Full Vector Studies of Polarity Transitions and Excursions Recorded in Long Sequences in the Koolau Volcano, O'ahu, Hawai'i.

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Paleomagnetic data are the unique source of observations to understand the geomagnetic transitions, and hence the geodynamo processes involved in field generation. However, because reversal and excursions occur over a few thousand years at most, it is a real challenge to acquire detailed paleomagnetic information about the reversing field. and the transitional field. There is a real need to obtain reversal records from long volcanic sequences to have a better understanding of the generation of the field. Therefore it is proposed here to study six long volcanic sequences varying in thickness from 300 to 800 meters that have registered several successive reversals and excursions of the paleofield recorded in the Koolau Volcano (O'ahu, Hawaii) covering about 0.7 Ma of geomagnetic field changes. These include detailed records of the successive Gauss-Matuyama (2.57 Ma), Cryptochron C2r.2r-l (ca. 2.42-2.44 Ma), Reunion(s), Huckleberry (2.137-2.086Ma) and Lower Olduvai (1.922 Ma) reversals. The objectives of this three year proposed study are primarily related to the knowledge of the field variations during periods like reversals and excursions but are also related to the critical time intervals that precede and follow reversals, particularly for the interval between 2.6 to 1.8 Ma years that is still not very well defined in terms of the Reunion(s) and the newly reported Huckleberry Ridge events. Reversal studies cannot be separated from the surrounding polarity intervals, and most records published so far were not long enough to provide complete information. These sequences offer the first exceptional opportunity to document field changes over a time period that includes several polarity intervals, thus documenting the field behavior over various time constants. This is a basic condition to extract and bring up significant constraints for theoretical and numerical modeling of the geodynamo.Intellectual merit of the proposed activity: The present proposal is focused on directional analyses and paleointensity studies in order to achieve determinations of the total field vector. The measurements will focus on the successive periods of polarity and their associated reversals as mentioned above. Preliminary experiments of paleointensity have been conducted on one sample per lava flow from several Koolau flows using a modified version of the Thellier-Coe technique. We have imposed a severe criteria to discard samples affected by magnetomineralogical transformations as well as magnetization components involving multidomain grains. Indeed it is important to deal with paleointensity values that rely on a minimum number of five to eight determinations per lava flow (as is the case for directions) in order to perform a suitable statistical treatment with appropriate error bars.Broader impacts of the proposed activity. The principal investigator and 4 undergraduate students will carry out the research project proposed here. The involvement of undergraduate students will be a priority in this project, from field work through lab work, analysis and publication. Data will be disseminated through publications, the web and community maintained paleomagnetic databases. This project will also provide stimulus to maintain the paleomagnetics and petrofabrics operation at UH which is used by a diverse group of students and researchers.--