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Excursions and paleointensity: Integration of magnetic and oxygen isotope stratigraphies

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A stack of relative paleointensity (RPI) data for the last 1.5 Myr utilizes 13 records, mainly from the North Atlantic but also including records from the South Atlantic and Pacific oceans. The stack differs from previous RPI stacks in that it utilizes only RPI records that have accompanying oxygen isotope data. The Match protocol of Lisiecki and Lisiecki (2002) is used to simultaneous optimize the correlation of RPI and accompanying isotope records thereby reducing the flexibility associated with correlation of RPI (or isotope) records alone. The resulting oxygen isotope stack has comparable resolution to the LR04 stack, and the RPI stack provides a useful reference template, with improved definition of 10,000-yr 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 lithologic contamination of some individual records. In this new stack, excursions and reversals occupy times of low geomagnetic paleointensity, indeed, all the more extreme RPI minima correspond to ages of adequately documented excursions or reversals implying that geomagnetic intensity has a threshold that triggers both excursions and reversals.

Although magnetic excursions were first recorded over 40 years ago, enhanced recovery of high sedimentation rate records from the deep sea has recently led to improved records. Artifacts of remanence acquisition and coring deformation have 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 the carrier of “excursional” remanence in some Arctic cores. The titanomaghemite apparently carries a partial self-reversed chemical remanence (CRM) 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 clearly recorded outside the Arctic and are an established characteristic of the Brunhes and Matuyama chrons, with about 7 excursions in each chron. The 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 “microchrons”. As excursions are certainly not unique to the Brunhes and Matuyama chrons, we might expect at least 5 excursions for each 1 Myr of reversal 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 perpetuate long enough (the uncommon case) for diffusion through the inner core to lead to long-lived field reversal (see Gubbins, 1999).

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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