Dating Techniques Year 10 Biology 2015

DATING FOSSILS Absolute dates: actual age of the specimen in years Relative dates: determine whether one sample is older or younger than another

Stratigraphy – RELATIVE DATING Study of layers or strata Principle of superposition Layers at the top are younger than the layers at the bottom Correlation of rock strata Matching of layers of rock from different areas

5 living fish sediment from river fish skeleton partly buried by sediment Sedimentation

6 more recent sediment collects older sediment becomes rock fish skeleton fossilised Layers of sediment

Earth’s crust movement 7 land raised above water level recent rock older rock Earth’s crust movement

8 fossilised skeleton exposed earth movements fracture rock Fractured rock

When rock strata become exposed, it can be assumed that, 9 When rock strata become exposed, it can be assumed that, in most cases, the lowest layers are the oldest* This means that the fossils of organisms preserved in the lowest layers represent animals and plants that lived many millions of years ago rock strata of increasing age Rock strata exposed at a cliff face. Each layer represents millions of years of deposition. * The movements of the Earth’s crust may cause the strata to become distorted. Sometimes the strata are vertical or even turned upside down with the oldest rocks appearing on top. It is usually possible to tell when this has happened. Rock strata

Fluorine Relative Dating Movement of fluoride ions in the water in the soil into the bone All fossils in a particular deposit should contain the same amount of fluoride The older the fossil the more fluoride it contains Eg: Piltdown Man

INDEX FOSSILS Index fossils: widespread & present for limited time on Earth Examples Trilobites and Ammonites

Determining the actual age of the fossil in years ABSOLUTE DATING Determining the actual age of the fossil in years Uses Radioactive dating Tree Rings

Decay of radioactive carbon-14 to nitrogen Radiocarbon Dating Decay of radioactive carbon-14 to nitrogen C-14 produced in upper atmosphere by cosmic radiation acting on nitrogen Decays at about the same rate 1:1012 (C-14:C-12) Requires at least 3 gms of organic material

Green plants use CO2 one atom in every million million (1012) is C-14 Radiocarbon Dating Green plants use CO2 one atom in every million million (1012) is C-14 Plant then eaten by animal  becomes part of animal Death – intake of C-14 stops but decay of C-14 continues at fixed rate Ratio of C-14 to C-12

Half-life of C-14 Over a period of 5730  30 years half of any given quantity of C-14 breaks down. HALF LIFE OF C-14

Problems with C-14 After 70,000 years amount of C-14 becomes negligible Material must be organic (contain carbon) Assumption that ratio of C-14:C-12 was constant – now know that it varies

Tree Ring Dating Concentric rings on the surface of a cut tree Each ring is one year Rings width varies according to favourable conditions Marker rings correlated with timber from ancient human structures

Tree-ring Dating

WHAT NOW? Read pages 48 – 52 Answer questions page 53 Q 1 – 13…