If you’ve ever wondered how exactly scientists produce reconstructions of past ocean and climate conditions using materials like calcite cave deposits and aragonite coral skeletons, the process is fairly straight forward. Generally, samples of the material are micromilled using a drill press, or for higher resolution sampling a computer controlled mill system. The press or mill is fit with a small drill bit (usually a diamond or carbide dental bit just like the ones that terrified us during childhood visits to the dentist).

The coral skeleton, or stalagmite is sampled along its axis of growth, to provide a sequential time series of powder samples. How fine (read: small) your sampling interval is will eventually determine how high-resolution your proxy reconstruction can resolve. Common chemical and isotope proxies in stalagmites and coral skeletons include δ¹⁸O (delta 18 Oxygen), which is the ratio of ¹⁸O to ¹⁶O in relation to an established standard (Vienna Pee Dee Belemnite for carbonate materials like calcite) as well as trace element ratios, such as Sr/Ca in coral aragonite. Isotope and chemical ratios are measured in the powder samples taken from the stalagmite or coral skeleton using  mass spectrometry, optical emissions spectroscopy, or another high precision elemental or isotopic analysis technique. These measurements can then be related back to climate and ocean processes, as the ratio of oxygen isotopes and trace elements in carbonate materials is related to environmental conditions they formed in. In the case of coral skeletons, δ¹⁸O is based on both the temperature and salinity of the waters the coral was growing in, while Sr/Ca is based on the ambient level of Sr in the ocean (which doesn’t change much on geologically short time scales) and the temperature of the water a coral grew in. In stalagmites, δ¹⁸O is affected by temperature and/or precipitation amounts. The amount of powder needed to conduct mass spectrometry and optical emission spectroscopy depend on the exact system being used, but generally are no more than 150 micrograms (150 millionths of a gram). That’s the equivalent of just 3 to 4 grains of sand!