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, 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.