Spires of tufa, reflected in the waters of Mono Lake, hold clues to climate changes of the past and future.
by Lauren Biron
Oct 22, 2010
The alkali flies of Mono Lake are small and black and form a living curtain that parts before a visitor’s stride.
Millions buzz along the shores, attempting to avoid the hungry California gulls or popping briefly underwater in bubbles of air – formed by their own hairs – to feed on algae.
Their alkali fly pupae are high in calories. The Kutzadika’a people who once lived here harvested and consumed them in great numbers – but geomorphologist Scott Stine doesn’t come to the lake to eat them.
He hopes to date them.
Parts of them at least – parts imbedded in one of Southern California’s most unique landscapes. As they mature from their adolescence, pupae combine calcium from their bodies with carbon from the water sloshing about in Mono Lake.
When an adult moves out of its pupal shell and into a lakefront apartment, it leaves behind a souvenir: a tiny drop of calcium carbonate. In this saline environment, flies leave their deposits on dramatic, inorganic spires and mounds that look like towers of gray, petrified sponges.
This is tufa. It is a porous form of limestone, looks uncomfortable to sit on, and holds a key to dating the rise and fall of Mono Lake over the past 60,000 years. When the past environment was wet or cold, the lake level rose; when it was warm or dry, the lake fell – indicators of climate change. And tufa holds valuable clues.
Exposed tufa towers show where Mono Lake once reached.
Stine and others showed climate scientists at the Comer Abrupt Climate Change Conference in Wisconsin this fall that Mono Lake won’t give up her history easily. And scientists consider mapping climate’s past crucial to predicting the future of a warming world that is expected to drain inland water resources in some of the thirstiest areas of the globe.
Tufa only forms under the water of the lake. It can form organically, when alkali flies grow up or algae settles down to colonize, or inorganically as a result of different waters (fresh streams and salty lakes, for example) mingling. Either way, carbon ends up in the tufa that geologists such as Stine date using radiocarbon techniques. Measuring how much the radioactive fraction of the carbon has decayed gives them an approximate age of when the tufa formed and a corresponding lake level.
But the carbon in tufa can lie.
“Very often we don’t have any plant detritus, so we’re stuck with dating tufa,” said Stine, a professor of geography and environmental studies at California State University, East Bay. “And we’re still trying to figure out the intricacies of tufa dating.”
Mono Lake has a “reservoir effect,” meaning it has a mass of old carbon that has lost much of its radioactivity. This carbon slips into the tufa deposits and creates incorrect readings during radiocarbon dating, causing modern tufa to appear ancient.
If a visitor rudely threw a Heineken bottle into the lake, a scientist could date the inorganic tufa on it in 50 years – only to find that the carbon says the bottle is 1,500 years old. And in case scientists want an additional challenge while trying to reconstruct lake levels, the reservoir effect – currently 1500 years – has changed through time.
That’s why Stine is at Mono Lake. He is a detective of past climate changes.
Tufa creeps out of Mono Lake.
Stine dates a variety of tufas and compares them with known events and lake changes from the past. He finds wood that has been preserved within tufa towers and compares the carbon date in the fossilized wood to the carbon date of the tufa.
Wood is the best organic material to use for carbon dating, since its carbon tells a reliable story. As Stine said, “wood dates true.” By comparing how different kinds of tufa relate to the correct dates found in wood, Stine can learn which kinds of tufa scientists can rehabilitate – compensating for error – and which kinds you have to throw in jail, disregarding them entirely.
The inorganically formed tufa, for example, will be given a life sentence of skepticism. “The vegetation,” Stine said, “is telling us that the tufa dates are wrong by a huge and unpredictable margin.”
The biologically precipitated tufa, though, might just need a helping hand. It does not date correctly, but it dates incorrectly in a consistent way, allowing for potential corrections.
Stine has not yet dated the tufa formed by alkali flies, but hopes to in the future – adding to the organic data for tufa made by algae. He predicts it to date the same way that algal tufa does: wrong, but predictably so. By finding the limits of tufa dating, Stine hopes to help those geologists who have no access to organic materials and are forced to use the questionable inorganically created limestone towers.
Of course, radiocarbon dating of wood and tufa has its own limits. Radiocarbon dating is only viable for dating materials less than 60,000 years old and wood decomposes over time.
Luckily, tufa contains trace amounts of other materials. Scientists Sidney Hemming and Xianfeng Wang discussed improved techniques that use the decay of uranium to thorium to date tufa towers. Their technique’s viable dating range? Up to 500,000 years, they told colleagues at the Comer Conference.
Combining these techniques, scientists hope to paint a more accurate picture of the past. “So much of what we are trying to understand involves dating,” said Stine. Known climate changes from the past can be used to make predictions about water level and environmental change in the future.
“We can read the story of the planet in ways that were never possible before,” said Penn State climate scientist Richard Alley at the close of this year’s conference.
Slowly, scientists are peeling back the layers of the Earth’s history – even if it is one abandoned fly pupa or algal remnant at a time.
