A stable isotope aridity index for terrestrial environments

Abstract

We use the oxygen isotopic composition of tooth enamel from multiple mammalian taxa across eastern Africa to present a proxy for aridity. Here we report tooth enamel ␦ 18 O values of 14 species from 18 locations and classify them according to their isotopic sensitivity to environmental aridity. The species are placed into two groups, evaporation sensitive (ES) and evaporation insensitive (EI bioapatite ͉ East Africa ͉ oxygen-18 ͉ mammals ͉ water use T errestrial responses to major climate changes, such as glaciations, orogenic events, and shifts in ocean circulation, are often characterized in terms of water availability or aridity (1-3). Although aridity proxies exist for different terrestrial settings (4-6), they are not applicable in every circumstance and additional proxies must be developed for further study of terrestrial environmental change. The 18 O composition of bioapatite has been used as a proxy for rainfall ␦ 18 O and seasonality of past environments (7, 8), but its utility in paleoenvironmental problems is limited by the complexity of climatic, environmental, physiological, and behavioral variables that influence bioapatite ␦ 18 O values. The correlation between bioapatite ␦ 18 O values and both meteoric water ␦18 O values and relative humidity demonstrate that animals have different isotopic responses to environmental change (7,9,10). In this study, we present tooth enamel ␦ 18 O data of 14 mammal species sampled from 18 locations in eastern Africa, which represent a gradient in environmental aridity. This data set shows that the tooth enamel ␦18 O values of some species vary with aridity whereas those of other species track the 18 O composition of meteoric water. We use the different isotopic responses of the sampled species as the empirical basis for an aridity proxy. ResultsThere is a marked increase in water deficit (WD) from the closed canopy Ituri Forest in the Democratic Republic of Congo to the arid shrublands of Lake Turkana in northern Kenya (Table 1, which is published as supporting information on the PNAS web site). WD in these regions negatively correlates to mean annual relative humidity (RH) (P Ͻ 0.01) and is used as a measure of aridity. Although previous studies compare bioapatite ␦18 O values with RH (6, 9, 10), RH data are not used here because they are not available for all study locations. (Fig. 1). Baboons from Mpala are the only mixed (C 3 -C 4 ) feeding baboons (otherwise C 3 browsers), and when not considered the baboon regression has a P value Ͻ0.001 instead of 0.14. enamel-mw of a second group, giraffids (Giraffa camelopardalis and Okapia johnstoni), oryx (Oryx beisa), dikdik (Madoqua kirkii), Grant's gazelle (Gazella granti), and buffalo (Syncerus caffer), increases with WD (P values range from Ͻ0.001 to 0.08) (Fig. 2 a-e). The regression coefficients (slopes) of the second group do not vary significantly from each other (P Ͼ 0.1, F test). DiscussionThe Evaporation Sensitive (ES)-Evaporation Insensitive (EI) Distinction. The regressions between WD and enamel-mw show t...

Dental microwear texture as a proxy for diet in xenarthrans

“…Additionally, Macropus taxa living today acquire most of their water from vegetation (e.g., Dawson 1995;Nowak 1999;Dawson et al 2004), consistent with other "evaporation-sensitive" taxa capable of tracking changes in water deficits (Levin et al 2006). Macropus δ 18 O bulk values from all horizons examined at Cuddie Springs are consistent with Macropus values from low rainfall regimes (Prideaux et al 2007; Fig.…”

Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia

“…A Kruskal-Wallis test indicates strong divergence in δ 18 O values, which in part, reflects differences in water dependence (34) between Au. afarensis and other mammalian families from the Hadar Formation ( …”

“…afarensis show that this species was distinct from many contemporaneous taxa in terms of drinking habits and/or other factors affecting oxygen isotopic composition (34). Fig.…”

Carbon and oxygen stable isotope compositions of late Pleistocene mammal teeth from dolines of Ajoie (Northwestern Switzerland)

“…The δ 18 O H2O values of meteoric water are controlled by fractionation processes during the atmospheric water cycle and water vapour transport, and, especially at mid-and high-latitudes, air temperature and humidity during precipitation are the major controlling factors (Dansgaard, 1964;Rozanski et al, 1993;Fricke and O'Neil, 1999). Bioapatite δ 18 O values of water-dependent, obligate drinking mammals record best meteoric water values and, hence, climatic conditions (Longinelli, 1984;Kohn, 1996;Levin et al, 2006).…”