SEASONAL AND EL NIÑO SIGNALS IN RADIOCARBON PROFILES FROM EARLY 20TH-CENTURY PERUVIAN BAY SCALLOP (ARGOPECTEN PURPURATUS) SHELLS

Isotope ratios from marine mollusk shell carbonate contain information about the water in which the shell formed. In the Pacific Ocean along coastal Peru, persistent southeasterly winds typically cause upwelling of nutrient-rich, cool, ¹⁴C-depleted water from below the thermocline. During an El Niño event, the thermocline deepens and marine upwelling thus shifts to nutrient-poor, warm, ¹⁴C-enriched super-thermocline water. This change in upwelling source water complicates ¹⁴C dating of marine systems, but can also provide an opportunity to study the history of El Niño thousands of years into the past, if ¹⁴C changes due to El Niño are reliably detectable in mollusk shells.

We have quantified intrashell ¹⁴C cycles in several approximately one-year-old Peruvian bay scallops (Argopecten purpuratus) collected in 1908 and 1926 from Callao Bay and Salaverry, Peru. In a shell from a scallop that lived at Callao Bay entirely during the non-El-Niño conditions of 1907–8, shell carbonate ¹⁴C age varied cyclically during ontogeny with a 292-¹⁴C-yr amplitude. In two similar A. purpuratus shells that grew during the 1925–6 El Niño event at both Callao Bay and Salaverry, ¹⁴C age also varied through ontogeny, but with smaller amplitudes of 172 and 143 ¹⁴C yr, respectively. Marine ¹⁴C reservoir correction (ΔR) minima for these three shells are similar (45 to 99 yr), while ΔR maxima vary from 372 yr for the non-El-Niño shell to 206 and 242 yr for the El Niño shells. Qualitative intrashell patterns of variation in ¹⁴C are similar to those in δ¹⁸O (a proxy for marine water temperature), although changes in ¹⁴C lag behind changes in δ¹⁸O by a period estimated by counting growth bands to range from 0 to 53 days.

We interpret the reduced ¹⁴C-age amplitude within the El Niño shells as showing suppression of the normally strong austral winter upwelling of cool, ¹⁴C-depleted sub-thermocline water. This observed variation in shell ¹⁴C content between normal and El Niño conditions may allow detection and analysis of El Niño events preserved in mollusk shells thousands of years into the past. We also note that in marine chronometric applications using mollusks, it is critical that uncertainties resulting from intrashell ¹⁴C variation be quantified, reported, and considered for meaningful high-precision marine ¹⁴C dating.