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SUMMARY:Excursions and paleointensity: Integration of magnetic and oxygen 
 isotope stratigraphies - Jim Channell\, University of Florida
DTSTART:20100525T153000Z
DTEND:20100525T163000Z
UID:TALK24108@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:A stack of relative paleointensity (RPI) data for the last 1.5
  Myr utilizes 13 records\, mainly from the North Atlantic but also includi
 ng records from the South Atlantic and Pacific oceans. The stack differs f
 rom previous RPI stacks in that it utilizes only RPI records that have acc
 ompanying oxygen isotope data.  The Match protocol of Lisiecki and Lisieck
 i (2002) is used to simultaneous optimize the correlation of RPI and accom
 panying isotope records thereby reducing the flexibility associated with c
 orrelation of RPI (or isotope) records alone.  The resulting oxygen isotop
 e stack has comparable resolution to the LR04 stack\, and the RPI stack pr
 ovides a useful reference template\, with improved definition of 10\,000-y
 r scale features relative to earlier stacks. Power at orbital periods that
  has been detected in many RPI records is virtually absent in the stack\, 
 supporting the contention that orbital power in RPI records is due to lith
 ologic contamination of some individual records.  In this new stack\, excu
 rsions and reversals occupy times of low geomagnetic paleointensity\, inde
 ed\, all the more extreme RPI minima correspond to ages of adequately docu
 mented excursions or reversals implying that geomagnetic intensity has a t
 hreshold that triggers both excursions and reversals.\n\nAlthough magnetic
  excursions were first recorded over 40 years ago\, enhanced recovery of h
 igh sedimentation rate records from the deep sea has recently led to impro
 ved records.  Artifacts of remanence acquisition and coring deformation ha
 ve led to numerous spurious “excursions” being added to the excursion 
 inventory.  This is particularly true for the Arctic oceans where lack of 
 traditional stratigraphic tools leads to a flexibility in the correlation 
 and labeling of “excursions”. New data implicate titanomaghemite as th
 e carrier of “excursional” remanence in some Arctic cores. The titanom
 aghemite apparently carries a partial self-reversed chemical remanence (CR
 M) acquired during sediment diagenesis\, a process facilitated by the low 
 concentration of labile organic matter in the Arctic oceans.  Nonetheless\
 , magnetic excursions (with durations apparently less than 1 kyr) are clea
 rly recorded outside the Arctic and are an established characteristic of t
 he Brunhes and Matuyama chrons\, with about 7 excursions in each chron.  T
 he better quality (least smoothed) excursion records exhibit close to 180 
 degrees of directional change\, and are globally manifest\, implying that 
 excursions should be considered as short-lived polarity chrons or “micro
 chrons”.  As excursions are certainly not unique to the Brunhes and Matu
 yama chrons\, we might expect at least 5 excursions for each 1 Myr of reve
 rsal history or more than 300 during the Cenozoic. The fact that excursion
  duration tends to lie close to estimates for the magnetic diffusion time 
 of the inner core favors the model in which outer core reversal must perpe
 tuate long enough (the uncommon case) for diffusion through the inner core
  to lead to long-lived field reversal (see Gubbins\, 1999). \n
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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