Paleoclimate
12C and 13C are measured as the isotope ratio δ13C in benthic foraminifera and used as a proxy for nutrient cycling and the temperature dependent air-sea exchange of CO2 (ventilation) (Lynch-Stieglitz et al., 1995). Plants find it easier to use the lighter isotopes (12C) when they convert sunlight and carbon dioxide into food. So, for example, large blooms of plankton (free-floating organisms) absorb large amounts of 12C from the oceans. The 12C was originally mostly incorporated into the seawater from the atmosphere. If the oceans the plankton live in are stratified (meaning that there are layers of warm water near the top, and colder water deeper down) the surface water does not mix with deeper waters very much, so that when the plankton dies it sinks and takes away 12C from the surface, leaving the surface layers relatively rich in 13C. Where cold waters well up from the depths (such as in the North Atlantic) the water carries 12C back up with it. So, when the ocean was less stratified than today, there was much more 12C in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species, coral growths rings, etc.
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