Quantifying Upwelling and Freshening in Nearshore Tropical American Environments Using Stable Isotopes in Modern Gastropods
To identify and quantify upwelling and freshwater influences in contrasting tropical ecosystems, we performed stable isotope analyses (δ13C and δ18O) on 13 serially-sampled modern Conus shells collected from coastal waters in the southwestern Caribbean (SWC, non-upwelling) and gulfs of Chiriquí (non-upwelling) and Panama (upwelling) in the tropical eastern Pacific (TEP). Most shells reveal seasonal variations in temperature and/or seawater δ18O in their δ18O profiles. Unusually high or low seasonal δ18O values measure the intensity of seasonal upwelling or freshwater input, respectively. To quantify upwelling and freshening signals, baseline δ18O 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 δ18O profiles reveal little or no upwelling in the SWC and Gulf of Chiriquí, and strong upwelling in the Gulf of Panama, as well as strong freshwater input in most areas. Dry-season δ18O values for Gulf of Panama Conus can exceed the baseline by as much as 2‰, equivalent to seawater temperatures approximately 9 °c lower than normal. in contrast, rainy-season δ18O values can be as low as 1.8‰ below the baseline, equivalent to salinities approximately 7 units lower than dry-season values. We use shell δ18O range (Δ18O) and δ18O-δ13C (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 Δ18O (2.3‰–3.2‰) and little O-C correlation. In contrast, the oligotrophic environments of the SWC lead to low (0.4‰) to moderate (1.6‰) δ18O and non-significant to positive O-C correlation. When applied to fossil shells, these methods can characterize the nutrient status of ancient ecosystems.
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Document Type: Research Article
Publication date: 2013-10-01
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