ing in water typically favor isotopically heavy oxygen and become isotopically heavier at lower temperatures. Isotopic ratios also are used to estimate the concentration of a chemical. When a chemical is common in the environment, a “favored” isotope will be used; shortage of a chemical leads to greater use of a less favored isotope. Marine photosynthesis increasingly favors the light isotope of carbon as carbon dioxide becomes more abundant, and this allows estimation of changes in carbon dioxide concentration from the isotopic composition of organic matter in oceanic sediments. Similarly, the growth of ice sheets removes isotopically light water (ordinary water) from the ocean, increasing the use of isotopically heavy oxygen from water in carbonate shells, which then provide information on the size of ice sheets over time. Stable isotopic values in organic matter also provide important information on photosynthetic pathways and so can afford insight into the photosynthesizing organisms that were dominant at a given location in the past.