Holocene radiometric dating

In certain areas a varve chronology can be established.This involves counting and measuring thicknesses in annual paired layers of lake sediments deposited in lakes that undergo an annual freeze-up.

The influence of humans is of world extent and is so profound that it seems appropriate to have a special geologic name for this time. The Holocene represents the most recent interglacial interval of the Quaternary period.

Charles Lyell proposed the designation Recent for the period that has elapsed since “the earth has been tenanted by man.” It is now known that humans have been in existence a great deal longer. The preceding and substantially longer sequence of alternating glacial and interglacial ages is the Pleistocene Epoch.

Work in this field by the University of Arizona’s Laboratory of Tree-Ring Research, by selection of both living trees and deadwood, has carried the year-by-year chronology back more than 7,500 years.

Certain pitfalls have been discovered in tree-ring analysis, however.

When the (radiocarbon-dated by burned wood), 70 cubic kilometres of debris were thrown into the air, forming the basin now occupied by Crater Lake.

The tephra were distributed over 10 states, thereby providing a chronological marker horizon.

The magnetized material to be studied can be natural, such as a lava flow; or it may be man-made, as, for example, an ancient brick kiln or smeltery that has cooled and thus fixed the magnetic orientation of the bricks to correspond to the geomagnetic field of that time. The wind may blow the ash 1,500–3,000 kilometres, and, because the minerals or volcanic glass from any one eruptive cycle tend to be distinctive from those of any other cycle, even from the same volcano, these can be dated from the associated lavas by stratigraphic methods (with or without absolute dating).

The ash layer then can be traced as a “time horizon” wherever it has been preserved.

A comparable eruption of Thera on Santorin in the Aegean Sea about 3,400 years ago left tephra in the deep-sea sediments and on adjacent land areas.

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