American Scientist

To the Editors:

In their interesting and well-reasoned article, "The Ancient Ceramics of West Mexico" (May–June), Robert Pickering and Ephraim Cuevas speculate that surface water, percolating into shaft tombs, could have been one source of manganese for the manganese oxide precipitated onto the surfaces of ceramic figurines by metal-reducing bacteria.

Although this is certainly a possibility in cases where shaft tombs are situated deep enough below the surface for water to acquire manganese in the parts-per-million range through percolation down the soil profile, it seems less likely for more shallow tombs (those less than 5 meters deep). Both diagenetic (trapped in mineral lattices of soil particles) and anthropogenic manganese (adsorbed to soil particles and in clay interlayers as a result of human activity) typically are resistant to leaching at shallow depths, although this depends partly on weathering rates, soil geochemistry and soil pH, among other factors.

In addition to water percolation, perhaps another source of manganese in the tombs was human tissues, which require manganese for bone metabolism and many enzymatic reactions. Necrophilous insects feasting on the remains of a tomb's occupants could have consumed elemental manganese and carried it with them when they came to rest on ceramic artifacts, such that the ultimate decay of the insects may have allowed their puparia to mineralize with the help of bacteria.

Christian Wells
Department of Anthropology
Arizona State University

Dr. Pickering replies:

I am grateful for Dr. Wells's thoughtful comments. I can envision an experimental tomb reconstruction that uses infested animal corpses, modern ceramic figurines, etc. to approximate the tomb conditions and to monitor the air quality of the tomb over a period of time. Perhaps that will give us a more definitive answer to the question. Clearly, there are still a number of pieces to the puzzle that we do not yet have.