Thank goodness for isotopes. The conventional wisdom about the history of glacier ice used to allow for four ice ages in the geologically recent past. Then, in the 1960s and 1970s, oxygen-isotope records from ocean sediments obliged us to increase the number of ice ages, to eight in the past million years (Ma) and many more in the past three or so Ma. It also became clear — although clear should be in quotation marks — that there has been an ice sheet’s worth of ice in Antarctica, apparently continuously, since about 14 Ma. More recently, it has become clear that major glaciation began in Antarctica around 34 Ma.
But there is an increasingly persuasive argument that clear should still be in quotation marks. Kenneth Miller and co-authors argue that the isotope records suggest episodic withdrawals of water from the ocean as far back as the later Cretaceous, 100 Ma ago. The only place to put the implied amounts of withdrawn water is into ice sheets.
The argument is appealing because it does away with a long-standing puzzle. Why hasn’t Antarctica been glacierized ever since it first drifted into place over the South Pole, where it has been sitting for the past 100 Ma? Miller’s answer is simple: it has.
Roughly two oxygen atoms in every thousand are of the heavier 18O isotope, with two more neutrons and therefore 2/16ths more mass than the lighter, more abundant 16O. (The superscript to the left of the elemental symbol O is the mass number, or number of neutrons and protons in each atom, of the isotope.)
Different isotopes of the same element are chemically indistinguishable. But any process that moves stuff around, such as evaporation, is likely to be sensitive to mass. It takes more energy to move heavier objects. Water molecules with an 18O instead of a 16O tend to lag behind in the liquid reservoir. Technically, they have a lower vapour pressure. So they also tend to condense out of the vapour phase more readily.
One result of this fractionation, on the scale of global glaciology, is that an ice sheet, necessarily fashioned out of ocean water, must be isotopically light (more 16O) and the ice-age ocean correspondingly heavy (more 18O). By coring the ice sheet, or the sediment that accumulates on the ocean floor, we get highly accurate and detailed records of — what?
There is a serious complication. Fractionation depends on the temperature as well as the isotope masses.
In ice-core records, the dominant influence is temperature. In ocean sediment cores, the signal due to sequestration of ice tends to be stronger. Unlike the ice sheets, the ocean does not suffer the preferential condensation of heavy oxygen that goes on as the evaporated water makes its way, cooling as it goes, to the site of snowfall. Ocean temperature is still a major confounding factor, however. The story is preserved in fossil micro-organisms, the shells of which are usually assumed to have the isotope abundances of the water in which they lived. But when it is colder the micro-organisms prefer water (and carbonate) molecules with more 18O.
Without information from some other source, therefore, Miller and his colleagues are tackling a problem that is underdetermined, with more unknowns than equations. They draw on several such sources, but the most important is a record of sea-level changes from New Jersey. Of course one sea-level record does not establish a case, but as they cannot resolve the lowest sea-level stands very well their estimates are conservative. The extra independent data turn the exercise into a kind of intellectual, if still speculative, triangulation: a heavy-isotope excursion is likely to be glacial if it coincides with a sea-level fall, and thermal if it does not.
I put the argument for Cretaceous glaciation of Antarctica on the persuasive side of the persuasive/convincing borderline. So many factors contribute to the way the world used to be — palaeogeography, greenhouse gas concentrations and short, sharp changes of sea level are just a few — and hardly any of the evidence remains. But work like Miller’s is a fine demonstration of how tantalizing the frontier of knowledge can be. And without the isotopes we would never get anywhere at this speculative frontier.