Climate across Cretaceous–Paleogene boundary

The Cretaceous Period (145–66 Ma), overall, had a relatively warm climate which resulted in high eustatic sea levels and created numerous shallow inland seas. In the Late Cretaceous, the climate was much warmer than present; however, throughout most of the period, a cooling trend is apparent. The tropics were much warmer in the early Cretaceous and became much cooler toward the end of the Cretaceous.[5]

70 million years ago in the Late Cretaceous, the Earth was going through a greenhouse phase. There was abundant CO₂ in the atmosphere which resulted in global warming. A theory was proposed that ocean circulation changed direction with two water masses in the Atlantic Ocean changing direction. One of the water masses sank to the ocean floor, took direction south, and ended up in the tropical Atlantic. The other water mass replaced the first water mass on the ocean surface around Greenland which warmed the Atlantic Ocean while the rest of the ocean cooled.[6]

Stratigraphic, faunal, and isotope analyses from the very Late Cretaceous (Maastrichtian) indicate some major events. 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. About 70.5 Ma, species richness increased by 43% which coincided with major cooling in the surface and bottom waters, which increased surface productivity.[7] Between 70 and 69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric CO₂ pressures with levels of 1000–1400 ppmV and mean annual temperatures in west Texas between 21 and 23 °C. Atmospheric CO₂ and temperature relations indicate a doubling of pCO₂ was accompanied by a ~0.6 °C increase in temperature.[8] At 67.5 Ma, species richness and surface productivity began to decline, coinciding with a maximum cooling to 13 °C in surface waters. The mass extinction over the last 500,000 years marks major climatic and moderate productivity changes. Between 200 and 400 kyr before the K–T boundary, surface and deep waters warmed rapidly by 3–4 °C and then cooled again during the last 100 kyr of the Late Cretaceous. The species richness declined during the late Cretaceous cooling and 66% of species were gone by the time of the K–T boundary event.

Across the K–Pg boundary, surface productivity decreased slightly.[9] A temperature gradient of ~0.4 °C per degree of latitude is proposed for North America across the K–Pg boundary. These data of terrestrial climates and ocean temperatures may have been caused by Deccan Traps volcanic gassing, leading to dramatic global climate change. 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.

The impact pushed atmospheric CO₂ levels up from 350 to 500 ppm to approximately 2300 ppm, which would have been sufficient to warm the Earth's surface by ~7.5 °C in the absence of counter forcing by sulfate aerosols.

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.[8]

The theory with the most support to date is for an impact by one or more asteroids. The Alvarez hypothesis, proposed in 1980, gave evidence for this. Luis Alvarez and a team of researchers found sedimentary layers all over the world at the K–T boundary that contained concentrated iridium that was much higher than other sedimentary layers.[13] Iridium is extremely rare in the Earth's crust, but it is very abundant in most asteroids and comets, as asteroids have a concentration of iridium of about 455 parts per billion while the Earth's crust typically contain only about 0.3 parts per billion. They interpreted it as debris from an impact that deposited around the globe.

They concluded that the asteroid was about 9.97 kilometers in diameter which would cause an impact with about the same energy as 100 trillion tons of TNT.[14] An impact of that magnitude would then create a large dust cloud that would block sunlight and inhibit photosynthesis for many years. The dust particles in the vapor-rich impact plume ejected from the crater and rose above the Earth's atmosphere, enveloped the Earth, and then descended through the atmosphere around the planet which blocked sunlight from reaching Earth's surface. Dust occluded sunlight for up to six months, halting or severely impairing photosynthesis, and thus seriously disrupting continental and marine food chains. This would then kill most plant life and phytoplankton which would also kill many of the organisms that depended on them to survive. Sulfuric acid aerosols were also ejected into the atmosphere which blocked about 20 percent of incoming sunlight. These sulfuric aerosols would take years to fully dissipate from the atmosphere. The impact site also contained sulfur-rich sediments called evaporites, which would have reacted with water vapor to produce sulfate aerosols. Sean Gulick, a research scientist at the University of Texas, postulated that an increase in the atmospheric concentration of the sulfate compounds could have made the impact deadlier in two ways: altering climate from sulfate aerosols in the upper atmosphere having a cooling effect, and generating acid rain from water vapor that can flush the lower atmosphere of sulfate aerosols. Earlier studies had suggested both effects might result from the impact, but to a lesser degree.[15]

Many other global catastrophes could have occurred as a result of the asteroid impact. Analyses of the fluid inclusions show that oxygen levels were very high during this time; this would support evidence for intense combustion. This concludes that global firestorms may have resulted from the initial incendiary blast. If global, widespread fires occurred, carbon dioxide content would have increased in the atmosphere, causing a temporary greenhouse effect once the dust cloud settled.[16]

The Deccan Trap eruptions were associated with a deep mantle plume. 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. The basaltic lava covered over 1,609,344 square kilometers of India under successive lava flow. Volcanic gases, mostly sulfur dioxide, were released during the massive eruption which contributed to climate change worldwide. The sudden cooling due to the sulfuric gases became a major stressor on biodiversity at this time. Rapid eruption of the vast Deccan Traps lava fields would have flooded Earth's surface with CO₂, overwhelming surface systems and sinks, triggering rapid K–T transition greenhouse warming, chemical changes in the oceans and the mass extinctions.[17]

Although iridium was a major basis for the Chicxulub impact theory, it was proposed that iridium could have come from the mantle plume volcanism. The Earth's core is rich in Iridium, and it is suggested that the mantle plume transported the iridium from the core to the surface during the eruptions. In fact, the hotspot volcano that produced the Deccan traps is still releasing iridium today.[18]

It is the current consensus of the scientific community that the Deccan Traps either only contributed to the extinction along with the Chicxulub impact, or that the Chicxulub impact was the main culprit in causing the extinctions. A direct link between Deccan volcanism and the mass extinction has remained obscure due to the lack of intertrappean marine sediments with age diagnostic microfossils that contain isotope data correlating the eruptions with the extinction.[19]

A theory for sea level fall in the Maastrichtian time period, the latest age of the late Cretaceous, has been proposed as evidence. It shows that sea level fell more at this time of the Cenozoic than any time during the Mesozoic. In rock layers at this time, the earliest layers represent sea beds, later layers represented shorelines, and the latest represented continental environments. The layers do not show distortion or tilting that is related to mountains, so sea level fall is most likely the cause.[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. This regression most likely would have caused a climate change by disrupting ocean currents and winds and therefore increased global temperatures.[21] Other consequences include the loss of epeiric seas and the expansion of freshwater environments. Although the expansion of freshwater was beneficial to freshwater vertebrates, marine environment species still suffered.[22]