Radioactive isotopes in dating fossils
Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere.
It is not possible to predict when an individual nucleus in a radioactive material will decay.The ratio of oxygen and hydrogen isotopes in the ice provides information about ancient temperatures; and the air trapped in tiny bubbles in the ice can be analyzed to determine the level of atmospheric gases such as carbon dioxide.Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past, and these sources of information can be combined to find the climate model that best fits all the available data.Evolutionary geologists consider that the top three metres (10 feet) of the Marlstone Rock Bed represent the whole of the Tenuicostatum Zone, the basal zone of the Toarcian Stage, Amongst the remaining quarries still ‘working’ the top of the Marlstone Rock Bed are the Hornton Quarries at Edge Hill near the village of Ratley, on the north-western edge of the Edge Hill plateau, some 10½ km (6½ miles) north-west of the town of Banbury (Figures 2 and 3).Building stone, known as ‘Hornton Stone’, has been quarried there since medieval times.
Together these index fossils have, in evolutionary reckoning, established the rock containing them as being Early Jurassic and about 189 million years old. Three samples of fossil wood were collected from the south wall of Hornton Quarries, one from immediately adjacent to the belemnite fossil (Figure 5) during the first visit, and two from locations nearby during the second visit.