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Unlike tooth enamel, soft tissues are constantly being made and remade during life.

Thus, their radiocarbon levels mirror those in the changing environment.

To determine year of birth, the researchers focused on tooth enamel.

with a standard deviation of plus or minus 120 years, the chances are two in three that that sample dates from between 1120 and 880 BC.

Here's how: Calculations based on one standard deviation of 120 years: 1000 120 = 1120 BC (Oldest date) 1000 - 120 = 880 BC (Most recent date) To increase the range of possible dates of a sample, archeologists may calculate the radiocarbon date to two standard deviations.

Archeologists use several methods to establish absolute chronology including radiocarbon dating, obsidian hydration, thermoluminescence, dendrochronology, historical records, mean ceramic dating, and pipe stem dating.

Each of these methods is explained in this section.

Calculations based on two standard deviations increases the possible date range, increasing the probability of the sample lying within this range to 95 percent.

Here's how: Calculations based on two standard deviations of 120 years (120 x 2 = 240) 1000 240 = 1240 BC (Oldest date) 1000 - 240 = 760 BC (Most recent date) As a rule, the more standard deviations used, the larger the probable date range for the sample and consequently, the higher the probability is for that sample to fall within the expanded date range.

Radiocarbon dates can be obtained from many types of organic material including charcoal, shell, wood, bone and hair.

The amount of carbon dioxide in the living organism is equal to that in the atmosphere.

Because the concentration of radiocarbon in the atmosphere has varied considerably over time, radiocarbon dates as far back as 7,000 years may be corrected by calibrating them against accurate dates from radiocarbon-dated tree rings and developing a master correction curve.

Archeologists use a statistical standard deviation to increase the range of dates for a sample that has been given a C14 date.

Over the past six decades, the amount of radiocarbon in people or their remains depends heavily on when they were born or, more precisely, when their tissues were formed.