Geoscience Reference
In-Depth Information
Harding considered several possible explanations for the location (and
elevation) of these trees. A large tectonic event might have lowered the
southern shoreline where the trees were found or raised the outlet of the
lake. Alternatively, an extreme climate event involving an extended dry spell
might have caused the lake level to drop, allowing the trees to grow along the
former shoreline. If the drought was followed by a return to wetter condi-
tions, the lake would have risen, submerging and drowning the trees. h e tree
stumps were minimally decomposed, suggesting that, whatever the cause, the
drowning had been rapid.
Harding found little evidence to support a sudden tectonic event; no visible
signs of this could be seen in the sediments or landscape. Moreover, submerged
tree stumps had been found in other nearby lakes, suggesting a more regional
cause and leading Harding to ultimately favor the drought hypothesis. But, in
the mid-1930s, he did not yet have the tools to date the trees, and he was forced
to put aside the mystery of these tree stumps for a later time. It would be three
decades before he was able to piece together the story.
In the years following Harding's original discoveries, scientists found
more tree stumps along the southern shore of Lake Tahoe, submerged at
depths ranging from two to twelve feet below the modern water surface.
Since Lake Tahoe is the largest and second-deepest alpine lake in North
America, only a very prolonged and severe drought could have lowered its
surface level enough to allow trees to establish and survive for centuries. But
when did this devastating drought occur? Fortunately, a method of dating
fossils and artifacts containing carbon (organic carbon or carbonate) by
measuring their radioactive content was invented in the 1950s. Using this
new technique, Harding himself had the gratii cation of i nally dating the
outer-most rings of the tree stumps to determine the time of their death.
Sampling three of them, he found that the trees had been l ooded and killed
approximately 4,800 years ago, in the mid-Holocene.
re-emerging tree stumps
h e late 1980s were marked by another historic drought: California's sec-
ond most severe drought on record. Lake Tahoe's surface level dropped
again to expose more tree stumps. h is time, it was an archaeologist, Susan
Lindstrom, who noticed the tops of trees sticking out of the water along
Tahoe's southern shore. Donning scuba gear, Lindstrom was able to i nd
