Stable-isotope microprofiling of wombat tooth enamel records seasonal changes in vegetation and environmental conditions in eastern Australia

Abstract

We present the first high-resolution carbon and oxygen isotope profiles of carbonate in modern wombat (genera Vombatus and Lasiorhinus) tooth enamel and demonstrate that the carbon isotope ratios ( δ 13C) clearly preserve seasonal changes in diet and local environmental conditions. Our baseline study of δ 13C in tooth enamel and bone collagen from wombats that inhabited semi-arid, savannah, temperate and alpine environments in eastern Australia documented an offset of 5.2 ± 0.4‰ and 12.9 ± 0.3‰ from the animal's diet, respectively. Marsupial wombats are obligate grazers, whose teeth grow continuously. Sequential micro-analysis of δ 13C and δ 18O along the length of wombat incisors yielded a time-series of isotopic variation. The microprofiles of wombat tooth δ 18O are highly variable between and within sites. Notwithstanding, the δ 18O values were relatively high in wombat teeth from arid and semi-arid sites, and significantly lower in those from southern temperate and coastal regions. In contrast, the δ 13C of wombat tooth enamel correlates well with the local environmental setting and the seasonality of C 3 and C 4 grass growth. In areas with marked seasonal changes in the proportions of winter C 3 and summer C 4 grasses, wombat microprofile δ 13C values varied by up to 8‰. Importantly, at sites with predominantly C 3 or C 4 grasses all year, variations in microprofile δ 13C values are small (< 1.9‰), indicating that the relatively large seasonality in δ 13C we observe in wombat teeth must be primarily controlled by changes in diet, rather than animal-specific factors. This study shows that microprofiling of δ 13C in wombat incisors, in particular, provides a high level of isotopic detail which is not available from time-averaged isotope records from bone collagen, or bulk analysis of isotopes in tooth enamel. Wombat teeth are common in the Quaternary fossil record and, as such, they have the potential to be outstanding archives of palaeoecological information. For example the δ 13C of fossil wombat teeth could be used to define the changing proportions of C 3 and C 4 grasses in wombat diets through the Quaternary, which in turn reflect changes in the seasonality of precipitation, summer temperatures, and possible ecosystem modification by human activities.