The core is then taken to the laboratory where scientists analyze the chemical composition of each year, band by band. In samples of Galapagos corals, scientists look at the ratios of Cadmium
(Cd) to Calcium (Ca) and Barium (Ba) to Calcium. A high ratio of Cd/Ca and Ba/Ca indicates that more upwelling has occurred because upwelled waters have more Barium and Cadmium.
Scientists also investigate
two isotopes of oxygen that are found in the individual coral bands. Different atom numbers of the same element are called isotopes. An isotope has a different number of neutrons from the common form of the element. For example, 12C (carbon 12) has
12 neutrons. Its isotope 14C has two more neutrons than the most common form of carbon (12C). Both being present in all sea water, the light oxygen isotope is called 16O and the heavy oxygen isotope is 18O (16+2). Because the coral builds its
skeleton using sea water and the elements dissolved in sea water, both oxygen isotopes 16O and 18O are found in the coral bands. Since there is a correlation between the number of 18O present in the coral skeleton and the temperature of the water,
less 18O present than 16O indicates that the air and ocean temperatures were warmer at the time the coral formed.
To measure the relative abundance of these oxygen isotopes, the core is first ground into powder. The powder is dissolved in a
strong acid, forming a carbon dioxide (C02) gas. The C02 gas is released as an ionized stream and passed through a strong magnetic field. Within the magnetic field, the lighter C02 (C16O2) molecules are bent more than the heavier ones (C18O2). After
the two types of C02 have been separated the scientists count the different molecules using a computer and determine a ratio of d18O to d16O. This complex process provides an accurate measurement of the environmental conditions the coral was exposed to during that year. Scientists use as many clues as possible to put the pieces of past climate together.
Sarah H. Dawso
NOAA Paleoclimatology Program
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