Absolute dating tells when man-made sites and artifacts were created.
Less than a century ago, when archaeologists, geologists and other scientists sought to determine the age of artifacts, fossils, buildings or geological events, they turned to "relative" methods to approximate a date sequence they hoped was meaningful. Yet as they tried for greater accuracy, significant advances in "absolute" dating techniques exploded in the early 1920s, with many of which are still in use today.
Absolute Dating Defined
Simply put, absolute dating refers to any method archaeologists and paleontologists use to estimate the age of any type of artifact or structure that has been modified or created by humankind. This process typically involves measuring and calculating what remains of the chemical and/or physical properties naturally present in everything from rocks to human remains, and the most widely accepted methods of absolute dating rely on measuring the radioactive decay of naturally occurring elements and isotopes within an object or artifact. Chemists have established the rate at which radioactive decay occurs for particular chemical isotopes and isotope sequences, so researchers now can estimate age as precisely as possible by examining how much radioactive material is left in a discovery.
Absolute Dating, Not Actual Dating
While these absolute processes cannot truly provide "actual" dates for when a certain thing was created, absolute dating provides a credible range of dates, a tremendous advantage over relative dating's limitation to age sequencing. Today, experts can more accurately date and place the discovery of artifacts, geological events and paleontological sites on both the global and human record.
Most experts concur that while superior to relative dating practices, even absolute dating cannot truly be "absolute"; therefore, site and artifact dating--even when absolute methodologies are used--can only be estimated within a few hundred years or so.
Carbon-14 Dating
Arguably the most significant archaeological and paleontological breakthrough of the 20th century, the discovery of carbon-14 dating makes it possible for these scientists to learn the age of artifacts containing organic matter--including all flora and fauna--that has been dead for several centuries to as many as 50,000 years. Present in all living things, when a plant or animal dies, it ceases to absorb the natural levels of carbon needed to sustain life; carbon that then begins to decay at a consistent radioactive rate. Measured in terms of the carbon isotope's half-life of just 5,730 years, the remaining levels of carbon-14 are commonly used to date artifacts, human and animal bones, and other dead organisms.
Radiometric Dating
When an artifact or fossil exceeds 70,000 years of age, the relatively short half-life of carbon-14 cannot adequately measure age; therefore, scientists must turn to the far more accurate method of radiometric dating. To reach back that far into the historical record requires using a more complicated isotope series with longer half-lives (e.g., uranium/lead or potassium/argon). Radiometric dating can be especially important to geologists, who have successfully used this tactic to date the Earth, as well as to paleontologists, who often employ radiometric dating to identify ancient fossils' place in the geologic record.
Dendrochronology
Dendrochronology--simply counting the number of rings in a tree's cross-section is an accurate and well-known means to establish the absolute date of wood younger than 4,000 years. A tree's distinct pattern of rings is a record of the rainfall it received over each year of its life, and since rainfall varies annually, as well as across a diversity of habitats, the ring patterns among trees living in the same region should be similar. Dendochronologists can match the annual rings on a sample from a specific site to what's known as the "master sequence" of the patterns identified on nearby trees' rings and accurately determine an absolute date for historic events and conditions.
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