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Gainesville, FL, United States

Meert J.G.,55 Williamson Hall | Levashova N.M.,Russian Academy of Sciences | Bazhenov M.L.,Russian Academy of Sciences | Landing E.,New York State Museum
Gondwana Research | Year: 2016

Novelty in the biological world is the culmination of genetic changes often triggered by the physical environment. The most radical phase of biological evolution took place during the Cambrian Evolutionary Radiation (CER). Prior to the CER, bacterial matgrounds and associated communities of Ediacaran organisms dominated the shallow seafloor. Near the end of the Ediacaran Period, ~ 550 million years ago, many soft-bodied biota went extinct. In the Early Cambrian, animals with the ability to burrow vertically altered the ecology of the seafloor and biomineralization became commonplace. Here we link the terminal Ediacaran extinction, the Cambrian substrate revolution and the diversification of biomineralizing organisms to changes associated with the reversal frequency of the Earth's magnetic field. Beginning around 550 Ma and continuing through much of the Cambrian, the Earth's magnetic field was rapidly reversing. Models, and limited paleointensity studies, indicate that rapid reversals are a feature of an overall weaker dipole. A weakened dipole reduces the dimensions of the magnetosphere that provides a barrier to incoming cosmic radiation. Here we show that the environmental effects of that collapse include increased dosing of UVB radiation into the shallow marine environment. Increased UVB radiation in the shallow marine environment provided selective pressure favoring organisms that could detect and avoid UVB damage by burrowing vertically, moving up or down in the water column, growing protective shells and other 'flight from light' mechanisms. These changes took place in advance of the CER, but effectively cleared the ecological space for the subsequent changes in the later Cambrian. © 2016 International Association for Gondwana Research.

Levashova N.M.,Russian Academy of Sciences | Bazhenov M.L.,Russian Academy of Sciences | Meert J.G.,55 Williamson Hall | Kuznetsov N.B.,Russian Academy of Sciences | And 3 more authors.
Precambrian Research | Year: 2013

The paleoposition of Baltica at the end of Neoproterozoic is of utmost importance for global paleogeography, but paleomagnetic data of Ediacaran age are very controversial. Neoproterozoic and Ediacaran clastic rocks are wide spread along the deformed eastern margin of Baltica. Paleomagnetic and geochronological studies were carried out at the several sections of the uppermost Zigan Formation of the Ediacaran Asha Series along with Paleozoic rocks from the same region. An ash bed interlayered in the upper part of the Asha series yields a zircon deposition age of 547.6±3.8Ma. With the aid of stepwise thermal demagnetization, a dual polarity high-temperature remanence was successfully isolated from red beds of the Zigan Fm, and its primary origin is indicated by the positive reversal test and regional consistency test. The overall mean direction of this remanence (declination D°=107.7 (287.7), inclination I°=-15.4 (15.4), radius of confidence circle α95°=4.8, N=36 sites) corresponds to a paleolatitude of 7.8°±2.5°, N or S. Geological data indicate that the study area was a part of the Baltic craton at least since the early Neoproterozoic, while paleomagnetic results on Paleozoic rocks from the westernmost zones of the Ural fold belt reveal not local and regional rotation with respect to Baltica. Also, several lines of evidence imply that the inclination shallowing in these rocks either absent altogether, or at worst less than 10°; hence the position of Baltica can be reliably reconstructed for time about 550Ma. The analysis of the existing paleomagnetic and geological data place Baltica to the east of Laurentia in tropical southern latitudes with the Uralian margin facing north in Late Ediacaran time. © 2013 Elsevier B.V.

Levashova N.M.,Russian Academy of Sciences | Bazhenov M.L.,Russian Academy of Sciences | Meert J.G.,55 Williamson Hall | Danukalov K.N.,Russian Academy of Sciences | And 3 more authors.
Gondwana Research | Year: 2015

The progress in understanding the evolution of the Earth during the Ediacaran-Cambrian is greatly hindered by the scarcity and inconsistency of paleomagnetic data for this time interval. In order to acquire new data and clarify the confusing situation, Upper Ediacaran clastic rocks of the Basu Formation were sampled at several localities in the westernmost parts of the South Urals that is the deformed margin of Baltica at least since the beginning of the Neoproterozoic. With the aid of stepwise thermal demagnetization, a dual-polarity high-temperature component (HTC) was reliably isolated from gray and maroon sandstones and siltstones at 34/49 sites. The HTC mean direction D°=55, inclination I°=-35 (k=31, α95°=4.5) corresponds to a paleolatitude of 19°±3°. The reversal and fold tests are positive for the HTC. The slump test on two meter-sized slumps shows that the HTC predates slumping in one case and is coeval with it, in the other, thus convincingly indicating the primary origin of the HTC. Also, we demonstrate that inclination shallowing is either absent altogether, or, at worst, less than 10°, in these rocks; hence the position of Baltica can be reliably reconstructed for time 560-575Ma. We reviewed paleomagnetic data with ages from 615 to 530Ma for Baltica and Laurentia and come to the conclusion that there is still no uncontestable scenario for the opening of the Iapetus Ocean that is based on non-controversial geologic and paleomagnetic data. © 2014 International Association for Gondwana Research.

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