The Earth?s magnetic field is still useful for navigation, but perhaps more importantly today it forms a protective shield against cosmic radiation that otherwise would lead to further depletion of the ozone layer that protects us from UV radiation and interfere with the satellite communications so essential for society. The stronger the field, the greater the shielding. That the Earth?s magnetic field has weakened profoundly, reversed polarity hundreds of times, and attempted to do so on many other occasions, is one of the great enigmas of science. There are a variety of ways by which geoscientists advance our understanding of the magnetic field, which is widely thought to be generated by the flow of hot molten iron that swirls around in Earth?s liquid outer core to generate a magnetic dynamo. These include: (1) creation and study of analog dynamos made of molten sodium or other (dangerous) electrically conducting metals, (2) supercomputer modeling of dynamo processes that produce polarity reversals and field excursions, and (3) amassing and interpreting observations from natural rocks that record the magnetic fields of Earth?s past. This study falls into the latter category, and we have proposed to greatly expand the set of observations that bear on understanding the last reversal of Earth?s magnetic field that took place about 770 thousand years ago. This grant will permit a collaborative team of scientists to acquire new age determinations using the potassium-argon radio-isotope decay clock, as well as new information on the intensity and direction of the field, as recorded in sequences of lava flows that erupted during the last full polarity reversal of Earth?s magnetic field. The information we plan to collect will, in turn, provide fundamental data from the real Earth?s past that can help to inform and validate approaches that rely on analog or supercomputer simulations of this process.
Reversals and excursions of Earth?s geomagnetic field create global marker horizons in sedimentary and volcanic rocks and polar ice. An accurate and precise chronology of these geomagnetic field instabilities is fundamental to quantifying the timing and rates of climate change, surface processes, and biological evolution. Moreover, knowing the structural and temporal evolution of reversals and excursions is essential to understanding the geodynamic process by which reversals occur. Advances in rock magnetic measurement techniques in both sediments and lava flows, astrochronologic methods applied to sediment cores, and radio-isotopic dating of lava flows during the past two decades, using the 40Ar/39Ar variant of potassium-argon dating, has produced a number of paleomagnetic records of the most recent polarity transition from the Matuyama reversed chron to the Brunhes normal chron. Given the importance of an accurate and precise age for the the Matuyama-Brunhes reversal, our objectives are to: (1) Greatly enlarge the set of 40Ar/39Ar ages from lava flows that bear on the timing of this reversal while improving the precision and accuracy of these age determinations, (2) Test the hypothesis that the discrepancy between the astrochronologic age of 773,000 years before present recorded in sediment cores and the 40Ar/39Ar age of lava flows reflects the accuracy of the very limited 40Ar/39Ar data available from the lava flows, and (3) Determine the paleointensity of lavas that record directional changes associated with the MB reversal and its precursor on Maui and in Chile. This project is an international collaboration between scientists in Japan, the Netherlands, Denmark, and the USA. We will train undergraduate and graduate students in both state-of-the-art radio-isotopic geochronology and paleomagnetic methods in the labs at UW Madison and UC Santa Cruz, respectively. A geochronology summer short course will be held at UW for graduate and undergraduate students. Moreover, The PI will work with staff in the UW Geology museum to create a display that highlights how geologists tell time, including the use of 40Ar/39Ar dating and astrochronology.
