Overview

The magnetic field is a fundamental part of the Earth system, which is tremendously important for our life too, as it shields the Earth form the dangerous Solar winds and affects satellite-based communication. The Earth’s magnetic field is generated by a magnetic dipole parallel to the spin axis. The polarity of this dipole has inverted several times in the past from currently “normal” (magnetic north pointing southward) to “reversed” (magnetic north pointing northward). The reversal process is still poorly understood. Still debated are (1) the total duration to accomplish a full reversal, thought to range between 15 and 10,000 years [e.g. Sagnotti et al., 2016], and (2) the actual mechanism of polarity flip, proposed to operate by either a gradual migration of the poles from north to south, or via variation of the magnetic field intensity [Tauxe, 2010].

It is widely accepted that sediments can effectively record the direction of the geomagnetic field thanks to a process called post-depositional remanent magnetization – PDRM [Kent, 1973]. By accumulating layer after layer, sedimentary rocks allow to track in time and space the flip of the geomagnetic field. The exceptionally well-preserved and continuous deep marine sedimentary sequence recovered by International Ocean Discovery Program (IODP) in the Philippine Sea [Arculus et al., 2015] offers a unique opportunity to analyse at a high temporal resolution the process of geomagnetic reversal. This 160 m-long sequence spans the last 25 Ma, and its age has been constrained precisely using biostratigraphic and magnetostratigraphic techniques based on the identification of 67 discrete reversals down-sequence.

A high-resolution analysis of the magnetic record of sediments from the Philippine Sea will allow to define how and how fast the magnetic field has flipped in the last 25 Ma. Duration of each reversal in sediments from the Philippine Sea compared with coeval reversals from other IODP cores recovered elsewhere will also provide important constraints on how geomagnetic field reversals operate not just in time but also in space and how we can link this fundamental process to the deeper geodynamo mechanism that generates the magnetic field in the Earth’s outer liquid core.

Methodology

Standard alternating field and thermal demagnetization experiments [Tauxe, 2010] will be employed to determine the polarity of the magnetization in the analysed sediments, using available palaeomagnetic equipment at Birmingham. The analysis will be applied to specimens from discrete intervals (~50 cm long) straddling each one of the 67 reversals identified in the sediments recovered at IODP Site U1438 [Arculus et al., 2015]. Samples for this study are already available at Birmingham. To obtain a high temporal resolution on the result, millimetre-thick slices of rock will be sub-sampled from the available specimens. Rock magnetics and scanning electron microscope analysis will then be used to define the nature and distribution of the magnetic carriers, and test the reliability of the recovered magnetic signal. Astrochronology [Hilgen, 1991] will then be employed to increase the age resolution of the analysed interval (down to thousands of years) using available magnetic susceptibility and major elements concentration data.

Training and Skills

CENTA students will attend 45 days training including a 10-day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to the student's projects and themes.

In addition to training in all different palaeomagnetic and rock magnetic techniques and data treatment, the successful candidate will learn the basics of astrochronology, consisting of time series analysis and astronomical tuning methods. Furthermore, he/she will be expected to attend workshops on astrochronology, which are periodically organised by Stephen Meyers (University of Wisconsin). He/she will also collaborate with scientists from the University of Birmingham and Utrecht University who are world-leading experts in paleomagnetism and astrochronology, gaining experience in a wide variety of geophysical techniques with application to geodynamics.

Timeline

Year 1: Background reading, learning the use of palaeomagnetic instruments and data analysis, as well as Astrochron software for time series analysis; sample preparation and paleomagnetic analysis of the top intervals of the sequence from IODP Site U1438 (Philippine Sea).

Year 2: Time series analysis and palaeomagnetic measurements of the remaining part of the sedimentary sequence. Rock magnetic and scanning electron microscope analyses.

Year 3: Palaeomagnetic measurements and time series analysis of sediments from the Gulf of Corinth recovered during IODP Expedition 381. This sequence is the ideal target to test the results obtained from the Philippine Sea as it is characterized by a high sedimentation rate, allowing a high temporal resolution on the PDRM process. Finalizing interpretation of paleomagnetic data and estimating the duration of geomagnetic reversals. Presentation of results at conferences, writing of thesis.

Partners and collaboration (including CASE)

The basics of astrochronology and the use of dedicated software (i.e. Astrochron) will be learnt by the student under co-supervision of Prof. F. Hilgen (Utrecht University) together with continuous inputs from the main supervisor Dr Maffione. Constant feedbacks revolving around the main process of post-depositional remanence magnetization (PDRM) will be provided by Prof. Wout Krijgsman (Utrecht University) who is a leading expert in magnetostratigraphy and problems associated to magnetization of sediments.

 

Further Details

Contact Dr. Marco Maffione (m.maffione@bham.ac.uk) for project specific information.  See CENTA web page for information on how to apply and general information (http://www.birmingham.ac.uk/generic/centa)