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 (δ 13 C and δ 18 O) on 13 serially-sampled modern Conus shells collected from coastal waters in the southwestern Caribbean (SWC, non-upwelling) and gulfs of Chiriquí (nonupwelling) and Panama (upwelling)

Low seasonality in central equatorial Pangea during a late Carboniferous highstand based on high-resolution isotopic records of brachiopod shells

“…The salinity reduction in the sampled portion of the Appalachian Basin was likely relatively minor during highstand times, as Neospirifer and associated marine fauna such as crinoids indicate near-stenohaline conditions (e.g., Stevens, 1971;Lebold and Kammer, 2006), but it was nevertheless sufficient to affect the isotopic composition of the water. In modern tropical settings with significant levels of terrestrial runoff, for example, along the Pacific coast of Panama, a 1.5‰ reduction in d 18 O of seawater similar to the ~1.5‰ d 18 O difference between brachiopods in this study and the Texas shelf specimens of Grossman et al (1993) equates to a drop in salinity by only ~6 psu (Tao et al, 2013).…”

“…Mii and Grossman (1994), Wang (1998), andPowell et al (2009) have documented fluctuations in Na, S, and/or Mg concentrations in serially sampled brachiopod shells corresponding to seasonal cyclicity. Similar fluctuations also occur in the shells of modern marine invertebrates (e.g., Tao et al, 2013). These changes may reflect differences in temperature, seawater chemistry, precipitation rate, or metabolism of the organism (Mii and Grossman, 1994;Wang, 1998;Powell et al, 2009).…”

“…Records of this type of enrichment have been noted in North American and Russian brachiopod shells from the arid Early Permian (Grossman et al, 2008). If, on the other hand, normal tropical conditions (moist, weakly seasonal) prevailed during glacial times, the isotopic records of seasonality should show relatively little variability, similar to records of modern gastropods in the Caribbean off of Panama (e.g., Tao et al, 2013). Due to the restricted nature of this portion of the late Pennsylvanian epicontinental sea, shells deposited during relatively moist climates may show depletion in d 18 O and d 13 C due to heightened freshwater influence relative to coeval strata in closer communication with the open ocean (cf.…”

“…In a restricted portion of the late Pennsylvanian epicontinental sea such as the Appalachian Basin, strong monsoonal seasonality would result in significant annual fluctuations in salinity and temperature of the seawater, changes that should be reflected in the isotopic composition and potentially traceelement ratios of brachiopod shells deposited in the region. Modern gastropod shells in equatorial marine environments show a clear isotopic record of seasonal drying associated with the much weaker present-day tropical monsoon (Tao et al, 2013). A year-round dry climate in such a setting would likely result in evaporative enrichment of d 18 O and d 13 C relative to more openly marine areas, as in the modern Red Sea (e.g., Ganseen and Kroon, 1991).…”

Variability of foraminiferal stable isotope ratios in Caribbean shallow waters of Panama: A modern framework for Neogene studies

“…All values of δ 18 O and δ 13 C are given in as reported by Grossman (2012): (Tao et al, 2013). The Conus variability is much less than the variability seen in the within-station foraminiferal data, which extend 200 m deeper.…”