Variability of monthly radiocarbon during the 1760s in corals from the Galapagos Islands

Abstract

Radiocarbon (Δ14C) measurements of monthly samples from a Galapagos surface coral are among the first data sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine. An average Δ14C value of -62‰ is obtained for 144 measurements of samples from monthly coral bands that lived from about AD 1760-1771 (±6 yr). High Δ14C values were found during January through March, when upwelling was weak or absent at the Galapagos Islands. Low Δ14C values were obtained mid-year during strong upwelling. The average seasonal variability of Δ14C was 15-25%o, which is greater than that at other tropical and subtropical locations in the Pacific Ocean because of intense seasonal upwelling at this site. Periods of sustained high Δ14C values were found during 1762-1763 and 1766. A spectral analysis revealed that the spectral density for the Δ14C data displays most of its variance at the 5-yr cycle, which is reflective of El Niño periodicity during the 20th century.