What information is hiding at the bottom of Lake Tahoe
Special to the Bonanza
December 16, 2005
Historic info about the climate and geology of the Tahoe Basin, the Sierra Nevada, and even the West Coast probably doesn't cover much more than a century or two. And yet the forces that shaped Lake Tahoe and the Sierra acted over thousands and millions of years. Geologists have been able to piece together much of the geological history by unearthing layers of sandstone and granite and analyzing their characteristics. But, much is still a mystery. And a lot of that mystery could be unraveled by studying the sediment that lies on the bottom of Lake Tahoe.
Under the auspices of the DOSECC (Drilling, Observations and Sampling of the Earth's Continental Crust) consortium, a group of scientists met last summer to plan a program to unlock the mysteries at the bottom of Lake Tahoe. Additional support for this effort was supplied by UC-Davis, UC-San Diego, Desert Research Institute, UNR and the USGS.
UC-Davis geology professor Ken Verosub is leading the effort to obtain cores of Tahoe's sediments. He points out that the sediments at the bottom of Lake Tahoe are estimated to be some 1,500 feet thick and contain materials as old as two-million years. Cores clear through the sediment layer would provide a rich source of data on Tahoe's geologic and climatic conditions during the Quaternary period. Cores of the top 10 to 30 feet could inform us about conditions during the Holocene period, the 12,000 years since the end of the last Ice Age.
What can we learn from such cores? This is the boundary transition zone for the North American and Pacific plates. Displacements in the sediment layers could provide information on seismic events, and perhaps about the formation of the Tahoe basin. Some seismological data are already available because the USGS has provided a topographic map of the surface of the sediments. But, deeper faults are still hidden.
We know that climate varies in cycles. Some cycles are several years long, like the current wet and dry periods at the lake. Some, perhaps, are centuries long, or even longer. For example, hurricane frequency cycles are thousands of years long. That's been recently confirmed by studies of marsh sediments in South Carolina.
Verosub explains that "understanding the climate record in Tahoe is key to understanding the climate record in the Sierras, in California, and the west coast. There's a societal relevance to that," he adds. "We're already having global change and we don't really know what that's going to produce for California. There are models that predict a decrease in rainfall. Others predict that there would be an increase, maybe even in Southern California.
"But we don't really know, and we don't have a good climate record. We have good climate records from the Pacific Ocean, but we don't have comparable records on land. The question is: how does the land connect and interact with the ocean, and how does that change with time? If we go into a much warmer period, will some of these boundaries between climate zones move and in what way will they move?"
"And even if we didn't have to worry about climate change," Verosub said, "there's this issue of long term droughts in California. I'm sure you know about the trees in Fallen Leaf Lake, and maybe in Emerald Bay. There are some in Lake Tahoe, and on the Walker River." It's believed that the trees grew along the lakeshore during extreme droughts several hundreds, perhaps a few thousand years ago when the lake level was much lower, and were submerged when the climate became wetter.
"There's a reconstructed rainfall history for the Sacramento Valley," Verosub said, "that's based on tree rings. It shows a roughly 70-year drought, just before Europeans came into the basin. If that record is right, it has nothing to do with global warming or any of these other factors. It says that somewhere in the natural variability of the system, we could get a 70-year drought. And, that's going to make that little thing we had in '79, look like nothing. So, we want to know what that variability is - over something like a 10,000 year record. And it would be interesting to look at the record in the lake and then go to some of the smaller lakes and bogs in the area and get those records as well."
Knowing how climate varied historically and prehistorically will help validate theoretical climate models that are being used to predict future climates in the Sierra. A way of judging how realistic these models might be is to see how well they can duplicate past climates.
There's much more to be learned with deep lake cores, and the prospect is exciting. A report is currently being prepared by interested scientists that will detail what all those things are. We'll review it when it's available.
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