Paleoclimatologists study the climate of the geological past. Using an innovative technique, new research from an international research team led by Niels de Winter (VUB-AMGC & Utrecht University) shows for the first time that dinosaurs had to cope with larger seasonal differences than ‘we never thought before.
De Winter: “We thought that when the climate warmed up like in the Cretaceous, in the time of the dinosaurs, the difference between the seasons diminished, much like the tropics today have less temperature difference between summer. and winter. However, our reconstructions now show that the average temperature has indeed increased, but that the temperature difference between summer and winter has remained rather constant. This leads to hotter summers and warmer winters. . “
To better characterize the climate during this period of high CO concentration2 concentration, the researchers used very well-preserved fossils of mollusks that lived in southern Sweden during the Cretaceous Period, around 78 million years ago. These seashells grew in the warm, shallow seas that covered much of Europe at the time. They recorded monthly variations in their environment and climate, like the rings of a tree. For their research, de Winter and his team used the “agglomerated isotope” method for the first time, in combination with a method developed by Niels de Winter.
Isotopes grouped together in combination with the VUB-UU method – a revolution in geology
Isotopes are atoms of the same element with different masses. Since the 1950s, the ratio of isotopes of oxygen in carbonate has been used to measure water temperature in the geological past. However, this forced the researchers to estimate the chemistry of seawater, as the isotopic ratio of seawater affects the isotopic ratio of the shell, resulting in higher uncertainty. About ten years ago, the “pooled isotope” method was developed, which does not depend on the chemistry of seawater and allows precise reconstructions. But the concentrated isotope method has a downside – it requires so much carbonate that temperature reconstructions at a more detailed level, such as seasonal fluctuations based on shells, were not possible.
De Winter has now developed an innovative method in which measurements of much smaller amounts of carbonate are artfully combined for temperature reconstructions. The agglomerated isotope method therefore requires much less material and can therefore be used for research on fossil shells, which, like tree rings, hold a great deal of information about their living conditions. The method also makes it possible to aggregate carbonates from successive summers (and winters) for better reconstitution of seasonal temperatures. For example, Winter found that water temperatures in Sweden during the Cretaceous “greenhouse period” fluctuated between 15 ° C and 27 ° C, more than 10 ° C higher than today.
The team also worked with scientists from the University of Bristol (UK) who are developing climate models to compare the results with climate simulations from the Cretaceous period. While previous climatic reconstructions of the Cretaceous were often cooler than these models, the new results agree very well with the Bristol models. This shows that the variations of the seasons and of the water chemistry are very important in climatic reconstructions:
“It is very difficult to determine the climate changes of so long ago on a seasonal scale, but the seasonal scale is essential for getting correct climate reconstructions. While there is virtually no difference between the seasons, reconstructions of mean annual temperature come out differently. when the difference between the seasons is great. It was believed that in the days of the dinosaurs the difference between the seasons was small. We have now established that there are larger seasonal differences. With the same average temperature over a year, you end up with a much higher temperature in the summer.
De Winter: “Our results therefore suggest that in mid-latitudes, seasonal temperatures are likely to increase with global warming, while the seasonal difference is maintained. This results in very high summer temperatures. The results provide new insight into the dynamics of a warm climate. at a very fine scale, which can be used to improve both climate reconstructions and climate predictions. In addition, they show that a warmer climate can also have extreme seasons.
The development has far-reaching implications for how climate reconstructions are carried out. It allows researchers to determine both the effect of seawater chemistry and that of the differences between summer and winter, verifying the accuracy of decades of temperature reconstructions. For his groundbreaking research, De Winter was nominated for both the annual EOS Pipette award and the 2021 New Scientist Science Talent.
The study by Winter and colleagues appeared in the journal Communications in Earth and Environment June 10.
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