The cliffs of Zumaia (Basque Country) contain a section of strata that reveals the geological history of the Earth in the period 115-50 million years ago (Ma). A team of researchers from the University of Barcelona and the Aragón University Institute for Environmental Sciences Research (IUCA-University of Zaragoza) has analyzed sediment and fossil rock samples there that were deposited during the so-called K / Pg limit (Cretaceous / Paleogene), between 66.4 and 65.4 Ma. In this interval, which divides the Mesozoic and Cenozoic eras, there was one of the five great mass extinctions that the planet has experienced.
The objectives of the study were to analyze the climatic changes produced just before and after this mass extinction and to find out their possible relationship with the great biological crisis. In addition, the scientists examined whether this climate change coincides on the time scale with its possible causes: the massive volcanism of the Deccan (India) – one of the most violent volcanic episodes in the geological history of the planet – and the orbital variations of the Earth .
“The particularity of the outcrops of Zumaia is that two types of sediments accumulated there – some richer in clays and others richer in carbonates – that today we can identify as strata of marl and limestone that alternate with each other forming rhythms”, he highlights researcher Sietske Batenburg, from the Department of Earth and Ocean Dynamics at the UB. ” This strong rhythmicity in sedimentation is related to the cyclical variations suffered by the orientation and inclination of the Earth’s axis in the rotational movement, as well as in the translational movement around the Sun. “
These astronomical configurations – the famous Milankovitch cycles, which repeat every 405,000, 100,000, 41,000 and 21,000 years – regulate the amount of solar radiation received, modulate the global temperature of our planet and condition the type of sediment that arrives. to the oceans. “Thanks to these periodicities identified in the Zumaia sediments, we have been able to establish the most precise dating of the climatic events that occurred around the time when the last dinosaurs lived,” explains Vicente Gilabert, another of the authors.
The isotopic analysis of carbon-13 on the rocks in combination with the study of planktonic foraminifera – microfossils used as high precision biostratigraphic indicators – has made it possible to reconstruct the paleoclimate and the chronology of the Zumaia sediments. More than 90% of the Zumaia Cretaceous planktonic foraminifera species became extinct 66 Ma ago, coinciding with a major disturbance in the carbon cycle and with an accumulation of impact glass spherules caused by the asteroid that impacted Chicxulub, in the Yucatan peninsula (Mexico).
Likewise, the conclusions of the study reveal the existence of three events of intense climatic warming – known as hyperthermal events – that are not related to the Chicxulub impact about 66 million years ago. The first, called LMWE and before the K / Pg limit, has been dated between 66.25 and 66.10 Ma. The other two events, after the mass extinction, are called Dan-C2 (between 65.8 and 65, 7 Ma) and LC29n (between 65.48 and 65.41 Ma).
There is an intense debate on whether or not the hyperthermal events mentioned were caused by the increase in volcanic activity in the Deccan, which emitted large amounts of gases into the atmosphere. “Our results indicate that all these events are in sync with extreme orbital configurations of the Earth known as maximum eccentricity. Only the LMWE, which produced an estimated global warming of between 2 and 5 ° C, appears to have a temporal relationship with a Deccan eruptive episode, suggesting that it was caused by a combination of the effects of volcanism and the last maximum eccentricity. from the Cretaceous ”, detail the experts.
The asteroid theory is confirmed
The global climatic changes that occurred in the late Cretaceous and early Paleogene — between 250,000 years before and 200,000 years after the K / Pg limit — were due to eccentricity maxima of the Earth’s orbit around the Sun.
However, the orbital eccentricity that influenced climate changes before and after the K / Pg limit is not related to the mass extinction of late Cretaceous species. The climatic changes caused by the maximum eccentricity and punctually increased by the volcanism of the Deccan occurred gradually on a scale of hundreds of thousands of years.
“These data would confirm that the extinction was caused by something completely external to the Earth system: the impact of an asteroid that occurred 100,000 years after this climate change at the end of the Cretaceous (the LMWE)”, points out the research team. “In addition, the last 100,000 years prior to the K / Pg limit are characterized by great environmental stability without obvious disturbances, and the great mass extinction of species occurred instantaneously on the geological time scale,” they conclude.
Text: University of Barcelona
