Paleo Slide Set: Climate and the Classic Maya Civilization
Oxygen isotope balance of closed basin lakes.

The raw material for paleoenvironmental studies is sediment that accumulates 
in an ordered manner through time and records changes in past climate conditions. 
The sediments are analogous to a magnetic cassette tape recording, and the challenge 
for paleoclimatologists is to "play back" the tape. Fossil pollen preserved in lake 
sediments are often used to reconstruct vegetation changes that can be influenced by 
climate. However, pollen cannot be used to reconstruct climate during the Classic Period 
because the Maya severely altered regional vegetation through clear cutting of the forest 
for agricultural purposes. It would be impossible to tell, for example, whether a given 
vegetation change was caused by climate or human agricultural activity. Because of this, 
scientists mainly relied upon geochemical (elemental and isotopic) evidence for climatic change.

One of the most important tools used to reconstruct the ratio of evaporation to precipitation 
is oxygen isotopes . Lake water (H2O) contains both the light isotope (16O) and heavy isotope 
(18O) of the element oxygen. When water evaporates, the lighter isotope (H216O) evaporates at 
a faster rate than the heavier isotope (H218O) because it has a higher vapor pressure. The 
reverse happens when water condenses. As long as evaporation equals precipitation over the lake, 
the lake is at a steady state and the ratio of 18O to 16O will be constant. However, if climate 
becomes drier and evaporation exceeds precipitation, the lake volume will be reduced and the ratio
of 18O to 16O in lake water will increase. Alternatively, under wet climatic conditions, the lake 
level will rise and the ratio of 18O to 16O will decrease. In closed basin lakes, the ratio of 18O 
to 16O in lake water is controlled mainly by the balance between evaporation and precipitation. 
The 18O to 16O ratio of lake water is recorded by aquatic organisms, such as gastropods and ostracods 
that precipitate shells of calcium carbonate (CaCO3). Scientists can measure the 18O to 16O ratio in 
fossil shells in sediment cores to reconstruct changes in evaporation/precipitation through time, thus 
inferring climatic change. 

Photo Credits:
David A. Hodell 
Department of Geology, University of Florida