Variability in coastal upwelling environments along the western Americas from nearshore geochemical and paleo-tracers.

摘要

Wind-driven coastal upwelling along eastern ocean boundaries supplies nutrient-rich waters to the surface ocean, fueling highly productive ecosystems. Climate processes which affect coastal winds and coastal water masses, such as the El Niño-Southern Oscillation, the Pacific Interdecadal Oscillation, or changes in solar insolation, cause upwelling systems to vary on times scales from years to millennia. An understanding of natural variability in coastal upwelling regions is essential for assessing potential climate change impacts and for formulating sustainable coastal resource management policies.; The nutrient-like trace metal cadmium (Cd) was an effective coastal upwelling tracer in the California Current, however this was not the case in the Peru-Chile Current, where the source waters for coastal upwelling may vary with upwelling intensity, leading to seasonally inconsistent upwelling tracer enrichments. Dissolved Cd:P ratios were lower in the southeast Pacific than in the northeast Pacific, perhaps due to an intense, shallow, and extensive (0°–40°S) oxygen minimum zone. Shore-based monitoring at eleven sites along the western Americas showed a close relation between Cd and upwelling-favorable wind forcing at the coasts of northern California (37.5°N), southern Baja California (23.3°N), and central Chile (36.5°S). Following the 1997–98 El Niño event nearshore Cd and nutrient enrichments at 37.5°N, 23.3°N, and 33.5°S were about 50% lower than usual, while at 23.4°S and 36.5°S salinity anomalies were >1.5 psu. Such large El Niño-related shifts in nearshore upwelling tracers should be large enough to be distinguished in paleo-environmental proxy records.; Modern and Holocene aragonitic mollusc shells (Protothaca staminea ) from a northern California coastal upwelling region were investigated as potential archives of paleo-upwelling/paleo-El Niño records. Mg/Ca and Sr/Ca ratios in a modern shell were highly correlated to water temperature, however possible ontogenetic trends and year-to-year differences in trace element incorporation suggest a population average should be used to define trace element-temperature relationships. The upwelling system at the tip of southern Baja California appears very promising for paleo-upwelling/paleo-El Niño reconstructions, since upwelling tracers there were not affected by nearshore or surface processes.