Paleomagnetic investigations of deep time: evolution and dynamics of the core, mantle and life

Thesis (Ph. D.)--University of Rochester. Department of Earth and Environmental Sciences, 2016. Study of the nature and evolution of Earth's interior is a critical aspect to understanding how life has flourished on Earth. Paleomagnetism, the study of the history of Earth's magnetic field,...

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Bibliographic Details
Main Authors: Bono, Richard K. (1988 - ), Tarduno, John A.
Format: Thesis
Language:English
Published: University of Rochester 2018
Subjects:
Online Access:http://hdl.handle.net/1802/31948
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Summary:Thesis (Ph. D.)--University of Rochester. Department of Earth and Environmental Sciences, 2016. Study of the nature and evolution of Earth's interior is a critical aspect to understanding how life has flourished on Earth. Paleomagnetism, the study of the history of Earth's magnetic field, provides a powerful set of tools to gain insight into the past development of Earth's interior. The geomagnetic field acts as a buffer against solar wind, protecting Earth's atmosphere from being stripped away by charged particles, and ultimately allowing for liquid water to be preserved on the surface. The paleomagnetic record allows for investigation into the thermal evolution of Earth's core and heat transport through the mantle over geologic time. This thesis presents the results of several studies through deep time, linking surface records of the magnetic field to activity and evolution in Earth's interior. Many of the results have implications, directly or indirectly, for the development/evolution of life on the planet. Plate motion is a reflection of mantle convection; understanding absolute plate motion requires a reference frame. It has been suggested that hotspots, surface expressions of long lived mantle plumes, may act as this reference frame. However, it has been demonstrated that hotspots are not fixed in position, complicating their use as a reference frame for plate motion. The archetypal example of this effect is the Hawaiian Hotspot. Chapter 1 details the results of a study on the Hawaiian Hotspot after its cessation of rapid motion (~47 millions of years ago, or Ma). Paleomagnetic study of cores drilled on Midway Atoll (~27 Ma) confirm that there has been little motion of the Hawaiian Hotspot after 47 Ma. These results also fail to support the suggestion that there has been significant true polar wander (motion of the entire Earth with respect to the spin axis) since 32 Ma. This result questioning true polar wander foreshadows work presented in Chapter 3. Moving back in geologic time, the Cretaceous represents a period where the increased surface expression of mantle activity (large igneous province volcanism and ridges) resulted in a greenhouse state. The Turonian (93.5-89.3 Ma) was marked by an ultra-warm period during which it has been hypothesized that a reduced thermal gradient and high atmospheric CO₂ resulted in an ice-free Arctic. Chapter 2 describes a new Cretaceous bird from the Canadian High Arctic, among one of the earliest records of modern birds in North America. It is hypothesized that the extraordinary volcanic activity (associated with ~95 Ma Strand Fiord volcanism and an associated large igneous province involving Alpha Ridge) produced an ultrathermal within an already warm greenhouse world. The associated warm Arctic environment had large freshwater lakes supporting a diverse population of freshwater fish, turtles, champsosaurs and birds. It is suggested that the widespread distribution and adaptability of these birds may have been factors in their survival through the cooler Late Cretaceous and into the Cenozoic. The Ediacaran into Cambrian (~635-521 Ma) marks a period of dramatic increase in the diversity and modes of life preserved in the faunal record, which has been called the "Cambrian Explosion". Understanding Earth’s structure and stability during this period provides a necessary context for the biotic and abiotic factors that could have led to this increase in the diversity of life. One hypothesis is that a rapid, 90° true polar wander event preceded the Cambrian Explosion. This idea was based in large part on the interpretation of contemporaneous paleomagnetic directions separated by 90° observed in the ~565 Ma Sept-îles Intrusive Suite, located in Quebec, Canada. In Chapter 3, this hypothesis is tested using single crystal paleomagnetic techniques. Study of oriented single plagioclase (the largest study using this approach to date) allows one to isolate the seminal magnetic carriers that are most likely to preserve magnetizations over a half billion years (or longer). Electron microscopy revealed single to pseudo-single domain Fe-oxide needles within plagioclase of the Sept-îles intrusives; compositional analysis determined these to be Mg-substituted magnetite (the first observation of Mg-substitution in natural Fe-oxides). Whole rock studies of Sept-îles intrusives revealed large, multidomain titanomagnetites. The latter document multiple exsolution events. A shallow, dual polarity direction carried by the single-domain bearing plagioclase crystals is interpreted to be primary in origin; the steep direction found in whole rock magnetic measurements is likely a modern overprint carried by softer, multi-domain grains. Thus, rapid true polar wander is not required to explain the paleomagnetic record and did not initiate the Cambrian Explosion of life. Chapter 4 continues study of the Sept-îles, further investigating the possible links between the observation of a rapidly reversing field, anomalously weak geomagnetic field strength, and the evolution of Earth’s core. Recent studies of the thermal and electrical conductivities of iron at core-like conditions have resulted in revisions to the time line for inner core nucleation. Prior to formation of a solid inner core, the geodynamo was driven solely by thermal convection (or possibly chemical precipitation of Mg or SiO₂ very early in Earth history); after nucleation, chemical convection associated with inner core growth becomes a dominant source of power for the geodynamo. New estimates of thermal conductivity suggest that the onset of inner core growth may have been as late as a half billion years ago. In this chapter, it is hypothesized that the rapidly reversing, weak geomagnetic field recorded in isolated plagioclase from the Sept-îles marks the formation of the inner core and the transition from a thermally-driven to chemically-driven dynamo. Furthermore, I speculate that the reduction in geomagnetic shielding would have eventually led to an increase in solar radiation to Earth’s surface, perhaps helping drive the large increase in faunal diversity observed at the start of the Cambrian. Understanding the start of the geodynamo is necessary to reconstruct the preservation of water and the start of life early in Earth’s history. Due to the increased activity of the young sun during the Hadean/Archean, the geomagnetic shield acted as a critical buffer preventing the stripping of the atmosphere by solar wind. Recovering paleomagnetic records from the Hadean/Archean is particularly challenging, given the rare preservation of rocks which can see through ubiquitous metamorphism and retain primary magnetizations. One place that might preserve a primary record of the Hadean/Archean magnetic field is the Jack Hills of Western Australia; metamorphic conditions indicate that detrital minerals hosted in the metasedimentary units could record these signals. Previous work shows that Jack Hills quartzite cobble conglomerates pass a conglomerate test, demonstrating that magnetite preserved in the cobbles retains an Archean remanence; further tests confirm these interpretations. These results have been challenged by the assertion that the Jack Hills have been affected by a pervasive 1 billion year old remagnetization which overprinted any primary remanence. However, the study asserting a pervasive overprint was compromised by several flaws; Chapter 5 discusses some of these concerns, in particular an inappropriate statistical treatment for one test which formed the crux of their interpretation. Guidelines for the proper interpretation of such records, including secular variation analysis, are provided. Further study of the Jack Hills cobble conglomerates presented in Chapter 6 focuses on lower unblocking temperature components of magnetization which may retain evidence for reheating after deposition. While the ensemble of directions isolated at low and intermediate unblocking temperatures are randomly distributed according to traditional paleomagnetic tests, distinct groupings are apparent in the data. Using a novel combination of spherical contouring and cluster analysis, records for a modern field overprint are observed at low temperatures. At intermediate temperatures, a cluster of directions is observed which is consistent with the direction reported from an inverse conglomerate test on Jack Hills metasediments, supporting the interpretation of a ~2.65 billion year old overprint. No evidence for a 1 billion year pervasive remagnetization was observed in samples collected by University of Rochester Paleomagnetic Research Group; intriguingly, this purported remagnetization was also not observed in the data presented in the study claiming a pervasive 1 billion year old overprint. The lack of a coherent direction at high unblocking temperatures, expected for the preservation of a primary magnetization, further supports the interpretation that select Jack Hills sedimentary components preserve primary magnetic signals.