Stable Isotope Profiling in Modern Marine Bryozoan Colonies Across the Isthmus of Panama

In the tropics, upwelling of cold, deep water is the principal source of major seasonal fluctuations in temperature. Along the tropical eastern Pacific (TEP) coast of the isthmus of Panama, seasonal upwelling induces corresponding drops in temperature. Upwelling does not occur along the southwestern Caribbean (SWC) coast of the isthmus. Our goal was to use these oceanographic differences to test the use of stable isotope profiles of free-living modern cupuladriid bryozoans as a method for quantifying paleo-seasonality. We determined O and C stable isotope values from micromilled carbonates profiled along the growth axis in three colonies of Cupuladria exfragminis Herrera-Cubilla, Dick, Sanner and Jackson, 2006 from the upwelling Gulf of Panama in TEP and three colonies of Cupuladria surinamensis Cadée, 1975 from the non-upwelling Bocas del Toro Archipelago in SWC. Pacific colonies had inter-colony δ¹⁸O_carb values ranging from –2.1‰ to –0.2‰ on the international Vienna Pee Dee Belemnite scale, whereas SWC colonies ranged from –1.7‰ to –0.6‰. Pacific colonies consistently reveal cyclical trends in δ¹⁸O_carb that are absent in the Caribbean colonies. Based on published measurements of temperature, salinity, and δ¹⁸O_sw, the ≤2.5 yrs of cyclicity seen in the Pacific colonies reflects a combination of seasonal freshening and seasonal upwelling of colder water. This preliminary study suggests the potential for more exploration of bryozoans as a source of paleoclimate proxies.