Q is for Quaternary
The Quaternary is a period of geologic time which includes the last approximately 2.6 million years of Earth’s history, including the present.
The advantage of working with materials from the Quaternary Period is that materials are so new that organic molecules are often preserved. With techniques of organic chemistry, it is possible to find fossilized leaves and isolate certain molecules, such as leaf waxes.
Waxes (and other organic molecules) contain both carbon and hydrogen, which makes it possible to examine both past vegetation and past precipitation.
Fornace, K.L., Whitney, B.S., Galy, V. Hughen, K.A., and Mayle, F.E., 2016, Late Quaternary environmental change in the interior South American tropics: new insight from leaf wax stable isotopes: Earth and Planetary Science Letters, v. 438, 75-85.
In this paper, the authors examine how air masses moved across South America during the Last Glacial Maximum (LGM), the most recent time that the Earth was covered with ice sheets. They take a drill core from a lake and extract leaf waxes at intervals along its length. Ages (in thousands of years) were established using radiometric dating and examination of the species of plant leaves that were preserved.
With this, the authors are able to show that there was a large change in the types of plants and the source of vapor (that later became precipitation) as the ice sheets melted back about 12 thousand years ago.