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.
David A. Hodell
Department of Geology, University of Florida
Click on above image to enlarge.
Download a zip file
of a full resolution TIF image