First name: Lily
Last name: O’Sullivan
Class Year: 2025
Advisor: Mark Brandon
Essay Abstract:
Stable isotope fractionation has become a common method for estimating the height of ancient topography. The atmosphere lifts as it flows over mountains, causing condensation, and fallout of rain and snow. The isotopic composition of the precipitation provides a record of the amount of atmospheric lifting. We investigate the contributions of orographic versus convective precipitation in the isotope composition of precipitation to test if significant convective precipitation distorts the relationship we see between topography and isotopic fractionation. We use modern river water samples from the Caribbean island of Dominica, in a region of heavy convection, in order to sample “mean-state” precipitation, meaning precipitation averaged over months to years of precipitation events.The mean-state is the relevant time frame to the study of paleotopography, as the precipitation isotope recorders used in these studies, such as volcanic glass, average precipitation over thousands of years.
We compare our observed fractionation values from rivers on Dominica to values generated by the Orographic Precipitation and Isotopes (Brandon M.T., 2022), a stratiform model. OPI predicts precipitation isotopes associated with steady stratiform flow of a saturated atmosphere over an arbitrary topography. We find that a stratiform flow model is capable of predicting isotopic fractionation in mean-state precipitation. Due to a good fit between observed and predicted data, we conclude that this calculation can accurately predict fractionation values for precipitation, even in an area with heavy convection. We broadly discuss the role of convective precipitation, the amount of precipitation and local temperatures on isotopic fractionation of precipitation, and the significance of these factors in estimating paleotopography.