#thermoluminesence

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Archaeological Dating

Babylonian “Astronomical Diary“ mentioning a lunar eclipse on 13 Ulūlū in the 5th year of Darius III [20 September 331 BCE], ten days before the battle of Gaugamela (British Museum, London; tablet BM 36390)

This post won't talk about relationships among archaeologists, but rather how they figure out when things happened. There are several methods they use, as well as dating systems. We'll go through them from the most recent to the farthest back.

With the advent of writing, people began recording things based on the reign of the current monarch and the calendar they used. Most calendars were lunisolar, combining the lunar month with an added month in irregular years to keep the various holidays in the appointed season. That these cultures were in contact with each other, through war and diplomacy, gives us an interlaced system of knowing when these reigns were. This allows us to figure out when things happened that were recorded that include a reference to the current monarch.

By Ángel M. Felicísimo from Mérida, España - Retrato de Julio César, Public Domain, https://commons.wikimedia.org/w/index.php?curid=91281949

It wasn't until the Julian calendar was implemented in 46 BCE that a definitive calendar that had a 'leap day' every four years and was 365 day year. That Rome came to control a large portion of the old world and traded with the far East, this calendar was helpful in calibrating other calendars to one we have a definitive date on. However the solar year is 11 minutes less than 365.25 days, leaving the calendar to drift one day off 1 day every 400 years. The Gregorian (our current) calendar corrects for this by skipping the leap day every century not divisible evenly by 4 (for example, 1700, 1800, and 1900 did not have leap days, but 2000 did). This lead to 'skipping' eight days in 1582, when it was initially decreed by Pop Gregory (making the 4th of October the 15th). This calendar was not accepted widely, some countries not adopting it until 1923 and some countries still using their own calendars along within their borders.

From the hoard known as the “Bodmer Papyri,” consisting of nine Greek papyrus scrolls, 22 papyrus codices and seven vellum codices in Greek and Coptic.

Other ways we figure out when things were written is the material it was written on and which writing system. Clay tablets, leather scrolls, papyrus codices (books) all give us clues as to a rough time period when a document was written when they're not dated. How letters are formed in particular writing systems also give us a clue to when things were written, for example, paleo-Hebrew resembled Phoenician until about 400 BCE, when the letters became less prong-shaped and more square.

The growth rings of a tree cut horizontally to the ground can be used to date the tree and wooden objects made from it. Ollikainen / iStock / Getty Images

Before writing, we have a few other ways of figuring out when things happened. One of those is carbon 14 dating. Carbon 14 is a type of carbon that has extra neutrons and gradually loses them and becomes carbon 12 (the 'normal' carbon) at a predictable rate. Carbon 14 is produced by an interaction between the solar winds and our atmosphere, similar to how the auroras are produced, so the rate it's produced is a bit variable, but we have found ways to recreate the 'wiggles' (actual scientific term) of its production. Tree rings let us know how much carbon 14 is in the atmosphere in a given year, letting us figure out these wiggles for 12594 years ago. All living things take in Carbon 14 by the act of breathing and eating other things that breathed. This intake stops when they die. Ice cores also give us a rough level of carbon 14, as well. Carbon 14 lets us date things to about 52800 years before the present, though prior to 13000 years ago, it's difficult to give a more exact level of carbon 14 in the atmosphere, which lets us know how much carbon 14 a living creature would take in. For water-dwelling creatures, which take in dissolved carbon 14, we don't have a solid record, so dates for them are approximated.

The right images shows fluorite glowing after being heated on a hotplate. Mauswiesel / CC BY-SA 3.0 / Wikimedia Commons

For things that can't be dated with radiocarbon dating but have been heated (like pottery), thermoluminesence is a method that can be used to date them. When certain minerals are exposed to the sun, they gather that energy into their crystal matrix. When they're heated (like firing pottery), they lose all this energy. With thermoluminesence, these items are exposed to heat (between 400-500°C), we can measure how much energy is given off and compare that to minerals that have never been heated. We can also tell when these these minerals last were in the sun (such as the interior blocks of walls and buildings that haven't been destroyed [think insides of the pyramids, for example]).

Curator Geoffrey Hargreaves inspects core samples from the Greenland ice sheet. They are stored in a freezer at -33F. The cores are vital to understanding changes in atmospheric carbon levels in the past. Roger Ressmeyer/Corbis/VCG / Getty Images

Other methods we have of dating things is figuring out what layers things are in. This gets complicated because things might have been buried in deeper layers, but we can get a rough date for things left in these layers. Decorations on pottery and how it's made also help us figure out when things were made as different civilizations made their pottery differently and decorated it though there were trade networks from very early in history.

When writing dates, archaeologists use a few systems, depending on what they're dating and where they're writing for. BCE and CE refer to 'before the common era' and 'common era', which is an attempt to be more culturally inclusive than BC and AD, even though they're calibrated to the same change over year. BP is 'before present', which is complicated as the present year changes every year. RCYBP and calBP refer to radiocarbon year before present, though the later refers to the calibrated (accounting for wiggles) date. TLYBP refer to dates calculated by thermoluminesence.

Resources:

What Does cal BP Mean? Accounting for Atmospheric Wiggles in Radiocarbon Dating

BP: How Do Archaeologists Count Backward Into the Past? What Do Archaeologists Mean by BP, and Why Do They Do That?

Introduction to dating documents

How Is the Age of an Ancient Manuscript Determined?