ‘D’ of HCO3− and growth rate of the concretion ‘V’ is: ‘L = D/V’. The characteristics of these facies are consistent with a relatively deep seabed depositional environment (a few tens of metres20–23), without rapid flow of water

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