On the other hand, the stable isotopes of other elements, like carbon and oxygen, can, in favorable cases, provide information about altitude, latitude, or the food that Ӧtzi ate. They also can help position Ӧtzi in regards to the watershed in which he was found. Oxygen atoms get deposited in the body when water is ingested. The oxygen isotope ratios in his body reflect the oxygen isotopic composition of rainwater falling when Ӧtzi was still alive and still drinking this water. Variation in the amounts of stable oxygen isotopes in each region occur because H218O preferentially precipitates out of clouds as air masses travel to higher elevations, further inland, and to higher latitudes (compared to H216O). This means that the remaining moisture at higher elevations, areas further inland and higher latitudes is depleted in oxygen-18 because it has already precipitated out in the process of moving to these locations. Ӧtzi was found in an area of large altitude variation. Areas north of the watershed in which he was found reflect precipitation from the Atlantic Ocean, a considerable distance away, and they are depleted in oxygen-18. The area to the south is supplied by the Mediterranean Sea, which is closer to the area in question, and the southern oxygen-18 abundances are higher than those to the north. We express these isotope ratios as a δ 18O value. This value expresses a ratio between the two stable isotopes; oxygen-18 and oxygen-16. It uses the intensities of oxygen-18 and oxygen-16 from mass spectrometer measurements to find 18O/16O sample value and then compares this ratio to a reference ratio (18O/16O standard). In the case of oxygen isotope ratios, the reference ratio is called the Vienna Standard Mean Ocean Water (VSMOW). It is calculated as follows:
