by Emil Morhardt
When did the first ice on Earth occur? About the only way to find out is to find old rocks with evidence of glaciation then determine exactly how old they are. A type of rock characteristic of glaciation is diamictite, a conglomerate-like mix of rocks with a large range of sizes held together with clay or mud that has been metamorphosed into mudstone. The large range of intermixed sizes in these deposits indicates lack of the size sorting that occurs in a river bed or floodplain, so some other source of disruption must have occurred, one of which is being scraped off and bulldozed along by a glacier. David Zakharov at the University of Oregon with a team of scientists from around the world (Zakharov et al., 2017) reasoned that if they could find examples of this type of rock that had formed near the equator, and could demonstrate that the water the rock encountered during formation came from glacial meltwater, then, they would have proven that at the time there were glaciers near the equator.
Such a mixture of rocks occurs in the Belomorian belt in Russia on the Baltic Shield which, although as far north as Norway now, was once nearly at the equator as the Earth’s tectonic plates moved about. At that time—the Paleoproterozoic, 2.45 to 2.22 billion years ago—there was also extensive tectonic rifting with hot magma intruding into the surrounding rocks. In the process it encountered water and the resulting hydrothermal interactions formed silicates, which incorporated oxygen from the water…and as the authors proceeded to show, this water almost certainly came from melting glaciers.
The researchers used a recently documented technique using properties of that oxygen to demonstrate that indeed these diamictites came from glaciers which were near the equator at the time. These are not only the earliest glaciers yet identified, their presence near the equator implies a largely frozen Earth.
The technique consists of measuring the relative amounts of two stable isotopes of oxygen, 18O and 17O, expressing the ratio in parts per thousand (‰ or per mil) and reporting it as delta18O. In these rocks the delta18O is very low (–27‰) and is characteristic of oxygen found in glacial water with a delta18O of –40‰, water which was deposited as rain at –40º C (which, as a matter of general interest is also –40º F, a glacial temperature and the only point at which the two temperature scales coincide.) The authors’ process is a little more complicated than this because they employ a further ratio of the two isotopes and a regression line derived from the different O18 concentrations to get all their observations on the same page, but it is the average delta18O that matters.
The result is an elegant demonstration that there were glaciers near the equator and hence a snowball-like Earth very early on.
Zakharov, D., Bindeman, I., Slabunov, A., Ovtcharova, M., Coble, M., Serebryakov, N., Schaltegger, U., 2017. Dating the Paleoproterozoic snowball Earth glaciations using contemporaneous subglacial hydrothermal systems. Geology 45, 667-670.