Evaluating the Sr-isotope alterations in shells used in Strontium Isotope Stratigraphy. (2014)
Undergraduate: Jonathan Perdomo
Faculty Advisor: Drew Coleman
Department: Biology
Dating ancient shorelines that coincide with periods of warmth on Earth help in predicting the current changes in sea level. Strontium Isotope Stratigraphy (SIS) is one method used for dating, which uses calcium-carbonate shells from organisms found in ancient shoreline deposits. The ratio of 87Sr/86Sr in a shell is measured by using a mass spectrophotometer, and then is compared to modern Sr-isotope ratios in seawater to determine the age. This method of shoreline dating depends on the shells maintaining their original 87Sr/86Sr ratios. By using entire powdered shell samples for analysis, we are assuming that the shell will be entirely homogenous in its original strontium isotope ratio. We predict that some of these calcium carbonate shells, especially the older ones, could have partly altered strontium isotope ratios due to diagenesis in an environment with different Sr-isotope ratios; this would lead to inaccurate dating when sampling the entire shell in the mass spectrophotometer. We are using a Micromill to drill linear samples of cross-sections of shells of different ages, from the outer to the inner, in order to test for changes in 87Sr/86Sr isotopes. We hypothesize that the outer parts of older shells, due to their prolonged contact with the environment and the seawater, will have different 87Sr/86Sr ratios than the inner, more preserved parts of the shell. If this is the case, sampling these whole shells for SIS will lead to inaccurate dating.