First Billion Years of the Geodynamo
The onset and nature of the geomagnetic field is important for understanding the evolution of the core, atmosphere and life on Earth. The geomagnetic field is generated in the liquid outer core, and hence is a probe of core conditions. The field also protects Earth from energetic particles streaming from the Sun (the "solar wind"); without this protective shield Earth might have developed into a dry and barren planet. A record of the early core geodynamo that generated the field is preserved in ancient silicate crystals from igneous rocks that contain minute magnetic grain inclusions. Our data indicate the presence of a geodynamo between 3.4 and 3.45 billion years ago, near the limit for the start of growth of the solid inner-core. While the magnetic field sheltered Earth's atmosphere from erosion at this time, the standoff of the solar wind was greatly reduced, and similar to that seen during modern extreme solar storms. These conditions suggest that intense radiation from the young Sun may have modified the atmosphere of the young Earth by promoting loss of light elements and water. Such effects would have been more pronounced if the field were absent prior to 3.45 billion years ago, as suggested in some hypotheses, or if an older geodynamo prior to inner core growth produced a weak field. In general, these considerations suggest the young Earth was more water-rich than today. The new frontier to learn more about these issues is obtaining geomagnetic field records that are more than 3.45 billion-years-old.
We are investigating the first billion years of geodynamo history and its implications for Earth evolution through the study of well-dated rock units from 6 ancient cores of continents (cratons). We are using a combination of existing methods of single silicate crystal magnetic measurements from in situ igneous host rocks and new approaches involving magnetic analyses of grains from sedimentary units. These measurements are achievable using highly sensitive magnetometers at the University of Rochester. Determination of the presence and strength of the geomagnetic field during the first billion years of Earth history is of broad interest to a range of scientists who study early Earth environments (atmosphere and biosphere) and the core. The investigation involves international collaboration with geologists from several countries, and multidisciplinary collaborations spanning astrophysics and space physics. Our program also integrates research and educational efforts. The study is contributing to graduate theses and undergraduates are receiving training in the field and the laboratory.
Archeomagnetism of Southern Africa: Implications for Longevity of the South Atlantic Anomaly
A broad low intensity area in Earth's recent magnetic field spans the southern Atlantic Ocean, Africa and South America. This is commonly called the South Atlantic Anomaly (SAA). The SAA allows a relatively close approach of Earth's radiation belts, affecting spacecraft operations. The low magnetic intensity decreases the efficiency of magnetic shielding in the region, which can influence atmospheric ozone. Many believe the SAA is linked to the dramatic decay of the dipole geomagnetic field intensity during the last 160 years, and the growth of an area of reversed magnetic field on Earth's core beneath South Africa. Some have even speculated Earth is heading toward a geomagnetic field reversal. But understanding these phenomena within the context of longer-term geomagnetic history has been limited by a lack of Southern Hemisphere archeomagnetic data (that is, data from archeological objects that were fired to high temperature and subsequently preserved a record of Earth's magnetic field as they cooled). Our research and education program is aimed at understanding the history of the geomagnetic field in the SAA region as recorded in southern Africa. The main focus of the work is the collection and subsequent analysis of archeomagnetic data from Iron Age burnt structures, and tests of models addressing how the nature of the boundary between Earth's core and mantle may be giving rise to recent changes seen in the geomagnetic field.
Archeomagnetic data from Iron Age sites of southern Africa (ca. 1000-1650 AD) show a sharp intensity drop at 1300 AD, at a rate comparable to modern field changes in the present-day South Atlantic Anomaly (SAA), but to lower values. The recurrence of low field values may reflect magnetic flux expulsion from the core promoted by the unusual core-mantle boundary composition and structure beneath southern Africa defined by seismology (specifically the African Large Low Velocity Seismic Province, or LLVSP). Because the African LLVSP is a longstanding structure, this region might be a steady site of flux expulsion, and perhaps the triggering site for geomagnetic reversals, on time scales of millions of years. If correct, this conceptual model is transformative because it suggests reversals do not initiate at random locations, but instead nucleate at core-mantle boundary sites that promote flux to leak upward. The model predicts modulation of the field on time scales of the lifetime of an eddy in the core flow, and will be tested by extending the archeomagnetic record of Iron Age southern Africa. The research program is integrated with undergraduate and graduate education and will be conducted with geologists and archeologists from South Africa, Botswana, and Zimbabwe.
The nature of the Ediacaran to early Cambrian geomagnetic field
The suggestion that the entire solid Earth rotated by 90 degrees, approximately 565 million years ago, and that this event sparked the explosion of life on the planet (when most existing animal phyla and classes first appeared), is one of the most controversial hypotheses in the geosciences. Ultimately, the veracity of this idea rests on how well the past geomagnetic field is recorded in ancient rock samples (the purview of the discipline of paleomagnetism). The PIs have developed a method for obtaining paleomagnetic data from geologic samples - known as the single silicate crystal approach - having higher fidelity than standard procedures. Preliminary data applying this method leads the team to an alternative hypothesis. Rather than recording a rotation of the entire solid Earth, the data reflect a geomagnetic field 565 million years ago that was in an unusual state: it was weak, and the poles reversed frequently. The team will test their hypothesis through a single silicate crystal paleomagnetic study of samples from three areas of southern Canada. Because the strength of the geomagnetic field is a principal factor defining shielding of the planet from the solar wind, this study will also help constrain the potential influence of energetic solar particles on the atmosphere and biosphere during this critical time in Earth history. The data collected will also be useful for testing if continental plate velocities were higher than uniformitarian assumptions, in the past. The work will support graduate students and undergraduates, who will acquire valuable skills though training in the classroom, field studies and in the laboratory.
The suggestion that the entire solid Earth rotated by 90 degrees during Ediacaran to early Cambrian times (approximately 635-530 Ma), and that this sparked the early Cambrian explosion of life, is highly contentious. This is not a question of whether such an event, known as inertial interchange true polar wander (IITPW), is theoretically possible, but rather whether it occurred. Ultimately, the veracity of the event relies on the fidelity of paleomagnetic data. The PI team has recently examined this enigma using single silicate crystal paleomagnetic analysis, a method that allows the isolation of single-domain magnetic carriers that are the best field recorders, capable of preserving remanences on billion-year time scales. In their study of the Sept-Iles (approximately 565 Ma) intrusion (Canada), they have not found support for IITPW. Instead, they find evidence that the geomagnetic field was reversing during cooling of the intrusion. Preliminary analyses further suggest unusually low paleointensities. These observations provide the basis for an alterative hypothesis: the Ediacaran to early Cambrian geomagnetic field was unusually weak, and reversed frequently. The team will study three areas in southern Canada to test their hypothesis. This work will also broadly constrain the boundary conditions for biotic evolution during the key Ediacaran to early Cambrian interval, and will have implications for the core, mantle and surface environment. Specifically,the investigators hope to constrain i. whether the geodynamo was in an unusual state (perhaps associated with inner core growth), ii. if the magnetopause standoff distance was reduced, allowing greater penetration of energetic solar particles, and iii. if continental plate velocities were higher than uniformitarian assumptions. The work will support graduate students and undergraduates, who will acquire valuable skills though training in field studies in southern Canada and in the laboratory. They will combine field excursions, classroom studies and summer programs to give students a comprehensive research experience.
Ultra-warm Arctic 90 million years ago
The geologic history of the Arctic region records past climatic conditions that can provide insight into modern conditions and the potential for future change. The focus of this proposed work is on an excursion within the Cretaceous greenhouse world to ultra-warm conditions, approximately 90 million years ago. Evidence for both high temperature and anomalous CO2 volcanic outgassing is found in the geology of the High Arctic. The former comes in the form of a spectacular assemblage of vertebrate fossils found on Axel Heiberg Island, including large-bodied crocodile-like champsosaurs, turtles and fish. The latter is inferred by the presence of continental flood basalts and the Alpha-Mendeleev Oceanic Ridge; these features may form one of Earth's most voluminous large igneous provinces. Multidisciplinary studies in the High Arctic improve understanding of the absolute age and paleotemperatures associated with the vertebrate fossils (carbon and oxygen isotope analyses) and the absolute age and duration of the anomalous volcanism (radiometric age and paleomagnetic analyses). With these data, we can test prior interpretations calling for high mean annual temperatures without seasonal ice, and whether contemporaneous pulses of volcanic activity could have contributed to the inferred warming by altering atmospheric CO2.
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).