Local water resource variability and oxygen isotopic reconstructions of mobility: A case study from the Maya area

Stable oxygen isotopic studies of migration and mobility using human tissue are based on the premise that δ18O values of potable water sources are relatively stable over time and vary predictably across the landscape. Local variability of δ18O is generally assumed to be negligible and negated by an averaging effect as oxygen is consumed primarily as imbibed liquid and incorporated into tissue. This study tests the assumption that variability of δ18O ratios of water from within a region are insufficient to impact the reconstruction of ancient mobility using isotopic measures of human tooth and bone. This case study focuses on the Maya area of Guatemala and Mexico, a region where many analyses of oxygen isotopic ratios in ancient human tissue has been conducted. 71 water samples were obtained over the course of five years, from precipitation, streams, rivers, lakes, aguadas (watering holes), civals (perennial wetlands), and caves. Unlike most baseline studies, the objective here was the repeated testing of a diversity of sources from the same location. δ18OVSMOW values from surface and meteoric water range from − 16.0 to + 3.6‰. The precipitation range is − 16.0 to 1.58‰ (n = 22) with the highest and lowest values measured at the same location (El Zotz, Guatemala). The surface water range is − 9.2 to 3.6 (n = 48), the lowest value is the Usumacinta River at Yaxchilan and the highest value is from a lake located 65 kilometers away. The δ18O values of local streams were remarkably stable over time. Major rivers, such as the Usumacinta River, have relatively constant values. Closed water basins (lakes, aguadas, and civals) demonstrated an evaporative effect and δ18O was variable both between basins and in individual basins over time. In the Petén region, mean δ18O of precipitation is statistically different than that of lake and aguada water. Local δ18O differences in consumed water likely explain the variability observed in human tooth enamel at Tikal and other Petén region sites. This case study demonstrates that local isotopic values may be more diverse than is generally assumed in studies of human mobility that rely on little to no baseline data. Future studies of mobility based on oxygen isotopic measures of ancient human tooth and bone should include measures of both meteoric water and a diverse range of surface and groundwater to develop more robust interpretations of oxygen isotopic values in ancient human tissue.