Climate changes locked inside microfossils

Fossilised remains of sea creatures are commonly found in rocks in the mountains of the Basque Country. So, at some time in the past, Euskal Herria was under the sea. For example, during the Palaeocene period, some 65-55 million years ago. The region was then subtropical, and similar in appearance to the Australian Coral Reef.

Along the Bizkaia and Gipuzkoa coast, around Eibar, in Irati and in Urbasa, for example, we can see Palaeocene outcrops at the surface. During that period there were collisions between the European and Iberian tectonic plates which pushed up earth mass that lay under the sea. These very collisions gave rise to the Pyrenees.

These Palaeocene rock outcrops are not at all common on the rest of the planet and, thus, in order to ascertain what happened during that period, researchers have an invaluable source of information in the Basque Country. Moreover, the area has another advantage: remains occur both of the sea crust and of the continental platform and its edge, given that the town of Zumaia at that time was submerged 1,000 metres below the sea while the Rioja Alavesa was above surface.

The importance of these rocks lies in the fact that, within them, remains which contain information on palaeoclimatic and palaeoecological changes that took place in the Palaeocene can be found: microfossils, for example. The data obtained from these miniscule creatures can prove to be very useful today in order to know about the evolution of global warming which is apparently taking place on Earth, just like now, the end of the Palaeocene saw a rapid rise in global warming.

Microfossils: data bank

A group of researchers at Leioa (the Bizkaia campus of the University of the Basque, Country) analysed microfossils, mainly planktonic foraminifers and calcareous nanofossils. These microorganisms lived in the earth’s crust at the bottom of the ocean and their fossils have been piling up over millions of years to the point of providing an unbeatable source of data.

These microorganisms are very sensitive to climatic or temperature changes and that is why some live in warm waters and others in cold. Thus, they are found in differentiated zones in the sea and so, if the sea temperature varies, these zones become modified and the microorganisms migrate with the changes from zone to zone. Thus, the fossil register for these microorganisms in any zone indicates the successive climatic changes that occurred during that era.

To analyse these microfossils it has to be taken into account that nowadays they form part of calcareous rocks or marls. For example, 80 % of the rocks formed during the Palaeocene at the bottom of the sea may be made up of these microfossils or, rather, of their shells.

In these analyses, researchers extract a small rock sample which is then broken up in water. Just one drop of this contains millions of microfossils. A drop is analysed under the microscope or with a magnifying glass, as the fossils are the approximate size of a few micras.

These investigations show up the different microfossil species found in the rock sample and the proportion of each are counted and analysed. In the rocks in the Basque Country more than 200 species of calcareous nanofossils and 175 of planktonic foraminifers have been identified. With the knowledge of which live in warm and which in cold waters, we can deduce what climatic changes happened in past times and have a good idea of what is likely to happen in the future.