Coring this way has yielded up to 13 metres of sediment core, which is carefully brought to the surface. Once on deck, or back in the lab, the core is split open and samples are taken from various layers, the youngest at the top. Quartz and clays have to be sieved out to establish the amount of dust, which has been deposited at each level in the core. Shells and skeletons of marine organisms, which form the majority of the sediment, are removed in several steps of chemical treatment. Age is established by radiocarbon dating or oxygen isotope stratigraphy on fossil shells of foraminifera found in the sediment.
Radiocarbon dating is performed on a single species of foraminifera.
Planktonic foraminifera are usually used for convenience because they are more abundant than the bottom-dwellers. An expensive dating method, it nonetheless is extremely reliable to about 40,000 years. Oxygen isotope stratigraphy is an alternative dating method, which is economical, and applicable back to a million years before the present. This technique is based on the way oxygen's two isotopes, O16 and O18, behave in the water cycle. O16, being lighter, evaporates more readily than O18.