John A. Tarduno
The 2017 Petrus Peregrinus Medal is awarded to John A. Tarduno in recognition of the creative and innovative character of his research, including his seminal studies on the evolution of the early Earth’s magnetic field.
John Tarduno has made important contributions across a wide spectrum of palaeo-, mineral and rock magnetism and their applications to diverse problems in the geosciences. These include terrane accretion, plate and hotspot motions, sulfate-reduction diagenesis in deep-sea sediments, and magnetic self-reversal in sea-floor basalt due to extreme oxidation of titanomagnetite. As a co-chief scientist of a 2001 Ocean Drilling Program cruise (Leg 197), Tarduno used palaeomagnetism to show a southward motion of the Hawaiian hotspot shattering our previous understanding, taught in many introductory Earth science courses and geology textbooks, that the Hawaiian–Emperor seamount chain was the result of a fixed mantle source of magma burning its way through a moving Pacific plate. However the most exciting of his contributions is his use of innovative palaeomagnetic techniques to characterise the geodynamo, especially the strength of the ancient field. Tarduno pioneered a method using single crystals of plagioclase feldspars from lava flows, the motivation being that the magnetic minerals encased in early-crystallising silicate mineral grains are often less easily altered during weathering, mild metamorphism, and laboratory heating than those that reside in the fine-grained matrix of the whole rock of the ancient field. This method is an experimental tour-de-force, involving series of heating experiments and magnetic measurements on tiny samples with very weak magnetic moments in a custom-designed, small-bore cryogenic magnetometer of unsurpassed sensitivity that he developed with the late William Goree of 2G-Enterprises. Applying these techniques to Archaean age crustal rocks from South Africa, Tarduno revealed the existence of an external geomagnetic field at 3.2 Ga and in a second study, a magnetic field at 3.45 Ga, and more recently a fingerprint in rocks from Australia of the geomagnetic field, possibly as early as 4.2 Ga. The discovery that the geomagnetic field was established in the first billion years of Earth history with a form that is approximately similar to the modern geomagnetic field has significance on multiple levels. Perhaps the most overarching is that they demonstrate that the flow of energy from the deep interior of the Earth – with mantle convection being the main agent – was, back then, comparable to what it is now. Tarduno’s career is peppered with significant contributions to Earth system science. Each study highlights a similarly patient, rigorous and creative approach to testing fundamental assumptions about planetary processes. His work presents a formidable challenge for understanding the evolution of the Earth as a planet.