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Seasonal cycles of manganese and cadmium in Galapagos coral

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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 Niño, 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 Niño 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 Full-size image (<1 K) and Full-size image (<1 K) 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 Full-size image (<1 K) signals, with higher ratios following seasonal upwelling, and less distinct seasonal cycles in Full-size image (<1 K) ratios one-half year out of phase with Full-size image (<1 K) variations. Average Full-size image (<1 K) 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.

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