Ongoing Projects
Neoarchean to Early Proterozoic evolution of Earth's core: Paleomagnetic tests using dikes and sills of the Zimbabwe craton
The history of Earth's magnetic field (paleomagnetism) recorded by magnetic minerals when rocks form provides a way to probe conditions in Earth's core in the past. Earth's magnetic field also shields the atmosphere from erosion by energetic particles streaming from the Sun (the solar wind), and thus may have played an important role in the evolution of the atmosphere. We will test two recent hypotheses concerning the development of Earth's core and atmosphere by sampling a magnificent set of igneous rocks (dikes and sills) preserved in Zimbabwe. The first hypothesis suggests that the onset of growth of Earth's solid inner commenced more than 2 billion years ago. By sampling the dikes and sills and investigating their paleomagnetic signature, we will test whether they record evidence for initial growth of Earth's inner core. Earth's atmosphere was somehow transformed about 2.3 billion years ago, from mildly reducing to oxidizing conditions. The second hypothesis suggests that this change was aided by removal of hydrogen from the atmosphere by the solar wind. We will test this hypothesis by gauging the past intensity of Earth's magnetic field (and hence its atmospheric shielding capacity) through paleomagnetic analyses. Our work could lead to a transformative change in how we relate deep Earth processes and evolution of the atmosphere. The research will be integrated with educational efforts, involving graduate and undergraduate students who will receive training in the field and laboratory. We will also undertake outreach activities to communicate our results to the local Rochester community and to the wider public.
Two recent hypotheses relate the nature of the geomagnetic field to fundamental aspects of core and atmosphere evolution. In the first hypothesis, inner core growth is postulated to occur prior to 2 billion years ago, as recorded by a lower quadrupole family contribution to Archean geomagnetic secular variation. This hypothesis in turn favors a small Phanerozoic core-mantle boundary heat flow. The second hypothesis relies on new paleointensity data and solar wind estimates for the Archean. Intense solar wind from the rapidly rotating young Sun is envisioned as stripping H from Earth's atmosphere, contributing to the transformation from mildly reducing to oxidizing conditions, potentially contributing to the ∼2.3 billion-year-old Great Oxidation Event. We will examine these ideas through paleomagnetic and paleointensity studies of a magnificent record of mafic dikes and sills exposed on the Zimbabwe craton. To test prior inferences on the nature of the paleosecular variation and its potential relationship to inner core growth, we will collect paleomagnetic directional data from these units, following a major U-Pb regional dating effort by our collaborators. We focus of three time windows spanning the Great Oxidation Event: 1.89-1.88, 2.51-2.41 and 2.58 billion-years ago. To examine the hypothesis of H-loss from the atmosphere, we will conduct paleointensity analyses using single silicate minerals; these values combined with estimates of solar winds will allow us to calculate magnetopause standoff distances that are needed to evaluate atmospheric effects.
An Active Vision Approach to Understanding and Improving Visual Training in the Geosciences
Field experience is a fundamental part of the training of student geologists, but practical considerations limit the numbers of students who can take part in extensive field programs. Moreover, little is known about how novice geologists acquire the visual skills of experts, raising questions about how best to develop teaching interventions. The 5-year project investigates differences between expert and novice geoscientists in the field and in a virtual semi-immersive display environment. The research team is composed of scientists and educators with expertise in perceptual learning, geology and geophysics, the recording and analyzing of eye movements, and large-field-of-view image capture of natural environments. They hypothesize that there are large differences between the eye-movement sequences of experts and novices, and that novices will show improvement during a field trip. The researchers will study similar groups in a virtual environment, hoping to gain additional insight into learning through comparisons of the data collected in the two environments. Their ultimate goal is to design a virtual semi-immersive environment that replicates the salient aspects of the field learning experience.
Pacific Hotspot Motion
We have conducted tests of the hypothesis of hotspot fixity using paleomagnetism. In these tests, paleolatitudes are compared with the latitudes of hotspots from which they are derived. From such comparisons we derive plots of hotspot-spin axis offset versus time. These data require large-scale motion between the Atlantic, Indian and Pacific hotspot groups during the mid-Cretaceous at a velocity of approximately 30 mm/yr (Tarduno and Gee, 1995).
The rate and magnitude of this hotspot motion suggests that many concepts of plate, hotspot and true polar motion should be rethought. We are presently examining these concepts through continued paleomagnetic studies on Pacific plate sedimentary and volcanic rocks. During an Ocean Drilling Program expedition (Leg 197) to the Northwest Pacific Ocean, we collected volcanic and sedimentary samples from 3 of the Emperor Seamounts to further examine the idea that the Hawaiian hotspot may have moving in the mantle during the formation of the Emperor chain (Tarduno and Cottrell, 1997).
For a reporting of the initial results of this effort, see Motion of the Hawaiian Hotspot: A Paleomagnetic Test, Initial Reports, Proceedings of the Ocean Drilling Program, 197
A wide range of investigations including plate circuit analyses, comparisons of the age progression of coeval hotspots on the Pacific plate and geodynamic modeling are consistent with paleomagnetic results that indicate motion of hotspots in Earth’s mantle during Late Cretaceous to Paleogene times, with important changes in the rate of motion near 50 Ma. In the Pacific, the change has been hypothesized to reflect plume dynamics and hotspot-ridge capture; in the Cretaceous the two long-lived Pacific hotspots with well-defined age progressive tracks (Hawaii and Louisville) were near ridges that subsequently waned. In the case of the Hawaiian hotspot, the ridge in question appears to have become extinct close to the time of the bend in the hotspot track. Testing whether a deeper component of Pacific mantle flow also changed near 50 Ma requires a higher resolution investigation of reference frames for absolute plate motion.
Mantle-circulation models demonstrate the potential for long-term stability of the Earth's spin axis in the presence of the necessary core-mantle heat flux to generate plumes. Polar wander, the rotation of the entire solid Earth, is a geophysically plausible process having important implications for our understanding of the history of the mantle and surface processes. To gauge polar wander, one needs paleomagnetic data from geographically widespread positions; an associated problem in evaluating polar wander has been the difficulty in obtaining robust paleomagnetic data from the region represented by the Pacific Ocean basin.
On the basis of interpretations of the skewness of marine magnetic anomalies from a small portion of the Pacific plate (between the Galapagos and Clarion fracture zones), Horner-Johnson and Gordon (2010) call for significant polar wander in a fixed hotspot reference frame (called “true polar wander”, or TPW) at ~32 Ma. We test this interpretation using paleolatitude data collected from Midway Atoll, one of the oldest outcrops of the Hawaiian-Emperor (H-E) seamount chain, with an age of 27.7 Ma (Dalrymple et al., 1977).
Our preliminary paleomagnetic analyses indicate a paleolatitude of 18.7° N. The latter is consistent with the present-day latitude of the H-E hotspot and suggests little (or no) cumulative polar wander since 27 Ma. Although we feel that it is unlikely that the ~5° of TPW reported by Horner-Johnson and Gordon (2010) occurred during the nominal ~4 million year window between 32 Ma and the formation of Midway Atoll, the rate in this case would have been ~1.2°/Myr, which disagrees with the polar wander rate modeled at 30 Ma of 0.15°/Myr (Schaber et al., 2009). We suggest instead that the marine magnetic anomaly skewness data reflect oceanic crustal formation processes rather than purely paleolatitude. Overall, our results are consistent with mantle-circulation models indicating that considerable core-mantle boundary heating can occur when polar wander is weak (Schaber et al., 2009).