Quantifying Upwelling and Freshening in Nearshore Tropical American Environments Using Stable Isotopes in Modern Gastropods

Abstract

To identify and quantify upwelling and freshwater influences in contrasting tropical ecosystems, we performed stable isotope analyses [delta]13C and [delta]18O) on 13 serially-sampled modern Conus shells collected from coastal waters in the southwestern Caribbean (SWC, non-upwelling) and gulfs of Chiriqui (non-upwelling) and Panama (upwelling) in the tropical eastern Pacific (TEP). Most shells reveal seasonal variations in temperature and/or seawater [delta]18O in their [delta]18O profiles. Unusually high or low seasonal [delta]18O values measure the intensity of seasonal upwelling or freshwater input, respectively. To quantify upwelling and freshening signals, baseline [delta]18O values free of seasonal upwelling and freshening have been calculated from average temperatures during rainy (non-upwelling) seasons and average salinities during dry (upwelling) seasons. Baseline-normalized [delta]18O profiles reveal little or no upwelling in the SWC and Gulf of Chiriqui, and strong upwelling in the Gulf of Panama, as well as strong freshwater input in most areas. Dry-season [delta]18O values for Gulf of Panama Conus can exceed the baseline by as much as 2%o, equivalent to seawater temperatures approximately 9 °C lower than normal. in contrast, rainy-season [delta]18O values can be as low as 1.8%o below the baseline, equivalent to salinities approximately 7 units lower than dry-season values. We use shell [delta]18O range ([delta]18O) and [delta]18O-[delta]13C (O-C) correlation to further identify upwelling and freshening environments and thus nutrient source and status. Eutrophic environments like the Gulf of Panama are characterized by high [delta]18O (2.3%o-3.2%o) and little O-C correlation. In contrast, the oligotrophic environments of the SWC lead to low (0.4%o) to moderate (1.6%o) [delta]18O and non-significant to positive O-C correlation. When applied to fossil shells, these methods can characterize the nutrient status of ancient ecosystems.