SR/CA RATIOS IN MODERN MEXICAN DIETS DOMINATED BY ALKALI-PROCESSED MAIZE: IMPLICATIONS FOR PALEODIETARY RECONSTRUCTIONS

The ratio, Sr/Ca, measured in human bone and teeth, is widely used in archaeological dietary reconstructions. Sr/Ca generally decreases with trophic level up a food chain because of the physiological preference by consumers for Ca over Sr, meaning that carnivores should display lower Sr/Ca than vegetarians. We find that this pattern can be reversed if diets are rich in alkali-processed maize. As part of a geochemical and Sr isotopic study of malnutrition in the Solis Valley, Mexico, we have examined elemental fractionations that occur during the production of the dominant food item, tortillas (55-70% of Solis dietary calories). Corn (Sr and Ca poor) and cal (a Sr-rich mix of CaO , Ca(OH)2, and residual calcite made by sintering marine limestone) are boiled together to improve maize digestibility, tortilla flour workability and flavor. Cal strongly influences both the isotopic and chemical characteristics of the processed corn and therefore the tortillas. We have simulated in the laboratory the process by which tortilla flour is made in Solis households. We have measured Sr and Ca of each ingredient and product in the several step process (in triplicate) for six separate samples of cal, determining the degree of mass balance at each step in the process. Sr/Ca in typical foods elsewhere (vegetables, 0.006 to 0.008 and meat, ~0.002) from Burton & Wright, 1995) are more than an order of magnitude higher than in Solis cal (0.00015), tortillas (0.00018) and deciduous teeth (0.00015 – 0.00018) from Solis. Since cal has such a low value of Sr/Ca, its use would make consumers of alkali-processed maize look to an archaeologist like high-end carnivores rather than vegetarians.