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Sir Nicholas Shackleton, 1937-2006

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 microfossils 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 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.

Another very important application was to place within a known timescale the fluctuating oxygen isotope signals that Emiliani and Shackleton had defined, by generating a long core record that extended to a reversal in the Earth’s magnetic field seen in sediment cores and known to be at 780,000 years ago.

This set the stage for the most important application of the oxygen isotope method: the reconstruction of the history of global ice volume through the ice ages. Milutin Milankovitch in the 1920s had hypothesised that ice ages were caused by changes in distributions of solar radiation at the earth’s surface in turn driven by changes in movement of the earth’s orbit.

Shackleton and his US co-workers Jim Hays (of the Lamont Doherty Geological Observatory of Columbia University, where Shackleton was appointed Senior Visiting Research Fellow in 1974) and John Imbrie (of Brown University), generated long climate records from different ocean regions and subjected the patterns to mathematical analysis. The result was the famous 1976 paper in Science (“Variations in the Earth’s Orbit: pacemaker of the ice ages”) where they showed that the three periodicities with which the earth’s orbit changes (100,000 years, 40,000 years and 21,000 years) were all present just as predicted.

This clear recognition of orbital control is also now revolutionising the whole of stratigraphy (the study of geological strata) because it provides in principle a means of correlating beds at separated parts of the Earth to a precision of 20,000 years at a time of hundreds of millions of years ago, and of determining precise “orbitally tuned” age-calibrated stratigraphies back to about 30 million years.

Shackleton also pioneered the use of carbon isotopes in palaeoclimate studies. Oxygen isotope determinations are made by extracting carbon dioxide gas from the microfossils and the method also generates data for carbon isotopes, which previously was not examined. Recognising that the carbon isotopic composition of the oceans is affected by the amount of carbon stored on land, he used carbon isotopes to assess the changing land reservoir of carbon between glacial and interglacial times.

He also applied the carbon isotope method to test an idea suggested by Wally Broecker of Lamont-Doherty Observatory to explain changes in the carbon dioxide content of the atmosphere between ice ages and today. The first reports had appeared from air trapped as bubbles in ice cores that carbon-dioxide concentrations at glacial times were about 190 parts per million compared to 280 parts per million in pre-industrial times. Broecker argued that the only way this could have occurred was by the transfer of carbon to the deep ocean by increased biological productivity.

Because this process preferentially enriches the lighter 12C isotope in organic matter that sinks to the sea floor, it should produce larger differences in the heavier 13C between surface and deep seawater at glacial times as recorded in the Foraminifera shells. Shackleton’s record, “Carbon Isotope Data in Core V19-30 Confirm Reduced Carbon Dioxide Concentration in the Ice Age Atmosphere”, published in 1983 in Nature, predicted how atmospheric carbon dioxide has changed over the past 100,000 years and was found to be very similar to that obtained from bubbles in the Antarctic Vostok ice core.

Shackleton worked with co-workers from over 20 countries and his contributions were invariably characterised by a mass of new high-quality isotope data collected with the help of his laboratory manager Mike Hall. One recent paper, “The Hundred Thousand-Year Ice Age Cycle Identified and Found to Lag Temperature, Carbon Dioxide and Orbital Eccentricity”, published in Science in 2000, is unusual in showing no new data, but is one of Shackleton’s most interesting and daring contributions. Having shown early in his career that the ice-volume component of the marine oxygen isotope records is dominant over temperature, there had been no way of knowing precisely what the contributions are.

By comparing ice-core and deep records in a complex manner, he was able to separate the two contributions and showed that the ice-volume component lags behind (it responds with a delay) the changes in carbon dioxide. In other words, changes in the ice sheets do not cause changes in atmospheric carbon dioxide. Shackleton’s analysis showed the reverse: carbon dioxide played a major role in causing the changes from glacial to interglacial conditions. This is an extremely profound analysis with potentially very important ramifications for our future climate.

Nick Shackleton was awarded many honours, including the Crafoord, Vetlesen and Blue Planet prizes and the Urey medal of the European Geophysical Association. In 1998 he was knighted. In 1985, he was elected a Fellow of the Royal Society, being awarded its Royal Medal in 2003, and in 2000 a Foreign Associate of the US Academy of Sciences.

As well as his many scientific accomplishments, Shackleton excelled in another area, that of music, which was almost as important to him as science. He was a very accomplished clarinet player. He combined both loves in the musicology concert held at each International Conference of Palaeoceanography. He was very attached to his college, Clare Hall, where he strongly supported music, art exhibitions and a Quaternary discussion group.

After his first marriage, to Judith Murray, ended in divorce, and the death of his second wife, Vivien Law, in 2002, Nick obtained great happiness from his partner Ingrid Pearson, who was with him and his family and friends when he died. Shortly before his death, he had initiated the foundation of the Sir Nicholas Shackleton Fund at Clare Hall to support Visiting Fellows in the general field of palaeoclimate research to continue his work in Cambridge.

Harry Elderfield. Reprinted with permission from The Independent, Obituaries, 8 February 2006