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About 94 million years ago was the second anoxic event, oxygen depletion of the ocean, which extinct ichthyosaurs and pliosaurs. Now scientists have found that it did not happen in one step, as previously thought, but two. An article in the Proceedings of the National Academy of Sciences says that between the two stages of reduction in the concentration of oxygen was a period of about 100 thousand years, when the oxygen content in the ocean and global carbon cycle is almost fully recovered.
In the Cretaceous period with the oceans occurred two anoxic (oxygen-free) events, during which the ocean has lost a significant amount of oxygen. The first of them — Aptian anoxic event (oceanic anoxic event 1), it happened about 120 million years ago and lasted from 1 to 1.3 million years, the second (OAE 2) occurred, according to various estimates, 91 or 94 million years ago, and was slightly shorter — about 800 thousand years. As a result of anoxia of the ocean there was a significant decrease in the oxygen concentration of a large number of deepwater areas, making the water uninhabitable for organisms that use oxygen for their vital functions, and consequently, led to mass extinction of marine animals. The majority of scientists connects the decrease of oxygen concentration with processes of global warming, an increase in the concentration of carbon dioxide in the atmosphere and activation of volcanism on the planet.
to clarify the time limits and sequence of geological phenomena during the second anoxic events, geologists from New Zealand and UK under the leadership of Matthew Clarkson (Clarkson Matthew O.) from the University of Otago analyzed the isotopic composition of uranium in oceanic and platform sedimentary rocks. Scientists have studied carbonate minerals collected in the Mediterranean and North seas and related to the investigated time interval. According to the data on the ratio of the isotopes uranium-235 and uranium-238 (supported by auxiliary data on the isotopic composition of carbon, oxygen and lithium), the authors evaluated the evolution of the decrease in the concentration of oxygen near the ocean floor. The usefulness of such analysis stems from the fact that water-soluble uranium IV are very sensitive to redox properties of the medium and oxygen-depleted water to recover uranium starts precipitating. The uranium-238 makes it much more readily so its concentration increases in the sediment and falls in the water.
Scientists have discovered that in the desired interval in the sediment content of the isotope uranium-238 is increased, but not once, but two. Over a time interval of about 100 thousand years between these drops the oxygen level is almost fully recovered. In addition, geologists were able to confirm that, in addition to lowering the oxygen concentration, also increased contents in the ocean the concentration of hydrogen sulphide. The obtained geochemical data scientists have compared with the results of biogeochemical modeling data, and linked changes in the isotopic content of uranium with possible dynamics in the global cycles of carbon and phosphorus. On the basis of these models the authors determined that during both stages of anoxic events growing concentration of carbon dioxide in the atmosphere and increased temperature.
Geologists have noted that the quantitative analysis of oxygen concentrations in ocean water and restore the exact dates anoxic event is still quite noticeable problem due to the fact that different models assume different mechanisms of this process. In particular, it is not clear how the oxygen content in the water near the ocean floor matched the water in less deep areas of the ocean. However, according to the obtained results it is possible to limit the concentration of oxygen and carbon dioxide in water at different periods of the entire event.
the Mass extinction of marine animals many scientists have linked changes in the chemical composition of ocean water and the global carbon cycle on the planet. For example, some geochemical models predict that following the mass extinction of marine animals can begin as early as the beginning of the XXII century. The greatest danger the animals with the largest body mass.
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