HR: 0800h
AN: U11A-0018    [Abstracts]
TI: Relationship between the Neoproterozoic snowball Earth and Cambrian explosion
AU: * Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technolofy, 2-12-1, Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Yoshihara, A
EM: yosihara@sci.u-toyama.ac.jp
AF: Toyama University, Gofuku 3190, Toyama-shi, 930-8555, Japan
AU: Isozaki, Y
EM: isozaki@ea.c.u-tokyo.ac.jp
AF: The University of Tokyo, 3-8-1 Komaba, Tokyo, 153-8902, Japan
AB: Origin of snowball Earth has been debated in terms of greenhouse gas (e.g., Hoffman and Schrag), obliqueness of Earthfs rotation axis (Williams, 1975), true polar wander (Evans, 2003), Galactic cosmic ray radiation (Shaviv and Veizer, 2003; Svensmark, 2006), or weakened geomagnetism (Maruyama and Yoshihara, 2003). A major difficulty for the greenhouse gas hypothesis is the on-off switch causing decrease and increase of appropriate amounts of CO2 by plume- and plate tectonics, and also in available amount of CO2 in atmosphere to be consistent with the observations. In contrast, the cosmic ray radiation models due to the star burst peaked at 2.5- 2.1 Ga and 1.4-0.8 Ga can explain on-off switch more easily than the greenhouse gas model. Cosmic ray radiations, however, must be modified by the geomagnetic intensity, fluctuating 150% to < 10% of the present-day level through geologic time. Our compilation suggests the idea of extensive glaciation appeared when the intensity decreased below 50% of the present-day value, as typically seen in the Neoproterozoic time. This proposes the idea of extensive cloudiness by increased cosmic rays in the Neoproterozoic to cause the snowball Earth. Time difference between the Neoproterozoic snowball Earth and Cambrian explosion is as large as 250 millions years, and this refuses their direct close-relationship. Role of frequent mass extinctions, i.e., 8 times during 100 m.y. from 585 Ma to 488 Ma, during the Ediacaran and Cambrian, has been proposed (Zhu et al., 2007). This frequency is one order of magnitude higher compared to that in the post-Ordovician time. Yet, the Cambrian explosion cannot be explained by mass extinction which replaced the vacant niches shortly after the mass extinction and never created a new animal with a new body plan. A new model proposed herein is derived from weakened geomagnetism and resultant extensive cosmic radiation to alter gene and genome for a long period over advancement of low magnetic intensity and cosmic radiations (Svensmark, 2006) from 1.2-0.8Ga. As to the new body plans of animals, it took an appreciably long time to prepare all 34 genometypes before the apparent Cambrian explosion. Geochemically extreme conditions and widened shallow marine environment on continental shelf by the return-flow of sweater into mantle in the Late Neoptroterozoic were the associated critical conditions to alter genomes during 1.2-0.52 Ga appreciably before the final consequence called the Cambrian explosion.
DE: 0429 Climate dynamics (1620)
DE: 0444 Evolutionary geobiology
DE: 0456 Life in extreme environments
DE: 5416 Glaciation
DE: 5440 Magnetic fields and magnetism
SC: Union [U]
MN: 2007 Fall Meeting