A study published in the journal Geology rules out that extreme volcanic
episodes had any influence on the massive extinction of species in the late
Cretaceous. The results confirm the hypothesis that it was a giant meteorite
impact what caused the great biological crisis that ended up with the
non-avian dinosaur lineages and other marine and terrestrial organisms 66
million years ago.
The study was carried out by the researcher Sietske Batenburg, from the
Faculty of Earth Sciences of the University of Barcelona, and the experts
Vicente Gilabert, Ignacio Arenillas and José Antonio Arz, from the
University Research Institute on Environmental Sciences of Aragon
(IUCA-University of Zaragoza).
K/Pg boundary: The great extinction of the Cretaceous in Zumaia coasts
The scenario of this study were the Zumaia cliffs (Basque Country), which
have an exceptional section of strata that reveals the geological history of
the Earth in the period of 115-50 million years ago (Ma). In this
environment, the team analyzed sediments and rocks that are rich in
microfossils that were deposited between 66.4 and 65.4 Ma, a time interval
that includes the known Cretaceous/Paleogene boundary (K/Pg). Dated in 66
Ma, the K/Pg boundary divides the Mesozoic and Cenozoic eras and it
coincides with one of the five large extinctions of the planet.
This study analyzed the climate changes that occurred just before and after
the massive extinction marked by the K/Pg boundary, as well as its potential
relation to this large biological crisis. For the first time, researchers
examined whether this climate change coincides on the time scale with its
potential causes: the Deccan massive volcanism (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 Zumaia outcrops lies in that two types of
sediments accumulated there—some richer in clay and others richer in
carbonate—that we can now identify as strata or marl and limestone that
alternate with each other to form rhythms", notes the researcher Sietske
Batenburg, from the Department of Earth and Ocean Dynamics of the UB. "This
strong rhythmicity in sedimentation is related to cyclical variations in the
orientation and inclination of the Earth axis in the rotation movement, as
well as in the translational movement around the Sun".
These astronomic configurations—the known Milankovitch cycles, which repeat
every 405,000, 100,000, 41,000 and 21,000 years—regulate the amount of solar
radiation they receive, modulate the global temperature of our planet and
condition the type of sediment that reaches the oceans. "Thanks to these
periodicities identified in the Zumaia sediments, we have been able to
determine the most precise dating of the climatic eepisodes that took place
around the time when the last dinosaurs lived", says Ph.D. student Vicente
Gilabert, from the Department of Earth Sciences at UZ, who will present his
thesis defense by the end of this year.
Planktonic foraminifera: Revealing the climate of the past
Carbon-13 isotopic analysis 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
chronology of that time in the Zumaia sediments. More than 90% of the
Cretaceous planktonic foraminiferal species from Zumaia became extinct 66 Ma
ago, coinciding with a big disruption in the carbon cycle and an
accumulation of impact glass spherules originating from the asteroid that
hit Chicxulub, in the Yucatan Peninsula (Mexico).
In addition, the conclusions of the study reveal the existence of three
intense climatic warming events—known as hyperthermal events—that are not
related to the Chicxulub impact. The first, known as LMWE and prior to the
K/Pg boundary, has been dated to 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).
In the last decade, there has been intense debate over whether the
hyperthermal events mentioned above were caused by an increased Deccan
volcanic activity, which emitted large amounts of gasses into the
atmosphere. "Our results indicate that all these events are in sync with
extreme orbital configurations of the Earth known as eccentricity maxima.
Only the LMWE, which produced an estimated global warming of 2-5°C, appears
to be temporally related to a Deccan eruptive episode, suggesting that it
was caused by a combination of the effects of volcanism and the latest
Cretaceous eccentricity maximum", the experts add.
Earth's orbital variations around the Sun
The global climate changes that occurred in the late Cretaceous and early
Palaeogene—between 250,000 years before and 200,000 years after the K/Pg
boundary—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 boundary is not related to the late Cretaceous mass
extinction of species. The climatic changes caused by the eccentricity
maxima and augmented by the Deccan volcanism occurred gradually at 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 late Cretaceous climate change (the
LMWE)", the research team says. "Furthermore, the last 100,000 years before
the K/Pg boundary are characterized by high environmental stability with no
obvious perturbations, and the large mass extinction of species occurred
instantaneously on the geological timescale", they conclude.
Reference:
Vicente Gilabert et al, Contribution of orbital forcing and Deccan volcanism
to global climatic and biotic changes across the Cretaceous-Paleogene
boundary at Zumaia, Spain, Geology (2021).
DOI: 10.1130/G49214.1