PROFESSOR SIR NICHOLAS SHACKLETON ; Professor of Quaternary Palaeoclimatology at Cambridge Who Identified Ice-Age Cycles
Elderfield, Harry, The Independent (London, England)
Nicholas Shackleton was a pioneer of the study of past climates, whose research identified ice-age cycles and the role of greenhouse gases, and provided important lessons in climate change.
The son of Professor Robert Millner Shackleton FRS, an eminent field geologist, he graduated from Clare College, Cambridge, in Physics in 1961 after which, as a result of what he later called "a series of random events", he followed his father into geology, but in a different direction. The random events were associated with the suggestion made, in about 1960, by the head of Geodosy and Geophysics at Cambridge University, Edward Bullard, that Harry Godwin, then head of the Sub-Department of Quaternary Research, should set up a laboratory in Cambridge to measure stable isotopes.
Nick Shackleton was the person selected to get the project started, and he stayed at Cambridge until his retirement as Professor of Quaternary Palaeoclimatology in 2004.
The reason for Bullard's suggestion related to the work of the Nobel Laureate Harold Urey. In 1947 Urey had published calculations which predicted that the heavy isotope of oxygen (18O) would be fractionated from its light isotope (16O) as a function of temperature. He suggested that this would provide a method to estimate temperatures in the geological past, from analysis of fossil shells composed of calcium carbonate minerals. He assembled a group of talented scientists who designed a mass spectrometer to test his theory and in the early 1950s demonstrated that it was correct.
Among this team was Cesar Emiliani who, because of his background in micropalaeontology (the study of microscopic fossils), went on to apply the techniques developed to tiny micro-fossils called Foraminifera recovered from deep-sea cores. Emiliani identified cycles of warm and cold sea-surface temperatures back to over half a million years' because of this work, Emiliani is often thought of as the founder of palaeoceanography.
The work carried out by Emiliani was extremely laborious and Shackleton realised that, to set up a successful laboratory, he needed to develop a mass spectrometer an order of magnitude more sensitive than that developed by Urey's team. He accomplished this as part of his thesis work and in 1967 he received his PhD for a dissertation entitled "The Measurement of Palaeotemperatures in the Quaternary Era".
Shackleton made oxygen isotope measurements on shells of fossil Foraminifera that lived in bottom waters and those from surface waters and, from a comparison, he saw a fatal flaw in Emiliani's work. The changes in isotopic composition were about the same, yet Emiliani had interpreted his results as an eight degrees change from the last ice age to today. This was clearly impossible: the deep- sea water temperature today is less than about two degrees Celsius' the dominant cause of oxygen isotope variations was not temperature, but was changes in the oxygen isotope composition of the oceans caused by removal of isotopically depleted water to form the ice sheets. In a spirit that typified Shackleton's generosity throughout his career he wrote in his 1967 Nature paper reporting this crucial result:
It should be emphasised that the time sequence which Emiliani has been able to obtain... remains of inestimable value... in a sense it is enhanced by the certainty that it is a time sequence for terrestrial glacial events rather than oceanographic events.
One such application for this time sequence was to identify the horizon of the last ice age in ocean cores worldwide which provided the temporal framework for a large US-driven project in which Shackleton participated, called Climap, to generate a global map of sea-surface temperature. The map was used by modellers to reconstruct atmospheric circulation in glacial times and as a boundary condition in models that explored changes in atmospheric temperature, of crucial importance for modelling future climate. …