While the mass extinction is well documented, there is much debate about the immediate and long-term climatic and environmental changes caused by the event.
[1] The terrestrial climates at this time are poorly known, which limits the understanding of environmentally driven changes in biodiversity that occurred before the Chicxulub crater impact.
Carbon isotope measurements of benthic foraminifera at the K–T boundary suggest rapid, repeated fluctuations in oceanic productivity in the 3 million years before the final extinction, and that productivity and ocean circulation ended abruptly for at least tens of thousands of years just after the boundary, indicating devastation of terrestrial and marine ecosystems.
[3] The K–Pg (formerly K–T) boundary is a thin band of sediment that dates back to 66 million years ago, and is found as a consistent layer all over the planet in over 100 known different locations.
In addition, mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates do not occur above this boundary, indicating extinction.
In the South Atlantic, planktic foraminiferal fauna and stable carbonate and oxygen isotopes from paleosol carbonate reveal two major events: late Cretaceous diversification and mass extinction at the end of the Cretaceous, with both events accompanied by major changes in climate and productivity.
This evidence shows that many of the species' extinctions at this time related to these climate and productivity changes even without the addition of an extraterrestrial impact.
It is unclear whether continental ice sheets existed during the Late Cretaceous because of conflicting ocean temperature estimates and the failure of circulation models to simulate paleoclimate data.
But in the latter part of the epoch, the temperatures warmed significantly, resulting in the absence of glaciated poles and the presence of verdant, tropical forests.
[12] The global climate of the Paleogene transitioned from hot and humid conditions of the Cretaceous to a cooling trend which persists proceeded today, perhaps starting from the extinction events that occurred at the K–T boundary.
Dust occluded sunlight for up to six months, halting or severely impairing photosynthesis, and thus seriously disrupting continental and marine food chains.
The impact site also contained sulfur-rich sediments called evaporites, which would have reacted with water vapor to produce sulfate aerosols.
Analyses of the fluid inclusions show that oxygen levels were very high during this time; this would support evidence for intense combustion.
If global, widespread fires occurred, carbon dioxide content would have increased in the atmosphere, causing a temporary greenhouse effect once the dust cloud settled.
The theory suggests that about 66 million years ago, the mantle plume at the Réunion hotspot burned through the Earth's crust and flooded western India with basaltic lava.
[19] A theory for sea level fall in the Maastrichtian time period, the latest age of the late Cretaceous, has been proposed as evidence.
[20] A massive fall in sea level would have greatly reduced the continental shelf margin which could have caused a mass extinction but for marine species only.
[22] Species that depended on photosynthesis suffered the most as the sunlight was blocked by atmospheric particles which reduced the solar energy that reached that Earth's surface.
For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary.
Omnivores, insectivores and carrion-eaters survived the extinction event, due to the increased availability of their food sources.