170 million years ago, in the middle Jurassic, the Serra d'Aire region, where today the important (and in lamentable degradation) deposit with dinosaur footprints from Pedreira do Galinha is located, was sea. A reef sea, azure blue, very shallow (a few meters), with warm and transparent waters, like that of the Caribbean, full of life and color, where the beaches were white with fine sand and white as snow.
The temperature and humidity of the air were those of today's tropical regions, allowing for lush vegetation in forests, along rivers and on swampy coastlines.
In these forests, dinosaurs dominated and enjoyed the necessary food. In the sea, molluscs, corals, algae and many other organisms adsorb carbon dioxide from the air and calcium dissolved in the water to build their calcareous shells and carapaces with them. [1].
After the death of these calcium carbonate builders, these skeletal parts were crushed by the agitation of the water and by the predation carried out by some animals, turning into sand and very fine mud.
By accumulating and hardening, these sediments gave rise to limestone layers (such as those that can be seen on the A1 motorway, crossing the Serra d'Aire) which contain, in its composition, the carbon dioxide in the air. time. If, on the slab that preserves the footprints, in the aforementioned deposit, we attack the limestone with an acid, there is a bubbling that is nothing more than the release of that gas trapped in it, and thus, when inhaling it, we are breathing part of the air that the great sauropods breathed here 170 million years ago.
This reasoning is valid for practically all occurrences of sedimentary limestone from successive geological epochs. This is because this sedimentary rock has always been formed, over Earth's time, by the imprisonment of carbon dioxide from the air by underwater living organisms (marine, lacustrine, river or others), in order to build their respective skeletal parts.
Thus, by causing effervescence on the limestone of the ground they walk on, anyone, anywhere in the world, can inhale part of the air from the time when the respective rock was formed.
For example, a resident of Lisbon, Rua Sampaio Bruno, where the remains of the natural landscape of this area of the city outcrop, can release the carbon dioxide trapped in calcium carbonate from the bryozoan fossils preserved in that reef seabed from about 23 millions of years, but proceeding in the same way in the Alcântara Valley releases an older gas, with around 95 million. In Peniche or Coimbra, in the Liásico limestones (lower Jurassic), still as an example, we can go back to 200 million years.
Author AM Galopim de Carvalho
Science in the Regional Press – Ciência Viva
[1] For example, molluscs initially form aragonite (unstable calcium carbonate) associated with conchiolin (a protein). After the animal's death, this carbonate transforms into its most stable polymorph, calcite.
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