The belief that certain features of coral skeletons are primitive may be misleading.

Skeletons of reef corals—if they retain their aragonite mineralogy, which is considered primitive—are considered reliable sources of information about past climates. However, new research by paleontologists from Poland and Switzerland has demonstrated the fallacy of this belief.

"There may not be a need to rewrite the textbooks, but we certainly need to reconsider many existing interpretations and be much more cautious in drawing conclusions. This is especially true since analyses of the isotopic compositions of aragonite corals are one of the foundations of paleoclimatology and climate reconstruction research," emphasized Prof. Jarosław Stolarski from the Institute of Paleobiology of the Polish Academy of Sciences, lead author of a publication on this topic in the journal "Scientific Reports." The publication was prepared by researchers from the Institute of Paleobiology of the Polish Academy of Sciences and the École Polytechnique Fédérale de Lausanne (Switzerland).

This refers to hexaradiate corals, common today and throughout the Cenozoic and Mesozoic eras, which build skeletons of aragonite, a variety of calcium carbonate. "If the skeletons of such corals are found with preserved aragonite mineralogy, they are generally considered diagenetically unchanged (i.e., not subject to physical or chemical changes after their formation) and retain the original isotopic signals of the environment in which they formed, allowing for the reconstruction of, for example, the temperature of ancient seas," the researcher explained in an interview with PAP.

He added that aragonite is an unstable mineral and very often transforms into calcite, a stable form of calcium carbonate. During this transformation process, the original isotopic signals recorded in the skeleton are erased. "Therefore, if we find calcite coral skeletons, with few exceptions, we can consider them metamorphosed. Aragonite skeletons, on the other hand, are considered remarkably well-preserved, with their original isotope composition," explained Professor Jarosław Stolarski.

However, the researchers decided to investigate this matter. This is especially true given that, as Professor Stolarski explained, the understanding of the mechanisms of coral skeleton formation has recently radically changed. To put it simply, he continued, the skeleton is not formed like a regular geological crystal by "precipitating" calcium carbonate, but by building it from nanometer-sized units with a high proportion of organic components. This makes the structure of such a biocrystal much more porous, and water can penetrate it quite easily.

The experiment involved placing the skeletons of present-day aragonite corals in a special device where they were kept at a temperature of 50 degrees Celsius in an aquatic environment with a completely different isotopic composition from the original seawater.

"After a week, we saw that although the skeleton itself remained entirely aragonite, the isotopic composition had changed radically, meaning that these isotopes from the water had penetrated the coral skeleton. Therefore, an isotopic exchange occurred, without any change in the mineralogical composition," Professor Stolarski pointed out.

What does this mean for paleontologists? "Let's apply this experiment to real life, to a real-life paleontologist's research. Imagine finding an aragonite skeleton – which he takes at face value, because it means it's perfectly preserved. He reads the isotopes and concludes, for example, that the water temperature during the coral's lifetime was 50 degrees Celsius. This may not be true, because the coral may contain isotopes from its environment, not its original one, but the one it inhabited for millions of years. Our results show that we need to be much more cautious in drawing conclusions for interpretations to be reliable," emphasized the paleontologist.

When asked whether this means that the results of all isotope studies on aragonite skeletons of corals and other organisms that create such skeletons should now be questioned, he denied it.

"It depends on the environment in which these fossils were preserved. If these are completely impermeable clay-type rocks, into which virtually no water from the external environment penetrates, there is a good chance that this isotopic signal is original. However, this will have to be proven. The mere preservation of the skeleton's aragonite mineralogy does not guarantee its 'perfect state of preservation,'" concluded Professor Stolarski. (PAP)

Science in Poland

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