amber

amber Amber is a light, organic substance that is generally yellow or orange in colour and may be transparent or cloudy. It is the fossilized resin from trees that lived millions of years ago Trees produce resin as protection against disease and insect attack. Some trees produce resin in large quantities, which seeps out of cracks in the bark and flows down the trunk. Resin is very sticky, and insects, other small animals and plant remains are readily trapped. The resin hardens and becomes incorporated into sediments, where it polymerizes to form amber; any inclusions are usually very well preserved.

Amber is composed of cyclic hydrocarbons known as terpenes; it has a hardness of 2–3 on the Mohs' scale, a specific gravity of 1.04–1.10, and a melting point of 200–380 °C. Amber is generally attractive to look at, warm to the touch, light, and easy to carve. These properties have made it desirable for jewellery since the Stone Age.

Amber is difficult to date and this is mainly done by studying the associated fossils in the sediments from which the amber comes. This, however, indicates only a minimum age because there is no way of knowing how long the amber took to get from the trees to where it was deposited. The associated fossils may give an incorrect age if the amber has been reworked. The oldest amber has been recorded from the Upper Carboniferous. The oldest amber that contains insects comes from the Lower Cretaceous.

Amber occurs in many parts of the world. The largest deposits are from around the Baltic Sea and in the Dominican Republic, which are generally considered to be Upper Eocene–Lower Oligocene and Lower–Middle Miocene in age respectively. Other significant deposits occur in Canada, the USA, Mexico, France, Spain, Germany, Sicily, Romania, Lebanon, Russia, China, Burma, Japan, and Borneo.

The insects and other inclusions that are trapped in amber provide evidence of past biodiversity, ecology, and biogeography. Although most species in amber are extinct, it is often possible to work out how they lived by comparison with their living relatives. There is a bias in what is preserved: many larger animals are strong enough to pull themselves out of sticky resin, whereas small and weak insects are more prone to be entrapped. Furthermore, insects that live on the resin-producing trees are more prone to entrapment than those that live in habitats away from the trees. Amber is crucial for our knowledge of small extinct insects that would not otherwise be fossilized. Of particular interest are specimens in which two different animals are caught interacting, such as parasites still attached to their host.

In recent years scientists have analysed air bubbles in amber to try to obtain evidence of past atmospheres. However, amber oxidizes, and oxygen in the bubbles would then react with the amber, which would affect the composition of the bubbles. Scientists have also attempted to extract DNA from the insects in amber. Some have claimed success, but independent attempts to replicate the results have so far been unsuccessful, which has led to suggestions that the recovered DNA may be recent contamination rather than ancient materials.

Andrew J. Ross