EXPLORING THE DELAYED OVERTURN IN CARIBBEAN FAUNA USING GASTROPOD STABLE-ISOTOPE PROFILES TO QUANTIFY SEASONAL UPWELLING AND FRESHENING OF COASTAL WATERS

Faunal overturn in the Southwest Caribbean (SWC) during the Late Neogene has been attributed to either the loss in regional productivity associated with the uplift of the Central American Isthmus (CAI; ~3.5 Ma) or a postulated decline in marine temperature associated with Northern Hemisphere glaciation (NHG) beginning ca. 3.3 Ma. Most of the taxa that went extinct were adapted to nutrient-rich environments uncommon in the SWC today, but their demise occurred 1-2 myr after the final closure of the CAI, which is coincident with the onset of severe NHG. Our aim is to determine which process was the root cause of the overturn of marine fauna in Late Neogene SWC waters through stable isotope analysis of gastropods. We present δ¹³C and δ¹⁸O data for 30 gastropod shells from Costa Rica and Panama ranging in age from 12 to 1.6 Ma that help identify sources of nutrients (upwelling and freshwater input) prior to, during, and after the final closure of the Central American Isthmus ~3.5 Ma.

Tropical marine surface waters are typically very stable, with conspicuous variability in temperature and salinity (and thus shell d¹⁸O) caused by either upwelling (cool temperature) or freshwater input (low salinity). Correlations between δ¹⁸O and δ¹³C provide another test of upwelling (‑) and freshening (+). Fossil specimen δ¹⁸O profiles do not show the strong upwelling and freshening signals found in modern Conus profiles from the Gulf of Panama in the Pacific (shell δ¹⁸O range > 2‰). However, specimens older than ~3.5 Ma show δ¹⁸O ranges greater than those of modern specimens (> 1‰), suggesting potential refugia where modest seasonal upwelling enhanced productivity. Baseline δ¹⁸O values were calculated for each shell based on geochemical data from deep-sea planktonic foraminifera. Profiles normalized to open-ocean marine conditions reveal the amount of upwelling and freshening experienced by the individual. This approach shows increasing freshening over the last 5 myr and almost no upwelling after 3.5 Ma. By 2.5 to 1.5 Ma, shells from the SWC recorded δ¹⁸O ranges and δ¹⁸O-δ¹³C correlations representative of modern oceanographic conditions at these localities, suggesting that nutrient-rich refugia were rare or absent by this time. This interval coincides with the delayed extinction which occurred 1 to 2 myr after final closure of the isthmus.