Microscopic grains of pollen are helping scientists to reconstruct the world’s past. The tiny grains can explain how the environment has been changed by man, and how the climate fluctuated thousands of years ago.
One oak tree releases more than 100 million pollen grains into the air every year. Some smaller plants are even more prolific — the common sorrel of waysides and woodlands emits an incredible 400 million grains annually.
Most windborne pollen ends up on the ground and decays in the soil in the presence of oxygen. But some falls into lakes or bogs, where it is preserved because peat deposits and the sediment at the bottom of lakes contain no oxygen. Some of the grains last for many thousands of years and fossilise. As new layers of sediment are formed, they trap pollen from plants growing at the time.
This fossil pollen provides a ‘book’ that enables palaeobotanists — scientists who study ancient plant life — to build up a picture of the vegetation, and hence the climate, of the past few thousand years. Pollen grains vary in size from 15 to 50 thousandths of a millimetre across, and have individual structures varying from plant to plant that can be identified under the microscope. The grains’ tough outer walls are preserved because they contain a decay-resistant protein. Fossil pollen is counted by taking a core sample (with a hollow cylindrical drill) from an organic deposit such as a peat bog. Then specimens are taken at regular intervals throughout the depth of the deposit and dated by radiocarbon dating.
The amount of pollen recovered in this way is very large. Samples taken have ranged from 20,000 grains per cubic centimetre from deposits made 11,000 years ago, to 650,000 grains per cubic centimetre a few thousand years later.
From this huge quantity a representative sample of about 1000 grains is analysed, and the proportions of the various plants are calculated. Scientists can see, for example, in what way plants colonised the northern lands after the last Ice Age about 12,000 years ago. One of the first trees found was juniper, which thrives in cold climates. As the weather became warmer it was replaced by birch, then oak and elm. A change to a moister climate brought alder.
It is also possible to see how people have influenced the vegetation by cutting down forests and growing crops. Pollen analysis carried out in 1987 on sediments from the Sea of Galilee (Lake Kinnereth), in northern Israel, showed that oak forests were cleared about 5000 years ago to make way for olive trees, grown for their fruit and oil. In the 3rd century AD the number of olive trees declined when the Jews left Palestine.
Picture Credit : Google
