Monthly Radiocarbon Variability during the 1760s in the Eastern Tropical Pacific

Monthly samples from a Galapagos coral that lived from AD 1760-1771 were analyzed for Δ14C. High Δ14C values were found for coral that grew from January to March, when upwelling is weak or absent at the Galapagos. Low Δ14C values were obtained mid-year during strong upwelling. The seasonal variability of Δ14C ranged from 10-28 ‰; this is greater than the seasonal variability at other tropical or subtropical locations owing to intense seasonal upwelling in the eastern equatorial Pacific. The Δ14C data suggest that three El Nino events occurred during this period. Δ14C values were high (average -64‰) during the upwelling seasons of 1762-64 relative to the other years (average -74‰), which resemble a short-lived climatic shift similar to that identified for 1976 by Guilderson and Schrag [1998]. The monthly Δ14C record revealed most of its variance at the 5-yr period, similar to ENSO periodicity during the 20th century, though the record is too short for these results to be conclusive. These data will be discussed in the context of a longer annual record of Δ14C being reconstructed from the same coral sequence.