Obsidian Hydration, Chemical Dating of Bones and Oxygen Isotope Analysis

Obsidian is a volcanic glass created when lava cools rapidly. It has been used for tool making by prehistoric populations due to its ability to form extremely sharp edges. Obsidian hydration dating is a method used to determine the age of these artifacts.

Mechanism

When a fresh surface of obsidian is exposed to the atmosphere, it absorbs water from the air. This water creates a hydration layer on the surface of the artifact. This layer grows at a predictable rate over time. Scientists measure the thickness of this layer using a microscope to estimate how long ago the artifact was produced.

Variables and Limitations

The rate of hydration is affected by the chemical composition of the obsidian and the surrounding environmental temperature. Obsidian from different volcanic sources hydrates at different speeds. Higher temperatures accelerate the rate of hydration, while lower temperatures slow it down. Calibration curves are necessary to account for these environmental differences.

Chemical Dating of Bones

Bones preserved at archaeological sites undergo chemical changes due to their interaction with the burial environment. These processes allow for relative dating and, in some cases, provide information about the history of the remains.

Fluorine Dating

Buried bones absorb fluorine from groundwater. Over time, the fluorine content in the bone increases. This method is relative, as the rate of absorption varies depending on the mineral content of the groundwater and soil at different locations. It was famously used to expose the Piltdown Man hoax by demonstrating that the jawbone and the cranium had different fluorine levels.

Nitrogen and Uranium Dating

Nitrogen content in bone decreases over time due to the decay of collagen. Measuring the remaining nitrogen can help estimate the age of a bone sample. Similarly, bones can absorb uranium from the soil. The concentration of uranium in bone increases with the duration of burial, providing another relative measure for site chronology.

Oxygen Isotope Analysis

Oxygen isotope analysis is a powerful tool for reconstructing past climates and studying the movement of ancient populations. It is based on the ratio of two stable isotopes of oxygen, oxygen-16 and oxygen-18, found in water and biological remains.

Principles of Analysis

The ratio of oxygen isotopes in precipitation changes based on global temperature and the volume of continental ice sheets. When the climate is colder, more oxygen-16 is trapped in ice sheets, leading to a higher concentration of oxygen-18 in the oceans. Marine organisms incorporate these isotopes into their shells. By analyzing these shell remains, scientists reconstruct past sea surface temperatures and global ice volumes.

Application in Bioarchaeology

Oxygen isotopes are also recorded in human teeth. As teeth form, they incorporate oxygen from the water consumed by the individual. Because the oxygen isotope signature of water varies by geographic region, analyzing these isotopes in tooth enamel allows researchers to identify if an individual migrated during their childhood.

Summary of Dating and Analytical Techniques

Additional Analytical Facts

• The hydration rind on obsidian is measured in microns. One micron is equal to one-thousandth of a millimeter. Collagen is a structural protein found in bone that is essential for radiocarbon dating. If collagen has degraded significantly due to soil acidity or environmental exposure, radiocarbon dating becomes impossible.
• Stable isotopes do not decay over time. Their ratios remain constant in biological tissues once formed, making them permanent chemical markers of the environment experienced during the time of tissue growth.
• Geochemical sourcing, often paired with obsidian hydration, involves analyzing trace elements to identify the exact volcanic quarry where the raw obsidian was collected. This provides direct evidence of ancient trade and procurement networks.
• Strontium isotope analysis is frequently conducted alongside oxygen isotope analysis to track human migration. While oxygen reflects water sources, strontium reflects the local geology of the area where an individual resided.

Mass spectrometry is the laboratory technique used to measure the precise ratios of stable isotopes in archaeological samples. It provides highly accurate data necessary for paleoclimatic modeling and reconstructing individual life histories.