Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales

Abstract

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a 'Diffusion-growth rate cross-plot' that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential.

Figure 1. Occurrence of tusk-shell concretion.

Spherical concretions formed around the mouth of a tusk-shell (Fissidentalium spp.) collected from the Kurosedani Formation of the Yatsuo Group distributed in Toyama Prefecture, Central Japan (Supplementary Fig. 1). (a) the diameter of the concretions is in the range 1 ~ 2 cm, with larger concretions occurring around larger tusk-shells. (b,c) cross section through a tusk-shell showing the internal texture, with porosity filled by precipitated carbonate (arrows in b). (d) aragonite layers forming a tusk-shell. (e) cross section showing a concretion formed around the mouth of a tusk-shell, which is in the centre of the picture. (f) SXAM Ca X-ray intensity in and around the cut surface of a concretion (e). A sharp boundary ‘L’ between the concretion and matrix is also defined by the Ca distribution. All photographs (a–e) shown here are taken by H.Yoshida.

Figure 2. Isolated concretion formation mechanism.

A spherical isolated concretion formed by the reaction between , a byproduct from the breakdown of fatty acid originating from the dead organism in the tusk-shell, and pore-water Ca, which was mostly derived from the surrounding matrix outside the concretion. Precipitation of these species as CaCO₃ occurred very rapidly and formed a hard concretion. The concretions continued to grow until the carbon of the organs was all consumed. Surrounding sedimentary layers have been bent by compaction after concretion formation was completed. The relationship between L (width of reaction front), D (diffusion coefficient of fatty acid in clayey matrix), and V (linear growth rate of reaction front) can be described as L = D/V.

Figure 3. Diffusion–growth rate cross-plot.

Relationship between effective diffusion coefficient (D; cm²/s) and growth rate of reaction front (V; cm/s) defined by dimension analysis. The field over which the tusk-shell concretions most likely formed is defined by the width of the reaction front (L = 0.2 cm) and the effective diffusion coefficient of in similar kinds of clay formation (e.g. Boom Clay2425). A very rapid minimum growth rate of the reaction front, in the order of 10⁻⁶ ~ 10⁻⁵ cm/s (weeks to months), is indicated.