Bottom of Lake Refines Carbon Dating Technique
This Died How Long Ago?
Counting thin white layers of dead algae, each less than a millimeter thick, the researchers calibrated carbon-14 dating back to 43,000 B.C.
Feb. 23 Each spring, tiny plants bloom in Lake Suigetsu, a small body of water in Japan. When these one-cell algae die, they drift down, shrouding the lake floor with a thin, white layer.
The time resolution is much better, says Hiroyuki Kitagawa, a research associate at the International Research Center for Japanese Studies in Kyoto, Japan.
Kitagawa, along with Johannes van der Plicht of Groningen University in the Netherlands examined the Suigetsu sediments and reported their findings in the latest issue of the journal Science.
A Carbon Primer
A much rarer form of carbon is created by particles from outer space raining down on the Earth. Take one atom of nitrogenseven protons, seven neutronslanguidly floating about in the upper atmosphere. Suddenly, a high-speed neutron slams into it, embedding itself into the nitrogen nucleus and kicking out one of the protons. For those doing the math, that leaves six protons, eight neutronsor one atom of carbon-14.
Carbon-14 is unstable. One of its protons eventually decays into a neutron and an electron, spitting out the electron. That transforms the carbon-14 back into nitrogen. If carbon-14 werent continually created, it would disappear.
Carbon, Carbon, Everywhere
This unstable carbon disperses throughout the environment. Plants photosynthesize it. Animals breathe it. Bigger animals eat plants and the smaller animals. Thus carbon-14 works its way into every living organism.
In 1947, chemist Willard Libby realized this provided a way to figure out how long something has been dead. Dead things dont photosynthesize, breathe or eat, so they dont replenish carbon-14.The longer something has been dead, the less carbon-14 there is.
That innovation won Libby a Nobel Prize and has enabled archaeologists and paleontologists to date the Dead Sea Scrolls, Egyptian mummies, bones of mammoths and other organic objects up to about 50,000 years old. By then, 99.7 percent of the carbon-14 has decayed away, making accurate dating difficult.
Calibrating the Clock
What complicates matters is that the level of carbon-14 levels has not been steady through time. During some periods, a particularly active sun pummels the Earth with more high-speed particles. Changes in the Earths magnetic field changes how many of those particles are deflected away. Changing temperatures may also alter the balance.
Ancient tree rings have allowed scientists to calibrate the carbon-14 clock back to 9400 B.C. The layered growth of coral reefs pushes the calibration back another 12,000 years, but runs into another problem. Deep ocean waters dilute the carbon-14 levels, making reefs seem older than they are.
Cores taken from Suigetsus lake bottom avoid that problem. Counting the thin white layers of dead algae, each less than a millimeter thick, gave the researchers the year, which could then be compared to the date obtained by carbon dating, back to 43,000 B.C.
Its an excellent piece of additional information, says Minze Stuiver, a carbon-dating expert at the University of Washington.
The new results dont radically change any carbon dates, just narrow the uncertainties of how long something has been dead.