Crétacé | EPFL Graph Search

Le Crétacé est une période géologique qui s’étend d'environ . Elle se termine avec la disparition des dinosaures non aviens, des ptérosaures, de la plupart des reptiles marins, des ammonites et de nombreuses autres formes de vie. Cette période est la troisième et dernière de l’ère Mésozoïque ; elle suit le Jurassique et précède le Paléogène. Sa fin est marquée par un stratotype riche en iridium que l’on pense associé à l’impact d’une météorite dans le Yucatan. Cette collision est considérée comme ayant participé fortement à l’extinction massive ayant entraîné entre autres la disparition des dinosaures. Néanmoins, la géologie montre que l'activité volcanique de grande ampleur commune aux cinq grandes extinctions avait déjà commencé avant l'arrivée du bolide. Le Crétacé est nommé d’après le latin creta, « craie », se référant aux vastes dépôts crayeux marins datant de cette époque et que l’on retrouve en grande quantité en Europe, notamment dans le Nord de la France (affleurant pa

The modern analogue of a Cretaceous coral with a calcitic skeleton

Anders Meibom, Jaroslaw Hubert Stolarski

It was argued that, in contrast to all known modern scleractinian corals that form aragonite skeletons, the original mineralogy of the Cretaceous "Coelosmilia" (ca. 70-65 Ma) was calcite during a period when the Mg2+/Ca2+ ratio of the seawater was presumably the lowest in Phanerozoic (Science 2007 318: 92-94). This is consistent with the idea that scleractinians extend only partial control over their skeletal mineralogy, because calcite is the preferred calcium carbonate polymorph (over aragonite) during abiotic CaCO3 precipitation in low Mg2+/Ca2+ seawater conditions. However, subsequently it was found that a number of different Cretaceous corals formed purely aragonite coralla in the same low Mg2+/Ca2+ conditions, suggesting instead that biological control over the skeletal mineralogy differs between taxonomically different corals (Geology 2017 45:319-322) and posing the question why was "Coelosmilia’s" skeletal mineralogy affected by the seawater chemistry changes and not that of the other corals. An unusual feature of "Coelosmilia" is the infilling of a significant portion of the calice with dense, banded skeletal deposits. They are crystallographically continuous with earlier formed layers of septa, wall and axial structures, but show distinct biogeochemical signatures. To date, no modern analogs of such corals were known, and it was tempting to argue that this skeletal infilling represents secondary, purely abiotic deposits (similar to marine cements, which would be calcitic in Cretaceous seas). Here we document a living scleractinian coral that exhibits a structural organization identical to Cretaceous "Coelosmilia", including dense calicular infilling that is a species-specific feature. Surprisingly, the inner parts of these skeletal elements are composed of fibrous calcite, whereas the rest of the skeleton is aragonitic. We suggest that this modern coral is phylogenetically related to "Coelosmilia" and capable of formation of strictly biologically controlled skeletal mineralogy, which can include both aragonitic and calcitic structures. This original bipartite skeletal organization may explain several diagenetic phenomena that we observe in "Coelosmilia", and is highly relevant for the proper geochemical sampling of corals with such skeletal organization.

2019

Aragonitic scleractinian corals in the Cretaceous calcitic sea.

Stéphane Laurent Escrig, Anders Meibom, Jaroslaw Hubert Stolarski

Changes in seawater chemistry have affected the evolution of calcifying marine organisms, including their skeletal polymorph (calcite versus aragonite), which is believed to have been strongly influenced by the Mg/Ca ratio at the time these animals first emerged. However, we show that micrabaciids, a scleractinian coral clade that first appeared in the fossil record of the Cretaceous, when the ocean Mg/Ca ratio was near the lowest in the Phanerozoic (thus a priori favoring calcitic mineralogy), formed skeletons composed exclusively of aragonite. Exceptionally preserved aragonitic coralla of Micrabacia from the Late Cretaceous Ripley Formation (southeastern USA) have skeletal microstructures identical to their modern representatives. In addition, skeletons of Micrabacia from Cretaceous chalk deposits of eastern Poland are clearly diagenetically altered in a manner consistent with originally aragonitic mineralogy. These deposits have also preserved fossils of the scleractinian Coelosmilia, the skeleton of which is interpreted as originally calcitic. These findings show that if changes in seawater Mg/Ca ratio influenced the mineralogy of scleractinian corals, the outcome was taxon specific. The aragonitic mineralogy, unique skeletal microstructures and ultrastructures, and low Mg/Ca ratios in both fossil and living micrabaciids indicate that their biomineralization process is strongly controlled and has withstood major fluctuations in seawater chemistry during the past 70 m.y.

Geological Soc Amer, Inc2016

The modern analogue of a Cretaceous coral with a calcitic skeleton

Anders Meibom, Jaroslaw Hubert Stolarski

2019