ENVIRONMENTAL SIGNALS FROM DONAX OBESULUS PROVIDE INFORMATION ON EL NIÑO-SOUTHERN OSCILLATION (ENSO) VARIABILITY

The far Eastern Pacific lacks long-lived organisms useful for paleoclimate reconstructions creating a need for novel archives sensitive to ENSO variability. Short-lived (<5 years) bivalves found in archaeological sites can provide “snapshots” of past climatic variability (i.e., seasonal range) rather than continuous, cross-dated time series (e.g., trees and corals). The short-lived intertidal bivalve Mesodesma donacium has been used to reconstruct ENSO in coastal southern Peru; however, M. donacium is functionally extinct north of ~16°S due to their vulnerability to El Niño events. We assess the short-lived (1–2 years) surf clam Donax obesulus as a complimentary archive to M. donacium in northern Peru. D. obesulus populations survive the warmer sea surface temperatures (SSTs) during El Niños, though they are vulnerable to colder SSTs associated with La Niñas. We collected live D. obesulus from the Nepeña Valley, Peru in 2012 (La Niña), 2014 (ENSO-neutral), and 2016 (El Niño) for a modern analog study. We analyzed the shells for stable isotopic (δ¹⁸O) and trace elemental (Sr/Ca, Mg/Ca, Ba/Ca) ratios and assessed their sensitivity to ENSO-related environmental signals; all but Ba/Ca represent seasonal cycles. D. obesulus δ¹⁸Oshell captures SST variability (average r² = 0.55 ±0.21 (n=18), RMSE = 2.8°C) with localized upwelling and advection likely influencing δ¹⁸Owater and shell δ¹⁸O. Ba/Ca anomalies coincide with chlorophyll-a and cold events in Mg/Ca and Sr/Ca. We sampled one M. donacium and six D. obesulus from a Nepeña Valley archaeological site (Caylán) to assess ENSO variability at ~2300 BP. Results from D. obesulus suggest a wider range of shell δ¹⁸O signals than previously suggested from using M. donacium alone. M. donacium mean δ¹⁸Oshell is enriched ~0.8‰ compared to preceding periods and ~1.1‰ compared to modern shells on the south coast of Peru, while D. obesulus mean δ¹⁸Oshell is enriched ~0.7‰ compared to modern representing 2.5–3.3°C cooler mean conditions and/or a shift in δ¹⁸Owater compared to modern ~2300 B.P. Both shell species suggest a La Niña-like mean state at 2300 BP compared to our modern shells. We find D. obesulus is a suitable compliment to M. donacium with the addition of trace elements for understanding past ENSO variability.