NOAA/WDS Paleoclimatology - Coastal Nepena Valley, Peru Modern Surf Clam Stable Isotope Data

The coastline of Peru lacks long-lived marine organisms useful for paleoclimatic reconstructions generating a need for novel archives. Short-lived (<5 years) bivalves are commonly found in geological and archaeological deposits and thus can provide "snapshots" of past climatic variability (i.e., seasonal range), similar to data obtained by individual foraminifera analysis, rather than continuous, cross-dated time series (e.g., trees and corals). Previous studies have found success using the short-lived intertidal clam Mesodesma donacium. However, M. donacium are vulnerable to die-offs from the warmer sea surface temperatures (SST) associated with El Nino events and are functionally extinct in northern Peru thus limiting the possibility of modern analog studies for that region. Here we investigate the short-lived (1-3 years) surf clam, Donax obesulus, commonly found in northern Peru, as a paleoclimate archive. Donax obesulus populations are able to survive the warmer SSTs present during El Nino years although they are vulnerable to colder SSTs associated with La Ninas. We assessed the environmental drivers underlying subannual d18O variability in D. obesulus from live collected shells from fish markets and coastal beaches near the Nepena Valley, Peru in 2012 (La Nina), 2014 (ENSO-neutral), and 2016 (El Nino). Forward modeling of pseudo-shell d18O reveals that SST variations are a dominant driver with secondary contributions from seasonally-varying seawater d18O (d18Osw). By accounting for varying d18Osw, we isolated the temperature dependent variable resulting in a paleotemperature equation for D. obesulus d18O. We verified our results with the d18O record of a D. obesulus shell collected in 2006. Our results suggest that the paleotemperature equation we developed is useful for reconstructing El Nino-Southern Oscillation (ENSO)-related climatic variations in this region and the pseudo-shell approach may be useful for understanding shell d18O in other locations.