The climate of the Cretaceous is less certain and more widely disputed. Average temperatures were higher than today by about 18 degrees F (10 degrees C). In fact, by the middle Cretaceous, equatorial ocean waters may have been too warm for sea life, and land areas near the equator may have been deserts, despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. Large volumes of organic matter accumulated because they were unable to decompose and were eventually deposited as black shale.
The extinction of nearly all animal species by the end of the Permian period allowed for the radiation of many new lifeforms. Large archosaurian reptiles that appeared a few million years after the Permian extinction dominated animal life during the Mesozoic era.
The climatic changes of the late Jurassic and Cretaceous provided for further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first birds and placental mammals also appeared. Angiosperms radiated sometime in the early Cretaceous period. As the temperatures in the seas increased, the larger animals of the early Mesozoic gradually began to disappear, while smaller animals of all kinds, including lizards, snakes, and perhaps the ancestor mammals to primates, evolved. The large archosaurs became extinct, while birds and mammals thrived.
During the early Mesozoic era, researchers do not conclusively predict high-latitude ice-free environments during the Cretaceous period. Various explanations have been proposed for the discrepancy, and subsequently incorporated into further modelling studies. Two of these include ocean circulation changes and the role of CO2. Of these, only an elevated atmospheric CO2 concentration could come close to reconciling the models with geological evidence. High levels of CO2 seem reasonable, considering the high global sea level and ensuing breakup of Pangea. In addition to increased outgasing of
CO2, the reduced continental area would result in a decreased rate of weathering of silicates and removal of CO2 from the atmosphere.
Unfortunately, there is little reliable evidence to support the CO2 model. M.A. Kominz and colleagues have estimated that average rates of the velocities of major tectonic plates were higher in the late Cretaceous and ocean ridge volumes were greater. In addition, Cretaceous sea-beds were dominated by calcite minerals, implying higher aqueous, and, consequently, atmospheric, CO2 concentrations.
sEE ALsO: Cretaceous Era; Jurassic Era; Triassic Era.
BIBLIOGRAPHY. H.C. Jenkyns, "Evidence for rapid climate change in the Mesozoic-Palaeogene Greenhouse World," Philosophical Transactions (Sep 15;361, 2003); Kominz M.A., Pisias N.G. "Pleistocene Climate: Deterministic or Stochastic?" Science (1979 Apr 13;204, 1979); The Meso-zoic Era, www.palaeos.com.
release. Despite this limited lifespan, it is a powerful greenhouse gas, having a global warming potential (GWP) of 23. This means that every kg. of methane emitted to the atmosphere has the equivalent forcing effect on the Earth's climate of 23 kg. of carbon dioxide over a 100-year period.
The global methane cycle is comprised of a wide range of sources and sinks. Major natural sources include wetlands, termites, the oceans, and release from hydrates (lattice-like structures of ice and methane, also known as clathrates, occurring in polar regions and in oceanic sediments). Recently, living vegetation has also been suggested as an important natural source of methane. Of known sources, wetlands dominate natural emissions, with between 100 and 200 million tons of methane emitted from these waterlogged soils each year. Wetland methane emissions arise from methane-producing bacteria known as methanogens, which are active in the anaerobic, carbon-rich environments common to wetland soils.
Fernando Herrera University of California, San Diego
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