ISOTOPIC ANALYSES OF FERAL HORSES FROM SHACKLEFORD BANKS, NORTH CAROLINA

The oxygen isotope ratios of teeth from modern equids have been shown to track the oxygen isotope ratios of local precipitation, and thus may serve as a proxy for paleoclimatic conditions. However, the accuracy of paleoclimatic reconstructions based on analyses of equid teeth is currently limited because the relationship between the isotopes of modern equids and their environment has not been precisely quantified. Furthermore, it has been demonstrated that the oxygen isotope ratios of individual horses from the same location can vary greatly when local surface waters are subjected to high rates of evaporation (i.e., in desert environments where precipitation is relatively low).

Here we examine the oxygen isotope variability in modern feral horses from an environment with relatively high precipitation and high relative humidity. Shackleford Banks is a barrier island off the coast of North Carolina, which supports a population of over 100 feral horses. Annual precipitation averages146 cm, while annual temperatures average 17 degrees Celsius. We analyzed the oxygen isotope ratios of tooth enamel from Shackleford horses as well as fresh water samples collected from local ponds on a monthly basis for one year. Water samples from the largest pond on the island (Mullet pond) averaged -3.3 +/- 0.6‰ (SMOW), which is similar the average value of precipitation (-3.8‰) measured by the International Atomic Energy Association at Hatteras, NC (located ~80 km to the north). Samples from smaller water holes appear to have undergone some evaporative enrichment and displayed oxygen isotope values that averaged 0.8‰. The oxygen isotope values of tooth enamel from the horses ranged from 24.5‰ to 29.4‰, averaging 27.0‰. This range of values is similar to the range observed in modern feral horses from desert environments.