Oxygen isotope fractionation between human phosphate and water revisited

Abstract

The oxygen isotope composition of human phosphatic tissues (δ¹⁸O_P) has great potential for reconstructing climate and population migration, but this technique has not been applied to early human evolution. To facilitate this application we analyzed δ¹⁸O_P values of modern human teeth collected at 12 sites located at latitudes ranging from 4°N to 70°N together with the corresponding oxygen composition of tap waters (δ¹⁸O_W) from these areas. In addition, the δ¹⁸O of some raw and boiled foods were determined and simple mass balance calculations were performed to investigate the impact of solid food consumption on the oxygen isotope composition of the total ingested water (drinking water + solid food water). The results, along with those from three, smaller published data sets, can be considered as random estimates of a unique δ¹⁸O_W/δ¹⁸O_P linear relationship: δ¹⁸O_W = 1.54(±0.09) × δ¹⁸O_P−33.72(±1.51) (R² = 0.87: p [H₀:R² = 0] = 2 × 10⁻¹⁹). The δ¹⁸O of cooked food is higher than that of the drinking water. As a consequence, in a modern diet the δ¹⁸O of ingested water is +1.05 to 1.2‰ higher than that of drinking water in the area. In meat-dominated and cereal-free diets, which may have been the diets of some of our early ancestors, the shift is a little higher and the application of the regression equation would slightly overestimate δ¹⁸O_W in these cases.

Introduction

In recent years, the study of prehistoric cultures has benefited from the use of stable isotope analyses. The majority of these studies have focused on the Holocene portion of human evolution, however, isotopic investigations may provide critical information about our earliest ancestors' diet and ecological setting (climate and environment). Such information plays a critical role in scenarios that seek to explain the evolution of early humans. For example, stable nitrogen and carbon isotope analyses of bone collagen (e.g., Ambrose and De Niro, 1986, Bocherens et al., 1991, Fizet et al., 1995, Richards et al., 2005) and carbon isotope analysis of bone and tooth carbonate (Lee-Thorp et al., 1994, Van der Merwe et al., 2003, Sponheimer et al., 2005) have been used to reconstruct the diet of some of our pre-Holocene ancestors. However, oxygen isotope compositions of human phosphatic tissues, which can be used to reconstruct climatic conditions (e.g., Fricke et al., 1995, Müller et al., 2003, Daux et al., 2005, Evans et al., 2006), identify foreigners in a population, assess the mobility of human groups (e.g., White et al., 2000, Dupras and Schwarcz, 2001, White et al., 2007), or reconstruct infant feeding behavior (e.g., Wright and Schwarcz, 1998), have not been applied, to our knowledge, to issues of early human evolution.

The ratio of oxygen isotopes in mammalian flesh, bones, and teeth reflects the origin of water imbibed as a liquid and ingested from food. The water contained in food has a complicated relationship with meteoric water and can be significantly enriched in ¹⁸O compared to meteoric water. The relative contribution of water entering the body as a liquid and from food varies from one species to another. Animals with low water turnover are expected to derive more water from isotopically-enriched food sources and less from drinking water than are those with high water turnover (Kohn et al., 1996). For the vast majority of terrestrial vertebrates, water turnover scales to body mass (e.g., Altman and Dittmer, 1968, Eberhardt, 1969). Therefore, the δ¹⁸O of the tissues of large animals should be less affected by their solid food consumption than is the δ¹⁸O of smaller animals. However, the influence of diet on the isotopic composition of the tissues may not depend only on water turnover but also on the proportion of water taken up as water contained in food. By way of example, larger herbivores consume plants that are highly hydrated (80 to 95% water by weight) and may contribute up to 50% of the total ingested water (from statistics of Agriculture and Agri-food Canada). As a result, large herbivores obtain a large proportion (up to 50% for wild herbivores; Kohn et al., 1996) of their oxygen from plants that are isotopically enriched. In contrast, the influence of food on tissue δ¹⁸O is insignificant in animals that are fed dry food (Luz et al., 1984).

Humans are medium size mammals with moderate water turnover. As we are mainly omnivores, we ingest less water from food sources than do herbivores. Therefore, the δ¹⁸O ingested by humans is strongly imprinted by the composition of our drinking water, which is strongly linked to environmental water (Longinelli, 1984, Luz et al., 1984, Levinson et al., 1987). Although there is a linear relationship between the oxygen isotope composition of human phosphate and the composition of meteoric water, the three previously-published fractionation equations differ in both their slope and intercept values (Longinelli, 1984, Luz et al., 1984, Levinson et al., 1987). This variation may result from the use of different analytical techniques among studies, small datasets that sample a restricted range of variation, or differences in the timing and duration of crown mineralization from one individual to another. Additionally, although not thought to be a big influence, diet may influence the oxygen isotopic composition of human tissues and contribute to the variation among studies. Although thought to be small, the effect of specific diets on δ¹⁸O_P is unknown. One can question the impact on δ¹⁸O_P of a vegetarian or a meat-based diet, the effect of cereal consumption, or the consequence of cooking food. If specific diets prove to have a sizable influence on δ¹⁸O_P, they would need to be considered when reconstructing paleoclimate from the isotopic signature of phosphatic tissues. Conversely, the δ¹⁸O_P of individuals of known isotopic context (δ¹⁸O_W) can provide information on their dietary practices.

In this study, we refine our knowledge of the oxygen isotope fractionation between water and phosphate in human tooth enamel. We provide a new set of oxygen isotope data for teeth of recent humans from 4°N to 70°N. We compare regressions between δ¹⁸O_P and the δ¹⁸O_W of their likely drinking water and of meteoric water. We test whether the differences among the previously published datasets (Longinelli, 1984, Luz et al., 1984, Levinson et al., 1987; this study) are statistically significant. We then investigate the effect of diet on δ¹⁸O_P by measuring the oxygen isotope compositions of water from raw and cooked vegetables, fish, and meat by modeling the impact of variable proportions of these constituents on δ¹⁸O_P.