Fire from the Sky

Modern wildfire regimes can be compared to the catastrophic global wildfires that proliferated following an asteroid impact 66 million years ago. As the asteroid struck Earth, immense heat was generated. The shockwave flattened forests and wildfires raged, covering Earth's surface in dust and soot.

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Rocks in the badlands of North Dakota are famous because the mudstones and sandstones hide traces of a very specific event— the disaster that occurred 66 million years ago and eliminated almost 75% of all species on our planet, including the non-avian dinosaurs. Most scientists agree that the cause behind this extinction was an asteroid that struck the carbonate platform of what is today the Yucatan Peninsula in Mexico. The crater is about 200 km in diameter and 2 km deep but is today filled in by younger sediments. It is named after the village located in the center of the crater, Chicxulub.  

Cretaceous-Paleogene boundary, N. Dakota
A shot from the so called badlands in North Dakota where the Cretaceous-Paleogene boundary is accessible. Right: Close up of the boundary clay with white ash layer left by the wildfires. Lower right: Fossil leaf Liriodendron.

The immense volume of rock excavated when the asteroid slammed into the carbonate platform in Yucatan, was ejected into the atmosphere as a spray of molten debris. Over the next few weeks, months, and years following the asteroid impact, all of these particles settled on the Earth’s surface and in the oceans. With time this formed a clay layer representing the boundary between the Cretaceous and Paleogene periods, and is commonly referred to as the K-Pg boundary. Close to the crater rim in Mexico and Belize, this ejecta layer can reach up to 30 metres thick. This same ejecta layer becomes thinner with distance from the impact site. It reaches a few centimetres thick in the US and Canada whereas in New Zealand it is only represented by a few millimetres of clay and is barely visible.

This boundary layer is enriched in the elements iron, nickel, chromium, iridium and even gold, which are mainly derived from the asteroid itself. The impacting body vapourized during the impact and became part of the dust layer that spread globally. However, there are also traces of the Yucatan gypsum- and limestone-rich bedrock, evidenced by the presence of sulfur and carbonates in the boundary clay at sites far from any ocean. It is rather incredible that we find traces of the rocks from Yucatan even in Denmark and New Zealand!

As the asteroid struck Earth, immense heat was generated both directly from the impact but also by the friction caused by the molten rock travelling through the atmosphere at supersonic speed. The initial shockwave flattened forests and wildfires raged leaving an ash-layer all over North America. Today this is visible as a whitish layer in the boundary clay and is detectable as far away as New Zealand.  An increased abundance of charcoal can be seen under the microscope in samples from continental sites, including North Dakota and New Zealand .  

Wildfires destroy ecosystems rapidly but a more long-term consequence is the dispersal of the microscopic, or even nano-sized soot particles. These hindered light from the sun reaching the Earth and, when forming the nuclei of aerosol particles, they prolonged the interval of darkness. Consequently, the lowered light levels at the Earth's surface led to photosynthesis shut down. Pollen analysis across the K–Pg boundary reveals a mass-death of vegetation globally with the subsequent dominance of fungi followed by the gradual recovery of plants. The initial recovery of the vegetation was represented by a succession of fern species, followed by recovery of conifers and finally the return of angiosperms (flowering plants), which were most severely affected. This recovery succession very much resembles the pioneer invasion of plants on new volcanic islands, or indeed the recovery of ecosystems following modern wildfires.

Recovery after fire
Recovery of ferns and fungi after the recent wildfire in Australia.

Global wildfires brought on by this catastrophe wrought major changes to terrestrial ecosystems. A different world appeared in the aftermath of the global firestorm. Over 40% of plant species disappeared, various groups changed in their relative abundance,  new plant species evolved and rose to dominate the Earth's vegetation, and even more dramatic changes occurred to the fauna. Such global calamities draw parallels with the rapid ecosystem changes caused by human activities and calls for global action to stem biodiversity loss and the establishment of reserves for the protection of remnant native forests.


Note: Poster image was used under open license from NASA image and video library. All other photographs were taken by the author. 


Further reading:

Alvarez, W., 1997., T. rex and the crater of doom. Princeton University press. pp.208.

Vajda, V., Raine, I. & Hollis., C., 2001. Indication of Global deforestation at the Cretaceous–Tertiary Boundary by New Zealand Fernspike. Science 294, 1700–1702

Vajda, V. & McLoughlin, S. 2004. Fungal Proliferation at the Cretaceous–Tertiary Boundary. Science 303, 489–490

Vajda, V., Ocampo, A., Ferrow, E. & Bender Koch, C., 2015. Nano particles as the primary cause for long-term sunlight suppression at high southern latitudes following the Chicxulub impact –evidence from ejecta deposits in Belize and Mexico Gondwana Research 27: 1079–1088.

Go to the profile of Vivi Vajda

Vivi Vajda

Professor, vivi.vajda@nrm.se

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