Seasonal cycles of manganese and cadmium in coral from the Galapagos Islands

Abstract

Manganese-to-calcium ratios in corals from the eastern and western Galapagos demonstrate regional differences in seasonal trace metal cycling. The variability of trace metal-to-calcium ratios within the Galapagos Islands points to their unique geographic setting as a major factor. This region is influenced by several major oceanic currents (e.g., the South Equatorial Current, the Equatorial Undercurrent, and the Panama, or El Nino, Current) and by extremely intense upwelling. Manganese-to-calcium ratios in a banded coral Pavona clavus from Isabela Island, the westernmost island in the Galapagos, have distinct seasonal cycles for the non-El Nino Southern Oscillation (ENSO) years 1946–1950, with lower ratios following intensified seasonal upwelling. Cadmium/calcium ratios show less distinct seasonal cycles. The near-moderate ENSO event in 1951 is marked by the disruption of seasonal cycles in MnCa and CdCa ratios. In contrast, corals from islands further east in the Galapagos (Hood Island, 1964–1973, Linn et al., 1990; San Cristobal, 1965–1979, Shen and Sanford, 1990), have stronger seasonal CdCa signals, with higher ratios following seasonal upwelling, and less distinct seasonal cycles in MnCa ratios one-half year out of phase with CdCa variations. Average MnCa ratios are lower for these corals from locations further east, indicating that Urvina Bay appears to have an additional localized source of Mn (Shen and Sanford, 1990). In general, these regional variations in seasonal trace metal cycling are consistent with coral stable isotope signals and with their geographic locations. These variations are important to consider in using coral records to reconstruct and interpret oceanographic events occurring prior to historical records.