Eastern Pacific El Niño-Southern Oscillation variability 400-200 BCE: perspectives from a dual-archive approach.

Reconstructions of the El Niño-Southern Oscillation (ENSO) suggest a decrease in intensity and variability during the middle Holocene, but disagree as to when modern conditions began. Most studies report an increase in ENSO intensity beginning around 3000-1000 BCE, but conflicting perspectives exist in eastern and western Pacific archives. We use δ¹⁸O signals, a proxy for sea surface temperature (SST) and salinity (SSS), from two short-lived (<5 years) bivalve (SLB) species, Mesodesma donacium and Donax obesulus, recovered from Caylán (9.18°S, 78.38°W), an archaeological site in coastal Peru to reconstruct ENSO variance. These two species exhibit complimentary tolerances to SST at both extreme ends of ENSO phases: D. obesulus being vulnerable to die-offs in the colder SSTs of La Niña events and surviving during El Niño warmth whereas M. donacium, on the other hand is vulnerable to the warmer SSTs of El Niños and thrives during La Niño cooling. We examine the variance of the annual range of δ¹⁸O in both species as a "snapshot" of ENSO variability. In doing so, we aim to fill a temporal gap in existing SLB reconstructions from coastal Peru. We analyzed ~monthly-resolved δ¹⁸O samples from 20 shells (12 D. obesulus, 8 M. donacium) from Caylán dating 400-200 BCE. We compared these samples to the variance of the annual range of SST of an in situ record from Chicama, Peru (7.69°S, 79.43°W) spanning the years 1925-2019. We found that the sampled shells likely represent all three phases of ENSO (La Niña, neutral, and El Niño), and the reconstructed variance of the annual range of SST from both species combined was similar at 400-200 BCE to that of the 20^th-21^st century in situ SST record suggesting modern like conditions unlike the quiescent in the mid-Holocene. We conducted a principal component analysis (PCA) on the shell population to understand which δ¹⁸O signals were driving the variance. Two D. obesulus shells, MA197-2 and MA197-27, exhibited the strongest loadings along principle component 1. Both of these shells' δ¹⁸O profiles represent temperature ranges >10°C, or equivalent to a major El Niño event akin to 1982-83 and 1997-98. Our results suggest that strong El Niño events dominated ENSO variability from 400-200 BCE, in agreement with other reconstructions suggesting increased ENSO activity during this interval.