Research interets

My research field is paleomagnetism, geomagnetism and environmental magnetism. I have used paleomagnetic data and other geophysical techniques to elucidate the structural and tectonic evolution of the Mediterranean/Alpine region. For this reason, my teaching role is in the field of structural geology and tectonics. Research activity in recent years can be subdivided into four categories:

1. The application of paleomagnetism to paleogeographies in mountain belts, notably in the Alpine-Mediterranean area. The objective has been to reconstruct the pre-deformational relative position of rock bodies (on scales from individual thrust sheets to continents). The field work has been carried out in Italy, Austria, Switzerland, Spain, Slovakia and Turkey.
2. The application of magnetic polarity stratigraphy to the generation of geologic time scales. Magnetic polarity stratigraphy (the record of geomagnetic polarity reversals in sediments and sedimentary rocks) is the preferred means of global stratigraphic correlation. The method can be used to correlate biostratigraphies, cyclostratigraphies, isotope stratigraphies and radiometric age determinations.
3. Environmental magnetism and geomagnetic paleointensity: the use of rock magnetic characteristics as sedimentological tools in recent sediments (e.g. magnetic granulometry) and the use of secular variation and paleointensities for high resolution correlation of recent sediments. Modern studies of past climate require millennial-scale correlation of climate-proxy records which usually cannot be provided by the stable isotopes, biostratigraphy or radiocarbon ages. Variations in the intensity of the geomagnetic dipole field, when recorded by sediments, appear to provide the desired means of global correlation.
4. Geomagnetism: high-resolution records of geomagnetic field behavior from deep-sea sediment drifts have revolutionized our knowledge of the geomagnetic field during the last ~2 million years. As a result, we have a clearer picture of the spatial and temporal characteristics of geomagnetic (secular) variation, and the morphology of polarity transition fields. The records are now being used to constrain computer-generated models of the geodynamo.

Projects for Graduate Students

1. Paleomagnetism in Italy

   The role of Adria (the continental lithosphere encircled by the peri-Adriatic mountain belts) in Alpine-Mediterranean tectonics has been debated for over 30 years. The ìAdria problemî remains at the forefront of publications dealing with the Alpine plate kinematics (see recent paper entitled: ìAlpine Plate kinematics revisited: The Adria problemî by Wortmann et al., 2001). Argand (1924) set the stage by advocating that Adria behaved as a promontory of the African plate. This idea has since been supported by paleomagnetic data from Adria, although the issue is complicated by the thrust sheet rotation of sedimentary cover on Adria and by the lack of adequate paleomagnetic data from Africa and Adria. A ìcuspî or ìhairpinî in the Jurassic and Cretaceous apparent polar wander path (APWP) for Adria (Channell, 1996) appears to fit with the recently-published ësyntheticí African APWP (Besse and Courtillot, 2002). The two APWPs are, however, not sufficiently well defined to establish coherency or document offsets that would imply relative rotation between Adria and Africa.

   This proposal will provide paleomagnetic poles for the critical Late Jurassic-Early Creteceous interval from both Adria and Africa. Proposed sampling sites on Adria (Italy) that may be affected by thrust sheet rotation (e.g. those in Umbria) have been chosen such that Late Jurassic-Early Cretaceous strata are continuously exposed within individual thrust sheets that can be seen to be not internally deformed. This will allow the shape of the Late Jurassic-Early Cretaceous APWP to be resolved. Comparison of these APWPs with those from Gargano (believed to be unaffected by thrust sheet rotation) with allow the amount of thrust sheet rotation elsewhere on Adria to be ascertained. For this study to succeed, precision of age control will be as important as precision in paleomagnetic mean directions. Many of the proposed sections were sampled in the 1980s to yielded polarity stratigraphies and biomagnetostratigraphic correlations for timescale purposes. A different sampling strategy will be used in these sections to determine the Late Jurassic ñ Early Cretaceous APWPs, and link these APWPs to the magnetostratigraphies and hence to the geologic timescale.

   Several hundred meters of Lower Cretaceous pelagic limestones exposed on the island of Maio (Cape Verde Islands) have very similar ìMaiolicaî facies to coeval strata on Adria. This provides the opportunity for obtaining an Early Cretaceous APWP for NW Africa that can be precisely linked to a polarity stratigraphy and hence to the geologic timescale. These data will augment the very sparse African data for this time interval and provide a means of comparison with the data from Adria and with the ësyntheticí African APWP of Besse and Courtillot (2002). Intercalated basalts and hyaloclastites at the base of the pelagic section on Maio provide the possibility of linking the polarity stratigraphy to absolute 40Ar/39Ar ages for much-needed timescale calibration.

   The proposed activity will help to establish whether the stable autochthonous core of Adria has rotated relative to Africa since Early Mesozoic. The question is critical to Alpine tectonics, and hence to understanding the evolution of the Alpine belt. The project will be integrated into the learning experience at the University of Florida. The students that participate in this project (during laboratory/field work) will come into contact with a wide range of geologic data pertaining to the evolution of the Alpine belt such as sedimentary facies analysis, earthquake seismology, and geophysical evidence for crustal structure. The educational experience will be enhanced by contact with international collaborators with whom the PI has collaborated in the past (e.g. E. Erba and G. Muttoni).

2. Emerging stratigraphies: paleointensity-assisted chronostratigraphies (PACs) in the North Atlantic

   Geomagnetic excursions (short-lived directional anomalies outside the ìnormalî range of secular variation) have a ~35 year history in paleomagnetic research. The first documented excursion was the Laschamp Event, detected in the Massif Central (France) in the 1960s. The short (few thousand-year) duration of geomagnetic excursions has meant that their recognition in volcanic sequences is highly fortuitous, and their recognition in sedimentary sequences requires both high sedimentation rates and high-resolution sampling. With the recent focus on high-sedimentation-rate drift sites as paleo-environmental archives, knowledge of the sequence of geomagnetic excursions has taken on new stratigraphic importance. There is currently no consensus, other than for the Laschamp Event at ~40 ka and the Blake Event at ~120 ka, on the sequence or age of geomagnetic excursions within the Brunhes and late Matuyama Chrons. Up to 15 excursions have been advocated for the Brunhes Chron and up to 8 for the late Matuyama Chron. At least some of these excursions appear to represent brief polarity subchrons (pairs of back-to-back full polarity reversals) that are manifest globally.

   For the last glacial cycle, geomagnetic paleointensity records from marine sediments have provided a means of global high-resolution correlation. Beyond the last glacial cycle, for example in the 320-400 ka interval, geomagnetic paleointensity records from the Iceland Basin (ODP Sites 983/984) and the sub-Antarctic South Atlantic (ODP Site 1089) provide a level of correlation unobtainable from stable isotope data alone. Due to the control of geomagnetic field intensity on cosmogenic isotope production, geomagnetic paleointensity data from marine cores can be correlated with cosmogenic isotope flux measured in ice cores, thereby providing a valuable means of correlation between ice cores and the marine climate records.

   The overall aim of IODP (Integrated Ocean Drilling Program) Expedition 303 (September to November, 2004) is to place North Atlantic climate records into a millennial-scale PAC chronology for the last ~2 Myrs. Such enhanced chronologies are required to gauge millennial-scale leads and lags among diverse climate signals from different parts of the globe. An additional important objective is to document the characteristics of the geomagnetic field over the last few million years. This will involve placing geomagnetic excursions, polarity transitions (reversals) and paleointensity into a PAC framework.

   Broader significance: As records of polarity reversal revolutionized stratigraphy in the1970s, by providing globally applicable time-lines, so geomagnetic excursions and paleointensity records are likely to revolutionize high-resolution, millennial scale, stratigraphy in the future. The investigation of millennial-scale climate change requires stratigraphic methods suitable for correlation at this scale. This project will help to provide the required reference template by combining excursional, paleointensity, and oxygen isotope records. From a geophysical perspective, the reference template will provide constraints for numerical modeling of (what is now believed to be) an inherently unstable geodynamo. Inadequate knowledge of the number, sequence, and age of excursions presently hamper efforts to produce realistic models. Graduate and undergraduate students will be involved in the research. The interdisciplinary nature of the research will be of great benefit to these students in their grounding as Earth scientists.

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