| 词条 | Mesoproterozoic |
| 释义 |
The Mesoproterozoic Era is a geologic era that occurred from {{Ma|Mesoproterozoic|Neoproterozoic}}. The Mesoproterozoic was the first period of Earth's history of which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic), but little is known about them. The continental masses of the Mesoproterozoic were more or less the same ones that exist today. Major events and characteristicsThe major events of this era are the breakup of the Columbia supercontinent, the formation of the Rodinia supercontinent, and the evolution of sexual reproduction. This era is marked by the further development of continental plates and plate tectonics. The first large-scale mountain building episode, the Grenville Orogeny, for which extensive evidence still survives, happened in this period. This era was the high point of the Stromatolites before they declined in the Neoproterozoic. The era saw the development of sexual reproduction, which greatly increased the complexity of life to come. It was the start of development of communal living among organisms, the multicellular organisms. It was an Era of apparently critical, but still poorly understood, changes in the chemistry of the sea, the sediments of the earth, and the composition of the air. Oxygen levels had risen to perhaps 1% of today's levels at the beginning of the era and continued rising throughout the Era. Clearly the era did see large quantities of organisms in at least some areas at some periods: The EIA/ARI Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States of June 2013 estimated around 194 trillion cubic feet of gas in place (ca. 44 trillion recoverable) and around 93 billion barrels of oil in place (ca. 4.7 billion recoverable) in the Lower Kyalla and Middle Velkerri formations alone of the Beetaloo Basin in Australia's Northern Territory.[1] Subdivisions{{unsourced|section|date=February 2013}}{{see also|Riphean (stage)|Jotnian}}The subdivisions of the Mesoproterozoic are arbitrary divisions based on time. They are not geostratigraphic or biostratigraphic units. The base of the Mesoproterozoic is defined chronometrically, in terms of years, rather than by the appearance or disappearance of some organism. This gives an illusory sense of certainty. Radiometric dating is a good tool, and gets better each decade.{{cn|date=February 2013}} This creates some problems. As a practical matter, radiometric dates have an error margin of 1-2%. That sounds good, but it means that two sites, both measured to be at the exact base of the Ectasian, might differ in age by over 50 My. Since the Ectasian is only 200 My long, these differences dramatically influence its timescale. And this accounts only for random error, ignoring systematic errors. Systematic errors can be caused by extraterrestrial events, by geochemical or biochemical sorting of isotopes, and human error. Thus far, biostratigraphy has usually proved considerably more exact. In addition, a thoughtful choice of biological marker can be used as a signal to expect a whole host of ecological changes. The difference between a Changhsingian and an Induan deposit isn't just a matter of a few years. The world changed significantly at the end of the Permian. By contrast, the transition from Calymmian to Ectasian has no meaning beyond calendar time. The usual reason given for the use of a chronometric system is that there is insufficient biological activity or geochemical change to find useful markers. That is a position which is now a little uncertain and is going to become increasingly tenuous over the next few years. For example, there are a number of good potential markers in the rise and decline of "Christmas tree" stromatolites, in the ebb and flow of banded iron formations, the appearance of stable carbon-13 isotope (13C) excursions, and so on. These have real meaning for the geologist and paleontologist. For that matter, they are not completely without biological markers. There has been considerable progress in studying and identifying fossil bacteria and Eukarya. The cyanobacterium Archaeoellipsoides is one relatively common form, apparently known from several species. It is probably related to the extant Anabaena and indicates the presence of significant free oxygen. Oxygen levels also had significant effects on ocean chemistry; continental weathering rates increased and provided sulfates and nitrates as nutrients. It would be remarkable if this didn't result in new populations of both bacterial and eukaryotic organisms. Since the presence of these cells would be tied directly to important geochemical events, they would make ideal organisms for biostratigraphy. For the time period from 1780 Ma to 850 Ma, an unofficial period based on stratigraphy rather than chronometry, named the Rodinian, is described in the geological timescale review 2012 edited by Gradstein et al.,[2] but {{As of|2017|02|lc=y}}, this has not yet been officially adopted by the IUGS. See also
References1. ^U. S. Energy Information Administration/Advanced Resources International, Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside of the United States, June 2013, p. III-46. The Lower Kyalla and Middle Velkerri formations are identified as Mesoproterozoic in references, p. III-49. {{clear}}2. ^{{cite book | title=The Geologic Time Scale 2012 | publisher=Elsevier | author=Gradstein, F.M. et al. (editors) | year=2012 | | volume=1 | page=361 | isbn=978-0-44-459390-0}} External links{{Commonscat|Mesoproterozoic}}{{Geological history|c|state=collapsed}}{{Authority control}} 3 : Mesoproterozoic|Geological eras|Proterozoic geochronology |
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