We develop and test a model to predict the geographic region-oforigin of humans based on the stable isotope composition of their scalp hair. This model incorporates exchangeable and nonexchangeable hydrogen and oxygen atoms in amino acids to predict the ␦ 2 H and ␦ 18 O values of scalp hair (primarily keratin). We evaluated model predictions with stable isotope analyses of human hair from 65 cities across the United States. The model, which predicts hair isotopic composition as a function of drinking water, bulk diet, and dietary protein isotope ratios, explains >85% of the observed variation and reproduces the observed slopes relating the isotopic composition of hair samples to that of local drinking water. Based on the geographical distributions of the isotope ratios of tap waters and the assumption of a ''continental supermarket'' dietary input, we constructed maps of the expected average H and O isotope ratios in human hair across the contiguous 48 states. Applications of this model and these observations are extensive and include detection of dietary information, reconstruction of historic movements of individuals, and provision of regionof-origin information for unidentified human remains. stable isotopes ͉ water ͉ anthropology ͉ forensics ͉ meteoric water T he carbon (␦ 13 C), nitrogen (␦ 15 N), and sulfur (␦ 34 S) isotope ratios of humans, other animals, and microbes are strongly correlated with the isotope ratios of their dietary inputs (1-5). The adage ''You are what you eat'' reflects the observation that there are limited differences (Յ1‰) between heterotrophic organisms and their diet in either the ␦ 13 C or ␦ 34 S values (6-8). These small isotopic differences arise because of fractionation events during metabolism; they also reflect that diet-derived carbon and sulfur are the only input sources into most heterotrophs. Although there are larger ''spacing'' differences in ␦ 15 N values (Ϸ3‰) between an organism and its dietary source (9), the isotopic relationships between organism and diet persist. ␦ 13 C, ␦ 15 N, and ␦ 34 S values provide limited geographic-based information about the origins of a food source. Hydrogen (␦ 2 H) and oxygen (␦ 18 O) isotope ratios of organic matter are more useful, because ␦ 2 H and ␦ 18 O values of precipitation and tap waters vary along geographic gradients (10, 11).Although differences in the ␦ 2 H and ␦ 18 O values of scalp hair have been noted in humans (12), less is known about dietorganism patterns of ␦ 2 H and ␦ 18 O values. Four potential sources can be important: dietary organic molecules, dietary waters, drinking waters, and atmospheric diatomic oxygen. Hobson et al.(13) provided evidence that ␦ 2 H values of drinking water were incorporated into different proteinaceous tissues of quail, although no mechanistic basis was proposed for this relationship. The ␦ 2 H values of bird feathers and butterfly wings (both are largely keratin) and water in the region in which the tissue was produced are highly correlated (14, 15). showed that Ϸ70% of the oxygen and Ϸ...

[1] Understanding links between water consumers and climatological (precipitation) sources is essential for developing strategies to ensure the long-term sustainability of water supplies. In pursing this understanding a need exists for tools to study and monitor complex human-hydrological systems that involve high levels of spatial connectivity and supply problems that are regional, rather than local, in nature. Here we report the first national-level survey of stable isotope ratios in tap water, including spatially and temporally explicit samples from a large number of cities and towns across the contiguous United States. We show that intra-annual ranges of tap water isotope ratios are relatively small (e.g., <10% for d 2 H) at most sites. In contrast, spatial variation in tap water isotope ratios is very large, spanning ranges of 163% for d 2 H and 23.6% for d 18 O. The spatial distribution of tap water isotope ratios at the national level is similar to that of stable isotope ratios of precipitation. At the regional level, however, pervasive differences between tap water and precipitation isotope ratios can be attributed to hydrological factors in the water source to consumer chain. These patterns highlight the potential for monitoring of tap water isotope ratios to contribute to the study of regional water supply stability and provide warning signals for impending water resource changes. We present the first published maps of predicted tap water isotope ratios for the contiguous United States, which will be useful in guiding future research on humanhydrological systems and as a tool for applied forensics and traceability studies.

The structural proteins that comprise approximately 90% of animal hair have the potential to record environmentally and physiologically determined variation in delta2H and delta18O values of body water. Broad, systematic, geospatial variation in stable hydrogen and oxygen isotopes of environmental water and the capacity for rapid, precise measurement via methods such as high-temperature conversion elemental analyzer/isotope ratio mass spectrometry (TC/EA-IRMS) make these isotope systems particularly well suited for applications requiring the geolocation of hair samples. In order for such applications to be successful, however, methods must exist for the accurate determination of hair delta2H and delta18O values reflecting the primary products of biosynthesis. Here, we present the results of experiments designed to examine two potential inaccuracies affecting delta2H and delta18O measurements of hair: the contribution of non-biologic hydrogen and oxygen to samples in the form of sorbed molecular water, and the exchange of hydroxyl-bound hydrogen between hair keratin and ambient water vapor. We show that rapid sorption of molecular water from the atmosphere can have a substantial effect on measured delta2H and delta18O values of hair (comprising approximately 7.7% of the measured isotopic signal for H and up to approximately 10.6% for O), but that this contribution can be effectively removed through vacuum-drying of samples for 6 days. Hydrogen exchange between hair keratin and ambient vapor is also rapid (reaching equilibrium within 3-4 days), with 9-16% of the total hydrogen available for exchange at room temperature. Based on the results of these experiments, we outline a recommended sample treatment procedure for routine measurement of delta2H and delta18O in mammal hair.

A semimechanistic model has recently been proposed to explain observed correlations between the H and O isotopic composition of hair from modern residents of the USA and the isotopic composition of drinking water, but the applicability of this model to hair from non-USA and preglobalization populations is unknown. Here we test the model against data from hair samples collected during the 1930s-1950s from populations of five continents. Although C and N isotopes confirm that the samples represent a much larger range of dietary "space" than the modern USA residents, the model is able to reproduce the observed delta(2)H and delta(18)O values given reasonable adjustments to 2 model parameters: the fraction of dietary intake derived from locally produced foods and the fraction of keratin H fixed during the in vivo synthesis of amino acids. The model is most sensitive to the local dietary intake, which appears to constitute between 60% and 80% of diet among the groups sampled. The isotopic data are consistent with a trophic-level effect on protein H isotopes, which we suggest primarily reflects mixing of (2)H-enriched water and (2)H-depleted food H in the body rather than fractionation during biosynthesis. Samples from Inuit groups suggest that humans with marine-dominated diets can be identified on the basis of coupled delta(2)H and delta(18)O values of hair. These results indicate a dual role for H and O isotopic measurements of keratin, including both biological (diet, physiology) and environmental (geographic movement, paleoclimate) reconstruction.