Athro, Limited Gubik Field Trip Section 20 or Section 65

Hiatella arctica Scale Bar is 1 cm

You collect a number of fossil shells of the bivalve Hiatella arctica that are clearly in situ in the sediments (not float), and bring them to a colleague who does aminostratigraphy.

Your colleague returns the following results:

Inferred Age: About 500,000 years before present.

[These value are published numbers from samples taken in this stratigraphic unit at skull cliff (See: Brigham,1985 and Brigham-Grette and Carter, 1992), though not in exactly this sampling trench. ]

Aminostratigraphy relies on the observation that all amino acids in organisms are left handed, and that over time in the environment, free amino acids derived from organisams gradualy return to a mixture of left and right handed. This process is dependent on both time and temperature. For example, the amino acid isoleucine is made by living organisms as L-isoleucine, but buried in sediments, it can change its conformation to D-isoleucine

During life, all of the amino acids that a bivalve incorporates from its soft tissues into its shell are left handed (all of the isoleucine is L-isoleucine). After the bivalve dies, the amino acids gradualy convert to both right and left handed forms (some of the L-isoleucine becomes the mirror image isomer D-isoleucine). This reaction runs in both directions, so eventualy (in a few million years) an equilibrium is reached (at about 1.3 to 1 for the D-isoleucine to L-isoleucine ratio). Thus, a sample that is mostly L-isoleucine is quite young, while a sample that contains a roughly even mixture of D- and L-isoleucines is quite old. This is expressed as a ratio of D-isoleucine/L-isoleucine, abbreviated aIle/Ile. A small ratio (near zero) indicates a young sample, a ratio closer to 1.3 indicates an older sample. However, the rate at which this equilibrium is reached also depends on temperature. It proceeds more rapidly at higher temeratures. Thus, getting a date from amino acid racemization is not a simple matter of pulling out some chemicals, finding a ratio, and getting a date.

Nome is some 10 degrees of latitude South of Skull Cliff, and over the last few million years has fairly consistently been about 8 degrees C warmer. The table below shows the difference in aIle/Ile ratios between samples that are considered to be of similar ages from Nome and Skull Cliff.

Thus obtaining a amino-acid racemization date from a sample is not a simple task. One needs to have samples from somewhere else that has a similar temperature history to compare with, and that other history needs to be calibrated with some other dating method.