Geoscience Reference
In-Depth Information
Earth scientists have devised a number of ingenious ways of assessing past
lake levels in closed basins. For instance, the discovery of ancient submerged
tree stumps rooted within some of these lakes has proved one of the most
straightforward methods. We know that most species of trees are not able to
germinate and grow with their trunks under water. h erefore, the existence
of tree stumps submerged in a lake today indicates that, in the past, when the
trees were living, the lake was much smaller than at present. A smaller lake
implies lower precipitation and greater evaporation—thus, a dryer climate
or a drought. By using the location of the tree stumps to estimate the size
of the smaller ancient lake, researchers can estimate past inl ow of streams
relative to evaporation. In addition, by counting the annual growth rings of
the submerged tree stumps, the duration of the past drought can be surmised.
h e death of the tree (when wetter conditions return, drowning the trees)
can be determined using radiocarbon dating of the outer growth rings.
At the opposite extreme, during wetter periods, the locations of ancient
lake shorelines can provide climate information. h e shorelines of once-large
lakes leave traces in the landscape in the form of wave-cut terraces, and these
can be used to determine the size of prehistoric lakes and their surface levels.
Furthermore, these terrace formations—and thus the time periods of the
larger lakes—can be dated, again with radiocarbon or other radiometric
techniques.
Recent technological advances have provided quantitative methods for
deriving past lake levels. h ese include the methods used by geochemists
on sediment cores taken from the bottom of lakes. Unlike tree stumps and
ancient shorelines, sediment cores have the potential for providing a con-
tinuous record of changing lake size, not just the extreme highs and lows.
Geochemists exploit the fact that, when a lake changes size during wetter
and drier periods, its chemistry changes, and these changes are recorded in
sediments or in the shells of organisms that precipitated from the lake water.
During drier climates, rainfall is reduced and more water evaporates of a
lake, thus concentrating the dissolved salts in the lake and causing certain
minerals, such as carbonate and gypsum, to precipitate out of the lake water.
h ese minerals then settle to the bottom, providing a layer indicative of a
drought. Similarly, certain forms of elements in lake water become more
concentrated during evaporation.
One such element that is commonly used by geochemists is oxygen.
Geochemists measure two forms, or isotopes, of oxygen (oxygen-16 and
oxygen-18) incorporated into calcium carbonate minerals that precipitate
