Earth scientists think it’s time to sync their geological clocks
For some scientists, measuring time is harder than clocking Olympic sprinters.
To date big events in Earth history, geologists need a clock that stretches back accurately far into the past. In fact, they do have two such clocks — one based on the steady radioactive decay of minerals, the other on the rhythmic swoops of Earth’s orbit.
But recently researchers have found that these timekeepers are out of sync, in ways that would horrify any Swiss watchmaker. It’s like having the clock on your coffeemaker 45 seconds or so ahead of your kitchen clock. Not the end of days, but annoying enough to demand a fix.
If researchers could get their clocks back in line, they would better understand the rela-tionships between crucial events in the planet’s past. It might seem like nitpicking to change a date from 251 million years ago to 252 million years ago, but it matters when you’re talking about the greatest mass extinction the planet has ever seen. If that “Great Dying” took place a million years earlier than thought, it lines up a lot better with huge volcanic eruptions in Siberia — strengthening the idea of a link between the volcanoes and the extinctions.
But two clocks are helpful only if they keep the same time. Which they don’t always, as many people learn the morning they have to switch their clocks to daylight saving time.
For their first clock, geologists look at the natural wind-down of radioactive elements: argon-40 decaying to argon-39 by losing a neutron, or various types of uranium decaying to lead by losing both neutrons and protons. Both of these processes happen at such a regular rate that rocks containing the right minerals with these elements can be dated incredibly precisely.
For their second clock, scientists analyze the rhythmic wobble of the Earth in its orbital cycles around the sun. Small changes in the planet’s orbit affect how rock layers are laid down to form the geological record — in ways that can be used to calculate at what precise point in the orbit the rocks formed.
To combine the two clocks, take layers that record the orbital changes, then look for tiny crystals within them that can be dated using radiometric methods. And boom: The two clocks can be linked, anchoring timescales that had been floating independently.
This ability makes earth scientists happy in ridiculously obscure ways. A paper in Geology in January, for instance, merges radiometric and astronomical dating to peg the interval between two specific geologic time points at 27.94 million years, plus or minus 160,000 years. And a famous warm period known as the Paleocene-Eocene Thermal Maximum began 55.530 million years ago, plus or minus 50,000 years, researchers report June 23 in Geochemistry Geophysics Geosystems.
These are the sorts of precision dates that eHarmony can only dream of. The matchmakers behind them are scientists working on a project called EARTHTIME. Their goal is to wrestle the clocks into perfect synchronicity. Sometimes, though, the dates don’t quite go as planned.
This spring, for instance, EARTHTIME researchers reported a puzzling discovery: The composition of the uranium that underlies many of the radiometric clocks can vary. Until now, scientists have used a single unshakable number to describe the ratio of uranium-238 to uranium-235 in today’s rocks. But a new survey revealed that that number actually varies a lot from rock to rock, a team led by the British Geological Survey’s Joe Hiess reported in Science in March.
Relying on the canonical number to calculate uranium decaying to lead could mean, in the worst-case scenario, that minerals are dated wrongly by up to 10 million years. That’s not like the coffeemaker and kitchen clocks being off by less than a minute. That’s like having half the clocks in your house being set to New York time and the rest to Tokyo.
Bringing these all into line will take a lot more work. EARTHTIME is developing new lab techniques and computer programs to wrest the secrets of time frozen in ancient rocks. Researchers are also figuring out ways to resolve puzzles like the inconstant uranium. In one extreme experiment, project scientists are building a special pipette system meant to be shipped from lab to lab. The idea is for all research groups to use the same equipment to run a particular analysis — in this case, studies of argon-40 to argon-39 — and eliminate any weird discrepancies among labs.
Once they’re done, scientists hope they’ll have the best clock ever made on Earth. In all the 4.567 billion years that this planet has been around.
SN Prime | July 23, 2012 | Vol. 2, No. 28
