As with lake sediments, the layers or stratigraphy of oceanic deposits and corals can give important clues to paleoclimates. Sea-floor sediments are primarily an accumulation of calcium carbonate- (CaCO3) based shells from organisms that once lived near the ocean surface. These organisms are often sensitive to changes in temperature and salinity, proliferating under optimal conditions and declining in unfavorable conditions.
Because the ocean surface is closely connected to sea-level climate conditions, the amount and type of shells found in the ocean core layers correlates well with atmospheric conditions when the organism died. Coral are small marine organisms that are also made of calcium carbonate, recording oceanic temperature conditions through their growth patterns. Like tree rings, coral thrive under certain conditions; coral are thicker during warmer, and thinner during colder, ocean episodes.
The calcium carbonate shells from ocean cores and coral are also source material for oxygen isotope anal ysis. This technique examines the ratio between two different isotopes of oxygen that are recorded in the skeletal remains of marine life. Ocean water contains two forms of oxygen molecules, the more common form with an atomic weight of 16 (16O) and the atypical variety with an atomic weight of 18 (18O). Water containing the lighter oxygen (16O) isotope evaporates more readily than its heavier counterpart. This means that during periods of extensive glaciation, the amount of 18O relative to 16O in ocean water increases as more of the lighter oxygen isotope is precipitated out as snow, becoming concentrated in glacial ice. Conversely, the 18O/16O ratio decreases during warmer interglacial episodes, when surface runoff returns the 16O previously stored in the cryosphere. Hence, oxygen isotope analysis is also useful for examining the precipitation layers of ice cores.
Taken from glaciated mountaintops and ice sheets, ice cores provide information on past climates in a variety of ways. Oxygen isotope analysis and the depth of snow accumulation in successive layers can give clues on temperature and moisture conditions. Major volcanic eruptions or meteorite impacts are often recorded as a significant layer of dust between ice layers; these episodes adversely impact surface insolation and global temperatures. Ice cores can also provide information on the past chemical composition of the atmosphere from small air bubbles trapped within. Depending on the length, ice cores enable reconstruction of past climates going back thousands of years.
SEE Also: Climate Forcing; Climatic Data, Atmospheric Observations; Climatic Data, Ice Observations; Climatic Data, Nature of the Data; Climatic Data, Proxy Records; Climatic Data, Sea Floor Records; Climatic Data, Tree Ring Records; Keeling, Charles David.
bibliography. Dennis L. Hartmann, Global Physical Climatology (Academic Press, 1994); L.D. Danny Harvey, Climate and Global Environmental Change (Prentice-Hall, 2000); Intergovernmental Panel on Climate Change, Climate Change 2001: The Scientific Basis (Cambridge University Press, 2001); Peter Robinson and Ann Henderson-Sellers, Contemporary Climatology (Prentice-Hall, 1999).
Petra A. Zimmermann Jill S. M. Coleman
Ball State University
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