Enamel δ18O Research Articles

Paleodietary change and its implications for aridity indices derived from δ18O of herbivore tooth enamel

A large body of research provides reconstructions of ancient water deficit—the difference between potential evapotranspiration and precipitation—using an aridity index derived from the δ18O values of tooth enamel from evaporation sensitive (ES) and evaporation insensitive (EI) mammalian herbivores. The former are typically ruminants that obtain most of their water from leafy dicots (the δ18O of their enamel reflects evaporation of leaf water), whereas the latter are non-ruminants dependent on surface water (the δ18O of their enamel reflects local meteoric water). Use of the index requires that fossil taxa are correctly classified as ES or EI. This study examines how paleodietary change can influence such classifications. In modern ruminant ungulates, dependence on drinking water is related to the proportion of grass versus dicots in the diet. This means that paleodietary change in ruminant ungulates should be associated with shifts in drinking behavior, altering enamel δ18O values and biasing the aridity index. Such bias is evident in estimates of water deficit derived from the enrichment of enamel δ18O in fossil Tragelaphini relative to Hippopotamidae from East African fossil sites spanning the last ~4.4Myr. Elevated grass consumption (measured by enamel δ13C) in the former is associated with reduced enrichment of enamel δ18O relative to the latter, translating to lower estimates of water deficit. In conducting such analyses of fossil ungulates, the diet of those ungulates must be screened to ensure consistency with the assumptions of the method.

Aridity and hominin environments

Aridification is often considered a major driver of long-term ecological change and hominin evolution in eastern Africa during the Plio-Pleistocene; however, this hypothesis remains inadequately tested owing to difficulties in reconstructing terrestrial paleoclimate. We present a revised aridity index for quantifying water deficit (WD) in terrestrial environments using tooth enamel δ18O values, and use this approach to address paleoaridity over the past 4.4 million years in eastern Africa. We find no long-term trend in WD, consistent with other terrestrial climate indicators in the Omo-Turkana Basin, and no relationship between paleoaridity and herbivore paleodiet structure among fossil collections meeting the criteria for WD estimation. Thus, we suggest that changes in the abundance of C4 grass and grazing herbivores in eastern Africa during the Pliocene and Pleistocene may have been decoupled from aridity. As in modern African ecosystems, other factors, such as rainfall seasonality or ecological interactions among plants and mammals, may be important for understanding the evolution of C4 grass- and grazer-dominated biomes.

Constraining paleohydrologic change during the Paleocene-Eocene Thermal Maximum in the continental interior of North America

Quantifying the relationship between carbon cycle perturbations and the hydrologic cycle in the geologic past is crucial to accurately modeling how future anthropogenic carbon emissions and resulting radiative forcing might affect the hydrologic cycle. Interpreting changes in proxy records for insight into paleohydrologic change is complex, and documented records of paleohydrologic response to past global warming are rare. We use the relationship between two independent proxy records, the stable isotope ratios of hydrogen in n-alkanes and oxygen in tooth enamel of Coryphodon, to examine paleohydrologic change in the continental interior of North America during the Paleocene-Eocene Thermal Maximum (PETM) hyperthermal ~56Ma. The Coryphodon δ18O record allows us to infer shifts in surface water isotope values through time by applying an empirical relationship between tooth enamel and precipitation δ18O values. Precipitation δ18O values increase by ~4‰ during the PETM, but n-alkane hydrogen isotope ratios show no directional change during the PETM. We explore multiple hypotheses that could explain the differences between the isotope records, including a change in apparent fractionation as a result of plant community change, a shift in the slope of the local meteoric water line, a change in the season of rainfall, or a shift in the season of lipid production. We model the changes that would be required to reconcile the isotope records for each hypothesis and evaluate the likelihood of each of the scenarios. We posit that the most likely hypothesis for the observed differences between the isotope records is a change in either the season of rainfall or the season of lipid production during the PETM.

Stable isotope dietary reconstructions of herbivore enamel reveal heterogeneous savanna ecosystems in the Plio-Pleistocene Malawi Rift

The Plio-Pleistocene expansion of Eastern Africa savanna ecosystems was a major driver for morphological and behavioral innovations in hominin evolution. Most evidence for hominin ecosystem reconstructions originates from the Eastern Rift in today's Somali-Masai Endemic Zone. We provide stable carbon (δ13C) and oxygen (δ18O) isotope data from 22 bovid, equid, suid, hippopotamid and elephant teeth from the southern part of the East African Rift and supplement these by δ18O values of present-day meteoric water. This region (Karonga Basin, Malawi) hosts remains of Homo rudolfensis and Paranthropus boisei. It is situated between hominin-bearing sites in Eastern and Southern Africa and therefore fills an important geographical gap for hominin evolution.Tooth enamel δ13C and δ18O data of 14 large-bodied mammalian herbivore species from Units 2 and 3 (ca. 4.3 to 0.6Ma) in the Chiwondo Beds provide information about foraging strategies and vegetation patterns. Our stable isotope data reflect C3-dominated ecosystems that permitted access to open C4-environments for migratory mammals. Many analyzed taxa show evidence for mixed diets with high abundances of C3-biomass consumption; only selected bovids were specialized grazers.δ18O of present-day meteoric water reflects rainfall amount and seasonality, elevation, and lake evaporation. δ18O values of Karonga Basin herbivore enamel are generally low and cover values similar to present-day drinking water, indicating both, evaporated and freshwater sources. Comparison of δ13C values from the Karonga Basin with other Eastern African herbivore communities reveals significant differences in dietary patterns with much more generalized mixed C3/C4, and pure C3 feeding behaviors in the Malawi Rift. Similarly, enamel δ18O values are lower than in most Plio-Pleistocene Eastern African herbivores, suggesting more humid conditions. Collectively, our data support models in which early hominin foraging activity included access to C3-dominated biomes, complicating traditional interpretations linking early human evolutionary innovations to a shift towards savanna habitats.

Stable isotopic composition of fossil mammal teeth and environmental change in southwestern South Africa during the Pliocene and Pleistocene

The past 5millionyears mark a global change from the warmer, more stable climate of the Pliocene to the initiation of glacial–interglacial cycles during the Pleistocene. Marine core sediment records located off the coast of southwestern Africa indicate aridification and intensified upwelling in the Benguela Current over the Pliocene and Pleistocene. However, few terrestrial records document environmental change in southwestern Africa over this time interval. Here we synthesize new and published carbon and oxygen isotope data of the teeth from large mammals (>6kg) at Langebaanweg (~5millionyears ago, Ma), Elandsfontein (1.0–0.6Ma), and Hoedjiespunt (0.35–0.20Ma), to evaluate environmental change in southwestern Africa between the Pliocene and Pleistocene. The majority of browsing and grazing herbivores from these sites yield enamel δ13C values within the range expected for animals with a pure C3 diet, however some taxa have enamel δ13C values that suggest the presence of small amounts C4 grasses at times during the Pleistocene. Considering that significant amounts of C4 grasses require a warm growing season, these results indicate that the winter rainfall zone, characteristic of the region today, could have been in place for the past 5millionyears. The average δ18O value of the herbivore teeth increases ~4.4‰ between Langebaanweg and Elandsfontein for all taxa except suids. This increase may solely be a function of a change in hydrology between the fluvial system at Langebaanweg and the spring-fed environments at Elandsfontein, or a combination of factors that include depositional context, regional circulation and global climate. However, an increase in regional aridity or global cooling between the early Pliocene and mid-Pleistocene cannot explain the entire increase in enamel δ18O values. Spring-fed environments like those at Elandsfontein may have provided critical resources for mammalian fauna in the mid-Pleistocene within an increasingly arid southwestern Africa ecosystem.

Standardized pork production at the Celtic village of Levroux Les Arènes (France, 2nd c. BC): Evidence from kill-off patterns and birth seasonality inferred from enamel δ18O analysis

The pig predominates the zooarchaeological assemblage of the second Iron Age settlement of Levroux Les Arènes, in a context characterised by intensive craft and trade activities. The pattern of pig mortality profiles (targeting two-year-old animals), standardized butchery cut-marks and the under-occurrence of ham-bearing bones suggest that there was standardized pork production potentially designed for trade. This hypothesis was investigated through enamel δ18O analysis, allowing the seasonality of pig births to be determined and the reasons for a particular age class in the kill-off patterns to be explained. Results reveal that pig births were spread across more than half a year, suggesting that slaughter was not seasonally constrained, but was performed to target a specific age class and weight/size at death, thus supporting the hypothesis of standardized pork production in Levroux Les Arènes. For the first time, the stable isotope approach – sequential δ18O analysis in pig incisors developed on a modern reference set – has been performed on an archaeological assemblage, after methodological adaptation. The results constitute the first zooarchaeological evidence of such a standardization of meat production from the second century onwards in Gaul.

Season of birth and sheep husbandry in late Roman and Medieval coastal Flanders: A pilot study using tooth enamel δ18O analysis

From the early Roman period, there is archaeological evidence for the exploitation of the Flemish coastal plain (Belgium) for a range of activities, such as sheep herding on the then developing salt-marshes and salt-meadows for the production of wool. During the early Middle Ages, this culminated in the establishment of dedicated ‘sheep estates’. This phase of exploitation was followed by extensive drainage and land reclamation measures in the high Medieval period, transforming areas into grassland, suited for cattle breeding. As part of a larger project investigating the onset, intensification and final decline of sheep management in coastal Flanders in the historical period, this pilot study presents the results of sequential sampling and oxygen isotope analysis of a number of sheep teeth (M2, n = 8) from four late Roman and Medieval sites (dating from 4th to 15th century AD), in order to assess potential variations in season of birth between the different sites and through time. In comparison with published data from herds of known birth season, incremental enamel data from the Flemish sites are consistent with late winter/spring births, with the possibility of some instances of slightly earlier parturition. These findings suggest that manipulation of season of birth was not a feature of the sheep husbandry-based economies of early historic Flanders, further evidencing that wool production was the main purpose of contemporary sheep rearing in the region. Manipulation of season of birth is not likely to have afforded economic advantage in wool-centred economies, unlike in some milk- or meat-based regimes.

Eocene–Oligocene latitudinal climate gradients in North America inferred from stable isotope ratios in perissodactyl tooth enamel

The Eocene–Oligocene transition (~34Ma) was one of the most pronounced episodes of climate change of the Cenozoic. In order to investigate this episode of global climate cooling in North America, we analyzed the carbon and oxygen stable isotope composition of the carbonate component of 19 perissodactyl (horse and rhino) tooth enamel samples from the Eocene–Oligocene rocks of the Cypress Hills Formation (southwestern Saskatchewan, Canada); we then compared the results with previously published data from the US Great Plains (Nebraska, South Dakota, and Wyoming).Average (±1σ) perissodactyl enamel δ13C values (vs. V-PDB) in the Eocene (−8.8±0.3‰) and Oligocene (−9.0±0.3‰) are indistinguishable, suggesting no major change in mean annual precipitation in Saskatchewan across the transition. The δ13C values in Saskatchewan indicate the presence of arid ecosystems and are slightly higher than those in the US Great Plains, suggesting drier conditions at higher latitudes. With respect to oxygen isotopes, average (±1σ) perissodactyl enamel δ18O values (vs. V-SMOW) in the Eocene (19.8±2.0‰) and Oligocene (20.1±3.6‰) are also indistinguishable, suggesting no change in the δ18O of meteoric precipitation across the transition in Saskatchewan. Enamel δ18O variability is much larger in the Oligocene vs. Eocene, indicating a large increase in temperature seasonality. This increase in enamel δ18O variability is much larger than that recorded in the US Great Plains, suggesting that higher latitudes are more sensitive to major episodes of climate change with respect to temperature seasonality. Finally, our data indicate no major change in the Oligocene vs. Eocene latitudinal gradient in local water δ18O in North America, which suggests no change in mean annual temperature gradients across the transition. This result supports the hypothesis that ascribes the climate change of the transition to a drop in atmospheric pCO2 because climate models show that this mechanism produces uniform cooling at mid-latitudes.

Diet and Habitat of Siwalik PrimatesIndopithecus, SivaladapisandTheropithecus

We present dental enamel stable carbon and oxygen isotope data, histological analyses of daily cross striations and perikymata, and microwear data of Late Miocene primates Indopithecus and Sivaladapis nagrii and an early Pleistocene primate Theropithecus delsoni, known from the Indian Siwaliks. The results indicate that the Late Miocene giant ape Indopithecus had a C3 diet, likely including nuts, seeds and fruits with hard cover, and most probably lived in a seasonal forest that experienced long dry and wet seasons. While the seasons changed as depicted by intra-annual variation in δ18O values, the C3 component of diet was similar throughout the year, as indicated by consistent δ13C values. Sivaladapis nagrii dental enamel δ13C and δ18O values, striations and pit percentages indicate that this small primate (3 kg) was a C3 mixed folivore-frugivore. Theropithecus delsoni dental enamel δ13C and δ18O values and microwear pattern indicate a diet comprising mainly C4 grasses or sedges.

Dinosaur teeth from the Jurassic Qigu and Shishugou Formations of the Junggar Basin (Xinjiang/China) and their paleoecologic implications

The Middle and early Late Jurassic Qigu and Shishugou Formations of the southern and central Junggar Basin yielded teeth of theropods (Theropoda indet.), sauropods (Eusauropoda indet.), and stegosaurs. The dinosaur assemblage of the southern Junggar Basin is less diverse and is represented by smaller forms than in the central part of the basin. The microwear of the teeth of Eusauropoda indet. resembles that observed in Camarasaurus and may have formed as a result of biting through resistant woody materials. Carbon and oxygen isotope data of the sauropod and theropod teeth indicate feeding within a C3-plant ecosystem in a continental setting. Differences in enamel δ13C and δ18O values between Eusauropoda indet. and the theropod teeth are comparable to those observed in other herbivorous and carnivorous vertebrates, and suggest at least partial preservation of original dietary signals.

Use of multiple oxygen isotope proxies for elucidating Arctic Cretaceous palaeo-hydrology

Abstract Stable oxygen isotope analysis of siderite and dinosaur tooth enamel phosphate from the Campanian–Maastrichtian Prince Creek Formation, Alaska, USA, are analysed to determine the palaeohydrology of the ancient Colville Basin north of the Ancestral Brooks Range. δ 18 O of freshwater siderites relative to V-PDB ranges between −14.86 and −16.21‰. Dinosaur tooth enamel δ 18 O from three different sites (Kikak–Tegoseak, Pediomys Point, Liscomb) range between +3.9‰ and +10.2.0‰. δ 18 O meteoric water are calculated from δ 18 O siderite that formed at seasonal temperatures ranging from −2 to 14.5 °C, with a mean annual temperature of 6.3 °C. At 6.3 °C, the δ 18 O w calculated from siderite ranged between −22.23 and −20.89‰ V-SMOW. Ingested water compositions are estimated from dinosaur teeth assuming body temperatures of 37 °C and local relative humidity of 77.5%, resulting in values ranging from −28.7 to −20.4‰ V-SMOW, suggesting consumption of meteoric water and orographically depleted runoff from the Brooks Range. The ranges in calculated δ 18 O meteoric water are compatible between the two proxies, and are mutually corroborating evidence of extremely 18 O-depleted precipitation at high latitudes during the Late Cretaceous relative to those generated using general circulation models. This depletion is proposed to result from increased rainout effects from an intensified hydrological cycle, which probably played a role in sustaining polar warmth. Supplementary material: Parameters used for generation of equations compared to Kohn (1996) can be found at http://www.geolsoc.org.uk/SUP18642

Diet, residential origin, and pathology at Machu Picchu, Peru

Pathological conditions in human skeletal remains provide a wealth of information about archaeological populations, but many are limited in their interpretive significance by their nonspecific etiologies. This study analyzes three common pathological conditions known to manifest in infancy and childhood in the skeletal population from Machu Picchu, Peru (N = 74) with published carbon, nitrogen, oxygen, strontium, and lead isotopic data (Turner et al.: J Archaeol Sci 36 (2009) 317-332; Turner et al.: Chungara: Revista de Antropología Chilena 42 (2010) 515-524) to distinguish early-life diet from residential origins as significantly associated with pathologies among the site's inhabitants. Analyses of variance indicate highly significant variation between enamel δ(18)O values, which serve as a rough proxy of local environment, and both cribra orbitalia (CO) and porotic hyperostosis (PH), generally understood to be markers of anemia. Results tentatively suggest that individuals manifesting these lesions may have lived closer to the arid coasts; however, no significant variation was found in parameters of diet (enamel δ(13) C(carbonate), dentin δ(13) C(collagen), dentin δ(15)N) by either CO or PH, suggesting that the primary factors causing anemia may have been more significantly related to residential origin rather than diet. Linear enamel hypoplasia (LEH) frequency significantly varied by both dietary and residential parameters, supporting models of LEH formation from a synergy of dietary and environmental factors. These results support previous research on the etiology of PH in the Andes; they also represent a useful approach to refining site-specific interpretations of pathological conditions in archaeological populations, and exploring etiological variation between populations.

Seasonality of birth and diet of pigs from stable isotope analyses of tooth enamel (δ18O, δ13C): a modern reference data set from Corsica, France

Since domestication during the mid 11th millennium BP in the Near East and the 8th millennium BP in China, the pig has played an important role in human diet and economy. The objective of this study is to evaluate the potential of stable oxygen and carbon isotopes sequential analyses of pig tooth enamel to investigate pig seasonality of birth and diet, as these are important parameters for the reconstruction of pig herding strategies in ancient societies.A pilot study was performed on a modern reference set composed of five free-range Corsican domestic pigs and four Corsican feral pigs. Tooth enamel from the mandibular incisors (I1, I2), molars (M2, M3) and canines (C) were sampled sequentially to map the stable oxygen and carbon isotope records in the pig mandible.The sequences of δ18O published in this paper may be used as a reference data set for births grouped in mid-autumn and late winter. The δ13C values measured in the wild pigs constitute the first reference set for a wild population living in a Mediterranean environment. Results show that among all sampled teeth (I1, I2, C, M1 and M2), the combination of the sequences of δ18O measured in the first and second incisors provides the most effective data for determination of birth seasonality. The sequences of δ18O measured in the second and third molars were too short and/or dampened to be interpreted in terms of seasonality. In domestic pigs, male evergrowing canines provide a one and a half year record, allowing observation of short-term seasonal variations in the isotopic composition of diet.Applied to archaeological assemblages, the joint stable isotope analysis of incisors and canines will enable the qualification of the seasonal rhythm of pig herding practices, including slaughtering strategies, in more concrete terms. These practices are directly linked to the modalities of pork production but also to the availability of food resources.

INVESTIGATING SEASONALITY AND SEASON OF BIRTH IN PAST HERDS: A REFERENCE SET OF SHEEP ENAMEL STABLE OXYGEN ISOTOPE RATIOS

Intra‐tooth sequential analysis of enamel δ18O is currently used to investigate birth seasonality in past animal populations, offering new insights into seasonal availability of animal resources, herd management and seasonality of site occupation. Reference data sets are still required to address two major difficulties: (1) that inter‐individual variability in the record of the seasonal cycle is affected by tooth size; and (2) that the season of birth cannot be directly estimated from the timing of tooth growth, because of a delay in enamel mineralization. We present a data set acquired on the lower second molar of 10 modern sheep from Rousay (Orkney) born within a few weeks of each other in April/May and submitted to the same environmental conditions until death. All sheep have recorded a sinusoidal pattern of δ18O variation spanning approximately a year. From the difference between the expected and the measured time sequence, the delay of enamel mineralization is estimated to be 5–6 months. The data set is then described using a model mainly based on a cosine function. The period, corresponding to the length of the M2 crown formed over a year, averaged 35.8 mm. A very slight variation of tooth growth rate with time and no attenuation of the isotopic signal towards the cervical margin of the crown could be detected in this data set. The lowest δ18O values, corresponding to the sheep's first winter, were tracked at a distance from the enamel/root junction that varied between 23.0 and 30.3 mm (xmin mean = 27.6 mm); the highest δ18O values, corresponding to the sheep's second summer, were between 6.3 and 11.6 mm (xmax mean = 9.9 mm). Most of the variability can be attributed to tooth size. When normalized on the period, xmin and xmax are 0.28 (± 0.05) and 0.78 (± 0.05) on average, meaning that the Rousay sheep have recorded the minimum and maximum δ18O values on average at 78% and 28%, respectively, of the end of the periodic cycle recorded in the second molar.

Stable isotopes of fossil teeth corroborate key general circulation model predictions for the Last Glacial Maximum in North America

Oxygen isotope data provide a key test of general circulation models (GCMs) for the Last Glacial Maximum (LGM) in North America, which have otherwise proved difficult to validate. High δ18O pedogenic carbonates in central Wyoming have been interpreted to indicate increased summer precipitation sourced from the Gulf of Mexico. Here we show that tooth enamel δ18O of large mammals, which is strongly correlated with local water and precipitation δ18O, is lower during the LGM in Wyoming, not higher. Similar data from Texas, California, Florida and Arizona indicate higher δ18O values than in the Holocene, which is also predicted by GCMs. Tooth enamel data closely validate some recent models of atmospheric circulation and precipitation δ18O, including an increase in the proportion of winter precipitation for central North America, and summer precipitation in the southern US, but suggest aridity can bias pedogenic carbonate δ18O values significantly.

Strengthening of the East Asian summer monsoon revealed by a shift in seasonal patterns in diet and climate after 2–3 Ma in northwest China

To examine climate variability in northwest China in the late Cenozoic and to test hypotheses regarding the development of C 4 ecosystems and the dynamics of the Asian monsoons, the carbon and oxygen isotopic compositions of 32 bulk and 368 serial tooth enamel samples from herbivores in the Linxia Basin (Gansu Province), ranging in age from 25 Ma to the present, were determined. The results corroborate and improve the record previously obtained from the area, showing that all mammals in the Linxia Basin lived in habitats consisting primarily of C 3 vegetation prior to 2–3 Ma and that C 4 grasses did not become a significant component of local ecosystems until the Quaternary. The data also show that shifts in climate to drier and/or warmer conditions after about 14, 9.5, 7, and 2.5 Ma, as indicated by positive δ 18O excursions in the bulk enamel-δ 18O record, were accompanied by increased seasonality; whereas negative δ 18O shifts in the bulk data after about 11, 6, and 1.2 Ma, which indicate shifts to wetter and/or cooler climate, were associated with decreases in seasonality. Intra-tooth δ 13C and δ 18O profiles reveal significant changes in the seasonal patterns of diet and climate after ~ 2–3 Ma. Prior to ~ 2–3 Ma, there was little or no seasonal variation in herbivores' diets and all herbivores fed on C 3 vegetation year around. After that time, the data show a significant seasonal variation in the diets of horses and bovids, ranging from a pure C 3 to a mixed C 3/C 4 diet (with C 4 plants accounting for up to ~ 60% of the diet). An inverse relationship (or negative correlation) between δ 13C and δ 18O values within individual teeth — a pattern characteristic of the summer monsoon regime — is observed in younger (< 2–3 Ma) horses and bovids but not in older fossils. These changes in intra-tooth isotopic patterns provide strong evidence for an enhanced monsoon climate since about 2–3 Ma.

Stable isotope evidence for seasonal consumption of marine seaweed by modern and archaeological sheep in the Orkney archipelago (Scotland)

The use of seaweed as fodder for domestic animals during winter has been suggested for the North Atlantic isles in prehistory with reference to a historically attested practice. We tested the use of stable isotope analysis in tooth enamel bioapatite to detect seasonal consumption of seaweed on modern sheep from the Orkney archipelago, with no contribution, seasonal contribution or exclusive contribution of seaweed to diet. There was no overlap between the δ13C of local terrestrial vegetation and marine plants available on the shore. Sheep relying on seaweed had enamel δ13C values as high as −3·1‰, differing from grazing sheep with enamel δ13C values comprised between −17·7‰ and −15·1‰. Seaweed consumption was also reflected in lower amplitudes of intra-tooth variation in enamel δ18O values (0·3‰ to 1·8‰ in seaweed-eating sheep; 3·6‰ to 4·1‰ in grazing sheep), likely due to ingestion of oceanic water. The protocol was applied to archaeological assemblages from the same archipelago. A winter consumption of seaweed was detected in one sheep from the Neolithic site of Point of Cott and one sheep from the Iron Age assemblage of Mine Howe. No contribution of seaweed to the sheep diet was shown at the Late Norse site of Earl's Bu.

Evidence of Cenozoic environmental and ecological change from stable isotope analysis of sirenian remains from the Tethys-Mediterranean region

Sirenians (sea cows: manatees and dugongs) have been primary consumers in tropical and subtropical shallow-water marine ecosystems for more than 50 m.y. Though fossils of the earliest sirenians have been recovered from the Caribbean, sirenians are thought to have originated in the Tethys-Mediterranean region, which underwent significant climate change over the course of the Cenozoic. Application of stable isotope analysis can provide new insight into how diet and habitat preferences were affected by climate change and how these ecological changes correlate with diversity patterns for this region. Tooth enamel was sampled from 57 specimens of sirenians from the Tethys-Mediterranean and adjacent seas spanning the Eocene to Pliocene. Enamel δ13C values across this time interval were consistently high and indicative of diets composed mostly of marine seagrasses. In contrast, enamel δ18O values showed significant change over time and across the region. Sirenians from southern sites (i.e., Egypt, Libya) had significantly higher mean δ18O values than specimens from most European locations, suggesting variation in salinity within the Tethys-Mediterranean. The range in enamel δ18O values increased from the Paleogene through the Neogene, but enamel δ13C values remained steady, indicating that changes in environmental conditions did not necessarily coincide with major changes in dietary preferences. Declines in sirenian taxonomic diversity in the late Miocene were likely due to the rapid climate and oceanographic changes during the latter half of the Cenozoic and their impacts on available dietary resources. Elevated δ13C and δ18O values in Mediterranean sirenians during the Messinian salinity crisis corroborate the hypothesis of ecophenotypic dwarfing in these animals.

The interactive effect of temperature and humidity on the oxygen isotope composition of kangaroos

Summary A strong relationship between the oxygen isotope composition (expressed as δ18O) of body water and relative humidity has been demonstrated for a number of mammalian herbivores with low drinking water requirements, including kangaroos. Consequently, it has been suggested that the oxygen isotope composition of preserved mammal remains may be used to reconstruct past relative humidity. Other physiological, environmental and ecological factors may also influence mammalian δ18O, thereby confounding the climatic signal, yet these factors have been rigorously examined in few taxa. We examined sources of variation in the δ18O of tooth enamel, assumed to reflect δ18O of body water, of kangaroos (Macropus spp.) collected throughout Australia. Relative humidity explained a large proportion of the variation in enamel δ18O, a finding that is consistent with previous studies. However, we also found a previously unreported interaction between mean annual temperature and relative humidity. At lower temperatures, the relationship between enamel δ18O and relative humidity was much steeper than at higher temperatures. This may be a consequence of the Peclét effect in plant leaves, whereby high transpiration rates diminish the 18O enrichment of bulk leaf water. It is likely that this interaction is also present in other herbivores with low drinking water requirements. We found little evidence that δ18O varied consistently between molars, suggesting that a ‘weaning effect’ is either absent or swamped by seasonal variation in precipitation δ18O. We suggest that the oxygen isotope composition of preserved kangaroo remains cannot be used to reconstruct relative humidity unless ambient air temperature can be reliably estimated.

A stable isotope aridity index for terrestrial environments

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 delta(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). Tooth enamel delta(18)O values of ES animals increase with aridity, whereas the tooth enamel delta(18)O values of EI animals track local meteoric water delta(18)O values, demonstrating that bioapatite delta(18)O values of animals with different behaviors and physiologies record different aspects of the same environment. The enrichment between tooth enamel delta(18)O values of ES and EI animals records the degree of (18)O enrichment between evaporated water (ingested water or body water) and source water, which increases with environmental aridity. Recognition of the ES-EI distinction creates the opportunity to use the (18)O composition of bioapatite as an index of terrestrial aridity.