Researchers can first apply an absolute dating method to the layer.
They then use that absolute date to establish a relative age for fossils and artifacts in relation to that layer. Anything below the Taupo tephra is earlier than 232; anything above it is later.
Paleomagnetism: Earth’s magnetic polarity flip-flops about every 100,000 to 600,000 years.
The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity.
Whenever possible, researchers use one or more absolute dating methods, which provide an age for the actual fossil or artifact.When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.Measuring carbon-14 in bones or a piece of wood provides an accurate date, but only within a limited range.
Says Shea: “Beyond 40,000 years old, the sample is so small, and the contamination risk so great, that the margin of error is thousands of years.
This includes factoring in many variables, such as the amount of radiation the object was exposed to each year.