dating the quaternary

• Short period of time so need to be precise with dates. 
• Once the organism dies the exchange between the organism and atmosphere stops and the amount of carbon 14 decreases through radioactive decay with a half life of 5700 years. 
• The carbon 14 method compares the amount of radioactive carbon 14 atoms to non-radioactive carbon 12. we know that this ratio was at the point the organism died so by working out the ration within the sample we can calculate age. 
• We can also use a geiger counter to measure the activity of the sample and a modern material. 

  • A modern wood has 250 counts per second, the old sample of the tree wood is 125 counts per second 
  • The activity has fallen by half so the wood must be one half life old. 
• Limits of carbon dating:

  • Short half life so activity fall fast 
  • Error of 200 years for ages up to 10000 years but increases after that 
  • Cannot date material older than 50000 years 
  • Useful in quaternary and archaeology but not anything older 
  • Cosmic radiation has varied over the last 50,000 years 
  • Post industrial increase in fossil fuel use has increased carbon 12 in atmosphere relative to carbon 14 (fossil fuels don’t have any 14 left)
  • Ancient wood can be contaminated by groundwater containing modern levels of carbon 14 
• Varves:

  • Fine sediment deposited in still water which makes a darker layer in the winter (made of clay)
  • Coarse sediment deposited in turbulent water which makes a lighter layer in summer 
  • Useful for dating as each pair represents one year 
• Varve limitations:

  • Assumes freezes and melts every year 
  • Climate is the same for all lakes in the region 
  • No gaps in sequence 
  • Constant source of sediment 
• Dendrochronology:

  • New growth of trees happens under bark 
  • Trees grow faster in spring and slows during summer and autumn 
  • Older trees give floating dates
  • Carbon 14 dating can obtain a certain date 
• Dendrochronology:

  • Like varves the width matches the length of the growing season of trees in the same region 
  • Dry years give narrow rings 
  •  trees that are old are best 
  • Time sequences using oak and pine go back 12,500 years from the present 
• Dendrochronology limitations:

  • Growth rings are better in temperate regions with defined seasons 
  • Can get two rings in one year if there is unusual weather patterns that make two growing seasons in a year 
• Isochronous marker beds:

  • Marker bed with distinctive composition of presence at a particular time and is very large
  • Like beds with a particular fossil 
  • Marine bands within delta sequences indicate marine transgressions 
  • Iridium layer at the end of the cretaceous 
• Volcanic ash:

  • Volcanic ash - short events have a particular composition and covers a wide area due to small size 
  • Volcanic ash can also be radiometrically dated to give an absolute age - helps date glacial deposits in Iceland 
  • Can be altered over time 
  • May be harder to identify a layer - especially of its older 
• Carbon 14 dating:

  • There are three carbon isotopes 
  • Most is carbon 12 
  • Carbon 13 is 1 % 
  • And carbon 14 is present in trace amounts 
• Carbon 14 dating:

  • Carbon 14 is made continuously in the upper atmosphere by cosmic ray impacts on nitrogen 14 
  • Carbon 14 disperses evenly through the atmosphere with oxygen to form carbon dioxide 
  • This is used in photosynthesis in plants, then is distributed through the food chain to animal and carbonate shells in the ocean 
• Carbon 14 is also decaying to nitrogen 14, however as plants photosynthesise and are ingested by animals they continue to absorb more carbon 14. 
• Carbon 14 is continuously exchanges between the atmosphere and living organisms, so the ratio of carbon 14 to other isotopes should be the same in living organisms as in the atmosphere. 
• Carbon 14 dating assumes that the rate of carbon 14 productions has remained constant in the upper atmosphere over the past 60000 years. Therefore we can predict amounts if carbon 14 in the atmosphere in the past. 
• Carbon dating has been used since the 1940s when libby presented his findings. He tested objects with known ages - trees or old artefacts. He was awarded the Nobel peace prize. 
• Items that can be carbon dated:

  • Wood 
  • Seeds 
  • Bones 
  • Leather 
  • Shells 
  • Soil 
  • Paper 
  • Water 
  • Blood 
• Dating pollen and seeds from peat bogs has allowed for a timescale for recent climate change. 

  • By identifying key species which are sensitive to temperature 
  • A detailed record has been compiled that extends to 50000 years 
• Varves:

  • Thickness of the paler coloured band depends on the length of the summer melt period 
  • Each lake in the same region should have the same pattern of longer summer layers and thinner shorter summer layers 
  • Different lakes can be linked and a longer period can be analysed than is seen in just one lake 
  • Lakes can be dated of you can link to lakes still producing varves today, the date is anchored 
  • No longer winter freezes in the UK, so no varves formed now 
  • So no actual ages but dates can be correlated to other lakes 
  • Theses are floating dates - how long but not when sediment built up