Thursday, June 4, 2009

Magnetic properties abstracts from AGU

The last few abstracts from these two session on geophysics and archaeology, looking at magnetic properties at two type of archaeological materials.

The image is unrelated, other than it is a cool representation of magnetic properties of thin films.


Magnetic Susceptibility of Ancient and Modern Potsherds Using a Fast, Cheap and Portable Probe
Glover, P W (, Université, Département de géologie et de génie géologique, Québec, QC G1V 0A6, Canada

It has been estimated that there exist over 100 million ancient potsherds in various collections worldwide, many of which have never been studied and for which the provenance is ambiguous or unknown. Indeed, many collections are extremely badly catalogued or completely mixed-up. We have been using a novel portable probe to measure the magnetic susceptibility and electrical conductivity of potsherds in the hope that this fast, cheap and portable measurement can provide data that will help to sort similar looking potsherds into sets in a manner which may help to define their provenance. The probe, which resembles a firearm, uses the Hall effect to make a non-destructive measurement on the potsherd. The probe is attached to an Dell Axim X51 PDA, which runs software that allows the measurement to be carried out and logged. Each measurement, which is made by pressing a button on the gun, takes only a few seconds. We have made measurements on three suites of ancient potsherds as well as a suite of modern potsherds that were created by using a garden centre and a hammer! In each case a set of 5 stacked measurements were taken on the inside and outside faces of the potsherd in two perpendicular directions. Potsherds which were either (i) so flat that the inside and outside could not be distinguished, (ii) so curved (radius of curvature less than 5 cm) that the probe tip could not approach the surface sufficiently closely, or (iii) smaller than the probe tip, were excluded from the suite of measurements. Each suite contained over 50 measureable potsherds. All measurements were completed within one day. In this pilot study we found that (1) each suite was represented by a normal distribution of magnetic susceptibility values, (2) the four different suites could be distinguished statistically on the basis of their magnetic susceptibility measurements, but (3) the distinction was not sufficiently powerful to separate all potsherds (i.e., there was a significant overlap of the susceptibility distributions). This seems to confirm that the method may be used to give additional information that can be used to help to provenance a potsherd, but the susceptibility measurement is not sufficient on its own. In addition, we found that (4) the electrical conductivity measurements depended upon the local conditions (mainly humidity) and was of no use in distinguishing between suites of potsherds. However, most interestingly, we found that (5) there is a statistically significant difference between the magnetic susceptibility measured on the inside face and that measured on the outside face for all three ancient suites of potsherd, but not for the modern potsherds. The reason for this is not currently known. One hypothesis is that the difference is due to the manufacturing style. Further studies are being planned to extend our database.

Magnetic Fingerprinting of Central Mediterranean Obsidian Source Groups
Weaver, I (, Department of Earth and Environment, Franklin & Marshall College, PO Box 3003, Lancaster, PA 17604-3003, United States
Sternberg, R (, Department of Earth and Environment, Franklin & Marshall College, PO Box 3003, Lancaster, PA 17604-3003, United States
Tykot, R H (, Department of Anthropology, University of South Florida, 4202 East Fowler Avenue, SOC 107, Tampa, FL 33620-8100, United States

The Central Mediterranean islands of Lipari, Palmarola, Pantelleria, and Sardinia are known to be the only sources of obsidian in that region (Tykot, 2002). The obsidian on each source island has a unique chemical composition (i.e., a fingerprint) that has allowed Tykot (2002) to provenance Neolithic obsidian artifacts from archaeological sites throughout the Central Mediterranean to these four sources. Thus, Tykot (2002) revealed material distribution patterns otherwise inaccessible to archaeologists. An exploratory study by McDougall et al. (1983) demonstrated the potential use of magnetic properties to distinguish Central Mediterranean obsidians. If each obsidian source group has a unique magnetic fingerprint, magnetic provenancing might serve as a quick, inexpensive, and non-destructive alternative to chemical provenancing. Our research continues the work started by McDougall et al.; we set out to characterize the Central Mediterranean obsidians and search for magnetic fingerprints that distinguish them. Our preliminary study of the magnetic properties of 30 geologic obsidian specimens from Sardinia A, B, and C subgroups and the three other Central Mediterranean island sources suggests that each source has a unique combination of magnetic properties that could allow magnetic provenancing of archaeological obsidians. For example, the combination of natural remanent magnetization (NRM; median [med] = 2.46 x 10-4 Am2/kg, interquartile range [IQR] = 9.77 x 10-4 Am2/kg) and magnetic susceptibility (low field, low frequency magnetic susceptibility [χlflf]: med = 27.0 x 10-6 m3/kg, IQR = 75.9 x 10-6 m3/kg) seems to have good potential to discriminate the groups. However, statistical analysis shows that there is significant overlap in the confidence intervals of these variables, a limitation which appears to be partially imposed at this point by the small sample size. Discriminant analysis of several magnetic variables also shows potential to assist in classification (i.e., provenancing) of Central Mediterranean obsidians. We have obtained discriminant functions that correctly classify over 80% of the obsidian specimens in leave-one-out cross validations using as few as three log- transformed magnetic parameters: NRM, χlflf, and saturation isothermal remanent magnetization (SIRM; med = 7.59 x 10-3 Am2/kg, IQR = 2.85 x 10-2 Am2/kg). Backfield coercivity of remanence, median destructive field, percent frequency effect, and saturation coercivity seem to contribute the least to group discrimination, yet their effects are statistically significant. Even though initial tests of discriminant analysis look promising, it should be noted that the small sample sizes and relatively large number of variables used in the discriminant analyses violate the test's assumptions and may make the resulting discriminant functions unusable; this will be addressed in future research. The results from our preliminary study suggest that the Central Mediterranean sources of obsidian might have characteristic and distinguishable magnetic fingerprints. However, many more geologic obsidian specimens must be measured to define the ranges of the source groups' magnetic properties and to further test this hypothesis.

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