Holocene Epoch

Chronology and correlation

The Holocene is unique among geologic epochs because varied means of correlating deposits and establishing chronologies are available. One of the most important means is carbon-14 dating. Because the age determined by the carbon-14 method may be appreciably different from the true age in certain cases, it has been customary to refer to such dates in “radiocarbon years.” Increasingly, however, as calibration data sets have become available, dates in radiocarbon years are being directly converted to calendar years. These dates, obtained from a variety of deposits, form an important framework for Holocene stratigraphy and chronology.

Radiocarbon years are calculated by examining the radioactive decay of carbon-14. This carbon isotope is generated when neutrons produced by collisions between cosmic rays and atoms in the upper atmosphere are captured by nitrogen atoms. Living tissue absorbs small amounts of carbon-14 through respiration and food ingestion. Carbon-14 continues to accumulate in an organism’s tissues until it dies. The carbon-14 then undergoes radioactive decay to become nitrogen, with a half-life of 5,730 years. Using this measure, scientists can estimate the age of a tissue in radiocarbon years from the amount of carbon-14 remaining in the sample.

The limitations of accuracy of radiocarbon age determinations are expressed as ± a few tens or hundreds of years. While many archaeological studies have relied on direct radiocarbon-calendar conversions, studies have shown that uncertainty between radiocarbon and calendar dates could still remain and that direct conversions could be subject to an offset error of 20–50 years. Since this prospect has the potential of impacting historical timelines in several fields, scientists recommend that researchers use other dating techniques, such as tree rings and sediment deposits, to verify radiocarbon-calendar conversions.

In addition to this calculated error, there also is a question of error due to contamination of the material measured. For instance, an ancient peat may contain some younger roots and thus give a falsely “young” age unless it is carefully collected and treated to remove contaminants. Marine shells consist of calcium carbonate (CaCO₃), and in certain coastal regions there is upwelling of deep oceanic water that can be 500 to more than 1,000 years old. An “age” from living shells in such an area can suggest that they are already hundreds of years old.

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. Because each year’s sediment accumulation varies in thickness according to the climatic conditions of the melt season, any long sequence of varve measurements provides a distinctive “signature” and can be correlated for moderate distances from lake basin to lake basin.

In some relatively recent continental deposits, obsidian (a black glassy rock of volcanic origin) can be used for dating. Obsidian weathers slowly at a uniform rate, and the thickness of the weathered layer is measured microscopically and gauged against known standards to give a date in years. This has been particularly useful where arrowheads of obsidian are included in deposits.

As noted elsewhere in this article, paleomagnetism is another phenomenon used in chronology. The Earth’s magnetic field undergoes a secular shift that is fairly well known for the last 2,000 years. 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.

Another form of dating is tephrochronology, so called because it employs the tephra (ash layers) generated by volcanic eruptions. The wind may blow the ash 1,500–3,000 kilometres (about 930–1,860 miles), 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. When the Mount Mazama volcano in Oregon exploded at about 7,700 bp (radiocarbon-dated by burned wood), 70 cubic kilometres (about 17 cubic miles) 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. A comparable eruption of Thera on Santorini in the Aegean Sea about 3,400 years ago left tephra in the deep-sea sediments and on adjacent land areas. Periodic eruptions of Mount Hekla in Iceland have been of use in Scandinavia, which lies downwind.

Finally, the measurement and analysis of tree rings (or dendrochronology) must be mentioned. The age of a tree that has grown in any region with a seasonal contrast in climate can be established by counting its growth rings. 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. Sometimes, as in a very severe season, a growth ring may not form. In certain latitudes the tree’s ring growth correlates with moisture, but in others it may be correlated with temperature. From the climatic viewpoint these two parameters are often inversely related in different regions. Nevertheless, in experienced hands, just as with varve counting from adjacent lakes, ring measurements from trees with overlapping ages can extend chronologies back for many thousands of years. The bristlecone pine of the White Mountains in California has proved to be singularly long-lived and suitable for this chronology; some individuals still living are more than 4,000 years old, certainly the oldest living organisms. Wood from old buildings and even old paving blocks in western Europe and in Russia have contributed to the chronology. This technique not only offers an additional means of dating but also contains a built-in documentation of climatic characteristics. In certain favourable situations, particularly in the drier, low latitudes, tree-ring records sometimes document 11- and 22-year sunspot cycles.