- Author: Faith Kearns
When we talk about water in California, we tend to focus on the past 100-150 years because it is the timeframe that corresponds with modern record keeping. At the same time, it's a relatively short period that doesn't yield much long range climate insight.
That's where other methods of looking into the past become important. For example, tree ring analysis has offered a much lengthier view – going back 1000 or so years. But, what about the deep past?
Matthew Kirby, a professor of geology at California State University-Fullerton, uses lake sediments to reconstruct California's water history over the past 100,000 years. He has a special focus on the most recent 11,700 years, also known as the Holocene or the time since the end of the last ice age.
Lake sediments can give a deeper view of history than tree rings, and are also often found in places in California where long-lived trees are uncommon. “Sediments accumulate in layers at the bottom of lakes and, like a history book, they record information,” says Kirby.
Lake sediments also contain information that tree rings do not. That includes pollens, which can be helpful for understanding ecological responses to changing water availability. Charcoal in lake sediments can also assist with fire history, and flood deposits can aid in understanding major flooding events.
Given the variability of California's water systems, this long view of history offers much needed perspective.
As an example, Kirby and his colleagues Sarah Feakins and Scott Mensing have studied the history of Zaca Lake, located near Santa Barbara. They compared their findings to tree ring studies by Edward Cook and others, and were happy to find that the results were comparable for the time period that all the studies covered.
“We have shown that our lakes capture many of the same features seen in tree ring records over the past 1000 years. This coherence between two separate water recording archives gives us a high level of confidence that lakes are the premier archive for understanding California's water history during the Holocene.”
So, what are they finding? Studies from several lakes across California provide evidence for a major drought lasting longer than 500 years, which occurred sometime around 2000-2500 years ago.
They are also finding that what happens in the tropical Pacific Ocean – El Niño, anyone? – has had a big influence on California's climate over the past 11,700 years. From this, says Kirby, “one might postulate that future water changes in California will be strongly forced by the tropical Pacific.”
A major objective of Kirby's research is to develop, assess, and explain the patterns of water change through time across California. He is particularly interested in what lake sediments might show about a well-known wet and dry pattern that dominates in the western U.S. – when it is wet in the north, it's generally dry in the south, and vice versa.
This precipitation “dipole” (see Wise, 2010) is presumed to be an important part of western U.S. climatology, but is not well understood beyond 500 years ago. Lake sediments can provide a deep time perspective on how the dipole responds to periods of warming, cooling, and other climate factors that impact California's water history.
Kirby cautions that “having preached the wonder of lakes, I should note that most do not contain an annually resolved chronology like that associated with trees. Consequently, most lake studies inherently represent a less ‘time constrained' record than trees for the past millennium.”
At the same time, California's water history cannot be reconstructed using single archives – whether tree rings or anything else. Multiple sources of information, such as lake sediments, are valuable, water-sensitive archives that need to be further developed to appreciate the range of natural water variability in California.