Paleo Slide Set: Coral Paleoclimatology
Comparison of coral cores and sea surface temperatures (SST).

Paleoclimatologists do not have the luxury of measuring past climates directly; 
instead, they use proxy signals that follow climatic signals closely yet indirectly. 
d18O, the ratio of heavy oxygen (18O) to light oxygen (16O), is a crucial proxy signal 
and the most important result of laboratory analysis. This ratio is determined by 
the ambient water conditions (i.e. sea surface temperature and, in some locations, 
fresh water influx and precipitation) at the time when a given portion of coral skeleton 
was formed.

This graph illustrates the close correspondence between the record of d18O obtained 
from coral cores from Punta Pitt, Galapagos and instrumental measurements of sea surface 
temperatures (SST) from Puerto Chicama, Peru (8S, 79W). Notice that red spikes in the 
d18O record match up with red spikes in the SST record and with the yellow zones 
that indicate ENSO warm phases. These red areas indicate periods when water temperatures 
were above the average. Remember that in the Galapagos, high water temperatures indicate 
the eastward movement of the Pacific warm pool and the reduction of upwelling activity 
in the eastern Pacific. As this graph shows, coral d18O data is nearly as accurate as 
instrumental data; moreover, coral records can cover the past 500-800 years of climate 
change, while instrumental records are only available for the last 50-100 years 
in many tropical areas.

Photo Credits:
Thomas.G. Andrews
NOAA Paleoclimatology Program