Question

Explain (giving at least one example) how stable isotopes of oxygen (i.e. 16O and 18O) can be used to measure the paleotemperature of the ocean, specifically based on calcium carbonate in “shells?”

Answer 1

Oxygen is one of most significant keys to deciphering past climate Oxygen comes in heavy and light varieties, or isotopes, which are useful for paleoclimate research. Like all elements, oxygen is made up of a nucleus of protons and neutrons, surrounded by a cloud of electrons. All oxygen atoms have 8 protons, but the nucleus might contain 8, 9, or 10 neutrons. “Light” oxygen-16, with 8 protons and 8 neutrons, is the most common isotope found in nature, followed by much lesser amounts of “heavy” oxygen-18, with 8 protons and 10 neutrons.

The estimation of paleotemperatures can be made thanks to the δ¹⁸O measurement of the mineral phase (carbonate) of fossil eggshells. Eggshells consist of 95% of calcium carbonate, the oxygen of which is directly derived from body water, which itself is derived from the water drunk by the bird just before and at the time of the egg formation . We consider that the water consumed by birds mainly corresponds to rainwater, also called meteoric water (mw). Between each of these stages, an isotope fractionation occurs . The values of these fractionations can vary according to a large number of physiological factors (e.g. the size of the birds, or even their more or less significant adaptation to an aquatic environment . In order to limit the errors linked in particular to the size of birds, the isotope fractionations used in the context of paleotemperature estimation from large fossil birds were calculated from the study of extant ostriches . A first isotope fractionation between body water (bw) and mineralized tissues (mt; eggshell) (Δ¹⁸O_mt-bw) was calculated by means of the δ¹⁸O measurements of the eggshell of numerous extant ostriches associated with the body water δ¹⁸O of hese same birds, measured from the analysis of the δ¹⁸O of the water extracted from the white and yolk of the eggs. Δ¹⁸O_mt-bw has a value of + 30%. Thus, the δ¹⁸O value of the bird body water can be calculated from the δ¹⁸O value of eggshells (δ¹⁸O_mt) according to equation [1.17]. It is important to note that δ¹⁸O_bw is reported relative to an international standard, different from the eggshell δ¹⁸O (δ¹⁸O_mt). I