Sir Nicholas Shackleton

With the recent death of Sir Nicholas Shackleton, paleoclimatology lost one of its brightest pioneers. Over the last ~40 years, Nick made numerous far-reaching contributions to our understanding of how climates varied in the past, and through those studies, he identified factors that are critically important for climate variability in the future. His career neatly encompasses the birth of the new science of paleoceanography to its synthesis into the even newer science of ‘Earth Systems’; he made major contributions to these evolving fields throughout his life, and his insightful papers are required reading for students of paleoclimatology.

Fundamentally, Nick was a geologist, with a research focus on changes in ocean chemistry recorded in the marine sediments and the calcium carbonate shells of tiny organisms (foraminifera) commonly found in them. Nick was among the first to recognize that changes in the oxygen isotope ratio(¹⁸O/¹⁶O) was not simply a function of temperature, as had been previously thought, but rather a reflection of global scale ocean chemistry which changed as ice built up on the continents during glaciations. This resulted from the fractionation of oxygen isotopes in water molecules, following evaporation from the ocean surface. As water vapor is carried towards higher latitudes, and condensation occurs, the precipitation that forms contains more of the heavy isotope (¹⁸O) which is thus returned to the ocean, leaving the vapor isotopically lighter. When precipitation forms as snow, and remains on the continents to form ice sheets, the overall composition of the world ocean gradually changes, becoming isotopically heavier (enriched in ¹⁸O) compared to periods when there are no ice sheets on the continents.

Benthic foraminifera (those forams living in the deep ocean where temperatures change very little) incorporated the isotopically heavier water into their structure as they formed their shells. Thus, by measuring the oxygen isotope ratios in benthic foraminifera, Shackleton effectively had a measure of how much ice had accumulated on land—a “paleoglaciation index”. Furthermore, because the deep ocean composition is fairly well mixed, benthic forams from all parts of the ocean recorded these changes more or less synchronously. Thus, the variations could be used to correlate marine records wherever they were recovered, providing a universal index of past earth history. Variations in the oxygen isotopes gave rise to what are now termed “marine isotope stages”; we are currently in isotope stage 1 (the Holocene) and the last glaciation is represented by isotope stage 2 (when the world ocean was more enriched in the heavy isotope). Notably, Shackleton (1969) was the first to make the (correct) identification of ‘isotope stage 5e’ with the Eemian interglacial identified in land-based pollen records (see figure). At that time (~125,000 years ago), the isotopic composition of the ocean indicated there was even less ice on the continents than there is today. This corresponded to higher sea-levels (~6m higher than today) largely because the Greenland ice sheet was much diminished. (It is now thought that there was a much smaller ice cap on the island at the peak of the last interglacial; these changes were brought about by orbital variations, whereas today there are concerns that higher levels of greenhouse gases may have a similar result).

Figure 1 from Shackleton (1969) showing the breakdown of the last 120,000 years into isotopic ‘stages’.

Earlier stages show the slowly evolving nature of glaciation (and the intervening interglacials) on the earth. Nick teamed up with Neil Opdyke, a paleomagnetist who was able to recognize (and date) reversals of the earth’s magnetic field, to provide a timescale for these changes in oxygen isotopes, and this provided a chronology that could then be used to understand the frequency of glaciations and rates of change. Once a fairly reliable timescale was established, it soon became apparent that there had been regular sequences of glaciations and interglaciations that were related to orbital forcing (changes in the earth’s position in relation to the sun, as elaborated by Milankovitch). This was described in a landmark paper in Science (1976) “Variations in the earth’s orbit: pacemaker of the ice ages”, co-authored with colleagues Jim Hays and John Imbrie. From this many more studies of orbital forcing, extending far back beyond the Quaternary period also evolved from the marine oxygen isotope stratigraphy. In particular, the CLIMAP (1981) project emerged, in which paleoceanographers mapped ocean conditions at the height of the last glaciation, by identifying in each sediment core the position where the isotopes in benthic forams were most enriched (indicating maximum ice on the continents). While many of the initial CLIMAP conclusions have been substantially revised, Shackleton’s isotope chronology remains as an essential tool in understanding earth history.

Having identified a universal chronometer for continental glaciation—not in the moraines and outwash deposits that provide the direct evidence of former ice sheets, but far away in the deep ocean basins—researchers were then able to better understand the history of other terrestrial records, such as the vast loess (wind-blown silt) deposits of China, and the emerging ice core records from Antarctica and Greenland. Furthermore, since the isotopic record in the oceans registered ice growth and decay on land, it effectively provided a proxy for sea-level changes that could be checked with sea-level terraces from areas where the land had risen (such as on the Huon Peninsula of New Guinea, and in Barbados), preserving a record of past sea-level changes in the coral reefs now exposed on land. By comparing the reef-based sea-level history with the benthic foram-based record, Shackleton (with colleagues John Chappell and others) was able to assess the extent to which deep ocean temperatures may have changed over time, which would have confounded the simple paleoglaciation index he originally formulated. Shackleton subsequently argued that changes in glaciation could not be fully explained by orbital forcing, and that feedbacks involving carbon dioxide must have played a critical role, thereby highlighting the importance of changes in the concentration of greenhouse gases in the atmosphere, in the past and in the future.

Nick Shackleton’s research, arising from his original Ph.D. thesis on isotopic variations of tiny marine organisms, had an enormous impact on earth sciences. He had the insight to understand the implications of his measurements and they literally transformed how we now view earth history. For his discoveries he received many honors and awards. He was a Fellow of the Royal Society, a Foreign Member of the U.S. Academy of Sciences, a Fellow of the American Geophysical Union, and numerous other societies also honored him. He was knighted for his “services to science”; with Willi Dansgaard (who made similarly far-reaching studies of oxygen isotopes in precipitation and applied these to ice cores) he received the Crafoord Prize (considered as earth sciences’ “Nobel Prize”); he was also awarded Japan’s Blue Planet Prize, the Milankovitch Medal, and most recently the Vetlesen Prize.

Though he always considered himself, “just a geologist”, Nick had many other talents, most notably in music. He was an accomplished clarinet player and had a passionate interest in the history of that instrument. He was considered a world expert for his knowledge of antique clarinets and he had his own unique collection. At Cambridge University, where he spent his entire career, Nick gave lectures on Quaternary geology and paleoclimatology, but he also taught the physics of music. Perhaps it was his knowledge of harmonics that played a critical role in his understanding of orbital forcing and its impact on earth history. At the tri-ennial ICP conference, Nick would always organise a Paleo-musicology concert where the many musically talented scientists (and occasionally their more-talented children) would entertain the rest of us. Nick was a true Renaissance man and he leaves a very important legacy to both musicology and the earth sciences. Besides all this, he was a nice guy and an inspiration to all who came into contact with him.