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.

Carbon dating
Living organisms breathe, eat and drink carbon-14. But once they die, no more carbon-14 comes in. So it starts decaying, letting archaeologists date objects up to 50,000 years old. (For Stongehenge, they dated organic matter in the silt below.) (

By Kenneth Chang
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.
At the bottom of Lake Suigetsu, thin layers of microscopic algae have been piling up for more than 45,000 years. Reading them like tree rings, scientists have improved their carbon-14 dating technique. (
The rest of the year, dark clay sediments settle on the bottom. The alternating layers of dark and light count the years like tree rings. That has allowed scientists to fine-tune a technique called carbon-14 dating, which is used to pin down dates for artifacts tens of thousands of years old.
     “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
How Much Carbon-14 Left?

Percent Left

Type in the years (without commas), press "Calculate" and the amount of carbon-14 remaining will appear.
Here’s how the dating method works: Carbon is the main building block of all living matter. The most common form, carbon-12, has six protons and six neutrons tucked within its nucleus. (The number is the sum of protons and neutrons)
     A much rarer form of carbon is created by particles from outer space raining down on the Earth. Take one atom of nitrogen—seven protons, seven neutrons—languidly 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 neutrons—or 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 weren’t 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 don’t photosynthesize, breathe or eat, so they don’t 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 Earth’s 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 Suigetsu’s 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.
     “It’s an excellent piece of additional information,” says Minze Stuiver, a carbon-dating expert at the University of Washington.
     The new results don’t radically change any carbon dates, just narrow the uncertainties of how long something has been dead.

Copyright 1998 ABCNews and Starwave
Corporation. All rights reserved. This material may not be published, broadcast,
rewritten, or redistributed in any form.