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Climate across Cretaceous–Paleogene boundary
The climate across the Cretaceous–Paleogene boundary (K–Pg or formerly the K–T boundary) is very important to geologic time as it marks a catastrophic global extinction event. Numerous theories have been proposed as to why this extinction event happened including an asteroid known as the Chicxulub asteroid, volcanism, or sea level changes. 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.〔Taylor, Kyle W.R., Christopher J. Hollis, and Rich D. Pancost. "Reconstructing Post Cretaceous/Paleogene Boundary Climate and Ecology at Mid-Waipara River and Branch Stream, New Zealand." Berichte Geol., 5 June 2011. Web. 10 Nov. 2012.〕 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.〔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.〕 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 foramifinera 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.〔Cowen, Richard. "THE K-T EXTINCTION." The KT Extinction. University of Berkeley, California, June–July 1999. Web. 10 Nov. 2012. .〕
==K–Pg boundary==
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. K and T are the abbreviations for the Cretaceous and Tertiary periods, respectively, but the name Tertiary has been replaced by "Paleogene" as a formal time or rock unit by the International Commission on Stratigraphy, and Pg is now the abbreviation. This boundary marks the start of the Cenozoic Era. Non-avian dinosaur fossils are found only below the K–Pg boundary which indicates that they went extinct at this event. In addition, mosasaurs, 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.〔Fortey, R. ''Life: A Natural History of the First Four Billion Years of Life on Earth''. Vintage. 1999. pp. 238–260.〕
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