http://www.abcnews.com/sections/science/DailyNews/carbon0220.html
By Kenneth Chang
ABCNEWS.com
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 rest of the year, dark clay sediments settle on the bottom. The
alternating layers of dark and light count the years like tree rings.
....
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 Earthās 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 Suigetsuās 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.
ćItās an excellent piece of additional information,ä says Minze
Stuiver, a carbon-dating expert at the University of Washington.
The new results donāt radically change any carbon dates, just narrow
the uncertainties of how long something has been dead.