Observations of the historical geomagnetic field and its secular variations from a wide variety of sources reveal transient and persistent geomagnetic features. Persistent features include the subdued secular variation in the Pacific relative to the Atlantic hemisphere, and regions of concentrated geomagnetic flux over Canada and Siberia at ~60° N latitude. Transient historical features include movement of the North Magnetic Pole and a growing region of reversed magnetic flux at the core/mantle boundary below the South Atlantic. Along with the present dipole intensity decay rate of 5% per century (five times the free decay rate), these observations have led some researchers to speculate that the geomagnetic field may be heading toward a reversal or an excursion. The question of whether the recently accelerated dipole decay is likely to continue is important to society because the magnetic field shields the earth from cosmic ray bombardment, with practical implications for telecommunications, human health, and global ecosystems. A recent proposition that the present rate of dipole decay is driven by the South Atlantic anomaly and is only a few hundred years old suggests that long-term changes could result from a series of transient centennial scale features, which are difficult to resolve in the short historical record. Understanding the interplay between transient and persistent geomagnetic features over longer time intervals in the paleomagnetic record is, therefore, likely to play a significant role in deciphering the dynamics of the geodynamo, as well as having implications for mechanisms of long-term production of cosmogenic isotopes in the atmosphere, which have been used to infer solar variability and its impact on past climate changes.
Recent paleomagnetic observations are now beginning to document transient features as abrupt sub-millennial and even sub-centennial paleomagnetic field behaviors. Paleomagnetic secular variation (PSV) records that record abrupt features are so-far limited to the Atlantic hemisphere (eastern North America, Iceland/Greenland, Europe/Middle East). Whether this reflects asymmetry of the field or a lack of available data is presently unclear. This project will test the spatial scale of abrupt geomagnetic behavior and beginning to assess PSV of the Pacific, while providing high quality data for the next generation of spherical harmonic models, through a detailed study of sediments from the Southeast Alaska continental margin. This opportunity arises because of recently collected new jumbo piston cores of extraordinarily high sedimentation rate (meters to 10's of meters per thousand years) with favorable magnetic properties, on cruse EW0408 of R/V Ewing. Paleomagnetic secular variation will be studied on eight out of 27 available jumbo piston cores using u-channel and discrete sample methods. Biogenic components suitable for radiocarbon dating are present, although severely diluted by the terrigenous sediment. Detailed chronologies based on radiocarbon will be developed using new techniques of precise accelerator 14C dating of ultra-small samples, developed at the Keck AMS Laboratory at UC Irvine. This study will, therefore, fill a significant data gap, test hypotheses on magnetic field variability and employing new methods for both high-resolution paleomagnetic measurements and fine-scale chronologic constraints that will substantially advance understanding of the dynamics of Earth's magnetic field. The data generated will contribute to differentiating the timescales of geomagnetic versus solar control on terrestrial cosmic ray flux and cosmogenic isotope production. This is significant for understanding both climatic and solar variability, and important to society because the magnetic field shields the earth from cosmic ray bombardment, with practical implications for telecommunications, human health, and global ecosystems. Regional PSV and relative paleointensity master curves will be developed and used as stratigraphic templates for a variety of marine and terrestrial paleoclimatic studies. Data will be contributed to databases and used to supplement ongoing spherical harmonic studies of the field from a region with little data. Radiocarbon results will contribute to understanding of changing reservoir ages in the North Pacific region, of interest to improving chronologies of past climate changes, and to understanding variations in the oceans carbon cycle. This project will support the training of both undergraduate honors students and graduate students.
