Three deep core sections (from 122m, 130m, and 139m) show distinct annual bands produced by the deposition of dust during the dry season (dry season dust layers are represented by triangles).
While annual bands provide accurate relative dating (the age of each ice band is known to be a year apart from directly adjacent bands), paleoclimatologists also search for absolute dates within a core chronology. The surface of the ice cap provides one absolute date. For example, the top layer of a core drilled in 1983 is known to date from 1983; scientists can then date deeper layers relative to the surface. Scientists also attempt to locate absolute dates deeper in the core to improve the
accuracy of the chronology. At Quelccaya, for instance, a thick layer of volcanic ash was found in a layer initially dated at 1598. Looking into historical records of colonial Peru, paleoclimatologists found that a massive eruption of the volcano Huaynaputina had occurred in 1600. Using the absolute date of 1600 for this layer, they were able to revise their chronology and improve its accuracy. Electrical conductivity measurements (ECM), particle concentrations, and the ratio of heavy to light oxygen molecules are other seasonally-variable core parameters that can be used along with visual stratigraphy in dating ice cores.
As the average thickness (l) of the annual layers in the slide demonstrates, annual layers thin significantly at depth due to increased pressure. While annual layers in the ice cliff shown earlier average 75 cm, annual layers at a depth of 139 m are just 4 cm thick and those at the core's base just 1 cm thick.
Byrd Polar Research Center, The Ohio State University
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