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The water
from which continental ice sheets grow derives from evaporation of ocean
water and its deposition at high latitudes as snow. Thus, as ice sheets grow,
sea level falls. Moreover, compared with sea water, the snow is enriched in
the lighter isotope of oxygen, 16O. So, as ice sheets grow, the ratio of 18O
to 16O in the oceans increases; the ratio is generally presented as the
standardized ratio delta-18O. Organisms that precipitate skeletons of calcium
carbonate (CaCO3) do so close to oxygen isotopic equilibrium with the waters
in which they grow, so the delta-18O of fossil skeletons provides a proxy
measure for ice-sheet size in the past. However, as the equilibrium delta-18O
of the CaCO3 also depends on temperature, an unambiguous interpretation of
ice-sheet size from fossil delta-18O requires additional temperature
information. Tripati et al[4] use an independent temperature proxy -- the
amount of magnesium incorporated into CaCO3 shells -- to isolate the effects
of changing ice volume
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