Seasonal and Long-Term Variations of Particle Fluxes Induced by Shifting ITCZ Location in the Northeast Pacific

In order to understand seasonal and long-term natural variation, particle fluxes were measured at a depth of 4,960m (10°30'N, 131°20'W) using a time-series sediment trap from July 2003 to June 2007 in the northeast Pacific. During the periods, sea surface temperature anomaly had been less than 1.5°C in the "NINO 3.4" region, indicative of no effect of strong ENSO events. Thus, particle fluxes for the study periods represent seasonal and long-term variations under the normal environmental conditions. Total mass fluxes varied from 3.48 to 36.87 mg/m²/day over the study periods with a distinct seasonal variation that agrees with that of chlorophyll-a at the surface layer: high in December-May and low in June-November. Biogenic components of the trapped particles consisted of 30 to 60% of calcium carbonate, 10 to 30% of biogenic silica, and 5 to 18% of organic carbon. Lithogenic component accounted for 10 to 30% of the total fluxes. All the components showed in-phase seasonal variations with that of total flux. However, contribution of each component varied depending on seasons. For instances, calcium carbonate contents were 2 times higher in December-May while organic carbon contents were higher in June-November. The high particle fluxes in December-May are attributed to the strong NE trade wind caused by southward shift of the Intertropical Convergence Zone (ITCZ). It resulted in deepening of surface mixed layer and weakening of thermocline, which induced more effective supply of nutrient-rich deep water to the surface. Total mass fluxes of July 2003-June 2004 (23.33 mg/m²/day in average) were two times greater than those of July 2006-June 2007 (10.09 mg/m²/day in average). Natural variability of annual fluxes during the normal environmental conditions exceeded the extents caused by the ENSO events reported in the previous studies.