Centennial-scale elemental and isotopic variability in the tropical and subtropical North Atlantic Ocean

摘要

The marine geochemistry of the North Atlantic Ocean varies on decadal to centennial time scales, a consequence of natural and anthropogenic forcing. Surface corals provide a useful geochemical archive to quantify past mixed layer variability, and this study presents elemental and isotopic records from the tropical and subtropical North Atlantic. A consistent method for stable lead isotope analysis via multiple collector ICP-MS is first presented. This method is then applied to western North Atlantic surface corals and seawater, constraining historical elemental and isotopic lead variability. Six stable lead isotope profiles are developed from the western and eastern North Atlantic, demonstrating consistent mixed layer, thermocline, and deep water variability. Finally, coralline trace element records, including cadmium, barium, and lead, are presented from the Cariaco Basin. First, a reliable method is developed for stable lead isotope analysis by multiple collector ICP-MS. This study presents new observations of the large (0.7% amu-l), time-dependent mass fractionation determined by thallium normalization, including preferential light ion transmission induced by the acceleration potential and nebulizer conditions. These experiments show equivalent results for three empirical correction laws, and the previously proposed [beta]Pb/[beta]TI correction does not improve isotope ratio accuracy under these conditions. External secondary normalization to SRM-981 provides one simple alternative, and a rationale is provided for this correction. With current intensities less than 1.5x10-12 A, external isotope ratio precision less than 200 ppm is observed (2[sigma]). Matrix effects are significant with concomitant calcium in SRM-981 (-280 ppm at 257 [mu]M [Ca]).