GP34B-01 INVITED
Excursions and Paleointensity: integration of magnetic and oxygen isotope stratigraphies
Although the study of magnetic excursions began 40 years ago, the documentation of magnetic excursions has gained strength in the last 10 years ago with increased and improved recovery of high sedimentation rate records from the deep sea. 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 high degree of freedom in the correlation and labeling of "excursions", and new data implicate authigenic greigite as the carrier of "excursional" remanence in some Arctic cores. Nonetheless, magnetic excursions with durations of a few kyr, are characteristic of the Brunhes and Matuyama chrons with about 7 excursions now firmly established in each polarity 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, thereby putting them at odds with a Poisson distribution of chron durations, implying that excursions are not part of a continuum of geomagnetic behavior with polarity chrons. The fact that excursion duration tends to lie close to estimates for the magnetic diffusion time of the inner core favors the model of Gubbins (1999) 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. A new stack of relative paleointensity (RPI) data for the last 1.5 Myr utilizes 11 records, mainly from the North Atlantic but also including records from the South Atlantic and Pacific oceans. IODP Site 1308 from the North Atlantic (re-drill of DSDP Site 609) is used as the correlation target as it possesses both high quality benthic oxygen isotope and RPI records over the entire 1.5 Myr interval. The stack differs from previous 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 degree of freedom 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. The coupling of RPI and oxygen isotope records, and the recognition of RPI minima that feature excursions, provides a new template for global stratigraphic correlation that promises not only to improve stratigraphic resolution, into the realm of interest for the study of "abrupt" climate change, but also to liberate (benthic) oxygen isotope data from their chronological role and allow their regional environmental characteristics to be fully utilized.