WFS News: A unified model of the formation mechanism of spherical carbonate concretions contain exceptionally well-preserved fossils

May 10th, 2018 Riffin

@WFS,World Fosssil Society,Riffin T Sajeev,Russel T Sajeev

Spherical, isolated carbonate concretions occur throughout the world in marine argillaceous sedimentary rocks of widely varying geological ages. These concretions are characteristically highly enriched in CaCO3compared to the surrounding sedimentary rock matrices and are typically separated from these rock matrices by sharp boundaries1,2,3. These sharp boundaries mean that the isolated carbonate concretions are readily identifiable and both geologists and non-scientists alike have been motivated to consider how such concretions could have formed.

Conceptual view of the features of spherical concretions. Ca profile and porosity distribution across the spherical concretion forming under rather stable conditions under which solutes diffused continuously, causing calcite precipitation until the organic source of carbon at the centre of the concretion was consumed.

Conceptual view of the features of spherical concretions. Ca profile and porosity distribution across the spherical concretion forming under rather stable conditions under which solutes diffused continuously, causing calcite precipitation until the organic source of carbon at the centre of the concretion was consumed.

It is also known that many of these concretions have various kinds of well-preserved fossils at their centres3. Isotopic analyses have been used to identify the source of carbon that formed the concretions4,5,6and to understand the diagenetic processes, including microbial activity, that occurred during sediment burial and concretion formation1,7,8. However, even though there have been many studies of concretions over several decades, there still remain questions regarding concretion formation9,10,11. The remaining questions include: How rapidly do concretions grow? Why are the concretions often spherical with sharp boundaries? and Why does the localization of Ca and CO3, and consequently concretion growth, stop? These questions can be answered by determining the relationship between the formation conditions of the spherical concretions, the mass transport processes in the sedimentary matrix, and the concretion growth rates. However, this relationship has yet to be described precisely.

Sampling locations and the characteristics in hand specimen of studied Japanese spherical carbonate concretions. (A) Teshio concretions (L1); (B) Yatsuo tusk-shell concretions (L2), and (C) Morozaki ‘ghost-shrimp’ concretions (L3). Teshio concretions from Hokkaido with rounded impressions in their centers suggesting the presence of a non-skeletal animal (arrows: A1). δ13C data also suggest an organic carbon origin. (D) X-ray diffractograms for each concretion shows a clear calcite peak. Index map is based on the data of Geospatial Information Authority of Japan website (http://www.gsi.go.jp/maps.gsi.go.jp). All photographs (A–C) shown here are taken by H.Yoshida.

Sampling locations and the characteristics in hand specimen of studied Japanese spherical carbonate concretions. (A) Teshio concretions (L1); (B) Yatsuo tusk-shell concretions (L2), and (C) Morozaki ‘ghost-shrimp’ concretions (L3). Teshio concretions from Hokkaido with rounded impressions in their centers suggesting the presence of a non-skeletal animal (arrows: A1). δ13C data also suggest an organic carbon origin. (D) X-ray diffractograms for each concretion shows a clear calcite peak. Index map is based on the data of Geospatial Information Authority of Japan website (http://www.gsi.go.jp/maps.gsi.go.jp). All photographs (A–C) shown here are taken by H.Yoshida.

For several decades, the formation of spherical concretions has been explained by diffusion accompanied by carbonate inter-conversion reactions12,13 and also by very slow three dimensional advection of water when cementation conditions are isotropic9,14. These processes give rise to a spherical morphology in homogeneous sediments or sedimentary rocks. The process is basically accepted, but previously proposed models cannot explain the steep chemical gradients (notably of Ca) that occur across the margins of almost all concretions, and also the constant calcite (CaCO3) concentration3,15 and constant porosity within the body of a concretion16,17, as determined by thin section analysis and porosity measurements3. Such a steep boundary occurs as the reaction 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 concretions3,18,19. In this case, the relationship between ‘L’, diffusion coefficient ‘D’ of HCO3− and growth rate of the concretion ‘V’ is: ‘L = D/V’. This relationship can be illustrated using a ‘Diffusion-growth rate cross-plot’, which was developed to explain detailed observations on concretions at a single locality in Japan3. Here, 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. The outcome of the analysis is also used to answer the above remaining questions about syn-genetically formed spherical carbonate concretions and to build on the previous study, which only investigated concretions from a single locality.

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@WFS,World Fosssil Society,Riffin T Sajeev,Russel T Sajeev

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One Response to “WFS News: A unified model of the formation mechanism of spherical carbonate concretions contain exceptionally well-preserved fossils”

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    June 3, 2024 at 8:32 am

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