How Are Fossils Dated? Layers, Isotopes and Index Fossils

Sedimentary rock forms as sand, mud and other material settles in layers, and unless something has turned the sequence over, each layer is younger than the one below it. That principle, first stated by Nicolas Steno in 1669, gives the order of events without giving any dates. A fossil in a lower layer is older than one above; a layer that cuts across others is younger than all of them; a fragment of one rock embedded in another is older than its host.

Geologists used this ordering to build the whole geological timescale during the nineteenth century, long before they could attach a single number to it. The Jurassic was defined by the rocks of the Jura Mountains and the fossils in them, and it was known to lie between the Triassic and the Cretaceous decades before anyone knew it began 201 million years ago.

A layer in England cannot be laid physically alongside one in Argentina, so the ordering has to be carried from place to place, and fossils themselves do the carrying. Some species were widespread, abundant, easily recognised and short-lived, existing for only a million years or so before going extinct. Find one of these index fossils in two rock sequences on different continents and the layers containing them are the same age.

Ammonites, coiled marine shells that evolved rapidly through the age of dinosaurs, are the classic example; their species change so fast that Jurassic and Cretaceous rocks can be placed to within a fraction of a million years by the ammonites alone. Tiny fossils, such as pollen and the shells of single-celled plankton, do the same job for rock cores drilled by oil companies. None of this gives an age in years. It gives a position in a sequence that runs the same way everywhere.

Numbers come from radioactive isotopes, unstable atoms that decay into other atoms at a fixed rate unaffected by heat, pressure or chemistry. Each isotope has a half-life, the time for half of any sample to decay, and by measuring how much of the parent isotope remains against how much of its product has accumulated, the time since the clock started can be calculated. The clock starts when a mineral crystallises and locks the atoms in.

Different isotopes cover different ranges. Uranium-238 decays to lead with a half-life of 4.5 billion years and dates the oldest rocks on Earth. Potassium-40 decays to argon with a half-life of 1.3 billion years and is the workhorse for volcanic rock of any age over a hundred thousand years. Carbon-14, famous from archaeology, has a half-life of only 5,730 years and is useless beyond about 50,000 years, which is why it plays no part in dating dinosaurs, despite what television sometimes implies.

The catch is that radioactive dating works on igneous rock, which crystallised from molten material, and fossils lie in sedimentary rock, which is made of older grains that were dated already when they formed elsewhere. So a fossil is dated by bracketing. Find a layer of volcanic ash below the fossil and another above it, date both, and the fossil lies between the two numbers. Volcanic ash falls are common in the geological record and each one is an instant marker across a wide area.

The 66-million-year date for the end of the dinosaurs comes from exactly this: a thin layer of clay found worldwide, rich in iridium from the asteroid, with datable volcanic minerals close above and below it, and index fossils that disappear at the same level everywhere. Several independent methods agree to within a few tens of thousands of years, which for an event 66 million years ago is agreement to one part in a thousand.

Every method has known limits, and the limits are what make the results trustworthy. Radiometric dates carry a stated uncertainty, usually under one percent. Minerals that were heated after forming can lose their argon and read young, which is why several minerals from the same rock are dated together. Layers can be folded or faulted so that older rock ends up on top, which the index fossils reveal. The methods are chosen so that each one's weakness is another one's strength. The tools, by what they give:

• •Layer order: which is older, no numbers
• •Index fossils: the same position in the sequence anywhere in the world
• •Uranium-lead: numbers for the oldest rocks, billions of years
• •Potassium-argon: numbers for volcanic layers above and below fossils
• •Carbon-14: numbers for organic remains younger than 50,000 years only

Fossils are dated by bracketing: the order of rock layers and the index fossils in them say what is older than what, and radioactive decay in volcanic layers above and below puts numbers on the sequence. Carbon-14 covers only the last fifty thousand years; everything older is dated by uranium, potassium and the layers themselves.