EVIDENCE FOR DIFFERING METHODS OF ALKALI-PROCESSING OF MAIZE IN PREHISTORIC MEXICO BASED ON 87SR/86SR IN BEDROCK, BONES AND TEETH

The nutritional value, flavor and texture of cornmeal are enhanced by boiling the corn in a high pH solution before grinding. In modern Mexico, this is achieved with "cal", a mixture of CaO and Ca(OH)2 made from sintered limestone. In some ancient cultures without access to limestone, wood ash was used instead. Aspects of the history of alkali processing can be deciphered from Sr isotopic data on bedrock, teeth and bones from modern and ancient Mexican populations. 87Sr/86Sr values in modern deciduous teeth from six Solis Valley villages in the Trans-Mexican Volcanic Belt, ~150 km NW of Mexico City, do not match the local landscape. Lying between 0.7058 and 0.7077, the isotopic values are higher than the local bedrock, soils, well waters (0.7038-0.7057), reflecting the importance (albeit variable) of the Sr-rich "cal" (0.7074-0.7075) in the diet. The cal data, coupled with the seawater 87Sr/86Sr evolution curve, are consistent with a late Cretaceous (Kr) limestone source for the cal. Published 87Sr/86Sr data (Price et al, 2000) for human bone from burials at Teotihuacan (Mexico City) and Monte Alban (Oaxaca) tell two distinct stories from the view of alkali processing of maize. At Monte Alban where the bedrock includes late-Kr limestone, bones and tooth enamel 87Sr/86Sr match late-Kr seawater (avg: 0.7075), consistent with the use of locally produced cal. At Teotihuacan however, human bone (and modern rabbit bone) 87Sr/86Sr values are surprisingly narrowly restricted at 0.7046, a value far below that of any marine limestone. Maize processing, if practiced, either employed wood ash from local vegetation or caliche produced by weathering of very local basaltic volcanic bedrock (possibly one single flow), but could not have involved the use of limestone.