The discovery of the Chicxulub crater dramatically enhanced the community’s ability to evaluate the environmental effects of an impact at the K-T boundary, because both the geographic location of an impact site and the target rocks involved in an impact can affect the environmental outcome. For example, rocks composed of the mineral anhydrite in the Chicxulub target sequence imply that sulfate aerosols were injected into the stratosphere, affecting the radiative budget of the atmosphere (heating the stratosphere while cooling the Earth’s surface) before settling to the troposphere where they were washed out as acid rain. Identifying the impact site and size of the crater was also important because the amount of debris ejected affects the environmental outcome. Any global, extinction-driving effects of an impact are largely caused by the interaction of this impact debris with the atmosphere.
Those global processes were driven by the production of a vapor-rich plume of material that rose from the crater, accelerated through the atmosphere, and expanded around the Earth in space, before raining back down through the atmosphere. The returning cloud of debris carried solid, molten, and vapor products that severely skewed environmental conditions. The environmental health of the Earth would not be the same for a very long time and most life was unable to cope with the changes. Some of the environmental consequences will sound similar to issues that are affecting the modern world, but the severity of the consequences was much greater following the Chicxulub impact event. A breakdown of the different types of environmental effects follows. Feel free to click on a keyword to jump down the page to your favorite topic.
Calculations of impact debris raining from space back through the atmosphere suggests that debris shock-heated the atmosphere, driving chemical reactions that generated nitric acid rain. Additional nitric acid rain may have also been produced by impact-generated wildfires (described in greater detail below). The acid rain may have fallen over a period of a few months to a few years.
Because the Chicxulub impact occurred in a region with rocks composed of the mineral anhydrite, which is a calcium sulphate mineral, sulfur vapor was also injected into the stratosphere. That sulfur, reacting with water vapor, produced sulphate aerosols and eventually sulfuric acid rain. This effect was completely unanticipated until the Chicxulub impact site was discovered. It may also be one of the more serious environmental consequences of that impact event.
Additional sulfur would have been liberated from the projectile, which also contained sulfur-bearing minerals. For smaller impact events, this source of sulfur can be one of the most important environmental consequences. (We refer readers to LPI’s website about Impact-Induced Perturbations of Atmospheric Sulfur for additional details.) In the case of the Chicxulub impact, however, the biggest source of sulfur and sulfuric acid rain was the target rocks, because of the thick deposits of anhydrite on the Yucatán Peninsula.
The combination of sulfuric acid rain and nitric acid rain produced by the Chicxulub impact event, like acid rain today, would have affected vegetation, effectively damaging the base of the continental food chain. The acid rain was not capable of acidifying large ocean basins, but it would have affected shallow freshwater lakes, ponds, and rivers on continents and shallow-water estuaries along the margins of continents with the sea.
There is geochemical evidence of these processes in the geologic record, where chemical leaching of K-T boundary ejecta has been measured. There is also evidence of enhanced continental erosion of sediments into the sea, although it is not yet clear whether the enhanced erosion and chemical weathering were caused by acid rain leaching of land surfaces or the denudation of plants from the land, the latter of which could have been caused by acid rain and many other impact-generated effects.
Evidence of impact-generated fires was recovered from K-T boundary sediments before it was understood how the fires were produced. Several types of carbon (fusinite, soot, pyrolitic polycyclic aromatic hydrocarbons, carbonized plant debris, and charcoal) indicated fire had swept over some continental regions. In addition to the soot, there is a biological signature that may reflect the recovery of plants in charred regions. In several locations in the western United States, ferns are the first plants to reappear after the impact, similar to their pioneering occurrence after forest fires today.
The distribution of those fires is still poorly understood. Although soot is found globally, it is an airborne particulate and, thus, not a good indicator of where fires were ignited. Model calculations have suggested a range of possibilities. The global distribution of iridium indicates ejecta was distributed globally, which potentially caused widespread atmospheric heating and fires. The discovery of the Chicxulub impact location allowed the ignition of fires to be explored in greater detail, which suggested that while heating may have occurred globally, threshold temperatures for generating fires may have had a restricted geographic distribution. Fires may have been generated in southern North America, for example, but the northern part of the continent may have been spared unless fires spread from the south. Several impact parameters, such as the trajectory of the impacting object, which is still not confidently known, will have affected the distribution of fire.
The amount of soot recovered from K-T boundary sediments implies the fires released huge amounts of greenhouse gases (carbon dioxide, carbon monoxide, and methane) that likely had a long-term effect on post-impact climate.
Dust and Aerosols in the Atmosphere
Calculations of an atmosphere choked with dust and sulphate aerosols from the impact event, and soot from post-impact wildfires, suggest surface temperatures fell and sunlight was unable to reach the Earth’s surface, shutting down photosynthesis. These calculations are consistent with the fossil record, which indicates the base of the marine food chain, composed of photosynthetic plankton, collapsed. Because a shutdown of photosynthesis also affects plants on land, herbivores and the carnivores that preyed on them would have been dramatically affected too. A shut-down of photosynthesis may have been the most severe of the impact’s environmental effects.
It would have taken a few hours to approximately a year for particles to settle through the atmosphere. The time depended on particle sizes. Relatively large ~250 micron diameter spherules found in some K-T boundary deposits would have settled out of the atmosphere within hours to days. However, submicron dust may have been suspended in the atmosphere for many months. Soot, if it was able to rise into the stratosphere, would have taken similarly long times to settle. Soot that only rose into the troposphere, however, would have been promptly flushed out of the atmosphere by rain. The dust, aerosols, and soot may have caused significant decreases in surface temperature of several degrees to a few tens of degrees.
The vaporization of the projectile and a portion of target rocks would have produced ozone-destroying chlorine and bromine. Additional chlorine and bromine were produced when vegetation was burned by post-impact wildfires. The amounts of those chemicals injected into the atmosphere were far more than that needed to destroy the ozone layer. The changes in nitrogen chemistry generated by the atmospheric heating described above also had the capacity to destroy ozone. The loss of the ozone layer may have lasted for several years, although it is uncertain how much of an effect it had on surface conditions. Initially, dust, soot, and nitrogen molecules in the atmosphere may have absorbed any ultraviolet radiation and sulphate aerosols may have scattered the radiation. The settling time of dust was probably rapid relative to the time span of ozone loss, but it may have taken a few years for the aerosols to be scrubbed from the atmosphere.
Greenhouse gases (carbon dioxide, water, methane) were produced from Chicxulub’s target lithologies and the impactor. Those gases may have caused greenhouse warming after the dust, aerosols, and soot settled to the ground. A lot of the carbon dioxide would have come from carbonate rocks on the Yucatán Peninsula. Those rocks, when vaporized, produce carbon dioxide. Water was liberated from the saturated sedimentary sequence and overlying sea.
It would have taken far longer for gases like carbon dioxide to settle out of the atmosphere than dust and sulphate aerosols, so greenhouse warming probably occurred after a period of cooling caused by the dust and aerosols. There are several estimates for the amount of heating, ranging from a global mean average temperature increase of 1 to 1. 5 °C, based on estimates of CO2 added to the atmosphere by the impact, to an increase of ~7.5 °C, based on measurements of fossilized leaves that grew after the impact event.
Summarizing the Environmental Effects
There are two relatively simple ways to think about the collective consequences of the environmental effects described above. First, one can think of them as a function of time after the impact event. In a diagram below, there are a series of environmental effects that were immediate and those that occurred over periods of months, years, and decades. The local and regional effects like fireball radiation, an airblast, and tsunamis occurred immediately. Other processes, like impact-generated fires, may have persisted for months. Even if the fires did not last that long, their secondary effects, like soot cooling, would have. A lot of the chemical effects in the atmosphere, such as sulphate aerosols, nitric and sulfuric acid rain, would have occurred over longer time scales. It probably took 5 to 10 years for the sulfuric acid rain to cease. Greenhouse warming would have persisted for decades if not longer. Some models estimate greenhouse warming persisted for thousands of years.
A second way to think about the collective effects is to imagine the experience at a single location, such as the Raton Basin of Colorado and New Mexico. In another diagram below, there is a curve that indicates how temperatures rose and fell after the impact event. When debris from the Chicxulub crater rained through the atmosphere, there would have been a series of temperature spikes. Those temperatures may have caused vegetation to burn. Afterwards, when the atmosphere was filled with dust, soot, and aerosols, temperatures would have dropped below pre-impact conditions. It would have been cold. Over much longer timescales, when the greenhouse gases began to dominate the environment, temperatures would have risen significantly.
For students wanting to learn more about these types of processes, a review article by Brian Toon and others (1997, Environmental perturbations caused by the impacts of asteroids and comets, Reviews of Geophysics 35, 41-78) is a good place to start, because it outlines the parameters that modify outcomes as a function of impact size. For an updated look at the specific effects of the Chicxulub impact, a review article by Kring (2007, The Chicxulub impact event and its environmental consequences at the Cretaceous-Tertiary boundary, in Palaeogeography, Palaeoclimatology, Palaeoecology 255, 4-21) will help.
Any other use of the information and images requires permission of David A. Kring and the LPI.
This web site is an updated version of information originally created for the University of Arizona Space Imagery Center's Impact Cratering Series.