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).

Shackleton 1969

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

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