Assessing El Niño Variability on the Coast of Peru Using Two Short-Lived Bivalve Species, Donax obesulus and Mesodesma donacium, From Archaeological and Modern Contexts

El Niño-Southern Oscillation (ENSO) has a tremendous impact on human occupation of the Peruvian coast, bringing anomalously high sea surface temperatures (SST), excessive rainfall followed by agricultural and subsistence failures resulting in the loss of human life. Past civilizations likely struggled during unusually intense ENSO episodes yet there are no written histories from pre-Columbian peoples. Long-lived organisms provide useful ENSO time-series reconstructions; however, these organisms are rare in coastal Peru requiring new approaches such as using short-lived organisms to reconstruct ENSO variability for select time intervals. Monte Carlo simulations suggest short-lived bivalves are better poised to reconstruct ENSO variability than single foraminifers (2–3 month lifespan) due to their longer lifespan (1–3 years). Reconstructions in central and southern coastal Peru using δ¹⁸O seasonal cycles from the short-lived bivalve Mesodesma donacium reveal a reduction in ENSO variability (δ¹⁸O seasonal cycle = ~1‰) at 2900 ±150 BP compared to modern M. donacium δ¹⁸O seasonal cycles (~2‰). M. donacium is vulnerable to die offs from warmer SST during El Niño events, possibly biasing its seasonal δ¹⁸O cycle towards La Niña conditions. Here we assess two species of short-lived bivalves (M. donacium and Donax obesulus) for reconstructing seasonal variability on the north-central coast of Peru using specimens from archaeological (2300 ±100 BP) and modern contexts. D. obesulus is a new proxy for assessing ENSO variability with a life span of 1 to 3 years and is a warm-water species that survives El Niño events yet may be sensitive to colder La Niña conditions. Initial δ¹⁸O results from a M. donacium archaeological specimen reveals a δ¹⁸O seasonal cycle of 1.1‰ confirming its ability to record seasonal signals and reduced ENSO variability compared to modern conditions for 2300 ±100 BP on the north-central coast of Peru.