Acceptance speech for the 2018 W.A. Johnston Medal
I am grateful to Jim Teller for thinking of me as a possible recipient of the Johnston Medal of CANQUA and to the senior members of the community who supported his nomination, namely Michael Lewis (BIO), John Shaw (GSC), Nat Rutter (U Alberta), Claude-Hillaire-Marcel (UQAM) and John Clague (Simon Fraser University). As a mathematically oriented geophysicist and climate scientist who continues to live intellectually, at least in significant part, on the extra-ordinary data from the last many decades of Quaternary research, I am especially touched by this recognition by CANQUA.
I was first attracted to work in this area in the fall of 1972, in the first week after arrival in Boulder Colorado to take up a Visiting Fellowship in the Co-operative Institute for Research in Environmental Science (CIRES) where my doctoral supervisor Colin Hines had organized a postdoctoral fellowship for me. Both Colin and several of the Permanent CIRES Fellows expected that I would work on problems involving atmospheric internal waves, on which I had also begun publishing during my time at the University of Toronto as a graduate student. Since my doctoral thesis was in fact written on the mantle convection process and associated plate tectonics, other members of the permanent fellowship were keen that I should continue to work on solid earth geophysical problems. It was in this spirit that I was approached by Christopher Harrison, a geodesist, who suggested that I look at a paper that had just appeared by Dick Walcott describing a compilation of radiocarbon dated relative sea level curves from Canadian sites that were once covered by the Laurentide ice sheet. Walcott had also written and discussed the implications of these data for mantle viscosity. He believed that these data, when taken together with the large free air gravity anomaly over the Hudson Bay region, required that the upper mantle-lower mantle viscosity contrast be high. Since the vigor of the convection process is strongly controlled by mantle viscosity, Chris was successful in capturing my attention! It was his further suggestion that I should speak to W.E. (Bill) Farrell who was also a Permanent Fellow at CIRES at the time and who had only a few years earlier completed his own doctoral thesis with Freeman Gilbert at Scripps on the problem of the surface mass loading of the elastic earth by ocean tides.
Within about 6 months of my initial year long stay at CIRES, I had devised a method whereby all of the software that had been developed by Freeman to apply to understanding of the Earth’s elastic gravitational free oscillations and which had been applied by Bill to the ocean tidal loading problem could be brought to bear on the problem of global glacial isostatic adjustment. The key turned out to involve a characteristic of linear viscoelastic field theories that is referred to in the literature as the “Correspondence Principle”. This makes it possible to map a linear viscoelastic problem onto an equivalent elastic problem to which a modification of Gilbert’s normal mode software (in the quasi-static limit) could be applied if the equivalent elastic solutions could be converted back to the viscoelastic solutions needed. The Peltier (1974) paper entitled “The impulse response of a Maxwell Earth” (Geophysical Journal of the RAS) described the needed methodology that enabled this last step to be taken. This led directly to the “Sea Level Equation” formalism that I have continued to develop, and that has since become central to all of the research on glacial isostatic adjustment and sea level change that has been ongoing over what is now approaching 50 years. As a first recognition of the scientific importance of this work by the Quaternary science community, the paper by Jim Clark, Bill Farrell and I was awarded the Kirk Bryan Award of the GSA in 1980. Jim Clark had been a doctoral student at the University of Colorado who was jointly influenced by Bill and I during my subsequent visits to CIRES from Toronto over the course of several summers. My interaction with John Andrews at INSTAAR, which was then in the same building as CIRES on 30th street, during these visits also had an important influence in my continuing development of the GIA models. Following the full year spent at CIRES, I had returned to Toronto to take up a faculty position at UofT in the Physics Department where I have since remained.
Over the subsequent years and continuing even to the present, I have continued to refine and apply the global model of the glacial isostatic adjustment process to rigorously address the problem of the depth dependence of mantle viscosity, one result of which has been the demonstration that the contrast in viscosity between the upper mantle and lower mantle is in fact low. The large free air gravity anomaly that is observed over the Hudson Bay region and that Walcott had imagined required that this contrast be high, was explicitly shown to be due to the action of the mantle convection process, not to uncompensated glacial isostatic adjustment as Walcott had assumed. This was demonstrated using the internal loading theory for the mantle convection process that Alex Forte (now at the University of Florida) had developed in his doctoral thesis with me, but it also required use of a model of the internal lateral heterogeneity of density in the mantle. This was produced using results obtained by Adam Dziewonski and his group at Harvard University based upon seismic tomographic imaging of the three dimensional structure of Earth’s interior. The paper describing this result by Peltier, Forte, Mitrovica and Dziewonski was published in GRL in 1992. It effectively undercut what had been the main argument for strong upper mantle-lower mantle viscosity contrast, a notion that has remained untenable since.
Over the previous and ensuing years I continued to work on the global problem of glacial isostatic adjustment and sea level change as well as on problems in atmospheric dynamics and ocean dynamics. Work in the latter areas was clearly required as my original position at Toronto was in the Atmospheric Physics group, later transformed to the current Earth, Atmospheric and Planetary Physics Group. My subsequent career has taken me to many places and involved interactions with many colleagues. I thought that for the current purpose there might be interest in my adding a few recollections of some of these events which involved persons whom many in the Quaternary science community would know or have known.
An especially interesting early event was the meeting in Stockholm, the papers presented at which were later compiled in the book “Earth Rheology, Isostasy and Eustasy” that was published in 1980. I was glad to meet Dick Walcott at this meeting whose paper had first stimulated my interest in the field of Quaternary research but who, by this time, had returned to New Zealand from his GSC position in Canada. I was also confronted at this meeting with the realization that the field was not heavily populated by persons like myself whose approach to understanding the earth in general, but Quaternary processes in particular, was “analytical”. Although I had extensive field experience as an undergraduate at the University of British Columbia, during which time I financed myself by working in the summers for a consulting geological engineer, staking mining claims in the Highland Valley of BC and later in the Northwest Territories (Uranium City, Fort Smith, Yellowknife), I had since transformed into a mathematical geophysicist. It was clear to me based upon the discussions at the Stockholm meeting that I would have to develop a deeper understanding of Quaternary “data” if I was to continue to make progress in understanding.
Skipping forward a few years takes me in memory back to a sabbatical spent at Cambridge University in 1987-1988 where my wife Claude and I were hosted by Nick Shackleton and his wife Vivien Law (later Lady Shackleton) at Clare Hall, where Nick was a Fellow. It was during this period that Nick offered me his newly analyzed oxygen isotopic data from the ODP 677 site in the Panama Basin. I didn’t do anything with this data until I got back to Toronto but once home a digital filtering methodology was developed which could be employed to adjust the map from depth to age in the 677 core in order to maximize the coherence between the isotopic data and orbital insolation forcing in both the obliquity and eccentricity-precession bands. The result suggested that the “dog’s leg” in the assumed depth-age map that was tuned to an assumed 730 kyr age of the Brunhes-Matuyama polarity transition should not exist. I presented this result at a meeting in Lamont at which Shackleton was in the audience. The result suggested that the Potassium Argon determined age of the transition was too young by about 50,000 years (!), a 7% error. This result was immediately checked by Ajoy Baksi, now emeritus at LSU in Baton Rouge Louisiana, using the more accurate Ar39/40 step heating method that had been developed by Derek York at Toronto. Using the same stack of volcanic rocks on the island of Hawaii that was the type section for the Brunhes-Matuyama transition, Ajoy determined its accurate age to be 780,000 years, as suggested by orbital tuning. The paper describing the original results obtained by orbital dating of the 677 core at Toronto were later checked by Andre Berger for a large number of other deep sea cores, and the Toronto results were only confirmed as correct for all cores. The result was published in a Shackleton, Berger and Peltier paper in the Proceedings of the Royal Society of Edinburgh in 1990.
In 1996 while spending the better part of the year as a visiting scholar with the Applied Mathematics Department at the University of Bristol, I was approached following a lecture there on the glacial isostasy problem, by a person who was involved with the BBC in developing a 10 part television series on Earth Science that was to be, and eventually was, called “Earth Story”. The producer of this series was to be Simon Singh, a PhD in high energy experimental physics from Oxford University who had decided to change fields and devote his efforts to science journalism and book projects. His book “Fermat’s Enigma” was a huge success. After the lecture at Bristol I was asked whether I would be willing to travel with Simon and a BBC film crew to Sweden where it was the intention to gather footage for an hour long segment of the “Earth Story” series that would deal with the importance of mantle rheology as this is constrained by the post glacial rebound process. A highlight of this trip was the visit to “Celcius rock” guided by my colleague Martin Ekman. There is a picture of Martin and I on Celcius rock in the Bay fronting the village of Lovgrunde on an island off the coast of Sweden taken by a BBC cameraman in my 1998 Reviews of Geophysics paper entitled “Postglacial variations in the Level of the Sea: Implications for Climate Dynamics and Solid Earth Geophysics”. In fact I have never seen the resulting “Earth Story” series from the BBC, but I did make contact with Simon about two years later when he presented a lecture at the Royal Ontario Museum. It was great fun reminiscing about our adventure together in Sweden.
A further career altering event occurred in 2003, one that began with a visit to the home of Gary Comer, a very wealthy individual who had once owned the American company “Land’s End” and sold it (twice) for large sums. Gary had befriended Wally Broecker who was encouraging Gary to get personally involved in climate science, which he did, to the benefit of many younger scientists whose careers he helped accelerate. After a nice dinner at Gary’s home outside Chicago, the “house” was equipped with a landing strip and a hanger housing multiple aircraft, we heard a seminar by Timothy Fisher on what was to be the target of our trip the next day to the Fort McMurray area of Alberta. Jim Teller of the University of Manitoba and Sid Hemming from Lamont were in the group as were George Denton, Tom Lowell and Tim, and of course Wally. The idea for the trip was to finally resolve the issue as to which way proglacial Lake Agassiz water took in making its way to the ocean, following diversion from its initial pathway to the south through the Mississippi River outlet to the Gulf of Mexico. This had become a major issue in the effort to understand the initiation of the Younger Dryas climate reversal. The following day we flew to Ft Mac on Comer’s private jet and once there the group used his accompanying helicopter and float plane to visit several locations that were thought to be important to solving the riddle. I do not recall anything even remotely definitive coming of this trip (I could be wrong on this) but once back in Toronto I was again interacting with my post doc Lev Tarasov (now professor at Memorial University) who was working with my model of glacial isostasy on explicit models of the glaciation-deglaciation process that included the deformation of the shape of the earth due to the changing weight of the ice load. It was natural given that the model included both ice sheets and Earth physics to try to better understand which way the water generated in deglaciation would have exited to the ocean. The model successfully captured the initial outflow to the Gulf of Mexico and the switch in direction that occurred following meltwater pulse 1A. However the switch was not to the east as Broecker had originally proposed but rather to the north into the Arctic Ocean through the MacKenzie River outlet. The Tarasov and Peltier paper entitled “Arctic freshwater forcing of the Younger Dryas cold reversal” was published in 2005 in Nature. Five years later it was followed up by the Murton et al (2010) paper, also in Nature, of which Jim Teller was a co-author, in which it was demonstrated that a high energy outflow into the Arctic Ocean had indeed occurred during the Younger Dryas interval. In work with post docs Guido Vettoretti and Marek Stastna, we showed that a freshwater outflow into the Arctic would have been as efficient at shutting down the Atlantic MOC and cooling the northern hemisphere as would an outflow directly onto the surface of the North Atlantic. A further paper has just appeared (in Nature Geoscience) in which the Arctic freshwater outflow has been recognized in Arctic Ocean sedimentary cores by Lloyd Keigwin of WHOI and others. Anne de Vernal and Claude Hillaire-Marcel had previously shown that there was no evidence for the freshwater outflow through the St Lawrence River system at YD time that Wally had originally proposed.
My only experience in going into the field with real Quaternaryists (if that is a word) occurred during a summer spent visiting the Earth Science Department of the University of Bergen in 2006, during which Jan Mangerud and John-Inge Svendsen lured me into accompanying them on a trip to a local bog from which they intended to extract a core using their Russian corer, the results from which were to help them resolve an issue related to the Younger Dryas. Unwittingly, but wanting to be a good sport, I offered to help and was given the task of “carrying the rods”. This task turned out to be more strenuous than I had imagined and I noticed that my friends were often smiling as they watched me sink knee deep into the bog, both on the way in and on the way out! Unfortunately they were able to catch my struggles on camera and now delight in showing the photograph whenever we find ourselves attending the same meeting. I’m told that they published a paper on this core in which I am acknowledged as an assistant!
I continue to find Quaternary research extremely fascinating and suggestive of the need for more and better models and “applicable mathematics”. My most recent work has included a series of climate dynamics papers on the Dansgaard-Oeschger oscillation process with Guido Vettoretti (who has just taken up a position at the Niels Bohr Institute of the University of Copenhagen), which appear to have finally explained this phenomenon for the first time in a mechanistic way. Futher new work is ongoing on the fundamental physics of the Heinrich event instability which is responsible for triggering these oscillations. Lots to do!
W. Richard Peltier