HR: 1340h
AN: B33C-0274    [Abstracts]
TI: Cycles in Fossil Diversity: Causes and Considerations
AU: * Rohde, R A
EM: rarohde@lbl.gov
AF: Department of Physics, University of California, Berkeley, CA 94720 United States
AU: * Rohde, R A
EM: rarohde@lbl.gov
AF: Lawrence Berkeley National Laboratory, Mailstop 50R5032 1 Cyclotron Road, Berkeley, CA 94720 United States
AU: Muller, R A
EM: ramuller@lbl.gov
AF: Department of Physics, University of California, Berkeley, CA 94720 United States
AU: Muller, R A
EM: ramuller@lbl.gov
AF: Lawrence Berkeley National Laboratory, Mailstop 50R5032 1 Cyclotron Road, Berkeley, CA 94720 United States
AB: Improvements in paleontology and geochronology have made it clear that the changes in biodiversity during the Phanerozoic (0-542 Ma) are not simply random but also contain strikingly periodic features. The most prominent and compelling of these is a 62 +/- 3 Myr cycle whose declines incorporate many of the mass extinctions in Earth's history. A second, statistically ambiguous cycle with a period of 140 +/- 15 Myr also occurs. Such periodicities almost certainly indicate that an as yet unknown cyclic physical process has been having a significant influence on the Earth's environment. Having searched records of sea level, climate, glaciation, isotopic shifts, volcanism, and other environmental changes, we have found no strong evidence of a matching 62 Myr cycle, though some similarities between the changes in diversity and episodes of volcanism and/or sea level change cannot be entirely ignored. By contrast, the 140 Myr cycle matches changes at the same frequency previously reported in climate, glaciation, and cosmic ray flux. The causes of these cycles are unknown though one can speculate on the kinds of astrophysical or geophysical processes that might be capable of maintaining periodic behavior on such long time scales. The leading candidates appear to be encounters with large scale galactic structure (e.g. spiral arms) and periodic pulses of plume formation from the core-mantle boundary. Encounters with spiral arms have previously been invoked to explain the 140 Myr changes in climate but the timing of such encounters is still very uncertain. In simulations and experimental models, plume formation can exhibit periodic modes and could thus lead to periodic episodes of major volcanism of the form frequently implicated in extinctions, though as yet no compelling evidence of periodic volcanism exists. Other scenarios, such as oscillations about the galactic plane or the presence of a solar companion star, are considered, but they are regarded as less likely. However, regardless of the mechanisms, the substantial changes in the biosphere accomplished by the 62 Myr cycle must have left environmental clues that will ultimately lead us to determining its origin.
DE: 3344 Paleoclimatology
DE: 1035 Geochronology
DE: 1630 Impact phenomena
DE: 1635 Oceans (4203)
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting