Climate across Cretaceous–Paleogene boundary

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The climate across the

Oxygen isotopes across the K–T boundary suggest that oceanic temperatures fluctuated in the Late Cretaceous and through the boundary itself. 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. Some researchers suggest that climate change is the main connection between the impact and the extinction. The impact perturbed the climate system with long-term effects that were much worse than the immediate, direct consequences of the impact.[3]

K–Pg boundary

The K–Pg (formerly K–T) boundary is a thin band of

plesiosaurs, pterosaurs and many species of plants and invertebrates do not occur above this boundary, indicating extinction. The boundary was found to be enriched in iridium many times greater than normal (30 times background in Italy and 160 times at Stevns, Denmark), most likely indicating an extraterrestrial event or volcanic activity associated with this interval. Rates of extinction and radiation varied across different clades of organisms.[4]

Late Cretaceous to K–Pg boundary climate

Late Cretaceous climate

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

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[citation needed].

Climate across the K–Pg boundary

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[

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]

Early Paleogene climate

The

arid climate than existed before the asteroid. This is most likely due to atmospheric dust reflecting sunlight for an extended time. 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. The warmer climate increased ocean temperatures leading to a proliferation of species such as coral and other invertebrates.[citation needed
]

A 2018 published study estimated that early Palaeogene annual air temperatures, over land and at mid-latitude, averaged about 23–29 °C (± 4.7 °C), which is 5–10 °C higher than most previous estimates.[10][11] Or for comparison, 10 to 15 °C higher than current annual mean temperatures in these areas, the authors also suggest that the current atmospheric carbon dioxide trajectory, if it continues, could establish these temperatures again.[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. This global cooling has been periodically disrupted by warm events such as the Paleocene–Eocene Thermal Maximum. The general cooling trend was partly caused by the formation of the Antarctic Circumpolar Current, which significantly cooled oceanic water temperatures. The Earth's poles were cool and temperate; North America, Europe, Australia, and South America were warm and temperate; equatorial areas were warm; and the climate around the Equator was hot and arid.[citation needed]

In the Paleocene, the Earth's climate was much warmer than today's by as much as 15 °C and atmospheric CO2 was around 500 ppmV.[9]

Mass extinction theories

The events at the K–Pg boundary were the influences of several theories on how the climate change and extinction event could have taken place. These hypotheses have centered on either impact events or increased volcanism or both. The consensus among paleontologists is that the main cause was an asteroid impact that severely disrupted the Earth's biosphere causing catastrophic changes to the Earth's climate and ushering in a new era of climate and life.

Asteroid impact

Main articles: Alvarez hypothesis and Chicxulub crater

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.

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

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]

Deccan Traps

Main article: Deccan Traps

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

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]

Sea level

A theory for

epeiric seas and the expansion of freshwater environments. Although the expansion of freshwater was beneficial to freshwater vertebrates, marine environment species still suffered.[22]

Species affected

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. Photosynthesizing organisms such as phytoplankton and plants started to die out which caused herbivorous species to suffer as well because of their heavy dependency on plants for food. Consequently, many predators became extinct as well.[20]

ammonites
) became extinct or suffered great losses. 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.

insectivores and carrion-eaters survived the extinction event, due to the increased availability of their food sources. Mammals and birds that survived the extinction fed on insects, worms, and snails, which then fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus and non-living organic material.[23]

See also

  • Timeline of Cretaceous-Paleogene extinction event research

References

  1. ISSN 1017-8880. {{cite book}}: |journal= ignored (help
    )
  2. ^ Wilf, Peter, Kirk R. Johnson, and Brian T. Huber. "Correlated Terrestrial and Marine Evidence for Global Climate Changes before Mass Extinction at the Cretaceous–Paleogene Boundary." Department of Geosciences, Pennsylvania State University, 26 Nov. 2002. Web. 10 Nov. 2012.
  3. ^ Cowen, Richard (June–July 1999). "The KT Extinction". University of California, Berkeley.
  4. ISBN 9780375702617
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  5. ISBN 9780785304432
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  6. ^ MacLeod, Kenneth (29 October 2011). "Cretaceous Climate Tied to Ocean Circulation". University of Missouri.
  7. ^ Li, Liangquan, and Gerta Keller. "Maastrichtian Climate, Productivity, and Faunal Turnovers in Planktic Foramifinera in South Atlantic DSDP Sites 525A and 21." Princeton University, 21 Mar. 1997. Web. 10 Nov. 2012.
  8. ^ a b Nordt, Lee, Stacy Atchley, and Steve Dworkin. "Terrestrial Evidence for Two Greenhouse Events in the Latest Cretaceous." Baylor University, 6 Oct. 2003. Web. 10 Nov. 2012.
  9. ^ a b Scotese, Christopher R. (10 November 2012). "Paleocene Climate".
  10. S2CID 135045515
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  11. ^ University of Bristol (30 July 2018). "Ever-increasing CO2 levels could take us back to the tropical climate of Paleogene period". ScienceDaily.
  12. ^ "Ever-increasing CO2 levels could take us back to the tropical climate of Paleogene period". University of Bristol. 2018.
  13. S2CID 31638639
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  14. ^ Fassett JE, Lucas SG, Zielinski RA, Budahn JR. "Compelling new evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone San Juan Basin, New Mexico and Colorado, USA" (PDF). International Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond, 9–12 July 2000, Vienna, Austria 1053: 45–46. 2007-05-18. 2001.
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  16. ISBN 9780195353273
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  20. ^ a b MacLeod N, Rawson PF, Forey PL, Banner FT, Boudagher-Fadel MK, Bown PR, Burnett JA, Chambers, P, Culver S, Evans SE, Jeffery C, Kaminski MA, AR, Milner AC, Milner AR, Morris N, Owen E, Rosen BR, Smith AB, Taylor PD, Urquhart E, Young JR (1997). "The Cretaceous–Tertiary biotic transition". Journal of the Geological Society 154 (2): 265–292.
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  22. ^ Archibald, David; Fastovsky, David. "Dinosaur Extinction". In Weishampel David B, Dodson Peter, Osmólska Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 672–684. 2004.
  23. ^ Sheehan Peter M, Hansen Thor A (1986). "Detritus feeding as a buffer to extinction at the end of the Cretaceous". Geology 14 (10): 868–870.
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