Carbon is present in all biological material including skeletal remains, charcoal and food residues and plays an integral role in the dating of materials, through radiocarbon dating. The ratio of different carbon isotopes naturally fluctuates over time, and, by analysing the composition of carbon dioxide (CO) in ancient air bubbles trapped in ice cores, a chronological record of these fluctuations can be constructed. Primary producers (such as grasses) absorb and sequester CO during photosynthesis, these plants are then eaten by consumers (such as cows, and later humans) which inherit this same CO signature. Therefore, by matching the carbon isotope ratios from a sample to ratios from the ice core record, the sample can be assigned to a broad period. After death, an organism no longer absorbs CO, Cs instability causes its concentration to decrease over time  The predictable rate at which this occurs is known as an elements decay rate.