AsianScientist (May 16, 2018) – A research group at Nagoya University, Japan, has developed a method to analyze how spherical rocks, known as concretions, are formed. Their work is published in Scientific Reports.
All over the world, spectacular fossils have frequently been found preserved inside solid, roughly spherical concretions. Geologists and casual observers alike have wondered why these hardened masses of carbonate formed around dead organisms, with round shapes and sharp boundaries with the surrounding material, typically in marine mud and mudstone.
Several important questions regarding concretions have long puzzled scientists. What conditions cause them to form? How long do they take to grow? Why do they stop growing? Why are they so distinct from the surrounding rock or sediments?
In the present study, researchers from Nagoya University have developed a method to analyze concretions using L-shaped cross-plot diagrams of diffusion and growth rate. With this method, they analyzed dozens of concretions from three sites across Japan and compared them with concretions from England and New Zealand.
“Until now, the formation of spherical carbonate concretions was thought to take hundreds of thousands to millions of years,” said Dr. Koshi Yamamoto of Nagoya University, who is a co-author of the study. “However, our results show that concretions grow at a very fast rate, over several months to several years.”
This rapid sealing mechanism could explain why some concretions contain well-preserved fossils of soft tissues that are rarely fossilized under other conditions.
“The concretions maintained their characteristics, with well-preserved fossils at their centers, or textures indicative of the original presence of organic matter. Simple mass balance calculations also demonstrate that the carbon fixed in the carbonate concretions came predominantly from the organs of organisms inside the concretions,” first author Dr. Hidekazu Yoshida explained.
All of the studied concretions were composed of calcite, with relatively consistent compositions that were distinct from the surrounding muddy matrix. Fine-grained, clay-rich sediments were found to be important to limit diffusion and permeability, as well as to slow the migration of solutes. These conditions would ensure that bicarbonate concentrations would rise high enough at the reaction front to cause rapid precipitation of calcium carbonate, with sharp boundaries from the surrounding mud.
This unified model for the creation of spherical concretions, which can be generalized by simple formulas, can be applied to interpret concretions from all over the world. The researchers noted that in addition to advancing our knowledge of this important preservation mechanism in the fossil record, this improved understanding of the rapid precipitation of calcite due to the presence of organic material may have practical applications in the field of sealing technology.
The article can be found at: Yoshida et al. (2018) Generalized Conditions of Spherical Carbonate Concretion Formation Around Decaying Organic Matter in Early Diagenesis.
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Source: Nagoya University.
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