The Brazil Mound Page 6

Obsidian is a natural volcanic glass, usually black, used by many prehistoric peoples to make arrowheads, knives, and other tools.

changes happened prehistorically, we can sometimes use them to date a site or a collection of artifacts: a site with only small arrow points is presumed to be later than a site with large dart points. Of course, it's usually not that easy, since many sites contain both , and often they're mixed together! At CA-SAC-43, time-sensitive artifacts included dart, spear, and arrow points; charmstones; and shell beads.

Obsidian Hydration

Comparison of artifact styles has been used by archaeologists to date sites since the early days of the discipline. Obsidian hydration dating , on the other hand, is relatively new. As obsidian sits in the ground, it absorbs microscopic amounts of water; the longer it sits, the more water it takes in. Archaeologists can look at a thin section of an obsidian tool or flake under the microscope and measure how much water it has absorbed, by the thickness of the band of water (the hydration rim ) within it. A tool with a thick rim will be older than one with a thin rim. These rims are measured in microns - one micron equals 0.000039 of an inch!

Several factors can affect the rate of absorption, including the type of obsidian used and the climate of the area (how hot the weather is, the amount of water in the environment, etc.). We do not yet understand these factors completely, or know how to compensate for them, and so obsidian hydration dating is still somewhat imprecise.

Radiocarbon Dating

All organisms, including plants, animals, and people, contain the natural element carbon . Carbon occurs in three different forms: ¹²C ("Carbon-12") and ¹³C ("Carbon-13"), which are stable; and ¹⁴C ("Carbon-14"), which is unstable or radioactive . Because it is unstable, Carbon-14 tends to break down, or decay, into the stable form Carbon-12, at a known rate. Once an organism dies, it stops taking in carbon, and eventually all the Carbon-14 in the organism will have decayed into Carbon-12. The radiocarbon method of dating measures the ratio of Carbon-14 to Carbon-12 in a dead organism, to determine how long it has been dead. This method is used to date animal bones, shell, charcoal, and other organic materials found in an archaeological site. Radiocarbon is one of the most precise dating methods available, and it is used by archaeologists and other scientists around the world.

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