Overall Research Interests:
My research specialty is the prehistoric variation of the Earth's magnetic field. This includes the pattern of directional and intensity (secular) variation during intervals of stable magnetic field polarity, the evidence for anomalous excursions, the sequence of magnetic field reversals, and the field geometry during transitions. All of these aspects of paleomagnetic field variability provide insights into the magnetohydrodynamic processes active in the Earth's core, which generate the geomagnetic field. One of my long-term goals is to define as best possible the space/time structure of the geomagnetic field, to then look for evidence in that structure of recurring, distinctive features of the field that may indicate coherent dynamic elements in the core dynamo, and finally to link the observed patterns of field variation to current theoretical studies of dynamo processes in order to better understand the fundamental dynamics of dynamo activity in the Earth's core.
This overall research focus has fostered the development of four other general research interests:
1) Rock Magnetism of Sediments: Any paleomagnetic study of sedimentary rocks must evaluate the processes that produce (or subtract from) natural remanence in sediments and estimate their relative times of remanence acquisition (or loss). These processes begin at the time of deposition and may continue for an extended time thereafter as the sediment de-waters, compacts, and chemically alters.
2) Quaternary Paleoclimate/Paleoceanography: The quantification and timing of continental and oceanographic environmental variability requires multidisciplinary efforts of several kinds. I have strong interests in such problems and believe that paleomagnetism/rock magnetism can provide unique perspectives on these problems. The pattern of secular variation can be used regionally to date Quaternary sediment sequences. The pattern of intensity variation and occurrences of excursions may ultimately be used to provide additional isochrons on a global scale. These dating tools can often dramatically improve the time resolution of sediment sequences and permit more detailed studies of the paleoclimatic and paleoceanographic variability recorded within them. By understanding the rock magnetic variability in sediments, one can estimate the role of (physical and chemical) environmental factors in the sediment magnetizations and try to infer paleoclimatic/paleoceanographic causes.
3) Microplate Tectonics: I have maintained an interest in the application of paleomagnetic/rock magnetic tools to practical problems in structural geology and tectonics. I have focused on issues relatiing to the evolution of the North American Cordillera, especially evidence regarding possible large-scale strike slip motions of crustal blocks (terranes) along the Cordilleran margin.
4) Time-Series Analysis: Time series analysis and related linear or nonlinear data processing techniques are necessary for considering the broad range of paleomagnetic field variation that has occurred in the geologic past. Specific techniques include 1-D and 2-D spectral analysis, scalar and vector statistical analyses, mathematical simulation modeling, vector waveform analysis, and both linear and nonlinear (dynamical) systems analysis.
Current Research Projects:
Paleomagnetic Secular Variation/Paleoceanographic Records for the Last 200,000 Years From Marine Sediments of the Northwest Atlantic Ocean: Over the last ten years, I have developed an integrated project to study longterm secular variation and rock magnetism recorded in high-sedimentation-rate (>20 cm/kyr) marine sediments from the Blake Outer Ridge and Bermuda Rise. This work is being carried out in an informal collaboration with Dr. Tom Johnson (University of Minnesota-Duluth) and Dr. Lloyd Keigwin (Woods Hole Oceanographic Inst.). I have helped to recovered over thirty 20-meter piston cores that are easily dated by carbonate stratigraphy and oxygen isotopes to sample the last 70,000 to 200,000 years. The pattern of secular variation, which is correlatable among the cores, illustrates several key features of longterm secular variation (including relationships to several excursions) and demonstrates that PSV can provide 5-10 times the temporal resolution for regional correlation compared to oxygen isotopes. This correlation potential, when linked to the rock magnetic studies of paleoceanographic variations and related studies of Johnson and Keigwin, has begun to provide an important new higher-resolution view of paleoclimatic/paleoceanographic variations in the northwest Atlantic Ocean for the last 200,000 years.
Time-Series Analysis of Paleomagnetic Secular Variation: A long-term goal is to provide better assessment of the space/time patterns of secular variation and excursions. One method for evaluating such field variability is time series analysis. I have recently summarized the observed field variability for North America over the last 10,000 years. I have developed directional secular variation time series for the last 3000 years on a global scale. At the same time, our North Atlantic studies (see above) are beginning to provide us a directional and paleointensity record of field behavior for the last 200,000 years. Time series analysis of all of these data sets are beginning to give us a coherent picture of field characteristics that are distinctive and recurring.
Paleomagnetic/Paleoceanographic/Paleoclimatic Studies of Late Quaternary Sediments from the Western Unites States - the Marine California Continental Borderland and Lakes of California/Nevada: I have recovered Holocene PSV records and rock magnetic records from marine sediments in four basins of the California Continental Borderland. I have also collected/studied sediment cores from Owens, Mono, and Tulare Lakes in California and Pyramid Lake in Nevada. The marine studies have been carried out in cooperation with Doug Hammond, Donn Gorsline, Bob Douglas, and Lowell Stott (all from USC). The lake studies involved Owen Davis (University of Arizona), Larry Benson (USGS-Water Resources), and Joe Smoot (USGS-Geological Division). The PSV records have been used to more accurately date these sediment sequences which are the subject of several detailed paleoclimatic and paleoceanographic studies, and especially to date, with a relative accuracy of less than 200 years, several early Holocene climatic fluctuations that are seen in both the California marine and continental environment. The rock magnetic studies have been used to interpret paleoceanographic or paleolimnologic changes within the marine and lake basins. My overall goal is to develop a detailed time-stratigraphic framework for late Quaternary marine and lacustrine sediments from the southwest USA based on PSV records, and to use this framework to carefully date regional paleoclimatic and paleoceanographic fluctuations that have occurred over the last 50,000+ years.
ODP Leg 172 - High Resolution Geomagnetic Field Variability for the last 1.1 Ma: A collaborative group (myself, Gary Acton - ODP, Brad Clement - Florida Int. U., Trevor Williams - UK, Makoto Okada - Japan) has helped recover more than 2.2 km of deep sea sediment from the western North Atlantic Ocean. These replicate sediment cores span the last 1.2 Ma and contain a continuous high-resolution record of field behavior. This composite record contains normal secular variation (both directions and paleointensity), at least 12 Brunhes excursions, another 5 Matuyama-aged excursions, the Brunhes/Matuyama polarity boundary and polarity transitions surrounding the Jaramillo and Cobb Mountain events. We expect to provide a significantly better perspective on the relationship between normal field behavior, excursions, and polarity transitions than has been possible before.
High Resolution Paleomagnetic/Rock Magnetic Studies of Late Quaternary Deep-Sea Sediments from Indonesia: In collaboration with Lowell Stott (USC) we have recently received eight 35-55 meter piston cores collected from deep sea sediments of Indonesia. Preliminary paleomagnetic and rock magnetic studies suggest that these sediment may be as good as our North Atlantic sediments as paleomagnetic recorders. We hope to provide the first high-resolution records of secular variation from Equatorial latitudes and look for excursions comparable to those we see in the Atlantic as indicators of the space/time structure of excursions.
ODP Leg 202 - High Reswolution Geomagnetic field Variability for the Brunhes Chron - I am working with Joe Stoner to recover very high resolution records of PSV, paleointensity and excursions for the last ~100,000 years from Southern Hemispher esites near the Chilean Continental Margin. We are also recovering medium resolution excursion and paleointensity records from the entire brunhes Chron from the eastern Equatorial Pacific OCean.
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