Bat Cave

Pleistocene Lake San Agustin and the Domestication of Maize

The viewer is standing within a cave looking out onto the vast, flat, treeless landscape of the playa. The cave walls are mostly silhouetted and form a ragged frame. Clouds cover much of the sky. Low mountains form the horizon. The playa is gray with a scattered cover of dry, yellow grass. In the foreground is a visitor log-in box.

Introduction

Geologic Time

Geology is the study of the planet earth- the materials it is made of, the processes that act on those materials, the products formed, and the history of the planet and its life forms since its origin. ~ USGS

01 / 08

Maize Time

"Humans grow maize and maize grows humans." ~ Michael Blake

Click on the "i" for information about each image. Click on each image to enlarge.

Isotope Time

The organic material in Bat Cave was some of the first excavated material to be dated using the C-14 radioactive carbon dating method.

The trackways at White Sands are inorganic and cannot be dated on their own. Instead, seeds found with the tracks were dated using the C-14 method.

Isotopes are atoms of a type with the same number of protons, but additional neutrons. The additional neutrons add mass. Example: The element Carbon [C] has 6 Protons. Carbon-12 has 6 protons + 6 neutrons. Carbon-14 has 6 protons + 8 neutrons. The ratio of C-14 to C-12 can be measured by various methods.

Isotopic Ratios are useful:

  1. Isotopic ratios can be measured in material to learn things about the conditions in which that material formed.
  2. When material is formed the system is closed. Therefore the isotopic ratio does not change...unless one isotope is radioactive.
  3. Radioactive isotopes decay at known rates. Therefore the measured ratio can tell us the time since the material was formed, or the organism died, and the system closed.
  4. Brilliant!
  5. But not simple. These methods give absolute dates, years before present, and the dates published may change as improvements to technology and methodology occur. Changes will also occur as our understanding about how isotopes exist and circulate within various earth systems improves.

Isotope uses mentioned in this StoryMap:

Geologic Time Scale. Rocks and minerals can be dated using various types of radioactive isotopes that are locked into the crystal structure of minerals upon formation.  Latest USGS Timescale .

Archeological Studies. Organic material can be dated using C-14, a radioactive isotope, that becomes incorporated into plant tissues and the tissue of those that eat plants and those that eat plant-eaters. Death closes the system, and the clock starts.

Climate Studies. Animals that make their shells from dissolved minerals in seawater lock stable, non-radioactive, oxygen isotopes into their shell structure. This tells us about the O-18/O-16 ratio of seawater, a proxy for glacial ice formation.

Finally...